Engineering Geology (BCV303)

MODULE 5
Modern tools and Geophysical methods
Aquifers
Aquifers may be defined as a formation that contains sufficient, saturated permeable
material to yield significant quantities of water to wells and springs.
Types of water bearing formation or
Classification of Rocks based on porosity and permeability
1. Aquifer: Geological formations which are both porous and permeable.
Example: Fine sand, coarse silt
2. Aquiclude: Geological formations which are highly porous but impermeable.
Example: Clay
3. Aquitard: Geological formations which are porous but less permeable.
Example: Sandy clay.
4. Aquifuge: Geological formations which are neither porous nor permeable.
Example: Granite rock

Rocks as aquifers
The various kinds of rocks posses’ variable water-bearing properties, depending chiefly
on their permeability and porosity. Of the three important types of rocks, the
Sedimentary rocks, generally, constitute the best aquifers, the metamorphic rocks and
especially the foliated ones, making moderate to good aquifers and the Igneous rocks
generally behaving as the poor aquifers.

SEDIMENTARY ROCKS AS AQUIFERS

Among the Sedimentary rocks, Gravels possess the highest water retaining as well as
water yielding capacities. This is truer in case of loose and weakly cemented coarse
gravels. In general, next to Gravel, the other Sedimentary rocks in their successive
order of decreasing water bearing capacity are loose sand, sandstone, limestone etc.
Shale (Clay) is the poorest in absorbing water, being impermeable although porous,
and hence classified as Aquiclude. Among sandstones, the water-bearing capacity
depends much upon their texture and nature of cementing material. Coarse-grained
sandstone may be good aquifers. Whereas fine grained sandstone may prove to be
the poorest aquifers. The water-bearing capacity of limestone depends much upon the
presence of solution channels, crevices, fissures and other such opening in the rock.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

Hence, fissured and cracks limestones may prove to be excellent aquifers and other
compacted limestones may prove to be totally unproductive

IGNEOUS ROCKS AS AQUIFERS

The intrusive igneous rocks like granites, syenites etc., are generally very compact and
dense and hence are non-porous. They are barren groundwater under normal
conditions. However when they are traversed by fissures or cracks, they may be
capable of holding some groundwater quantities. Even these cracks and fissures die
out with depth, and as such, there is absolutely no possibility of getting any
groundwater in these rocks at depths greater than 80-100mts. The extrusive igneous
rocks also exhibit great variations in their water-bearing properties. Basic igneous
rocks like basalts are generally rich in cavities and contraction cracks; and as such may
become permeable and sources of groundwater. Acidic igneous rocks like rhyolites
may or may not contain groundwater, because such rocks although generally possess
interstices, but may be filled up with ash and other material, and hence the
uncertainty.

METAMORPHIC ROCKS AS AQUIFERS

Non-foliated metamorphic rocks like Marble and Quartzite are generally
impermeable, except along the original bedding, if the same is not completely
destroyed during metamorphism. Foliated metamorphic rocks like Slates, Schist,
Phyllites and sometimes even Gneiss may contain some good amount of Groundwater
due to their being highly fractured.

Types of Aquifers

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

Aquifers can be of Four types:

1. Confined aquifer:

If the water within an aquifer is confined i.e., held under pressure by an overlying
impervious strata, the aquifer is known as confined aquifer. The confining bed has
a significantly lower hydraulic conductivity than the aquifer.

Artesian aquifer is a confined aquifer developed in suitable geological situations

where the piezometric surface is above the groundwater level. A well in such an
aquifer flows without pumping. The imaginary surface to which water rises in well
tapping the artesian aquifer is known as piezometric surface.

2. Unconfined aquifer:

If the aquifer is exposed to atmosphere or possesses a free surface open to the

atmosphere, it is termed as unconfined aquifer. There will be a water table in an
unconfined aquifer. Sometimes called water table aquifer. The term phreatic
aquifer is also frequently applied to denote an unconfined aquifer. Unconfined
aquifers usually receive recharge water directly from surface, from precipitation or
from a body of surface water (e.g. a river, stream or lake) which is in hydraulic
connection with it.

3. Perched aquifer:
A special case of an unconfined
aquifer is the perched aquifer that
occurs whenever an impervious
(or relatively impervious) layer of
limited horizontal area is located
between the water table of a
phreatic aquifer and the ground
surface

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

4. Leaky aquifer:

It is completely saturated aquifer bounded above by a semi pervious and below by

an impervious layer. A semi pervious layer is defined as a layer which has low
permeability. Lowering of piezometric levels in leaky aquifer for examples by
pumping will generate a vertical flow of water semi pervious layer into the pumped
aquifer. Horizontal flow component in the semi pervious layer is negligible since it
is very low permeability.

AQUIFER PARAMETERS

1. Porosity
Porosity is nothing but the ratio between the total Voids or Pores to the total volume
of the rock or material.

2. Permeability

Permeability is the ability of a formation to transmit water through its pores. It can
be defined as the flow per unit cross sectional area of the formation when subjected
to a unit hydraulic head per unit length of flow and has the dimension of velocity.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

3. Specific yield:

Specific yield is the water removed from unit volume of aquifer by pumping or
drainage and is expressed as percentage volume of aquifer. Specific yield depends
up on grain size, shape and distribution of pores and compaction of the formation.

Sy = (Wy / V) *100

Where, Sy = Specific yield in %

Wy = Volume of water drained by gravity

V = Total Volume of aquifer.

4. Specific retention

Specific Retention is the percentage (%) of total volume of the saturated Aquifer
which will be held/retained in a unit volume of saturated Aquifer by molecular and
surface tension forces against the force of Gravity after full Gravity Drainage.

Sr= 100 Wr / V

Where, Sr = Specific retention

Wr =the volume occupied by retained water V= Total Volume of rock or soil.

It should be noted that fine grained materials yield little water, whereas coarse-
grained materials permit a substantial release of water and hence serve as aquifers.

Porosity = Specific yield + specific retention

5. Storativity

Storativity(S) or Storage coefficient is the volume of water that an Aquifer takes into
storage per unit surface area of the Aquifer. The amount of water per unit volume
of a saturated formation that is stored or expelled from storage owing to
compressibility of the mineral skeleton and the pore water unit change in head.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

Storativity (S) = Volume of water / (Unit area X Unit Head change)

S = m3/ (m2 X m)

Storativity is a non-dimensional parameter.

6. Transmissibility
Amount of water that can be transmitted horizontally through a unit width by the
full saturated thickness of the aquifer under a hydraulic gradient.

T= Kb

Where, T = Transmissibility

K = hydraulic conductivity

b = thickness of the aquifer.

GROUNDWATER EXPLORATION:
Groundwater exploration is the investigation of underground formations to
understand the hydrologic cycle, know the groundwater quality, and identify the
nature, number and type of aquifers. A detailed knowledge on groundwater
resources enables its sustainable use, where the regional distributions of resources,
the hydraulic characteristics of the aquifer as well as the regional and temporal
variations of the water quality are important factors.

There are different groundwater exploration methods.

1. Electrical Resistivity Method:

Electrical resistivity methods, electromagnetic methods, self-potential methods and
induced polarization methods are the important categories of electrical methods.
All geological formations have a property called electrical resistivity (ρ) and this
resistivity is expressed in the units of Ohm-meters (Ωm).

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

This method makes use of the fact that water increases the conductivity of
rocks and thereby decreasing their resistivity.

The various geological factors which influence the electrical resistivity of the
subsurface formations are: Mineral content (most of the rock forming minerals have
high resistivity except Sulphide minerals), Compactness, Moisture (moisture may
occur in the rocks), Salinity of moisture, Texture of the rock (fine grained rocks show
a higher resistivity compared to coarse grained ones)

Resistivity, or inverse of electrical conductivity, is a measure of the capacity of a

rock to allow an electric current through it. Dry rocks and sediments, dense,
compact and non-porous rocks will offer a greater resistance to the electric
current compared to loose, porous and wet, saturated materials.

There are two variations of the resistivity method.

1. Profiling Method (Lateral exploration)

2. Sounding method (Vertical Electrical Sounding[VES])

Electrical Profiling/ Traversing:

It is done to detect lateral changes in resistivity which reflects the subsurface

lithology in a large area up to the same depth, whereas

Electrical Sounding:

It (also known as Vertical Electrical Sounding VES) is done to determine vertical

changes in resistivity which reveals the changes in Lithology at a particular place
with increasing depth.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

In profiling or sounding, there is scope for electrode arrangements to be made in

different ways in the field as such arrangements are called “electrode
configurations”. The two main methods which are commonly used are: Wenner
configuration and Schlumberger Configuration.

The Wenner configuration:

This method was developed by Wenner in 1915. In this configuration, the outer
electrodes, C1 and C2 are used to send current into the ground and the inner
electrodes, P1 and P2 are used to measure the potential. The electrodes are inserted
in the ground and connected in a circuit to a source of electrical energy. As the
current is introduced, it travels from one electrode, passes through the material and
leaves the ground at the other current electrode. In between the current will meet
resistance from the material and there will be a drop in its potential. This drop in
potential is measured by potential electrodes. The important feature of this setup
is that the distance between any two successive electrodes is equal. The apparent
resistivity measured in the Wenner method is given by

ρa = 2πa R=2πa (V/I)

Where, a = Electrode separation

V = Potential difference measured I = Current sent into the ground

ρa = apparent resistance (ohm-mts)

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

In this system, all four electrodes are shifted and moved simultaneously for each
reading starting with small separation of one to two meters and expanding up to
hundreds of meters.

1. Seismic Method:

Geophones

 Seismic reflection methods provide information of geologic structure thousands of

meters below the surface (deep seated strata), whereas seismic refraction methods
of interest in groundwater studies go only about hundred meters deep.

 This method involves the creation of small shocks at the earth’s surface either by
the impacting of a heavy instrument or by a small explosive. The elastic waves
caused by shock travel downwards into various rock layers and are refracted back
to the surface from junctions between different rock layers.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

 The waves are picked up at various points on the ground surface by geophones and
recorded. This record shows when energy commenced and when it was picked up
at the surface.

 By knowing the arrival time of different waves at different distances from the
source, velocity of propagation of wave through each rock layer can be calculated.
Seismic waves depend on the media through which it is passing velocities are
greatest in solid igneous rocks and least in unconsolidated materials.

 A time travel curve (time versus distance from source to geophone) is drawn and by
knowing the distance where change in slope is indicated, the depth to the rock layer
is computed.

 Characteristic seismic velocities for a variety of geological materials are employed

to identify the nature of alluvium or bedrock. In coarse alluvial materials, seismic
velocity increases from unsaturated to saturated zones. Consequently the depth of
water table can be mapped. Changes in seismic velocities are governed by changes
in elastic properties of the formations. In sedimentary rock the texture and geologic
history are more important than the mineral composition. Porosity tends to
decrease more velocity, but water content increases it.

Factors affecting coefficient of permeability

Following are factors effecting permeability of soils.
1. Size of soil particle

2. Specific Surface Area of Soil Particle

3. Shape of soil particle

4. Void ratio

5. Soil structure

6. Degree of saturation

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.

 Engineering Geology (BCV303)

7. Water properties

8. Temperature

9. Adsorbed water

10.Organic Matter

Important Questions
1. Define Aquifer and explain the types of Aquifers.
2. Explain the Aquifer parameter.
3. Explain the factors affecting coefficient of permeability.
4. Explain the principle of electrical resistivity method and with neat
sketch describe the function of resistivity meter.
5. Explain seismic method and its application in Civil Engineering.

Prof.Balappa Navalayi., Department of Civil Engineering, KLECET, CHIKODI.