75
2.
Subsurface of Groundwater
Availability of aquifer and its characteristics can be ascertained by exploratory
tubewell. Small diameter holes are drilled by means of direct rotary rig to ensure
collection of samples of geologic formations. If thick good aquifer materials are
obtained, the small hole can be reamed to a bigger diameter production well.
The exploratory tubewell is suitably developed by air-compressor and
ascertained the yield by using pumps of varying r.p.m. Chemical quality, draw
down at a given discharge is also recorded. Such an exercise is needed for
future well development in the area. However, if the test well fails to encounter
any good aquifer the well is abandoned.
Resistivity Logging
Within an uncased well, current and potential electodes can be lowered to
measure electrical resistivities of the surrounding media and to obtain a trace of
their variation with depth. The result is a resistivity (or electric) log. Such a log is
affected by fluid within a well, by well diameter, by the character of surrounding
strata, and by groundwater. Electrical logging is very useful in delineating fresh
water aquifer from saline aquifers and in coastal or salinity affected areas, this
method must be applied before screening the well to be used as production
well.
Aquifer performance - Tests
Aquifer parameters such as transmissivity, storage coefficient are determined
by Aquifer Performance Test (APT) in the field condition. This need a pumping
well with one or more observation wells to study the effect of pumping on
water-table or piezometric surfaces. Darcy's law is utilized for arriving at
different equations to determine the aquifer parameters such as
transmissibility, storage co-efficient etc. in the field in different conditions of water
flow such as steady flow under confined conditions, unsteady flow and
non-equilibrium conditions etc. The basic assumptions in applying the
mathematical equations are:
1)
The aquifer is isotropic and homogenous.
2)
The aquifer has infinite areal extent.
