oa Ground Water Geophysics

In the exploration for, and the extraction and management of<br>ground water resources, surface geophysics is used for three<br>general objectives. These are:<br>(1) Delineation of aquifers and aquitards, and mapping<br>geologic features impacting movement and storage of<br>water in such strata (e.g., fracture and shear zones,<br>intrusive dikes).<br>(2) Mapping water quality. The economic value of ground<br>water resource depends on the concentration of the<br>dissolved solids in the water. In many coastal<br>aquifers fresh water resources of high value are<br>underlain by brackish and saline ground water of lower value.<br>(3) Wellhead protection, such as delineation of prime<br>recharge areas.<br>More than 90 percent of practicing exploration geophysicists<br>are employed by the oil and gas industry in the search for<br>hydrocarbons, and by far the dominant geophysical methods<br>employed is marine and land reflection seismics. The application<br>of geophysics to ground water is more balanced in terms of<br>techniques utilized, mainly because seismic properties are not<br>much influenced by water quality. On the other hand, electrical<br>resistivity is highly influenced by TDS, and electrical and<br>electromagnetic methods compete on equal footing with seismics<br>for their fair share of geophysical surveys in ground water<br>investigations. Case histories will illustrate the contribution<br>of surface geophysics to various ground water studies.