Geophysical Investigation:

Geophysical Investigation is the study of the structure of the earth’s crust by physical methods for
the location and surveying of minerals, it is an integral part of geophysics.

Geophysical investigation techniques

 Seismic method.
 Gravity method.
 Magnetic method.
 Electrical method.
 Radioactive method.
 Well logging method.
 Electromagnetic method

Seismic Method:
Investigating based on the analysis of elastic waves generated in the earth by artificial means. The
elastic waves produced during sudden disturbance is called as Seismic wave. These seismic waves
are recorded using the instrument Seismograph and the record obtained is Seismogram.
It is an important geophysical prospecting applied in exploration of oil and natural gas deposits,
deep ground water exploration, depth estimation, geotechnical problems. Seismic prospecting can
be done by two methods
1. Reflection method
2. Refraction method

Types of seismic waves

Seismic wave are classified in to the following
1. Compressional or Longitudinal or primary waves (P waves)
2. Shear or Transverse or secondary wave (S wave)
3. Surface waves (L wave)

Seismic Reflection Method:

Seismic Reflecting Method is a method of prospecting that uses the principles of seismology to
estimate the properties of the Earth's subsurface from reflected seismic waves.
 The method requires a controlled seismic source of energy
 Seismic waves generated… dynamite, mechanical impact, earthquakes.
 Reflected at formations having different physical properties.
 The reflection are recorded by detecting instruments (geophone, hydrophone) responsive to
ground motion.
 Geophone which converts ground motion into an analogue electrical signal. In water,
hydrophones are used, which convert pressure changes into electric al signals.
Principle of seismic reflection: seismic waves are generated by a surface source, are reflected at
boundaries between rock layers, and are detected and recorded by receiver at the surface.

Seismic surveys use vibration induced by a sound generating equipment to provide a picture of
subterranean rock formations found at depth [30,000 feet below ground level (BGL)]. This is
accomplished by generating sound waves downward into the earth's crust which are reflected
through various boundaries between different rock strata. On land, the sound waves are generated
by small explosive charges embedded in the ground or by vibrator trucks, referred to as thumpers
which shake the ground.
The human ear can barely hear the thump, but the frequency generated penetrates the earth's crust.
The echoes are detected by electronic devices called geophones which receive the reflected sound
waves and the data are recorded on magnetic tape which is printed to produce a two dimensional
graphic data through which illustration of the subsurface can be done.

(1) seismic sensors,

(2) seismic exploration station,
(3) shooting point,
(4) shot point,
(5) direct wave,
(6) reflected wave

Calculation of depth
The geometrical relation pertaining to a single geophone and a single reflecting layer is shown in
the figure
S is the shoot point, x is the distance to the geophone, V1 is the velocity in the first layer, T is the
arrival time of the reflected wave. The depth Z can be calculated from the equation

Application:

 Near-surface applications – to understand geology at depths of up to approximately 1 km, used for

engineering and environmental surveys, as well as coal and mineral exploration, for geothermal
energy surveys.
 Hydrocarbon exploration: Hydrocarbon exploration used by the hydrocarbon industry to provide a
high resolution map of acoustic impedance contrasts at depths of up to 10 km within the subsurface.
 Crustal studies – investigation of the structure and origin of the Earth's crust, through to the
Mohorovicic discontinuity and beyond, at depths of up to 100 km.
Electrical Methods of Geophysical Surveys for Civil Engineering Investigations.

Electrical resistivity method:

This method is based on the principle of Ohm’s law and universally employed for groundwater
prospecting, required for various water supply schemes.

Principle:

All materials will conduct or resist current. If they conduct current, it will be in various proportions,
based on their composition and moisture content present. The conductivity of any rock / soil is the
reciprocal of resistivity. Knowing the resistivity values, different rocks strata present in the earth’s
crust is inferred and their aquifer characteristics are studied.

Equipment used:

 Resistivity meter
 Two current electrodes & two potential electrodes
 Power pack
 Cables, hammers etc.,

Types:
i) Wenner electrode array
ii) Schlumburger electrode array

Procedure:
In both the methods all the four electrodes are erected firmly into the ground and a known current
(I) is sent into the ground through the two current electrodes (C1 & C2) and the potential difference
(V) between the two potential electrodes (P1 & P2) is measured.
In the case of Wenner configuration of electrodes, all the four electrodes are equally spaced where as
in schlumburger configuration, the potential electrodes are closed spaced and current electrodes are
placed further apart.

Wenner Array:
a = electrode spacing
C1, C2 = current electrodes
P1, P2 = potential electrodes
Formula used:

Where ρ A = apparent resistivity in ohm-m

a = electrode spacing
v = potential difference between 2 potential electrodes in millivolts / volts
I = current sent in Ampere / milli amps

Resistivity of some materials

Material Resistivity ( Ω-cm)

Massive rock > 400

Shale and clay 1.0

Seawater 0.3

Wet to moist clayey soils 1.5 - 3.0

Application:
 The electrical methods have been successfully employed in delineation of ore bodies occurring at
shallower depths.
 Determination of Depth to the bed rock: as for instance, in important projects like dams, buildings
and bridge foundations, where it would be desirable that the structure should rest on sound hard
rocks rather than on overburden or soil.
 Location of geological structures: like folds; buried valleys, crushed and fractured zones due to
shearing and faulting.
 Location of Aquifers: and other water bearing zones which could be easily interpreted on the basis
of known resistivity values of moistures rich rocks and dry rocks.
MAP:
Map is a picture or representation of the Earth's surface, showing how things are related to
each other by distance, direction, and size. Maps are a way of showing many things about a
portion Maps are a two dimensional representation of the surface of the earth and its features.
Maps are a kind of shorthand language media with two main purposes:
1. To convey detailed information about a specific area, and
2. To indicate the position of the specific area relative to other parts of the earth of the earth's
surface on a flat piece of paper that can be carried and transported easily.

MAPPING

Cartography or map-making is the study and practice of crafting representations of the

Earth upon a flat surface.

The various features shown on a map are represented by conventional signs or symbols. For
example, colours can be used to indicate a classification of roads. Those signs are usually
explained in the margin of the map, or on a separately published characteristic sheet.

Geological Map:

A geologic map or geological map is a special-purpose map made to show various

geological features. Rock units or geologic strata are shown by colour or symbols to indicate
where they are exposed at the surface. Geologic maps are our most important and complete
compilation of information about the solid Earth we live on.

By mapping different rock types and structures, geologists can determine the relationships
between different rock formations which can then be used to find mineral resources, oil,
and deposits.

Geological maps play a vital role in mining / quarrying operation and it serves as a valuable
tool for a mining engineer.
Geological maps are further classified as
i. Hydrogeological maps : geological maps incorporated with ground
water details
ii. Contour maps : Maps with elevation details
iii. Soil maps : Maps with soil details
Application:
i) They application in mining / quarrying operations to exploit minerals successfully &
profitably
ii) Hydrogeological maps find application in locating groundwater sources for water supply
projects.
iii) Contour maps find applications in knowing elevation details required for laying
pipelines, construction of roadways in hills, etc.,
iv) Soil maps find application in knowing soil types, required for foundation design of civil
engineering structures.