Geology Reference
In-Depth Information
in the weathered granite and in semi-confined conditions in the fractured
granites. The depth to water level varies from 11 m to 20 m. The yield of
the bore-wells range from 1000 gph to 5000 gph. The high yielding bore-
wells are either recharged by the irrigation tanks or tapping the deeper
fractures. The yield of the bore-wells in the vicinity of the dolerite dykes is
high as they are tapping thick fractured zone. The area comprises thin soil
cover of sandy loam and clayey soils and is underlain by granites. These
granites are medium to coarse grained and of pink and grey colour.
GEOPHYSICAL METHODS
Several geophysical methods are available for groundwater exploration. Most
of the available geophysical methods have been applied here to study the
aquifer system of a hard rock granitic terrain. A brief description of the
methods is given here.
Electrical Resistivity Technique
Electrical resistivity technique is the most commonly applied method among
all the geophysical methods for groundwater exploration, because of the
large variation of resistivity for different formations and the changes that
occur due to the saturated conditions. Resistivity is defined as the resistance
offered by a unit cube of material for the flow of current through its normal
surface. If
L
is the length of the conductor, and
A
is its cross-sectional area,
then the resistance is defined as
L
A
R
=
where
is constant of proportionality and is called as resistivity. In MKS
system the unit of resistivity is ohm-metre (m).
In the case of an inhomogeneous medium, the resistivity measured is
called as the apparent resistivity. The apparent resistivity of a geologic
formation is equal to the true resistivity of a fictitious homogeneous and
isotropic medium. Resistivity of rock formations varies over a wide range,
depending on the material, density, porosity, pore size and shape, water
content and quality, and temperature. There are no fixed limits for resistivities
of various rocks: igneous and metamorphic rocks yield values in the range
10
2
to 10
8
ohm m while sedimentary and unconsolidated rocks, 1 to 10
4
ohm.m.
Generally for measuring the resistivities of the surface formations, four
electrodes are required. A current of intensity
I
is introduced between one
pair of electrodes, called as current electrodes, named as A and B and some
times as +
I
and -
I
denoting source and sink respectively. The potential
difference produced as a result of current flow is measured with the help of
another pair of electrodes, called as potential electrodes and represented by
