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The measured data are thus interpreted in terms of subsurface resistivity and thickness of various 30 3 Delineation of Fresh Groundwater Zone on Island layers. In order to interpret the vertical electrical sounding data the following assumptions are made: • subsurface consists of layers of finite thickness, • layers are homogeneous and isotropic having uniform thickness, and • boundary planes separating each layer are horizontal. There are different methods to convert the measured data into subsurface apparent resistivity and layer thickness.

One of the methods is directly to drill a bore hole and delineate the saturated zone. However, the drilling in many cases may not be possible due to logistic, economic or other restrictions. Therefore, indirect methods to obtain the information about subsurface conditions are often being used. S. 1007/978-3-319-50073-7_3 25 26 3 Delineation of Fresh Groundwater Zone on Island Geophysical methods are amongst them (Zohdy et al. 1974). These methods can be carried out at number of desired places on ground surface.

11). The parameters thus arrived are as follows: T ¼ 467:5 m2 =d and S ¼ 1:1 Â 10À3 The application of above “Type Curve” method requires sufficient aquifer response during pumping phase. However, due to insufficient aquifer response during short duration pumping phase, the method is limited in its application. Therefore, pumping phase data and recovery data are considered together for the interpretation using numerical method (Rushton and Redshaw 1979; Singh and Gupta 1986; Gupta and Singh 1988).