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Geophysical survey

Geophysical survey is one of the modern techniques for groundwater exploration. The resistivity survey which is popular because of its high resolution power to delineate subsurface features as well as geological boundaries.

The main principle of the electrical methods of exploration for groundwater is based on the concept of resistivity. All geological formation posses a property called resistivity which determines the ease in which the electrical current flows through them.

Electrical profiling investigations are conducted in order to trace lateral boundaries of litho logical units having different electrical properties. Profiling is useful in areas where the sub-surface formations are horizontal or nearly horizontal and posses’ sufficient resistivity contrast. Resistivity profiling method can differentiate less permeable areas from more permeable areas on the basis of apparent resistivity measured in the field.
Resistivity sounding
Resistivity sounding method is used for depth investigations. This method is referred to as the vertical electrode sounding (VES). During the geoelectrical measurements, the distances between the current and the measuring electrodes are gradually increased while the centers of the layout remain at a fixed point. Apparent resistivity’s computed from the current strength and the potential difference readings are used to estimate the resistivities and thickness of individual.

Subsurface   rock   layers.      The   measured   resistance   value   can   be   correlated   with vertical geological sections.
Electrode configuration
There are number of ways of setting up of current and potential electrodes, in the exploration of groundwater by electrical resistivity methods. The choice of an array and the distance between the electrodes is very important for obtaining the best possible information of the subsurface geology of a given area. In practice, different electrode configurations have been adopted, the Schlumberger electrode configuration is used in this area. Because this method in particular has practical, operational, and interpretational advantages over the other methods.
Schlumberger electrode configuration
This is a four electrode symmetric system in which the inner electrodes (MN) are placed very close to each other and the distance between them is kept very small compared to current electrode distance (AB), usually less thean1/5th of the current electrode distance. The apparent resistivity values obtained with this array are attributed to the midpoint of the configuration, which is called the observation point ‘O’. This configuration has been deployed in the study area.
Aquifer test
An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" (drawdown) in observation wells. Aquifer testing is a common tool that hydrogeologist use to characterize a system of aquifers, aquitards and flow system boundaries.

A slug test is a variation on the typical aquifer test where an instantaneous change (increase or decrease) is made, and the effects are observed in the same well. This is often used in geotechnical or engineering settings to get a quick estimate (minutes instead of days) of the aquifer properties immediately around the well.

Aquifer tests are typically interpreted by using an analytical model of aquifer flow (the most fundamental being the Theis solution) to match the data observed in the real world, then assuming that the parameters from the idealized model apply to the real-world aquifer. In more complex cases, a numerical model may be used to analyze the results of an aquifer test, but adding complexity does not ensure better results (see parsimony).

Aquifer testing differs from well testing in that the behaviour of the well is primarily of concern in the latter, while the characteristics of the aquifer are quantified in the former. Aquifer testing also often utilizes one or more monitoring wells, or piezometers ("point" observation wells). A monitoring well is simply a well which is not being pumped (but is used to monitor the hydraulic head in the aquifer). Typically monitoring and pumping wells are screened across the same aquifers.
General characteristics
Most commonly an aquifer test is conducted by pumping water from one well at a steady rate and for at least one day, while carefully measuring the water levels in the monitoring wells. When water is pumped from the pumping well the pressure in the aquifer that feeds that well declines. This decline in pressure will show up as drawdown (change in hydraulic head) in an observation well. Drawdown decreases with radial distance from the pumping well and drawdown increases with the length of time that the pumping continues.
The aquifer characteristics which are evaluated by most aquifer tests are:
Hydraulic conductivity the rate of flow of water through a defined cross sectional area, at a unit of hydraulic gradient. In English units the rate of flow is in gallons per day, the cross sectional area is one square foot.
Specific storage or storativity: a measure of the amount of water of a confined aquifer will give up for a certain change in head;
Transmissivity the rate at which water is transmitted through a unit thickness of an aquifer under a unit hydraulic gradient.
Additional aquifer characteristics which are sometimes evaluated, depending on the type of aquifer, include:
Specific yield or drainable porosity: a measure of the amount of water an unconfined aquifer will give up when completely drained;
Leakage coefficient: some aquifers are bounded by aquitards which slowly give up water to the aquifer, providing additional water to reduce drawdown;
The presence of aquifer boundaries (recharge or no-flow) and their distance from the pumped well and piezometers.
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