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For the development of groundwater resources in a given area, knowledge of the occurrence and movement of groundwater is required. In addition, the hydrological parameters of the aquifers are needed to decide the location and types of wells. The geological conditions of the area will influence the occurrence and movement of groundwater.
Hydrogeology refers to the study of the laws of occurrence and movement of groundwater, its chemistry and its relation to the geologic environment. Groundwater prospecting and exploration helps in locating groundwater and understanding some aquifer properties.
Methods for exploration, of groundwater may be classified as follows:
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1. Surface:
(i) Geological field reconnaissance, and
(ii) Geophysical methods.
2. Subsurface:
(i) Geophysical methods,
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(ii) Test drilling, and
(iii) Interpretation of well data.
Geological reconnaissance helps in understanding the geology of the area in relation to the possible occurrence of groundwater. A study of structural geology in conjunction with stratigraphy to locate possible water bearing formations is required. Stratigraphy helps in locating the position and thickness of water bearing formations and confining beds.
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Gravel, sand, sandstone and limestone are best water carriers even though all such deposits may not yield significant quantities of water. Clay formations, shales and crystalline rocks does not yield enough water for exploitation.
For detailed information professional geologists need to be consulted:
1. Surface Geophysical Methods:
Using geophysical methods, working on the earth’s surface it is sometimes possible to estimate where groundwater occurs and also under special conditions, information on water quality. These methods are not always successful but are less costly than subsurface investigations.
These methods have been originally developed for petroleum and mineral exploration. Although several methods are available for the purpose, two methods viz., the electrical resistivity and seismic refraction methods have been successfully used for groundwater investigation. Only the principles of these methods will be given here. Detailed procedures for the tests are to be obtained from literature on geophysical prospecting.
i. Electrical Resistivity Method:
In this method the electrical resistivity of the formations is measured by passing a current. The resistivity (P) of a material is given by-
Where, R is the resistance, A is the area of cross-section and L is its length. Its units are ohm-m2/m or simply ohm-m. The resistivity values obtained are interpreted to understand the nature of formations.
The procedure involves measuring the potential difference between two electrodes resulting from an applied current through two other electrodes (C1 and C2) outside but in line with the potential electrodes.
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Current electrodes consist of metal rods driven into the ground. Potential electrodes are porous cups filled with a saturated solution of copper sulphate to prevent electrical fields forming around them. When the distances AB = CD = BC = a, (known as Wenner arrangement) the apparent resistivity is given by
A D.C. current is applied between the electrodes and for different spacings of electrodes, values of V and I are noted. When the spacing between electrodes is increased, a deeper penetration of the electrical field occurs and a different apparent resistivity is obtained. The apparent resistivity is plotted against electrode spacing, for various spacing at one location and a smooth curve is plotted.
Comparing the nature of this curve with curves obtained under similar conditions, the nature of the strata and also the depth of the waterable is obtained. Curves obtained with different strata and known watertable conditions are published and available in geophysics literature (referred to as Mooney’s Curves).
ii. Seismic Refraction Method:
In this method, a small wave is created at the earth’s surface, either by using a dynamite charge or by the impact of a heavy instrument. The time required for the sound or shock wave to travel known distances is measured. These measurements are interpreted to determine the nature of substrata.
Seismometers, also known as geophones or detectors are used to record the shock waves. They receive the shock waves and convert the vibrations into electrical impulses which are subsequently recorded on an oscillograph along with the time of the initial explosion.
Fig. 6.6 (a) indicates how a spherical wave travels outwards from the shock point. Its speed is governed by the material through which it is passing. Velocities are greater in solid rock formations than in unconsolidated materials.
As the waves are intercepted by watertable they are refracted and propagated back. From the observations recorded, a time-distance graph as shown in Fig. 6.6(b) is plotted. From this graph, the velocities vi and vi in the upper layer and the aquifer are calculated. The depth to the watertable D is calculated using the following formula –
Where, S is the intercept shown in Fig. 6.6(b). Good results are obtained up to 75 to 100 m, though satisfactory results are reported up to 700 m depth.
2. Subsurface Geophysical Methods:
Subsurface geophysical methods consist of test drilling and obtaining what are known as well logs or borehole logs. Test drilling (also referred to as trial bores) consists of drilling small diameter bores for the purpose of obtaining geological and other information relating to the aquifer.
Information about each stratum and its depth as well as water levels and water samples for water quality analysis are obtained. The test bores can also be used as observation wells for measuring water levels or for conducting pumping tests to determine aquifer properties.
Geophysical measurements to provide a continuous log from surface to the bottom of the bore can be conducted. The geophysical well logging is an accurate and convenient way of obtaining many subsurface data. This provides information about the nature of formations in their nearly undisturbed condition.
This information can be used for understanding the subsurface conditions as well as for the design of tubewells.
There are several methods for well logging such as:
(i) Resistivity logs,
(ii) Spontaneous potential logs,
(iii) Radioactive logs,
(iv) Acoustic logs,
(v) Temperature logs and
(vi) Calliper logs.
In resistivity logging, the property of electrical conductivity by different earth materials and water is used. To obtain a resistivity log an electrode is suspended on a conductor cable and lowered into the uncased bore hole. An electric current is allowed to flow from the electrode in the well to another electrode which is placed near the top of the well and the resistance to the flow of current between the two electrodes is measured.
The logging tool is lowered down the bore hole and the recorded variation in resistivity is obtained in the form of a resistivity curve. Interpretation of the resistivity curve will give the nature of the strata encountered. Resistivity measurements, their interpretation and other well logging methods require appropriate knowledge of geophysics. Details of geophysical aspects of groundwater are given in Sharma and Chawla (1977) and Roscoe Moss Company (1990).