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Electrical Resistivity Method
The electrical resistivity of a rock formation limits the amount of current passing
through the formation when an electrical potential is applied. Resistivity of rock
formations vary over a wide range, depending upon the material, density,
porosity, pore size and shape, water content and quality, and temperature. There
are no fixed limits for resistivities of various rocks; igneous and metamorphic
rocks yield values in the range 102 to 104 ohm-m. In relatively porous
formations, the resistivity is controlled more by water content and quality within
the formation than by the rock resistivity. The interpretation of the resistivity curves
needs experience and expertise which can be developed after working for few
years in different areas.
Seismic Refraction Method
The seismic refraction method involves the creation of a small shock at the
earths surface either by the impact of a heavy instrument or by exploding a
small dynamic charge and measuring the time required for the resulting sound,
or shock wave to travel known distances. Seismic waves follow the same laws
of propagation as light rays and may be reflected or refracted at any interface
where a velocity change occurs. Seismic reflection methods provide
information on geologic structure thousands of feet below the surface, whereas
seismic refraction methods - of interest in groundwater studies - cover only a
few hundred feet. The travel time of a wave depends upon the media through
which it is passing; velocities are greatest in solid igneous rocky and least in
unconsolidated materials.
Air Photo Interpretation
Proper interpretation of aerial photographs of a region can provide information
on the terrain characteristics, vegetation, land form use, drainage pattern,
alluvial planes, even buried channels. Groundwater maps prepared from such
observations can predict areas of most and least promising for ground water
supplies. Such maps aid in selecting test drilling sites, reduce costs of
groundwater investigations and assist in locating industrial plants requiring large
water supplies.
