In surface geophysical methods of exploration, certain physical properties and parameters of subsurface formations and contained fluids are measured by instruments located on the surface. Among various methods of Geophysics, the electrical methods are most commonly employed due to its simple data acquisition, processing and interpretation with less expenses. The main objectives of the geophysical surveys are to provide the following information:

• Thickness of weathered and fractured zones and depth to massive basement rocks, delineation of solution cavities in Karst formations.

• Structural and stratigraphic condition controlling ground water occurrences.

• Potentials of aquifers on the basis of correlation studies.

The problems of the use of geophysical methods in the assessment and management of ground water may be classified into two categories: soft rock area problems and hard rock area problems

    Soft rock areas

The application of geophysical methods in soft rock areas for ground water is well established. A stepwise detailing in soft rock areas will be as follows:

• Schlumberger vertical electrical sounding at prospective sites.

• Recommendation of the borehole points.

• Electrical logging of the borehole after the drilling for detailing of the water well.

• Planned utilization of available ground water after water quality analysis.

• Recommendation of a sustained yield throughout the year and over the years.

    Hard rock areas

The stepwise plans given for the soft rock areas have been founded to be quite suitable for hard rock areas also. However, the integration of geophysical methods and techniques will be much useful to delineate interested ground water zones in the subsurface formations. A step-wise detailing in hard- rock areas will be as follows.

• The Gradient resisitivity profiling or Wenner resistivity profiling should be carried out to delineate the probable points for drilling. The probable point must lies on the sharp anomalies of different profiling.

• For obtaining the probable points, Horizontal Loop Electro Magnetic (HLEM) profiling at two frequencies and Very Low Frequency (VLF) method may also be used.

• To carryout vertical electrical sounding (VES) at the probable points for further delineation of the aquifer zones.

• The sounding data must be interpreted manually consequently the data should be inferred by Computer aided Software’s.

• The Interpreted results must be standardized with the nearby available borehole data or with local geology.

• Kink analysis and Factor analysis are very important tool for semi qualitative interpretation of the sounding data.

• The densely populated areas where there is lack of space for spreading, the Pole-Dipole resistivity sounding technique may be used to delineate the sub-surface formation accurately.

• On the basis of Vertical Electrical Sounding results carried out water shed/basin the Vertical Electric Cross-section and Horizontal Electric Cross-section may be prepared to understand the subsurface formation in detail.

• The geo-electrical layer parameters enables determination of Total Longitudinal Conductance (C) and Total Transverse Resistance (Z) rather more precisely than any other parameters. These parameters are very useful in identifying potential ground water bearing zones in the area under investigation.

Geophysical well logging, also called borehole geophysics, includes all the techniques of lowering sensing devices in a borehole and recording some physical parameter that may be interpreted in terms of rocks their contained fluids and construction features of wells. The depth wise sequential record of physical parameters such as resistivity, temperature etc. constitutes a geophysical well log. The main objectives of the Geophysical well logging are to provide the following information:

• Identification of the position of water table and of parched zones of saturation.

• Determination of bed contacts and bed thickness.

• Stratigraphic Correlation.

• Determination of aquifer parameters.

• Evaluation of quality of formation water.

• Determination of depth of casing.