Geophysical Prospecting for Groundwater

Abstract

Geophysical methods are based on the measurement of physical properties of subsurface lithosphere units. The gravity and magnetic methods are based on the measurement of density and susceptibility contrast, respectively. In ground water exploration, these surveys are required generally to determine bedrock depth contours in different aquifer-bearing structures such as faults, folds, veins, and dykes. Seismic method is based on the measurement of velocity contrast and used mainly to locate the overburden thickness. Within the overburden, the particular velocity range determines the groundwater-bearing zones. Electrical and electromagnetic methods are based on the measurement of electrical resistivity or conductivity of subsurface minerals and rocks. Electrical method is very popular in groundwater prospecting. Here, current and potential differences are measured between two points (probes) when the ground is energized by another two current electrodes. Here, the ratio of voltage and current multiplied by geoelectric constant provides the apparent resistivity. Interpretation with curve-matching techniques provides the true resistivities and respective thicknesses. Different range of resistivities indicate different rock types or minerals. Thus, the sand with water contents is determined. The electromagnetic method is based on the principle where primary wave is sent through the transmitter to the ground, and depending on the intensity of signals, it encounters the depth. The secondary wave generated inside the ground is received by the receiver with time. From these secondary waves, the apparent resistivities are calculated. Presence of groundwater then understood with resistivity range.