7. Use of Electromagnetic Methods for Groundwater Studies

Inductive electromagnetic survey methods are now widely used to map near-surface geology by mapping variations in the electrical conductivity of the ground. Such variations generally are caused by changes in soil structure (porosity), clay content, conductivity of the soil water, and degree of water-saturation in the soil. The frequency-domain methods which can be used to map terrain conductivity are very low frequency (VLF, including VLF resistivity), the audio-magnetotelluric and controlled-source audio-magnetotelluric methods, and Slingram (including ground conductivity meters and borehole induction loggers). The time-domain methods include those using either fixed- or moving-loop configurations. The advantages and disadvantages of each technique are presented in sufficient detail to allow the prospective user to decide which method is most appropriate for a particular groundwater problem. Case histories are presented which illustrate the usefulness of some of the electromagnetic methods for groundwater exploration, mapping industrial contaminants in groundwater, mapping groundwater quality, and mapping soil salinity in connection with the growth of crops.