HEM System

Helicopter Electromagnetic Data Processing, Analysis and Presentation

System for Oasis montaj v6.2

TUTORIAL and USER GUIDE

www.geosoft.com
The software described in this manual is furnished under license and may
only be used or copied in accordance with the terms of the license.

Manual release date: February 10, 2005.

Written by, Nancy Whitehead with sincere thanks to Tanya Elliott and Nick
Valleau. Please send comments or questions to info@geosoft.com

Program Copyright © Geosoft Inc. 2005. All rights reserved.

Geosoft and Oasis montaj are registered trademarks of Geosoft Inc.

GEOSOFT, Oasis are trademarks of Geosoft Inc.

Windows®, and Windows NT™ are either registered trademarks or

trademarks of Microsoft Corporation.

Geosoft Incorporated
8th Floor
85 Richmond St. W.
Toronto, Ontario
M5H 2C9
Canada
Tel: (416) 369-0111
Fax: (416) 369-9599

Web Site: www.geosoft.com

E-mail: info@geosoft.com
Contents

Geosoft License Agreement 1

Finding More Help Information 3

Contacting Technical Support 3

HEM System Overview 4

Tutorial 1: Working with the HEM System 5

Before you begin 5

Creating a project 5

Loading the HEM Menu 6

Creating a New Database 7

Importing Data into a Database 8

Spreadsheet View 10

Tutorial 2: Set Up Your HEM Project 11

Setting HEM Parameters 11

Saving or Loading HEM Parameters 12

Generating Resistivity and Depth Nomograms 13

Generating Conductance and Depth Nomograms 15

Keeping track of your data in the HEM System 17

Keeping Track of Channel Names 17
Keeping Track of Single and Multiple Lines 18

Tutorial 3: Pre-Processing Data 19

Filtering Data 19

Displaying Filtered channels 21

Tutorial 4: Applying Drift Corrections 22

 Creating Zero Level Tables 22

Apply Drift Correction 25

Displaying Levelled channels 26

Creating Subset Database 27

Splitting subset data into lines 28

Create a Line Path Plot 29

Tutorial 5: Calculating Apparent Resistivity 32

Resistivity Lookup Table Method 32

Resistivity Inversion Method 33

Gridding Resistivity Data Results 34

Tutorial 6: Picking Anomalies 36

Picking Anomalies 36

Re-Labeling Anomalies 38

Apparent Conductance Calculation 38

Export Anomalies to ASCII File 40

Tutorial 7: Plotting Results 42

Classify Anomalies 42

Anomaly Symbol Plot 43

Understanding Profile Plotting 47

Profile Naming Conventions 47

Suggestions for Optimizing Plotting 47

Single Profiles 48

Multi-channel Profiles 50

Tutorial 8: Archiving Data 52

Appendix 54
 1

Geosoft License Agreement

GEOSOFT agrees to supply the Licensed Program(s) as specified in my purchase order. Geosoft shall grant me a non-
transferable, non-exclusive license to use the Licensed Program(s), subject to the Terms and Conditions herein contained.
Should there be a separate signed agreement between you and Geosoft, or between your company and Geosoft, pertaining to
the licensed use of this software, that agreement shall take precedence over the terms of this agreement.

DEFINITIONS:
In this Agreement:
"Licensed Program(s)" means the actual copy of all or any portion of Geosoft’s proprietary software technology, computer
software code, components, dynamic link libraries (DLLs) licensed through the Geosoft license server, including any
modifications, improvements or updates provided by GEOSOFT.
“Effective Date” is the date the Geosoft license is installed. This date is recorded by the Geosoft License server when the
Licensed Program(s) is installed.
"Services" means the Services described on Section 4.
"Termination" means the occurrences contemplated by Section 6 and 7.

LICENSE:
GEOSOFT grants to me a non-transferable and non-exclusive license to use the Licensed Program(s) for my own purposes
whereby the Licensed Program(s) are being used only by myself, on one computer, at any one time.
Title and all intellectual property rights in and to the License Program(s), including, without limitation, copyright, trade secrets
and trade marks, shall remain with GEOSOFT. I agree to refrain from raising any objection or challenge to such intellectual
property rights, or from assisting or causing or permitting other(s) to do so, during the term of the Agreement and thereafter
I may not assign this Agreement or any part thereof or sub-license the rights granted herein, or lend, rent, time-share, sell or
lease the software without the prior written consent of GEOSOFT.
I may not attempt to reverse engineer, de-compile or disassemble the software.
I may not make any attempt to circumvent the License Manager that controls the access to the software use.

TERM:
The Term of this Agreement shall commence on the Effective Date and shall continue until termination, as described in Section
6.

SERVICES:
(i) According to the terms of my initial purchase, GEOSOFT shall make available to me, without additional fees such
corrections and improvements to the Licensed Program(s) as may be generally incorporated into the Licensed Program(s) by
GEOSOFT. (Normally this will be for a period of twelve (12) months).
(ii) GEOSOFT has a strong commitment to customer service and product support. GEOSOFT offers me, subject to applicable
Service Charge(s), continuing support in the form of email or telephone advice and other assistance in problem diagnosis and
the correction of errors or faults in the Licensed Program(s) during the life of this License. When a problem occurs which
appears to be related to errors or faults in the Licensed Program(s), I may contact GEOSOFT and GEOSOFT will make an
honest effort to solve the problem. However, GEOSOFT cannot guarantee service results or represent or warrant that all errors
or program defects will be corrected. Also it is to be noted that each Licensed Program is designed to operate on a Windows
NT (sp 6 or later), Windows 2000 or Windows XP platform.
(iii) Further, if I request service relating to the modification of the Licensed Program(s) to meet a particular need or to conform
with a particular operating environment, GEOSOFT may, at its discretion, modify the Licensed Program(s) to meet these
particular needs, subject to applicable Services Charge(s). However, all intellectual property or other rights which may arise
from such modifications shall reside with GEOSOFT.

PROTECTION AND SECURITY OF LICENSED PROGRAM

I agree that all additions, modifications, revisions, updates and extensions to the Licensed Program(s) shall be subject to all of
the terms and conditions in this agreement.
I acknowledge that all copies of the Licensed Program(s), provided by GEOSOFT or made by me pursuant to this Agreement,
including, without limitation, translations, compilations, partial copies, modifications, derivative materials and/or updated
materials, are proprietary, and the property of GEOSOFT, and may not be distributed to any other persons, without
GEOSOFT’s prior written consent.
I will not provide or otherwise make the Licensed Program(s) available to anyone in any form without GEOSOFT's prior written
consent.
TERMINATION:
This agreement shall terminate upon the termination date, if any, specified in your purchase agreement with Geosoft.
This agreement may be terminated only upon thirty-days prior written notice to GEOSOFT.
GEOSOFT may terminate this Agreement upon prior written notice effective immediately if I fail to comply with any of the terms
and conditions of this Agreement.
This Agreement shall terminate automatically upon the institution, or consenting to the institution of proceedings in insolvency
or bankruptcy, or upon a trustee in bankruptcy or receiver being appointed for me/us for all or a substantial portion of my/our
assets.

EVENTS UPON TERMINATION:

I shall forthwith discontinue use of the Licensed Program(s), on the day Termination shall occur and agree not to resume such
use in the future without written authorization from GEOSOFT.
I shall uninstall and remove all software from my computer. Within thirty days after Termination, I shall destroy all physical and
digital copies of the Licensed Program(s). This obligation relates, without limitation, to all copies in any form, including
translations, compilations, derivatives and updated materials, whether partial or complete, and whether or not modified or
merged into other materials as authorized herein.

WARRANTY:
GEOSOFT does not warrant that the functions contained in the Licensed Program will meet my requirements or will operate in
the combinations which may be selected for use by me, or that the operation of the Licensed Program will be uninterrupted or
error free or that all program defects will be corrected.
Each Licensed Program shall be furnished to me in accordance with the terms of this Agreement. No warranties, either
express or implied, are made to me regarding the Licensed Program.
THE FOREGOING WARRANTIES ARE IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING,
BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OR MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.

LIMITATION OF REMEDIES
I agree to accept responsibility for the use of the programs to achieve my intended results, and for the results obtained from
use of said Program(s). I therefore accept complete responsibility for any decision made based on my use of the
aforementioned Licensed Program(s).
In no event shall GEOSOFT be liable for any damages arising from performance or non-performance of the Licensed
Program(s), or for any lost profits, lost savings or other consequential damages, even if GEOSOFT has been advised of the
possibility of such damages, or for any claim against me by any other party.

GENERAL:
I agree that this Agreement is a complete and exclusive statement of the agreement with GEOSOFT.
This Agreement supersedes all previous Agreements with respect to the Licensed Programs, with the exception of a current
signed Technical Service Agreements.
GEOSOFT is not responsible for failure to fulfill its obligations under the Agreement due to causes beyond its control.
Should any part of This Agreement for any reason be declared invalid, such declaration shall not affect the remaining portion
which shall remain in full force and effect as if this Agreement had been executed without the invalid portion thereof.
The relationship between the parties is that of independent contractors. Nothing contained in this Agreement shall be deemed
to constitute or create a partnership, association, joint venture or agency.
The provision of this Agreement shall be binding upon me and GEOSOFT and my respective successors and permitted
assigns.
This Agreement will be governed by the laws of the Province of Ontario and applicable laws of Canada.

YEAR 2000:
The Licensed Programs have been tested to conform to DISC PD2000 1:1998 Year 2000 Conformity Requirements
(www.bsi.org.uk/disc/year2000/2000.html), with the exception of clause 3.3.2, paragraph b. Section 3.3.2 paragraph b) requires
that inferences for two-digit year dates greater than or equal to 50 imply 19xx, and those with a value equal to or less than 50
imply 20xx. The Licensed Programs will recognize all two digit years as 19xx. This is to prevent errors importing historical data
that pre-dates 1950. All dates that follow 1999 must use four digit dates in the Licensed Programs.
 3

Finding More Help Information

There are several other functions included in the Oasis montaj help system that may
be useful to your work. The entire documentation for the system is available through
the online help system. This electronic library of information enables us to constantly
update the information and provide you with the most up-to-date information
available.

The best way to find information in this system is to use the Search tab to perform a
full-text search of all help topics.

Contacting Technical Support

The list below provides contact information for Geosoft Technical Support around the
world.

North America Europe and North Africa

Geosoft Inc., Geosoft Europe Ltd.
85 Richmond St. W., 8th Floor 20/21 Market Place, First Floor
Toronto, Ont., Wallingford, Oxfordshire
Canada United Kingdom
M5H 2C9 OX10 OAD
Tel +1 (416) 369-0111 Tel: +44 1491 835 231
Fax +1 (416) 369-9599 Fax: +44 1491 835 281
Email: tech@geosoft.com Email: tech.eu@geosoft.com
South America Australia and Southeast Asia
Geosoft Latinoamerica Ltda. Geosoft Australia Pty. Ltd
Praça Floriano 51 / 19º Andar 350 Hay Street
CEP: 20031-050, Centro Subiaco, WA
Rio de Janeiro, RJ, Brasil Australia, 6008
Tel: (55-21) 2532-0140 Tel +61 (8) 9382 1900
Fax: (55-21) 2532-7197 Fax +61 (8) 9382 1911
Email: tech.sa@geosoft.com Email: tech.au@geosoft.com
South and Central Africa
Geosoft Africa Ltd.
Buren Building, Second Floor
Kasteelpark Office Park
c/o Nossob & Jochemus Streets
Erasmuskloof X3, Pretoria
Tel: +27 12 347 4519
Fax: +27 12 347 6936
Email: tech.za@geosoft.com
4 HEM System Overview

HEM System Overview

Geosoft’s Helicopter Electromagnetic (HEM) system is designed to assist in the
processing, analyzing and presenting of Inphase and Quadrature electromagnetic data
acquired by helicopter surveys.

In this manual, we will guide you through the key steps necessary to install and begin
working with the HEM system. We will also walk you through the necessary
procedures you will need to know to process, analysis and present your HEM data.

The procedures in this manual will show you how to:

• Tutorial 1: Working with the HEM System (page 5)
• Tutorial 2: Set Up Your HEM Project (page 11)
• Tutorial 3: Pre-Processing Data (page 19)
• Tutorial 4: Applying Drift Corrections (page 22)
• Tutorial 5: Calculating Apparent Resistivity (page 32)
• Tutorial 6: Picking Anomalies (page 36)
• Tutorial 7: Plotting Results (page 42)
• Tutorial 8: Archiving Data (page 52)
This tutorial will walk you through all of these processing steps using an example
dataset (hemdemo.xyz) found in the “Oasis montaj/Data/hem” directory.

For more information on working with HEM data we have included HEM Data
Processing – A Practical Overview by Nick Valleau in the Appendix, page 54. This
paper is available on the Geosoft web site at:
www.geosoft.com/resources/papers/index.asp
 Tutorial 1: Working with the HEM System 5

Tutorial 1: Working with the HEM System

In this section describes how to begin working with the HEM system in Oasis
montaj. The topics discussed in this section include:
• Creating a project
• Loading the HEM menu
• Importing data

Before you begin

This tutorial uses sample data provided on the Oasis montaj CD. This data
(hemdemo.xyz) is located in the “Oasis montaj|Data|hem” directory. Before you
begin the tutorial, copy the data file to a working directory such as D:\Tutorial.

Creating a project
In order to access the QC QA menus in Oasis montaj, you must have an open
Project. An Oasis montaj "Project" encompasses every item in your working project;
from the data files in your project (databases, maps, and grids), to the tools used
(including auxiliary tools such as histograms, scatter plots etc.), to the project setup
including the menus you have displayed and whether you are working on a map or
profile and the state in which you left it the last time you used it.

The project also controls your working directory. Projects are saved as (*.gpf) files. If
you open an existing project from a directory, the system assumes that all your
project files are located in the same directory. To streamline your work, as well as
keep it organized, you may wish to make sure that your project file is in the same
directory as the other files you want to use. We recommend that each project you
work on have its own project (*.gpf) file. If you use a number of applications or add-
on tools in Oasis montaj that have different menus, you can use the project to display
only the menus you require.

The Project Explorer tool enables you to browse as well as open any project item.
The Project Explorer has two tab windows, the Data window that includes all data
files included in the project and the Tools window that organizes and maintains the
project tools. To access the Tools window click the Tools bar on the bottom of the
Project Explorer and click the Data bar on the top to return to the Data window.
6 Tutorial 1: Working with the HEM System

Important Note: Workspace files (*.gws) used in Oasis montaj prior to version 6.0 can
be easily converted to Project files (*.gpf) simply by opening them in
Oasis montaj 6.0. On the Open Project dialog (File|Project|Open)
select File of Type as "Workspaces (*.gws)" and when asked if you
want to convert the old workspace into a new Oasis montaj project
file, select "Yes". The workspace file will be converted to a project file
and all associated workspace information will be transferred to the
new project file. In addition, the workspace file will remain untouched
so that it can be opened in previous versions.

T O C REATE A P ROJECT :

1. Start Oasis montaj.

2. On the File menu, select Project and then select New. The New Project dialog is
displayed.
Note: Oasis montaj assumes that your data is in the directory containing this project
(i.e. D:\Tutorial).
3. Specify a name and directory for the project. For example, name the project
(HEMDemo) and specify the working directory as D:\Tutorial.
4. Click the [Save] button. The system saves the project and indicates it is open by
adding menus to the Menu bar, adding buttons to the Standard Short-cut bar and
by displaying the Project Explorer window. These are visual clues indicating that
you are ready to start working with the system.

Loading the HEM Menu

Before you can start working with the HEM system, you must load the HEM menu
file (HEM.omn) to your Oasis montaj menu bar. If you require more detailed
information on modifying menus, refer to the Oasis montaj Online Help System
(Help|Help Topics).
 Tutorial 1: Working with the HEM System 7

T O L OAD THE HEM MENU

1. On the GX menu, select Load Menu or click the load Menu icon ( ) on the
main toolbar. The Load Menu dialog will be displayed.
2. Select the (HEM.omn) from the list of files and click the [Open] button. The
system adds the HEM menu on your menu bar.
3. After completing these steps, you are now ready to start using the HEM system. If
you require more details about Oasis montaj capabilities, please refer to the
Oasis montaj Quick Start Tutorials, which can be found on the Help|Manuals
and Tutorials menu.

Creating a New Database

One of the fundamental technologies in Oasis montaj is its unique database
architecture. This architecture is designed to let you rapidly create and import data of
many kinds (ASCII and binary) into high-performance Oasis montaj databases. After
data is imported into a database, you have numerous options for editing, visualizing,
processing and performing other tasks.

An additional feature of Oasis montaj is the ability to add compression to your

database. You can choose either to compress for speed, compress for size or no
compression at all. Which type of compression you use is entirely up to you. It all
depends on which type better suites your needs. For example, you would more than
likely compress for speed if you have a lot of hard drive space available. However, if
space is limited, you may wish to compress for size.
8 Tutorial 1: Working with the HEM System

T O C REATE A N EW D ATABASE

1. On the Data menu, select New database. The Create New Database dialog is
displayed.

2. Specify a New database name (HEM.gdb). Specify the Maximum lines/groups

and Maximum channels/fields. The defaults are 200 and 50, which is fine for the
purposes of this tutorial, but generally; we recommend that you specify a number
that is representative of the final estimated project size. This strategy ensures that
you have enough space available in your project while not consuming excessive
storage space. The size can be changed later via the Data|Maintenance menu,
should your project expand in scope.
3. Specify Compress for SPEED in the Compression box and click the [OK]
button.
4. The system creates a new database with the database name that you specified and
opens a spreadsheet window with empty channel header cells and data cells.

Importing Data into a Database

There are a number of options for importing Geosoft and third-party formatted data.
In this tutorial, you will import and work with standard Geosoft XYZ files, default
templates and import modes. For more information about how templates and import
modes work refer to the Help system.

T O I MPORT D ATA INTO D ATABASE

1. On the Data menu, select Import and then click Geosoft XYZ. The Import XYZ
data dialog is displayed.
 Tutorial 1: Working with the HEM System 9

2. Using the [Browse] button locate the XYZ data file (hemdemo.xyz) from your
working directory.
3. In the Import template box, specify (hemdemo) and then click the [Template]
button. The Import .\hemdemo.XYZ dialog will be displayed.

4. To edit the settings for each channel, select (highlight) the channel of interest
(TIME) and make your changes in the Source Data and Output Channel boxes to
the right.
5. In the Source Data box, select a Format for your time as (HH.MM.SS.SS) and
then in the Output Channel box, specify a Name for the channel as (TIME). For
more information on the parameters, click the [Help] button.
10 Tutorial 1: Working with the HEM System

6. When you have completed modifying your data, click the [OK] button to return
to the Import XYZ data dialog box. Click the [OK] button to import the data and
display it in your spreadsheet window.

Note: An asterisk * character (or blank for text strings) is used to indicate that the
point has no valid data and is a dummy place holder. A double asterisk **
indicates that the data are too wide for the spreadsheet column. To change the
width of a column, place the cursor on the dividing line between the column
headers. The mouse becomes a double arrow (see image above). Click on the
left mouse button and drag the line to the right to increase the column width.
Release the mouse button when done.

Spreadsheet View
When you create or open a database, you see a spreadsheet view. The Spreadsheet
view is your “window” to the Oasis montaj database and it also provides you with
flexibility in setting up your working environment. All data is stored securely in the
underlying database — you simply decide which data you want to display in the
spreadsheet and keep all other data in the background, hidden from view.

The spreadsheet is organized in rows, channels (columns) and lines. Rows and
columns work similar to standard spreadsheets in that you can edit and delete them as
needed. For more information on Spreadsheet views, see the online help (Help|Help
Topics).

The data that you have just imported has already undergone some pre-processing.
This includes lag corrections, merging of location and altimeter data and removal of
the calibration portion of flights.
 Tutorial 2: Set Up Your HEM Project 11

Tutorial 2: Set Up Your HEM Project

Before you can start processing your data, you must configure your HEM project so
that the system will work with the dataset. There are three main steps in setting up a
project:
• Set Parameters
• Generate Resistivity and Depth Nomograms
• Generate Conductance and Depth Nomograms
An additional menu item, Load Parameters that enables you to select previously
saved parameters.

The (hemdemo.xyz) data file contains data (6.2 m coil separation) for the following:
• 985 Hz, vertical coaxial inphase and quadrature (cx985i, cx985q)
• 7001 Hz, vertical coaxial inphase and quadrature (cx7000i, cx7000q)
• 385 Hz, horizontal coplanar inphase and quadrature (cp385i, cp385q)
• 6606 Hz, horizontal coplanar inphase and quadrature (cp6600i, cp6600q)
• 34135 Hz, horizontal coplanar inphase and quadrature (cp34ki, cp34kq)

Setting HEM Parameters

The HEM parameters that you must define include the number of frequencies,
channel names, frequency values, and coil separations and orientations. Note that the
HEM system can accept up to 10 frequencies. Coil orientation can be horizontal
coplanar, coaxial, or vertical coplanar.

T O S ET P ARAMETERS

1. On the HEM menu, select Project setup and then select Set parameters. The
Number of Frequencies dialog is displayed.

2. Specify the No. of frequencies as (5), as the hemdemo dataset has five
frequencies. Click the [OK] button. The Frequency #1 dialog is displayed.
12 Tutorial 2: Set Up Your HEM Project

3. Using the dropdown lists, select the In phase channel as (cx985i) and the
Quadrature channel as (cx985q). Specify the Frequency (Hz) as (985), the Coil
separation (meters) as (6.2), and Coil orientation as (Coaxial).
4. Click the [OK] button. The system accepts the parameters for Frequency #1 and
displays the dialog for the next frequency (Frequency #2).
5. Specify the Frequency parameters for all five frequencies, as shown in the table
below:
In Phase channel Quadrature channel Hz Coil Coil Orientation
Separation

cx7000i cx7000q 7001 6.2 vertical coaxial

cp385i cp385q 385 6.2 horizontal coplanar

cp6600i cp6600q 6606 6.2 horizontal coplanar

cp34ki cp34kq 34135 6.2 horizontal coplanar

6. Continue this process, clicking the [OK] button until the parameters are set for all
frequencies.

Saving or Loading HEM Parameters

The HEM system is designed to save project parameters in a special initialization

(*.INI) file. Initialization files function as processing templates that enable you to
store filename and processing information, and re-use it as required.

For example, most datasets from the same HEM bird will have the same frequencies
and coil separation. You can use an (*.INI) file for a given HEM system and load this
into the system when required.

T O S AVE P ARAMETERS

1. On the HEM menu, select Project setup and then select Save Parameters. The
Save parameters to an INI file dialog is displayed.
 Tutorial 2: Set Up Your HEM Project 13

2. Specify a Parameter filename (HEMDemo.INI). The system saves your defined

parameters in the (*.INI) file in your project directory.
T O L OAD P REVIOUSLY S AVED P ARAMETERS

1. On the HEM menu, select Project setup and then select Load Parameters. The
Load parameters from an INI file dialog is displayed.

2. Using the [Browse] button, select a Parameter file name that you want to load.
Click the [Load] button. The system loads the parameters.
3. Check that the new parameters are in the system.

Generating Resistivity and Depth Nomograms

For each frequency, a pair of resistivity* and depth** (to the half space) nomograms
are generated when you run the HEM|Project setup...|Resistivity nomograms... menu
item. In total, ten nomograms (grid files) are generated with one command. The
generation of these nomograms is based on a uniform conductive halfspace model.
You can view these nomograms by displaying the corresponding grids on maps.

The resistivity nomogram grids have an X axis of ASinh(in-phase)*, Y axis of

ASinh(quadrature)* and Resistivity values are in Log10. Depth is plotted linearly.

*Asinh scaling is used so that the values both above and below zero can be stored.
Asinh(x) = log(x + sqrt(1+x*x)), which behaves like –log(-x) for x<<0, log(x) for
x>>0, and linearly around x=0.
Note: The returned depths values are the apparent distances from the bird to the top
of the conductive half space. The apparent thickness of the overlying resistive
layer is obtained from the returned depths by subtracting the true bird height.
Remember too, that the bird height must first itself be calculated, usually from
radar altitude.
The generated nomogram grid files use the following name convention.
14 Tutorial 2: Set Up Your HEM Project

• Resistivity nomogram grids use the following naming convention: “R” +

“Frequency” + “1st letter of coil orientation”. For example, R985C.GRD is a
resistivity nomogram grid for 985 Hz, coaxial coil orientation and R385H is a
resistivity nomogram grid for 385 Hz, horizontal coplanar.
• Depth nomogram grids use the same convention except that the 1st letter of the
file name is 'D' instead of 'R'.
T O G ENERATE AND D ISPLAY R ESISTIVITY AND D EPTH N OMOGRAMS

1. On the HEM menu, select Project setup and then Resistivity nomograms. The
system calculates the nomogram grids and displays the Creation of nomogram
grids dialog.

2. This dialog lists the names of the new Resistivity and Depth nomogram grids that
have been created and stored in your project directory. Click the [OK] button to
continue.
3. To view a nomogram, select the Grid|Display Grid|Single grid menu item. The
Place a grid on a map dialog is displayed.

4. Select the Grid name by clicking the [Browse] button and locating the grid file
(R385h.map) in your project directory.
5. Click the [New Map] button and the grid will be displayed on a new map in your
project.
6. You can repeat steps 2 to 4 and select the map (D385h.map) to display the two
maps as shown below.
 Tutorial 2: Set Up Your HEM Project 15

Generating Conductance and Depth Nomograms

For each frequency, a pair of conductance and depth (to the body) nomograms will be
created after running the HEM|Project setup|Conductance nomograms menu option.
This process simulates conductance and depth nomogram grids based on the thin
vertical plate model. The plate is defined to be 1000m in length along the strike and
500m in extent. The calculated nomogram depths are distances below the Tx and
must be adjusted for flight height.

For each frequency, this process creates a database with four channels of data: X, Y,
Conductance and Depth. The X channel represents in-phase and Y channel
quadrature. The data is then gridded to create conductance and depth nomograms.
Note that conductance is stored as Log10 values, and that the in-phase and quadrature
values have been transformed using the formula:

z -> log(z + sqrt(1+z*z))

This is an arc-sinh type transform, which handles negative inputs. To get the true in-
phase and quadrature values in ppm, use the forward transform:

z -> (exp(z) - exp(-z))/2.

You can view these nomograms by loading the corresponding grids into maps. Note
that conductance is plotted in Log 10 units and depth is plotted linearly.

Note that since the field calculations are performed with the receiver coil directly
above the plate, both the horizontal coplanar and vertical coplanar systems are null-
coupled, and give no response (in fact, the vertical co-planar system is always null-
coupled to a semi-infinite plate when flown perpendicular to the plane of the plate).
16 Tutorial 2: Set Up Your HEM Project

A warning message is issued if one or more of the preset coil orientations are not
coplanar, to the effect that only coaxial system nomograms are produced.

The generated nomogram grid files use the following name convention.
• Conductance nomogram grids use the following naming convention: “C” +
“Frequency” + “1st letter of coil orientation”. For example, C500C.GRD is a
conductivity nomogram grid for 500 Hz, coaxial coil orientation.
• Depth nomogram grids use the same convention except that the 1st letter of the
file name is 'H' instead of 'C'.
T O G ENERATE C ONDUCTANCE AND D EPTH N OMOGRAMS

1. On the HEM menu, select Project setup and then select Conductance nomograms.
The Creation of nomogram grids dialog is displayed.

2. This dialog lists the names of the new conductance and depth nomogram grids
that have been created and stored in your project directory. This dialog also
informs you that: Conductance nomograms are created only for coaxial coils;
other systems are null-coupled over plate.
3. Click the [OK] button to continue.
4. To view a nomogram, click the Grid|Display Grid|Single grid menu item. The
Place a grid on a map dialog is displayed.
5. Select the Grid name by clicking the [Browse] button and locating the grid file in
your project directory.
6. Click the [New Map] button and the grid will be displayed on a new map in your
project.
 Tutorial 2: Set Up Your HEM Project 17

Keeping track of your data in the HEM System

The HEM System automates a large number of steps in HEM processing, analysis
and presentation. From experience, we have identified two common sources of
potential errors in the system. If the following situations are not applicable, please
contact Geosoft technical support for further assistance.
• Keeping track of channel names
• Keeping track of single and multiple lines

K EEPING T RACK OF C HANNEL N AMES

It is important to keep track of the channel names you are using as you progress
through the processing cycle. This is especially important during:
• Filtering
• Drift correction – both finding zero-levels and applying corrections.
Results obtained at these stages are used later in anomaly picking. We recommend
using a book-keeping approach at first, until you gain familiarity with the system.
Most problems typically occur when applying corrections since this process requires
reading a table based on previously entered channel values. If you have problems at
this point, you may want to check that channels are not duplicated or missed in this
table.

When you run the filter GX (HEMFILT) you can apply filters to one channel or a
group of channels. The result (filtered data) will be saved into new channels, which as
a default has the original input channel names + “_F”.

When running the drift correction GX (HEMLEVEL) for each input channel to be
levelled this GX creates a temporary channel to contain the level data that is
18 Tutorial 2: Set Up Your HEM Project

interpolated from the data of the related field in the lookup table. It subtracts the data
in the temporary channel from the data in the input channel to be levelled. The result
is then saved in a new channel, which as a default has the input channel name + “_L”.
The temporary channel is automatically deleted.

When picking anomalies the new channels will have a default of the input channel
name + “_A”.

K EEPING T RACK OF S INGLE AND M ULTIPLE L INES

There are several points in the system in which you have the option of selecting the
lines you want to process. These occur at the filtering and single profile parameter
stage. This functionality enables you to select only a portion of your data for initial
processing and parameter selection.

Please note that when you select specific lines for processing, the system changes the
selection status in the database. For example, if you select particular lines for
filtering, say L10 in the HEMDEMO database, you should notice that the system de-
selects all other lines.

If you want to process all data, you must remember to re-select all lines.
 Tutorial 3: Pre-Processing Data 19

Tutorial 3: Pre-Processing Data

Once the project is set up, the next step is usually to examine the data. The Profile view is your
"graphical window" to the Oasis montaj database. You can display profiles of one or more variables
in your database simply by selecting the channel header, right-clicking and from the popup menu
selecting, Show Profile. The profile appears directly below its corresponding database in a profile
window. You can have up to five "panes" with 32 variables in each window.

A key point to note about profile windows is that they are linked dynamically to their corresponding
database. When you select a value or range of values in either the database or profile window
respectively, they are also highlighted in the other window. This capability keeps you in touch with
your data and gives you an interactive means of accomplishing quality control or analysis tasks.

You can display both your In Phase (cp385i) and Quadrature (cp385q) channels in the same profile
panel. Then with your cursor over the Profile window, right click and from the popup menu select
Panel options. The Panel Options dialog will be displayed, check ( ) the Same scale for all profiles
in panel option, and both profiles will be set to the same scale.

Filtering Data

Once the data has been examined the next steps are usually to define best spike removal, and noise
filtering parameters, and then to pre-process the data using a combination of a non-linear filter (to
remove spikes) and low pass filter (to clean background noise). This section assumes that you have
already examined the data to select filter parameters and therefore, only describes the pre-processing
sequence.
20 Tutorial 3: Pre-Processing Data

T O F ILTER D ATA:

1. On the HEM menu, select Filter. The HEM Filters dialog is displayed.

2. Complete the parameters as shown in the dialog above. The following table provides an extended
description of selected parameters and the rational for entering certain values:
Non linear filter. YES to apply non-linear filter and NO not to, use default (YES).
Filter width (data points) Maximum width of the noise in data points. Features that are
wider than this width will not be changed. Use default (3).
Filter tolerance Only noise of greater amplitude than this tolerance will be
changed. Use default (10).
Lowpass filter YES to apply low pass filter and NO not to. Use default (YES).
Cut-off wavelength Short Wavelength cutoff in fiducials. Default is 0.0. Specify (40)
fiducials.
Channels to filter Use default (list of eight comma separated channel names
specified in the project setup). This configuration processes all
the channels in one step.
Note that you can also apply the above filter parameters to one
channel initially and evaluate the results before applying to all
channels.
Lines to filter For the HEMDEMO project, leave this field blank. The system
will process all currently selected lines.
When processing your own data, please note that you can use this
field to select any subset of your data. Simply type a valid line
selection string, or a group of comma separated line selection
strings. This is useful for testing filters on 1 or 2 lines first.

3. Click the [OK] button to filter the data. The system will filter the data and ten new data channels
containing the filtered results will be created. Note that, HEM “filtered” channels use the
following naming convention: “original channel name” + “_F”.
 Tutorial 3: Pre-Processing Data 21

Displaying Filtered channels

You can display your filtered channels with the original data channels, and examine whether the
filtered results are satisfactory.

T O D ISPLAY F ILTERED C HANNELS

1. On the Data menu, select Channels|Display all. All of channels in your database will be
displayed in the spreadsheet window, including the newly created “*_F” channels.

2. Show a profile of cx7000i and cx7000i_F with same scale for flight 55. Set the range of the X-
Axis, 40600 to 41200 and the Y-Axis range as, –130 to –60. The output should look like this:
22 Tutorial 4: Applying Drift Corrections

Tutorial 4: Applying Drift Corrections

Drift (or zero level) corrections, which compensate for DC shift in background values of inphase and
quadrature channels with time, are accomplished in two steps in the HEM system.
• Creation of a zero level tables
• Applying drift correction to data
Once the data has been corrected for drift (or zero level) it is recommended that you create a subset
database, containing the filtered and levelled data. The raw data can be archived for future reference.
Flight lines can then be split into lines and a line path plot can be displayed.

Creating Zero Level Tables

It is assumed that there is a zero signal at high altitudes so several high altitude stretches are flown:
at the start of, during and end of a flight. These sections of a flight are used to create a zero level
table.

T O C REATE Z ERO L EVEL T ABLES

1. Select (highlight) the HEM.gdb. Deselect all the flight lines except for L5601.

2. Then, display the profiles of several (or all) of the inphase and quadrature channels for flight
L56. Examine the profiles and data to determine the background component. This is where there
is zero signal or no ground response. Note that it is often useful to display altimeter data in a
separate profile window to help in selecting the area of highest altitude or least signal.
 Tutorial 4: Applying Drift Corrections 23

3. Highlight a range of data points in the profile window. (It does not matter which channel is
highlighted.) In this example above, data points from fiducial 0 to 2000 are highlighted.
4. On the HEM menu, select Drift correction and then select, Pick zero levels. The Pick zero levels
dialog is displayed.

5. In the Table box, specify a name for the new zero table file as (56hemzero). Then using the
dropdown list, select the Save mode as (Insert). Note that, if there is no file called
(56hemzero.tbl) in your current working directory, both modes will do the same and create a
table file with the first row of data. However, if a file with the name (56hemzero.tbl) exists in
the current directory, the mode “New” will over-write the file and the mode “Insert” will insert
the new data into the existing file and sort the datasets in the file using the TIME field.
24 Tutorial 4: Applying Drift Corrections

6. Using the Time reference channel dropdown list, select (TIME). We will leave the Zero level
channel(s) to the default – a list of the10 unfiltered inphase and quadrature channels.
7. In the Level to set box, specify the default (0.0). Note that, if you leave the value as 0.0, the
system averages the values over the range of selected data. If you use a non-zero value, the
system averages the values over the selected range and then adds your specified value (i.e.
applies a level shift to averaged data). This could be used later to add non-zero level to refine
drift corrections. For more detailed information, click the [Help] button.
8. Click the [OK] button and the HEMZERO dialog is displayed. This dialog indicates that the
(56hemzero.TBL) table file was successfully created. Click the [OK] button to continue.

9. The table file is saved in your project directory. You can open this table file in any text editor
(i.e. Notepad) and view the results. At this point the table file will contain one row of data, as
shown below.
Note: To view the 56hemzero.tbl table file, on the Edit menu, select Edit ASCII file. Using the
[Browse] button, locate the file in your project directory and then, using your default text
editor, open and view this file, as shown below:

10. Now highlight data points from fiducials 21700 to 22200 and repeat Steps 4 to 8. Keeping all of
the settings the same in the Pick zero levels dialog.
11. The table file (56hemzero.tbl) will be updated to now include two rows of data.

12. Highlight data points from fiducials 32000 to 32600 and repeat Steps 4 to 8 and then highlight
data points from fiducials 56100 to 56500 and repeat Steps 4 to 8.
13. The table file (56hemzero.tbl) will be updated and will now have four rows of data (see below).
 Tutorial 4: Applying Drift Corrections 25

14. This process (Steps 1 to 12) should be repeated for each flight line. You should create a new
table for each line and ensure that only that line is selected when picking the levels. Also note,
you should include the flight information in the zero table names (e.g. 56hemzero.tbl) to ensure
the correct table is used on the correct line.
Note: Four zero table files (flt55_zero.tbl, flt56_zero.tbl, flt57_zero.tbl and flt58_zero.tbl) are
provided with this tutorial. You can use these table files, or create new zero table files
following the procedure above.

Apply Drift Correction

The next step in the process is to remove the drift corrected values from the dataset. For consistency
we will use the zero table files (flt55_zero.tbl, flt56_zero.tbl, flt57_zero.tbl and flt58_zero.tbl)
provided with this tutorial. However, you can also use the tables you created in the previous step.

T O A PPLY D RIFT C ORRECTIONS :

1. Open the database (HEM.gdb). Deselect all the flight lines except for L55.
26 Tutorial 4: Applying Drift Corrections

2. On the HEM menu, select Apply Drift correction. The system displays the Apply HEM drift
correction dialog.

3. Using the Table browse button ( ) locate and select the zero table file (flt55_zero.tbl).
4. From the Channel(s) to level dropdown list, note that the default is a list of comma separated
channel names specified in the project setup stage. However, in this case, we want to level the
“filtered” data rather than the original raw data therefore, you MUST edit this list. To edit, type
“_F” at the end of each channel name displayed in the list.
5. For the Field name(s) in the table dropdown list, this is the same list of comma separated
channel names specified in the project setup stage, we will accept the default names as this list
corresponds with the field names in the table file. Then, from the Time reference channel
dropdown list, select (TIME).
6. Click the [OK] button. Ten new channels are added to the database. All of these new channels
will have “_F_L” in their names. Note that, HEM levelled channels use the following naming
convention: “name of channel to be levelled” + “_L”. For example, cp385q_F is the (filtered)
channel to be levelled, and cp385q_F_L is the corresponding levelled channel. These channels
will be used to pick anomalies, calculate resistivity and plot at a later stage of this tutorial.
7. Repeat Steps 1 to 6 for each of the flight lines. Ensure that the correct line is selected with the
correct table file.

Displaying Levelled channels

You can display your levelled channels with the original data channels, and the filtered data
channels.

T O D ISPLAY L EVELED C HANNELS

1. On the Data menu, select Channels|Display all. All of channels in your database will be
displayed in the spreadsheet window, including the newly created “*_L” channels. You can view
the profiles of the original, filtered and levelled data at the same scale to observe the changes.
 Tutorial 4: Applying Drift Corrections 27

Creating Subset Database

After the drift corrections have been completed it is a good idea to subset the data to a new database,
the raw data can then be archived for future reference. From this point onward we will only be
working with the filtered, levelled EM data therefore before we begin you should remove the raw
and filtered channels from the currently displayed spreadsheet (select each channel header, right-
click and from the popup menu, select Remove column). Then, during export to the subset database
we will only export the “displayed” channels.

TO EXPORT DATA TO SUBSET DATABASE

1. Ensure that all flight lines are selected.

2. On the GX menu, select Run GX. The Run a GX dialog is displayed. Using the [Browse] button,
locate the GX file (dbsubset.gx). Click the [OK] button and the Create new subset database
dialog is displayed.
28 Tutorial 4: Applying Drift Corrections

3. Complete the fields as shown above. The following table provides an extended description of
selected parameters and the rational for entering certain values.

Subset database Specify a database name – hemlines.gdb in this case.

Lines Export the Selected lines

Channels It is a good idea to export only the levelled, filtered EM channels as well as
X, Y and auxillary information, since further processing will not use the raw
EM data. In hemdemo.gdb remove the raw data channels from display as
well as the levelled (_L) channels and then choose to export only the
Displayed channels.
Remove mask Continued processing is only relavent for EM data that has a survey location
dummys specified. The database can be windowed to exclude points without a
location by specifying the Mask channel below as X or Y and setting this
parameter to All dummies
Mask channel X

Compression None
Type

4. Click the [OK] button and a new database (hemlines.gdb) will be created and displayed in your
current project.

Splitting subset data into lines

Now that we have a new subset database (hemlines.gdb) that only includes the filtered and levelled
data, we can now split the data into lines for further processing.

TO SPLIT DATA INTO LINES

1. Make sure the database (hemlines.gdb) is selected; this is the database you will work with from
here onwards.
2. On the Utility menu, select Split line and then Split on line channel. The Break up a line based
on a line dialog is displayed.
 Tutorial 4: Applying Drift Corrections 29

3. Using the Line to break dropdown list, select line (L5501). Then, using the Line reference
channel dropdown list, select (line).
4. Click the [OK] button. The line L5501 will be broken into component lines based on line
numbers in the reference channel line.
5. Repeat Steps 1 to 4 for the remaining flight lines (L5601, L5701, and L5801) and save the
database changes.

Create a Line Path Plot

We recommend that at this point you display the lines on a map before further processing of the
data.

T O C REATE A N EW M AP

1. Open and select the database (hemlines.gdb).

2. On the Mapping menu, select New map, and then New map from X,Y. The Data range to map
dialog box will be displayed.
3. Click the [Scan data] button to have the system query the database for the data ranges and report
the Minimum and Maximum X,Y co-ordinates.

4. Click the [Next] button. The Create a New Map dialog will be displayed.
30 Tutorial 4: Applying Drift Corrections

5. Enter a Map name (linepath) and click the [Scale] button. The system automatically determines
a scale that will best fit the data on the default map template. Once the “best-fit” scale has been
determined you can then round the scale to a more useful value (70000).
Note: If no scale is specified, a scale will be chosen which will fit all of the data within the default
map template.
6. To change the default map template, click the [Templates] button. The Map Template Manager
dialog is displayed. From this dialog you can select from a list of predefined map templates, or
you can create a [New] template or [Modify] a predefined one. For more detailed information,
click the [Help] button on the Map Template Manager dialog.
6. Click the [Finish] button and a new blank map (linepath.map) will be displayed.

T O C REATE AL INE P ATH P LOT

1. On the Mapping menu, select Line Path. The Line path plot dialog will be displayed.
 Tutorial 4: Applying Drift Corrections 31

2. You can leave the defaults as they are. Click the [OK] button and the system displays the survey
lines on your map.

3. The line path of your survey area is displayed on the map. If you zoom into the map and examine
the tie-lines, you will note that they are the 49xxx lines, and can be deselected for gridding
purposes.
32 Tutorial 5: Calculating Apparent Resistivity

Tutorial 5: Calculating Apparent Resistivity

In this section, we will use the channels (cp6600i_F_L and cp6600q_F_L) to demonstrate how to
calculate apparent resistively. The procedure described below can be used to process data in any
filtered or levelled EM channels.

Resistivity Lookup Table Method

Use the Lookup Table Method to calculate apparent resistivity or apparent depth values for HEM
data by using a gridded look-up nomogram.
1. On the HEM menu, select Resistivity and then select Grid lookup. The Look up value from a
nomogram dialog is displayed.

2. Complete the fields as shown above. The following table provides an extended description of
selected parameters and the rational for entering certain values.
Look-up type Select the type of value to look up from the dropdow list, resistivity or
depth. In this case (Resistivity).
Inphase channel Select from the dropdow list the inphase channels to use (cp6600i_F_L).

Quadrature channel Select from the dropdow list the quadrature channel to use
(cp6600q_F_L).
Output channel Specify the output channel (this is a new channel to be created). Type
(Res6600).
Nomogram File This should be the resistivity nomogram file for the inphase quadrature
channels created at the project setup stage. The nomogramfile is called
R6600H.GRD where R stands for resistivity, 6600 is the frequency and H
stands for horizontal coplaner coil orientation.
Unit scaling Normal or Log 10. We are using the default setting of (Normal).

Threshold This is a minimum cutoff inphase and quadrature value below which data
 Tutorial 5: Calculating Apparent Resistivity 33

valuesare likely too noisy or erroneous to be reliable. The default setting

for this entry is not to apply the threshold (empty entry). We use a value
of (0) in this example.
3. Click the [OK] button. The Res6600 channel is created and added to the database
(hemline.gdb).
4. To display the Res6600 channel (if it is not displayed), right click on an empty channel header,
select List from the popup menu. From the list, select the Res6600 channel. The channel will be
displayed in the spreadsheet window.
5. Repeat the above process to create an apparent depth channel using the Look-up type as Depth
the Output channel as Depth6600 and the Nomogram file as D6600H. Subtracting the bird
altitude (radar_alt-33) will give the apparent depth to the conductive half space.

Resistivity Inversion Method

The Inversion Method is used to invert resistivity values for HEM data by using the uniform
halfspace model. This is a single-pass inversion to a half-space model for an HEM system. It does
not rely on a look-up table or nomogram file, but calculates forward models as required, sacrificing
speed for flexibility and accuracy. The inversion works by finding the half-space resistivity which
minimizes the error in a least-squares sense between the input and calculated In-phase and
Quadrature values.
T O C ALCULATE R ESISTIVITY USING THE I NVERSION M ETHOD :

1. On the HEM menu, select Resistivity and then select Inversion. The Invert Resistivity Value
dialog is displayed.
34 Tutorial 5: Calculating Apparent Resistivity

2. Complete the fields as shown above. The following table provides an extended description of
selected parameters and the rational for entering certain values.
Height channel (m) This is the height of the HEM bird above ground. Select (radar_alt).

Inphase reference
Select from the dropdow list the inphase channels to use
channel (cp6600i_F_L).

Quadrature
Select from the dropdow list the quadrature channel to use
reference channel (cp6600q_F_L).

Output resistivity
Specify the output channel (this is a new channel to be created). Type
channel (Res6600Inv).

Frequency (Hz)
Frequency that corresponds to the reference channels picked above.
Type (6600)

Coil separation (m)

Coil separation that corresponds to the reference channels picked above.
Type (6.2)

Coil orientation
Coil orientation that corresponds to the reference channels picked
above. Select (Horizontal coplanar)

Fractional error in
When the fractional error in the output resistivity value reaches this
inversion value the inversion stops. Use the default of 1% (0.01)

Threshold for
The forward model solution has errors on the order of 1 ppm. Below
inversion (ppm) this level the output resistivity will not calcuated. Keep this at (1.0)

Invert
You may choose to invert the In-Phase and Quadrature components
alone, or together. Choose (In-phase and Quadrature)

Error Calculation
If one component is close to an order of magnitude larger than the
other, it will dominate the inversion. If this is the case you can choose
Log Scaling for the errors. For this data use the default (Unscaled).

Starting value for

Optional, leave blank for this demo.
Inversion

3. Click the [OK] button. The Res6600Inv channel is created and added to the database
(hemline.gdb).

Gridding Resistivity Data Results

Once apparent resistivity has been calculated they can then be gridded and displayed as images. For
more detailed information on Oasis montaj Gridding see the online help (Help|Help Topics) or the
Oasis montaj Quick Start Tutorials (Help|Manuals and Tutorials).

T O G RID D ATA

1. Open and select the database you wish to process (hemlines.gdb). Using the Line Selection Tool
(right-click on the Fiducial header cell and from the popup menu, select Selections|Selection
Tool) deselect all the tie lines (L49011 to L49091).
 Tutorial 5: Calculating Apparent Resistivity 35

2. On the Grid menu, select Gridding|Minimum curvature |Dialog controls. The Minimum
Curvature Gridding dialog is displayed.
3. Using the Channel to grid dropdown list, select the (Res6600Inv). In the Name of new grid file
box, specify (Res6600Inv).
4. Then, in the Grid cell size box, specify a grid cell size. For this tutorial we will leave specify a
cell size of (20). This should normally be ½ to ¼ the nominal sample interval. If not specified,
the data points are assumed to be evenly distributed and the default cell size will be. ¼* (sqrt
(grid area / #data points)).
5. Click the [Advanced>] button to specify additional parameters and click the [<Back] button to
return to the Minimum Curvature Gridding dialog. Click the [OK] button grid the data.
6. The Minimum Curvature grid (es6600Inv.grd) will be created and displayed in your current
project.
36 Tutorial 6: Picking Anomalies

Tutorial 6: Picking Anomalies

In this section we will demonstrate how to pick anomalies, rename selected anomalies for
classification and plotting, calculate conductance, and then export the anomalies to a file.

Picking Anomalies
Anomalies are picked automatically and placed in new channels in the database. The naming
convention is similar to other HEM processes, where the new channels have the same name as the
starting channels with the addition of “_A” added to the end of the channel name.

T O P ICK A NOMALIES :

1. On the HEM menu, select Anomalies and then select Picking Anomalies. The Anomaly picking
dialog is displayed.

2. The default for the Channels to pick anomaly is a list of channel names given at the Project
Setup stage. However, for this example, we will pick anomalies in channel (cx7000i_F_L and
cx7000q_F_L). To do this, type these two channel names separated by a comma.
3. We will leave the Lines to pick anomaly parameter to the default, which is blank. You can type
any valid line selection strings (comma separated) however, if you leave the entry empty all the
currently selected lines are used. Then, for the Format of results, select (Values) from the
dropdown list.
4. For the Base level specify (0.0), as the minimum value above which highs may be accepted as
anomalies. Then, for the Minimum amplitude, specify (20.0).
Note: For a point in the channel to be picked as an anomaly, its value must be a peak value that is
greater than the ‘Base level’ and the difference between this peak value and the low value of
the adjacent readings at the left and right troughs of the channel profile must be greater than
the ‘Minimum amplitude’.
5. Click the [OK] button and the HEMPICK2 dialog is displayed. This dialog informs you on the
number, in this case 422, anomalies found.
 Tutorial 6: Picking Anomalies 37

6. Click the [OK] button. The system locates multiple anomalies and stores them in two new
anomaly channels (cx7000i_F_L_A and cx7000q_F_L_A). (Note that, the “_A” has been added
to the file names to signify that these new channels contain Anomaly locations for the cx7000i
and cx7000q channels that have in turn been filtered “_F” and levelled “_L”.)
7. Display symbols of the 7000q_F_L_A channel with profiles of the related inphase and
quadrature channels to view the anomalies found. The following example shows the levelled
inphase and quadrature channels in profile format and shows the anomalies (plotted as symbols)
for line L40441.

7. After analyzing the results, you may want to add anomalies to or delete anomalies from the
current list. To do this, edit the anomaly channel (7000q_F_L_A). Experiment by adding an
anomaly to Line L40441, fiducial 18409. Then, delete the last anomaly (fiducial 20381). Note
the symbol is automatically displayed or removed from the profile view.
38 Tutorial 6: Picking Anomalies

Re-Labeling Anomalies
After automatically picking and manually editing anomalies, we need to sort anomalies in each
channel and label, (or re-label) them alphabetically. This label channel can then be used in anomaly
classification and plotting.

T O R E -L ABEL A NOMALIES :

1. On the HEM menu, select Anomalies and then select Re-labelling Anomalies. The Re-labelling
anomalies dialog is displayed.

2. Using the Channel to relabel dropdown list, select (cx7000q_F_L_A) and in the Output channel
name box, specify (cx7000q_Anom).
Note: The Output channel name default (entry left empty) is to use the name given in the Channel
to relabel plus “_C” to form a new name for the anomaly label channel.
3. Click the [OK] button and a dialog is displayed with the number (285) of valid labels found.

4. Click the [OK] button and the system creates an anomaly label channel CXQ1_Anom
containing multiple anomalies. Your results should be similar to the ones shown below.
• Line L40441 –7 anomalies (A,B,C,D,E,F,G)
• Line L40571 — 8 anomalies (A,B,C,D,E,F,G, H)
Anomalies in the channel are numbered alphabetically starting from A in each line.

Apparent Conductance Calculation

The apparent conductance calculation samples a conductance nomogram at specified inphase and
quadrature locations and creates a new channel that contains the conductance data.

Note that, since the field calculations are performed with the receiver coil directly above the plate,
both the horizontal coplanar and vertical coplanar systems are null-coupled, and give no response (in
fact, the vertical co-planar system is always null-coupled to a semi-infinite plate when flown
 Tutorial 6: Picking Anomalies 39

perpendicular to the plane of the plate). A warning message is issued if one or more of the preset
coil orientations are not coplanar, to the effect that only coaxial system nomograms are produced.
Note: The conductance nomograms were generated in Tutorial 2: Setting up your HEM project
page 15.
T O C ALCULATE A PPARENT C ONDUCTANCE :

1. On the HEM menu, select Anomalies and then select Conductance. The Lookup conductance
from a nomogram dialog is displayed.

2. Complete the fields as described in the following table that provides an extended description of
the parameters and the rational for entering certain values.
Inphase reference Select from the dropdow list the inphase reference channel
channel (cx7000i_F_L).
Quadrature reference Select from the dropdow list the quadrature reference channel
channel (cx7000q_F_L).
Anomaly reference Select from the dropdow list the anomaly reference channel
channel (cx7000q_Anom)
Output conductance Specify the output conductance channel (Cond7000).
channel
Nomogram file Browse to find the nomogram file (C7001C.GRD).
Conductance units Select (Normal) from the dropdow list. Note that, we used coaxial inphase
and quadrature channels and a coaxial nomogram to estimate conductance.
This is due to the fact that under the horizontal coplanar coil configuration,
there is no response when the coils are on top of the thin vertical plate,
therefore nomograms were generated.
3. Click the [OK] button. The system creates a conductance channel (Cond7000) that you can
display in the profile window using a symbol format.
4. The above procedure can be repeated to find apparent depth to top of the body. Choose a
different Output channel (Depth7000), the correct nomogram file (H7001C) and use Log10 for
the conductance units.
40 Tutorial 6: Picking Anomalies

Export Anomalies to ASCII File

For reporting purposes, you may want to export a copy of the anomalies and their classifications in
table format.

T O E XPORT A NOMALIES :

1. On the HEM menu, select Anomalies and then select Export anomalies to ASCII file. The Export
XYZ data dialog is displayed.

2. Specify the name of the new XYZ data file (HEManom) and click the [Template] button. The
Export XYZ template dialog is displayed.

3. In the Template Name box, specify (HEManom.o0).

4. To export anomalies only, add or remove channels to configure the template so that it appears as
shown above.
5. De-select the “Include Dummies” check box so that the file is as compact as possible.
 Tutorial 6: Picking Anomalies 41

6. Click the [OK] button. You are returned to the Export XYZ data dialog. Click the [OK] button,
the system creates the anomaly file and saves it, in XYZ format, in your working directory.
42 Tutorial 7: Plotting Results

Tutorial 7: Plotting Results

There are a number of classification and plotting options available for HEM data:
• Classifying anomalies
• Plotting anomaly maps
• Plotting a single profile
• Plotting multiple profiles

Classify Anomalies
The HEM system enables you to classify anomalies found into eleven groups (levels)
based on the values in a channel and range of values for each level.

T O C LASSIFY A NOMALIES :

1. On the HEM menu, select Plot and then select Classify anomalies. The HEM
classification of anomalies dialog is displayed.

2. From the Anomaly channel dropdown list, select (CX7000q_Anom) and from the
Reference channel dropdown list, select (CX7000q_F_L).
3. In the Classification channel box, specify a new channel name as (CLASS).
 Tutorial 7: Plotting Results 43

4. In the Minimum value box, specify the minimum (base level) used in picking
anomalies as (0.0).
5. In the Level 1 maximum value box, specify (50), in the Level 2 box, specify (100),
in the Level 3 box, specify (150), in the Level 4 box, specify (200) and in the
Level 5 box, specify (250).
6. Keep Level 6 to 10 maximum values entries empty. The system will then group
anomalies with Cx7000q_F_L values above 250 into “Level 6”. Therefore, we
will not have any anomalies in Levels 7 to 11.
7. Click the [OK] button. The system will create a CLASS channel.

8. Display the CLASS channel by right clicking on an empty channel header,

selecting List from the popup menu, selecting CLASS from the list, and clicking
the [OK] button.
9. You will see the first anomaly in Line L40441 to have a classification of 5, (based
on the reference channel – cx7000q_F_L in this case). You may want to also
display the Conductance channel and check the corresponding value.

Anomaly Symbol Plot

In this section we will demonstrate how to draw anomaly symbols with postings onto
a base map. We will first run through the basic steps of creating a base map we will
call (Symbolplot.map).

T O C REATE A B ASE M AP :

1. Select (highlight) the current database (hemlines.gdb).

2. On the Mapping menu, select New map, and the New map from X,Y. The Data
range to map dialog box will be displayed. Click the [Scan data] button to have
the system query the database for the data ranges and report the Minimum X,Y and
Maximum X,Y co-ordinates.
3. Click the [Next>] button. The Create a New Map dialog will be displayed. Enter
a Map name (Symbolplot) and click the [Scale] button. The system automatically
determines a scale that will best fit the data on the selected map template.
Note: Once the system determines a scale that will best fit the data based on the
selected map template, you can then round the scale to a more useful value
(70,000).
4. Click the [Finish] button and a new blank map (Symbolplot.map) will be
displayed.
Note: If no scale is specified, a scale will be chosen which will fit all of the data
within the selected map template.
5. On the On the Mapping menu, select Base map, and the Draw base map. The
Base map layout dialog box will be displayed.
44 Tutorial 7: Plotting Results

6. Accept the default parameters, and click the [Next>] button. The Figure style
base map dialog is displayed.
7. Accept the default parameters, and click the [Next>] button. The Figure titles
dialog is displayed. Specify a title for your map (Anomaly Symbol Plot), and
click the [Finish] button to display a base map on the open map sheet.
T O P LOT A NOMALY S YMBOLS :

1. Deselect all line except L40441.

2. Select (highlight) the current map (Symbolplot.map).

3. On the HEM menu, select Plot and the select Anomaly map. The HEM classified
symbol plot dialog is displayed.

4. Complete the fields as shown above. The following table provides an extended
description of selected parameters and the rational for entering certain values.
Classification Specify the classification channel name (CLASS) created at the
channel previous stage.
Symbol font name HEM system comes with a anomaly font file called
(HEMANOM.GFN). This file is the default setting in this entry
which we use.
 Tutorial 7: Plotting Results 45

Symbol size Use the default size of (1.8) cm.

Symbol color Default is black for both outline and fill. We use (black outline &
red fill) to make the symbols more interesting.
Angle This angle is used to rotate symbols (and postings) so that they are
aligned with survey lines. Default is (0.0) which we use.
Symbol and In each of these 11 entries, we need specify a symbol number (the
annotation number in the GFN file) and a text string with a comma in
between. The text is used for annotating the legend. The default
text strings in these 11 entries describe the symbol types used.
5. Click the [OK] button. The HEM symbol plot: post and legend dialog is
displayed.

6. Specify the four channels to post, how to draw the posting text (such as size,
offset to symbols, etc.) and titles for legend as shown above. The following table
provides an extended description of selected parameters and the rational for
entering certain values.
Channel to post at top Default is an empty entry which means no right corner of
right corner of symbol symbol posting at this position. Select (CX7000q_Anom)
because we want to post anomaly number at this corner. Note
that the term “top right corner” refers to the position before the
symbols and postings are rotated.
Channel to post at top Select in (Depth7000).
left corner of symbol
Channel to post at Select in (cx7000q_F_L).
bottom right corner of
symbol
46 Tutorial 7: Plotting Results

Channel to post at Type in (cx7000i_F_L).

bottom left corner of
symbol
Plot legend Specify (Yes).
7. Use the defaults values for the remaining entries on this dialog and click the [OK]
button. The anomaly symbol map is created and displayed in your project.

Note: This example only shows data for a single line since only one line was
selected in the database.

Zoomed view of the Symbol Plot for Anomaly “B”

 Tutorial 7: Plotting Results 47

Symbol Plot Legend detailing which channel is plotted at which corner

Understanding Profile Plotting

The HEM system profile plotting is designed to handle plotting of either single or
multiple profiles.

When you plot a single profile, the procedure is to enter plotting specifications (line,
plotting scale, channel, etc.) via the Single profile dialog box. The system then stores
your specifications in a script (*.gs) file with default filename “_protmp_.gs”. When
the Single profile menu item is run, it first reads the script file for plotting instructions
and then determines how to plot the profile.

When you plot multiple profiles, the procedure requires an additional step, namely,
you must manually create the Geosoft Script file that identifies all profiles to plot,
profile offsets, labels and other parameters. When you are finished, you then select
the Multi-channel profile plotting menu item. This then reads your script file and uses
the Single profile dialog to generate all stacked profiles according to your
specifications.

Profile Naming Conventions

You have the option of plotting all lines or a subset of lines. When you plot lines, the
system creates a series of map (*.map) files that start with “SP_” plus the line name.
For example, if the line number is L100, the profile plot name will be SP_L100.map.

Suggestions for Optimizing Plotting

Profile plotting normally takes some effort to determine the best settings for final
presentation. We suggest that you experiment with the HEM|Plot|Single profile…
menu item to determine the basic parameters that work best for your data.

The first time you work with data, you may want to run the option with the defaults
and no scale factors. The system will then calculate starting scales, which you can
view by accessing the menu option again and looking at the values in the dialog box.
This is a quick way of starting to work.

In addition, you can take advantage of the (_protmp_.gs) file created in the system.
You can use this as a template for creating your own script file for plotting multi-
channel profiles. The basic template contains 10 parameter lines with multiple
parameters on each line. If you want to plot multiple profiles, copy the parameter
48 Tutorial 7: Plotting Results

lines from the previous profile and change the vertical offset, scales and other
parameters as required.

An efficient way to learn about parameters is to examine the script file generated the
first time you run the Single profile dialog box. You may want to copy this file and
then experiment with the copy to determine the effect of varying different parameters.

For additional information, click the [Help] button on the Single profiles parameters
dialog. Parameter information, application notes, and links to other topics, including a
link to an example of a GS Script File are available in the help.

Single Profiles
When plotting single profiles, you must provide certain basic parameters (horizontal
axis reference and channel to plot). Note that, you may want to use your Y channel as
the horizontal axis reference channel for testing before plotting all lines, for each
north-south survey line. When starting to plot profiles for a project, you may also
want to select a specific line.

T O P LOT S INGLE P ROFILES :

1. On the HEM menu, select Plot and then select Single profile. The Single profile
parameters dialog is displayed.
 Tutorial 7: Plotting Results 49

2. Set parameters as shown above. Note that, initially we will leave the Horizontal
and Vertical map scales blank.
3. Click the [OK] button. The system creates the script file (_protmp_.gs) in your
project directory and creates and displays the new map (sp_l40441.map) with the
profile as shown below. Note that, if you select multiple lines, the system creates
multiple maps but only displays the first map in the sequence. This
implementation is designed to save both memory and plotting time especially in
the case where you have many lines to plot. To see the other maps, you must open
them individually (Map|Open map).

4. The script file (_protmp_.gs) created when you ran the Single profile menu
option, should look as below.
50 Tutorial 7: Plotting Results

Multi-channel Profiles
For the purposes of this tutorial, we have provided a starting script file called
(HEMDEMOnew.gs). This script file plots the inphase and quadrature profiles for
all five frequencies, the resistivity for frequency 6600 and the altimeter data for this
dataset.

T O P LOT S TACKED P ROFILES :

1. On the HEM menu, select Plot, then select Multi-channel profiles. The Multi-
channel profile parameters dialog is displayed.

2. Complete the fields as shown, remembering to change the Prefix of map names
field, at the bottom of the dialog, to “Demo”. This prevents the system from
overwriting any previously generated profile maps.
3. Click the [OK] button. The stacked profile map (Demo40441.MAP), as shown
below, will be displayed in your project,.
Tutorial 7: Plotting Results 51
52 Tutorial 8: Archiving Data

Tutorial 8: Archiving Data

For your convenience, the HEM system includes an Archive option that enables you
to export your data to Geosoft XYZ format file (includes line header followed by one
or more columns of data).

T O A RCHIVE D ATA:
1. On the HEM menu, select Archive. The Archive XYZ data dialog is displayed.

8. In the Archive data file box, specify the name of the new archive file
(HEMD_ARC), in XYZ format. The Reference channel (X) provides the system
with a means of determining how to locate the start of channels and determine
their fiducial spacing.
9. Click the [Template] button. The Export XYZ template dialog is displayed.
 Tutorial 8: Archiving Data 53

10. Specify the template name (HEMD_ARC.o0) and then select the parameters as
shown above (Note that, when archiving in the future you can include any
additional channels (anomaly classifications, conductance, etc.) that you may
require).
11. Click the [OK] button. You are returned to the Archive XYZ data dialog. Click the
[OK] button. The system creates an ASCII file and stores it in your project
directory. This file contains the header information and column data. All date,
flight, line and time information will be in columns so that the data resembles a
flat ASCII file. In addition, dummies (placeholders) will be removed so that the
file is as compact as possible, as shown below.
54 Appendix

Appendix

HEM Data Processing – A Practical Overview

by Nick Valleau
Exploration Geophysics (2000) 31, 584-594

HEM data processing - a practical overview

Nicholas C. Valleau1
Key Words: HEM, AEM, EM, electromagnetic, airborne, data processing, frequency-domain, resistivity, conductance

Paper presented at the 14th ASEG Conference & Exhibition, Perth, March 2000.

ABSTRACT INTRODUCTION

The theory of frequency-domain helicopter electromagnetic There are a growing number of geophysicists working with
(HEM) surveys is relatively straightforward, but in practice there frequency domain helicopter electromagnetic (HEM) data, in both
are many issues to deal with, both in hardware development and in survey contracting and exploration organisations. More
the software to handle the data processing and analysis (DPA). commercial HEM systems are also available worldwide than in the
past. A practical overview of the data processing and analysis
This paper gives an overview of standard HEM data processing (DPA) procedures for HEM surveys is not available in the
and analysis techniques. This is not new technology, and is literature, to the author's knowledge.
intended only to fill a perceived gap in the literature by
summarising existing basic DPA procedures for typical HEM This paper provides a brief, informal review of basic data
datasets. processing techniques specific to HEM data. This is not a
comprehensive review; theoretical information, detailed
Assuming that the inphase (IP) and quadrature (Q) data for each algorithms, and advanced procedures such as modeling and
frequency has been calibrated to parts per million (ppm) during interpretation are not included. However, it does provide enough
data acquisition, the main procedures are as follows. (when combined with adequate software tools) to enable a
geophysicist to carry out the routine HEM data processing of any
A. DATA PROCESSING dataset.

• Apply standard geophysical pre-processing techniques such The procedures described assume an understanding of
as parallax (lag) corrections. frequency-domain EM theory and knowledge of airborne
• Remove non-geological noise from the raw data using geophysical data processing. The most common HEM coil
appropriate filters. configurations in regular use are horizontal coplanar and (vertical)
coaxial. Several frequencies are recorded, each as inphase (IP) and
• Apply drift corrections (zero level corrections). Remove quadrature (Q) components of the secondary electromagnetic field
system and instrumentation drift from all EM channels. at the receiver coil, in parts per million (ppm) of the primary field
• Archive data and analysis results along with complete at the receiver.
documentation of all archived parameters and necessary
metadata. Practical Examples

B. DATA ANALYSIS For each data processing task described below, an example is
given. Four flights (45 lines) of raw data from a small HEM
• Calculate apparent resistivities and depths. survey in Australia's Northern Territory are used for these
• Pick anomalies for conductive target body location and examples. The survey was flown for Ashton Mining (WA) Pty.
analysis. Requires an EM interpreter, but automatic picking Ltd. by Geo Instruments Pty. Ltd., using a Geotech-built
provides a first pass. Hummingbird system. The system has five frequencies of
operation with a transmitter-receiver coil separation of 6.2 m and
• Analyse anomaly targets. Calculate apparent conductances nominal bird altitude of approximately 35 m. (Bettina Townrow,
and depths to conductors. 1999, personal communication):
• Presentation of results; various combinations of:
Horizontal Coplanar Configuration:385 Hz, 6606Hz, 34135 Hz
❏ Standard geophysical maps - survey lines etc.; Vertical Coaxial Configuration: 985 Hz, 7001Hz
❏ Apparent resistivity and depth maps;
Software used for processing the data consists of standard and
❏ Profile maps of IP, Q, resistivity, anomalies;
specialised tools for HEM data, built in Geosoft Inc.'s Oasis
❏ Classified anomaly symbol maps with annotations montaj data processing environment.
(e.g. Identification letters, apparent depths, etc.);
❏ Detailed multichannel profile plots for each survey line. OVERVIEW
❏ Conductivity depth sections. The primary steps in the DPA sequence for HEM data are
❏ Anomaly report. discussed in the following sections. It is best to begin processing
HEM data on a flight-by-flight basis, splitting flights into
individual survey lines only after noise and drift corrections have
1
Geosoft Australia Pty. Ltd. been applied to the data. Prior to carrying out the HEM-specific
32 Richardson Street,
West Perth, WA 6005, Australia
data processing described in this paper, some calibration and
Phone: 61 (8) 9322 8122 routine pre-processing work are required.
Fax: 61(8) 9322 8133
Email: nick.valleau@geosoft.com.au

584 Exploration Geophysics (2000) Vol 31, No. 4

Valleau HEM data processing - a practical overview

Calibration before detailed processing can begin. Before proceeding with

noise removal, I removed the ferry portion of the flight (en route to
Normally the survey contractor converts the inphase (IP) and survey area) and windowed the data to a reasonable dynamic range
quadrature (Q) EM data from millivolts to parts per million (ppm) to enable visualisation.
during data acquisition. The calibration factors are usually
determined empirically on the ground using a "known" source NOISE
(coil). This process contains potential sources of error, and it is
possible that calibrations drift with time. Inaccurate calibration is Noise is often a significant issue in HEM data, where signal to
a potential source of quality problems in a HEM dataset. noise ratios can be quite low, especially in resistive terrain.
Fortunately, small calibration errors should have little impact on Common noise sources are electronics, vibration, or movement of
finding anomalies and mapping geology for mineral exploration. or within the EM bird. Fortunately, most system noise can be
If absolute EM values are required, then more sophisticated removed through filtering. Longer wavelength noise, if present, is
calibration is required. a greater concern as it may be hard to distinguish and filter from
geological responses.
HEM data processors should be aware that survey contractors
normally convert vertical coaxial HEM data from negative to The two main types of non-geological noise observed are
positive values. As well, horizontal coplanar data from Dighem individual spikes and lower amplitude, high-frequency noise.
HEM surveys prior to 1 January 1999 are calibrated to half the Spikes, if present, must be removed first so that they are not
theoretical EM amplitudes (Wait, 1982; Greg Hodges, 1999, smoothed into apparent anomalies by low-pass filtering. This can
personal communication). be done manually or with a nonlinear, median or other spike-
removal filter. A low-pass filter can then be used to remove high-
Pre-Processing frequency noise, leaving only geological wavelengths. The
wavelength and amplitude of the noise can vary with EM
Routine pre-processing is required for all airborne geophysical frequency and with time, so the processor should examine all
data, and must be carried out prior to beginning the HEM-specific frequencies carefully to decide on wavelengths to be removed, and
processing. Common requirements include: test the filters applied.

• Correct GPS locations and apply parallax (lag) corrections; Practical Example
• Merge auxiliary data with the HEM database (if required),
such as locations, radar altimeter, magnetics; For this survey, profile data was examined to determine the
typical size (width and amplitude) of spikes and the wavelength of
• Calculate HEM bird altitude from aircraft radar altimeter. background noise. A quick assessment found background noise in
all frequencies, fairly irregular but with typical wavelengths of 10-
As well, an experienced eye should routinely examine data for 20 fiducials (1-2 seconds). Typical noise amplitudes are in the
problems at all stages of processing. The raw data can quickly be range 0 to 5 ppm, depending on EM frequency. Figure 2 shows
assessed by visual examination in profile form, or gridded. For some excessive noise in this dataset. Occasional large-amplitude
HEM, two important aspects of data quality are noise and signal spikes are one to three points wide.
drift.
These wavelengths of noise can be removed without affecting
Practical Example the geological information in the data. A nonlinear filter (Naudy
and Dreyer, 1968) was applied to all EM channels, to remove
Figure 1 shows preprocessed HEM data for one complete spikes with a maximum width of three points and minimum
flight. For simplicity, only the HEM data is shown, but it is also amplitude of 10 ppm. This was followed by a low-pass filter with
useful to display bird altitude and other information at this stage. a wavelength of 40 fiducials (4 seconds). Examination of the
There is non-survey data on these flights that must be cleaned up filtered data against the original was deemed satisfactory (Figure 2).

Fig. 1. Profile display of all 10 raw HEM data channels against fiducials for Flight 55 of the sample dataset (inphase and quadrature channels from
each of the 5 frequencies). Profiles are scaled to view the useful EM data only. On the computer, colours are useful to distinguish the individual
channels more readily.

Exploration Geophysics (2000) Vol 31, No. 4 585

Valleau HEM data processing - a practical overview

DRIFT (ZERO LEVEL) CORRECTIONS

Unfortunately, drift often does not vary linearly with time. This
The data base level in EM channels varies, or "drifts", over can make corrections very difficult since the high-altitude
time. This is thought to be largely due to temperature variations, measurements cannot be readily interpolated to yield accurate drift
and can be caused by either the EM system or instrumentation. corrections over the whole survey. This is the primary cause of
Due to the high sensitivity of these systems, the amplitude of drift line-to-line "leveling" busts (similar in appearance to aeromagnetic
over a survey flight can be greater than system noise levels, or even leveling problems) that often appear in apparent resistivity maps.
signal levels. Drift is probably the most significant source of poor Problems also arise if the aircraft is not high enough to be out of
data quality in HEM surveys, especially if it is nonlinear with time ground effect, so that the observed "zero levels" still contain some
or cannot be measured accurately. secondary field from the ground.

It is necessary to remove drift from all EM channels. The The applied drift corrections are normally checked by
procedure during the survey is to fly to high altitude (usually 400 m calculating apparent resistivities from the corrected data at each
or higher) at the start and end of each flight, and often in the EM frequency, and displaying the gridded resistivity as an image.
middle of the flight between survey lines. At high altitude there is Remaining drift or other data integrity problems show up as line-
no ground response, so the secondary field signal should be zero. to-line "leveling" busts. In this case, the best approach is for an
The reasonable assumption is made that the drift at high altitude is EM expert to work with the calculated resistivity results in
the same as that at flight altitude. To carry out "leveling" or "drift problem areas and estimate the error in IP and Q values for each
corrections" in post-processing, the processor looks at the high- channel, at locations between high-altitude zero levels. These
altitude flight data segments to determine the signal levels that error values can be added to the table of "zero levels" for each EM
must be subtracted from the secondary field data to yield zero channel, and the drift corrections reapplied to the whole flight.
values. These "zero levels" are tabulated and then interpolated by Several iterations of this process may be required in difficult areas,
time across the entire dataset to provide an adjustment for every which can be very time-consuming.
data point, for every EM channel.

Fig. 2. Detail of raw HEM data in ppm (vertical coaxial 7000 Hz inphase), showing spike and background noise, with filtered version superimposed.
Nonlinear and lowpass filters were applied, as described in the text. X axis is fiducial counter in tenths of seconds.

Fig. 3. Profiles of horizontal coplanar 6600 Hz quadrature channel for Flight 55, raw and after drift (zero level) corrections. The high altitude flight
segments used for selecting zero levels are indicated, as well as locations of the interactively selected zero levels. X axis is fiducial counter in tenths
of seconds.

586 Exploration Geophysics (2000) Vol 31, No. 4

Valleau HEM data processing - a practical overview

New hardware technology has reduced drift problems and end of the flight, we would have left significant drift errors (up
somewhat in recent years, and new software tools allow easier to 30 ppm) in the data.
selection of zero levels, and application of the corrections. Good
survey practices and frequent high-altitude flight segments during This process was repeated for the other three flights in the
a survey can also reduce problems considerably. dataset. Few line-to-line leveling problems are visible in apparent
resistivity images calculated from the leveled data (see next
Practical Example section), so the drift corrections for this 6600 Hz horizontal
coplanar data look quite reasonable.
To demonstrate drift corrections, we will work with a single
EM channel. In practice, all EM channels must be drift corrected After completing the drift corrections, the leveled survey data
for each flight, and the software is designed to work on all and auxiliary information from each flight was split into separate
channels at once. I have chosen an example of excessive drift survey lines in a new database for subsequent processing. The
found in this dataset (Figure 3, top profile). The four segments leveled flight data was archived in case it is needed again in the
where the EM signal is low and inactive are the high-altitude future.
portions of this flight. Several survey lines were flown between
each high-altitude segment. Also, in each high-altitude segment a APPARENT RESISTIVITIES AND DEPTHS
small spike of signal is present. These internal calibration signals
will be ignored for this process. While the immediate goal of HEM surveys is often to locate
prospective anomalies, apparent resistivity maps created from the
To select zero levels for this flight, I examined each high- data are also extremely valuable for geological mapping and
altitude segment closely. I interactively selected a range of data interpretation. Apparent resistivities are also used for checking
unaffected by noise, internal calibrations or ground effects. By data integrity, particularly drift problems as discussed in the
selecting a range of data, rather than a single data point, the data is previous section.
averaged over a period of time to reduce the effects of local noise.
Altimeter data is often helpful in picking optimal zero levels Apparent resistivities can be calculated from each frequency
because you can use the maximum height reached as a guide to the used in the survey. However, the horizontal coplanar coil
best data to pick. configurations are most often used for resistivities, because they
have maximum coupling to horizontal layering. Most of the
A table was created with an entry for each EM channel (the standard resistivity data and maps provided by HEM survey
average ppm value over the selected range), and for the average contractors use one of two calculation methods, or variations of
time (fiducial) value for that range. This was repeated for each them. Both methods assume a uniform conductive halfspace and
high-altitude segment of the line, yielding a table with fiducials in work independently at each data point, for each frequency. The
one column, and corresponding signal levels (zero levels) in two methods are:
separate columns for each EM channel. Table 1 shows the table of
zero levels for Flight 55. On this flight there were four high- 1. Pseudolayer method. Inphase and quadrature data (or total
altitude sections, so there are four selected zero levels. The amplitude and phase) are the input, while apparent
locations of the zero level picks are shown in Figure 3. resistivity and apparent distance to the conductive halfspace
are output, as used by Dighem systems historically (Fraser,
To apply the drift corrections, the observed zero levels from 1978). Principal advantages include:
Table 1 were then subtracted from each EM channel. At each point
during the flight the correct value to subtract was interpolated • the apparent resistivity is usually more accurate since
between the known corrections, based on the time at that point. resistive surface layers are ignored in the calculation;
The bottom profile of Figure 3 shows the results of the drift • the apparent depth to the conductive halfspace, a
correction. As expected, the high-altitude survey data now all byproduct of the calculation, is itself a very useful
contain values close to zero, and all other values have been interpretive tool (after subtraction of bird altitude). It
adjusted accordingly. provides some mapping of the resistive surface layer
thickness, if present.
It is very useful to have the two high-altitude segments in the
middle of this flight because it can be seen from the top profile in
Figure 3 that the drift is nonlinear. At the start and end of the flight 2. Altitude-amplitude method. EM signal amplitude and bird
the signal levels are close to zero, whereas in the middle two altitude are the input, and apparent resistivity is output
sections they have significant positive values. If we had applied (Cheesman, 1998). Assumes that the conductive halfspace
drift corrections based on the high-altitude data only at the start is at the earth's surface. This method is normally more
tolerant of drift problems and poor data quality, requiring
less rigorous data corrections to provide useful results.
/ GEOSOFT TABLE - Flight 55 Zero Levels However, it can be misleading, particularly in areas of
/ resistive cover.
/ time cp6600i cp6600q
1043.6 26.42422535 11.68676056 All calculated resistivities and depths using these methods are
2971.4 53.34983871 41.25552419 "apparent" values, since actual ground conditions are not a
4907.7 73.71405941 16.24762376 uniform conductive halfspace. If the earth was indeed a uniform
6116.5 90.13040936 2.942748538 halfspace and the EM data was perfect, both methods would give
/ End of Data. the same results. Since this is never the case, comparisons of
results from the two techniques are very useful for interpretation.
I recommend using both methods on all datasets where possible.
Table 1. Zero level table for use in applying drift corrections to Flight
55. Normally all EM channels would be included; only the 6600 Hz
channels are shown here. The time column is measured in fiducials Calculated apparent resistivities from the two techniques can be
(1 fiducial = 1 second). gridded and displayed as images. The first resistivity maps created

Exploration Geophysics (2000) Vol 31, No. 4 587

Valleau HEM data processing - a practical overview

for an area are examined for leveling busts and coherency. resistivity with depth. Generally, higher frequencies provide
Microleveling of resistivity data for final grids can sometimes be information on shallower features. Thus, areas with significant
successfully used for aesthetic purposes, but requires careful resistive cover according to the depth map, correlate with resistive
consideration because of the nonlinear relationship between the IP areas in the 34 kHz resistivity map. The similarity between the 34
and Q channels and the calculated resistivities. It is preferable to kHz pseudolayer resistivity and the 6600 Hz amplitude-altitude
correct the EM channel drift as described earlier, rather than the resistivity reflects the fact that the pseudolayer method at the
calculated resistivities. higher frequency is responding to very shallow features, while at
the lower frequency the pseudolayer method sees further through
There are many refinements and control parameters for these the resistive upper layers than the amplitude-altitude approach.
methods, particularly with regard to handling or avoiding areas of
low signal strength. One approach for the amplitude-altitude ANOMALY PICKING AND TARGET ANALYSIS
algorithm, for example, is to use only the quadrature signal rather
than total amplitude, in areas where the inphase response is very For mineral exploration, the prime goal of most HEM surveys
low. More advanced modeling can be done as well. is anomaly detection. Anomalies are selected and analysed,
usually starting with determination of apparent conductance and
Use of apparent resistivities from different frequencies yields depth to the causative body.
significant information on the variation of resistivity with depth.
Lower frequencies have greater ground penetration than the higher Anomaly picking normally requires an EM interpreter.
frequencies. Automatic picking provides a first pass, which is then interactively
edited by adding or removing anomaly picks on each survey line,
Practical Example while examining the EM profiles. Many simple (or complicated)
algorithms can be used for the automatic picking. By common
Apparent resistivities for the horizontal coplanar coil convention, the final selection of anomalies is alphabetised along
configurations were calculated along all survey lines using the each survey line for identification.
drift-corrected IP and Q data, with both resistivity methods.
Figure 4 shows profiled results for the 6600 Hz data. At each selected anomaly, apparent conductances and depths to
conductors are then calculated. Most basic methods of doing this
While interpretation of the results is beyond the scope of this assume that the causative body is a vertical thin sheet conductor
paper, it is worth the reader examining the relationships between oriented perpendicular to the survey line, in a uniform resistive
the two calculated resistivities, the bird altitude and the calculated halfspace. Conductance is the conductivity-thickness product of
apparent depth. As well, note that the pseudolayer method often the thin sheet, a useful measure since it is difficult to distinguish
calculates the top of the conductive halfspace to be higher than the conductivity from thickness of the sheet using this simple model.
actual ground surface. This reflects either a problem in the EM Depth is calculated from the HEM bird to the body, so bird altitude
data or the fact that the "halfspace" is not uniform in conductivity. must be subtracted to yield depth from ground surface to the top of
the body.
I then gridded the apparent resistivities and depth data for all
survey lines; these images are displayed in Figure 5 for two of the Vertical coaxial coil pairs are normally used for this calculation
frequencies. Again, note the general agreement between the two since they are maximum-coupled with such bodies. In fact the
resistivity calculation methods and the rough inverse correlation horizontal coplanar configuration has no coupling at all with such
between apparent depth and the amplitude-altitude resistivity. a body when the HEM bird is directly above the thin sheet.

In general, all of these maps provide different information, so The EM interpreter will generally classify the anomalies after
all are useful for a complete interpretation. Apparent resistivities this process, commonly using the calculated conductance as a
from different frequencies yield information on the variation of measure. Like the anomaly picking, automatic anomaly

Fig. 4. Profiles of leveled 6600 Hz IP and Q data for line 40691. In the second panel are calculated apparent halfspace resistivities using the
pseudolayer and amplitude-altitude calculation methods. The third panel shows elevations of HEM bird, ground surface and apparent top of the
conductive halfspace from the pseudolayer method. X axis is fiducial counter in tenths of seconds.

588 Exploration Geophysics (2000) Vol 31, No. 4

Valleau HEM data processing - a practical overview

Fig. 5. Maps of apparent resistivity and apparent depth (bird to conductive halfspace) of the survey area, calculated from two horizontal coplanar
frequencies, using both calculation methods. The colour schemes used for the resistivities and depths are shown, crosshairs are spaced 1 km apart.

Exploration Geophysics (2000) Vol 31, No. 4 589

Valleau HEM data processing - a practical overview

classifications should then be examined by the interpreter and conductance and apparent depth to top of the body (thin sheet), as
possibly reclassified. For example, the interpreter may use special well as classifications and anomaly ID for each anomaly.
classifications such as "culture" or "surficial". Final classifications
are used in presenting the anomalies as various symbols in maps PRESENTATION AND ARCHIVING OF RESULTS
and profile displays. More complicated classification schemes
might combine multiple channels of information in some manner. As with all geophysical data, there are many potential ways to
present HEM survey information. Images of the HEM data itself
As with resistivities, the simple conductance model used is (IP and Q) are rarely displayed in plan maps because EM signal
rarely correct, so the results are always inaccurate. It is strength varies with bird height so that such maps are difficult to
nonetheless a useful approach to quickly analyse and classify interpret. HEM survey parameters, instrumentation details and
observed anomalies in an automated fashion. An EM interpreter other information are normally plotted in the margins (not
should examine the data to ensure that prospective anomalies are included here for space reasons). Survey lines are commonly
not overlooked. Advanced EM modeling methods can also be included on all maps.
applied to this data to yield more accurate information including
body extent, strike, dip and other parameters, especially from the The most common presentations are various combinations of:
vertical coaxial coils.
• Apparent resistivity maps for horizontal coplanar
Practical Example frequencies (Figures 5 and 7). Very useful for geological
mapping and interpretation. Resistivities are usually
Automatic picking was applied to the leveled Coaxial 7000 Hz displayed using an inverted colour scheme with conductive
inphase and quadrature channels. In this simple example, all peaks areas appearing "hot" (red) and resistive areas "cool" (blue).
were picked which had a minimum data value of zero and a • Profile maps of IP and Q data, often superimposed on
minimum anomaly amplitude of 20 ppm above the local resistivity or other (e.g. magnetic) maps. Figure 8 shows an
background. Figure 6 shows the automatically picked targets for example. These maps are useful for the anomaly interpreter.
these two channels on one survey line.
• Classified anomaly symbol maps with annotations, often
In this dataset there were 296 automatic picks using the superimposed on resistivity or other colour maps; symbols
quadrature data. I decided to use the quadrature channel picks as usually represent conductance classifications or interpreted
my starting point, rather than combining the anomalies from all information such as culture. Figure 7 includes an example.
channels. I then manually added two more anomalies into the Annotations generally include anomaly identification,
database anomaly channel for this line (marked as triangles in apparent depth and other parameters such as apparent
Figure 6), and deleted one. This editing is normally done based on conductance or EM data values at that location (Ontario
all EM channels, but for simplicity I have only displayed the 7000 Geological Survey, 1990).
Hz data in this Figure. The final picks were sorted alphabetically • Simple colour symbol maps, proportional symbol maps and
along each line. similar presentations can quickly highlight important
anomalies, based on apparent conductances, apparent
The final selection of anomalies in the database was then fed depths, interpreted classifications or other parameters.
into the Geosoft apparent conductance algorithm, which uses an
implementation of the PLATE forward modeling approach • Multichannel profile plots for each survey line at full
developed at the University of Toronto (Dyck et al., 1980). horizontal map scale (Figure 9). Symbols and anomaly ID
Automatic classifications were then applied. In this example I letters match those in the classified anomaly maps. These
specified conductance breakpoints of 0.5, 0.75, and 1.0, to give plots are also very effective in colour, to help distinguish
four classification levels. Figure 6 shows the resulting apparent channels.

Fig. 6. Profiles of 7000 Hz IP and Q data along line 40441. Automated anomaly picks and manual modifications are shown. The second panel shows
the calculated apparent conductance and depths (from surface to top of a thin sheet model) for each anomaly. Below the anomaly ID, the bottom
panel shows anomaly classifications based on conductance.

590 Exploration Geophysics (2000) Vol 31, No. 4

Valleau HEM data processing - a practical overview

Fig. 7. Example HEM survey map presentation for part of survey area. Included are survey lines, colour image of apparent resistivity, and classified
anomaly symbols with annotated ID, apparent conductance and depth, and 7000 Hz Q value. Full-scale maps would normally have more anomaly
symbols, additional survey specifications and other information in the margin.

Exploration Geophysics (2000) Vol 31, No. 4 591

Valleau HEM data processing - a practical overview

• Anomaly report including anomaly ID (for cross- SUMMARY

referencing to maps and profiles), location, conductance,
classification and other information (Table 2). HEM data processing can be fairly complex, especially with the
• Conductivity depth sections along survey lines, based on the number of data channels involved. Good data management,
fact that lower frequencies penetrate deeper on average than careful attention to detail, and well-designed software tools are
higher frequencies. Requires very accurate drift corrections required. HEM expertise and time is needed for detailed
and the calculation of depths at which to plot the resistivity interpretations and often to assist in various processing steps,
for each frequency, at each data point. Useful approaches especially if iterative drift corrections are required.
are the Sengpiel (1988) and differential parameter (Huang
and Fraser, 1996) resistivity sections. To improve HEM data quality, the most important area to look
at is drift. If HEM equipment improvements can eliminate system
Proper archiving of the data and processed information is drift, or even make it linear with time, it would be a major leap
critical, along with complete documentation. Documentation must forward in the industry's technology. In the meantime, drift
include standard items such as descriptions of archived data corrections are optimised with as many high-altitude flights as
channels, HEM system specifications, and details of the work feasible to sample the drift adequately for a given system. System
done. Useful additions include nominal bird height, aircraft height, noise is usually a less significant source of problems than drift or
line spacing, line direction, and a list of final flight and survey line calibration issues, as much of this noise can be removed efficiently
numbers. through filtering.

Analysis of resistivities from multiple frequencies and using

both the pseudolayer and the amplitude-altitude approaches will
yield more geological information than any single resistivity map.
In general, it is useful to have a wide range of frequencies. For
conductive areas, use lower HEM frequencies to maximise
penetration. For resistive areas, use higher frequencies to get more
detail in the near surface.

There are many possible presentations of HEM survey results.

Selection of the most appropriate presentations yields the greatest
benefit from the investment in the data. Resistivity maps are
excellent for geological interpretation, while anomaly analysis is
very effective for finding (or eliminating) drill targets. It is useful
to look at many combinations of these various maps, and combine
them with other available information such as magnetics, satellite
imagery, known geology, etc.

ACKNOWLEDGEMENTS

I thank Ashton Mining (WA) Pty. Ltd. for allowing the use of
this survey dataset, and the staff at Geo Instruments Pty. Ltd. for
supplying the HEM data. The Geosoft Technical Solutions group
allowed the use of the specialised HEM processing tools
developed by them. I must also acknowledge the numerous
unnamed contributors to my practical HEM education, particularly
during my years at Dighem Surveys and Processing Inc. Thanks
to Chris Bishop for reviewing the manuscript.

REFERENCES
Cheesman, S., 1998, HEMRES2 GX documentation (online Help for the amplitude-
altitude resistivity inversion algorithm in Oasis montaj software): Geosoft Inc.

Dyck, A.V., Bloore, M. and Vallee, M.A., 1980, User manual for programs PLATE
and SPHERE: Research in Applied Geophysics, 14, University of Toronto.

Fraser, D.C., 1978, Resistivity Mapping with an airborne multicoil electromagnetic

system: Geophysics, 43, 144-172.

Huang, H. and Fraser, D.C., 1996, The differential parameter method for
multifrequency airborne resistivity mapping: Geophysics, 61, 100-109.

Naudy, H. and Dreyer, H., 1968, Essai de filtrage non-lineaire applique aux profiles
aeromagnetiques: Geophys. Prosp., 16, 171-178.

Ontario Geological Survey, 1990, Airborne electromagnetic and total intensity

magnetic survey, Sturgeon Lake - Savant Lake area: Map 81505.

Sengpiel, K.P., 1988, Approximate inversion of airborne EM data from a multilayered

ground: Geophys. Prosp., 36, 446-459.

Wait, J.R., 1982, Geoelectromagnetism: Academic Press Inc.

Fig. 8. Profile map of 6600 Hz quadrature data. At full scale both IP

and Q data would normally be plotted together, in different colours or
line types.

592 Exploration Geophysics (2000) Vol 31, No. 4

Valleau HEM data processing - a practical overview

Fig. 9. Multichannel profiles for survey line 40391. Scales are optimised for different channel groupings. This example includes IP and Q for all
channels, apparent resistivities, classified anomalies, and bird height. Labeling and scales are designed to be efficient for the interpreter rather than
overly detailed.

Table 2. Section of an anomaly report for the first few lines

of this survey. Parameters chosen to be included in the
report are anomaly ID (alphabetical by line), anomaly
classification, location, selected apparent conductances,
resistivities and depths as calculated during the processing,
and bird altitude. EM data channels or other parameters
might be included. Fiducial (time) is often a useful addition.

Exploration Geophysics (2000) Vol 31, No. 4 593

Valleau HEM data processing - a practical overview

SUGGESTED ADDITIONAL READING

Grant, F.S. and West, G.F., 1965. Interpretation theory in applied geophysics:
McGraw-Hill Inc.

Telford, W.M., Geldart, L.P., Sheriff, R.E. and Keys, D.A., 1976, Applied Geophysics:
Cambridge University Press.

Developments and Applications of Modern Airborne Electromagnetic Surveys,

Proceedings of a U.S. Geological Survey Workshop Oct. 7-9, 1987: U.S.
Geological Survey Bulletin, 1925.

Proceedings of the AEM 98 International Conference on Airborne Electromagnetics,

1998: Expl. Geophys., 29 nos. 1 & 2.

Practical Geophysics II for the Exploration Geologist (Chapter 6), 1992, compiled by
Richard van Blaricom: Northwest Mining Association.

Oasis montaj - HEM System User Guide, 1997, by Greg Hollyer and Huanjin Wang:
Geosoft Inc.

594 Exploration Geophysics (2000) Vol 31, No. 4