Advances in electromagnetic methods for groundwater studies

Abstract

In the past decade, both inductive electromagnetic survey instrumentation and associated interpretive techniques have become refined to the point that electromagnetic techniques are widely used for geological mapping as well for the direct detection of conductive ore bodies. Electromagnetic survey techniques have been particularly successful in exploration for potable groundwater, for measuring salinity levels in aquifers and monitoring coastal saline intrusion, and for mapping soil salinity in connection with crop growth. Regardless of the techniques employed, it is the terrain conductivity that is measured, and it is a particular advantage of electromagnetic techniques that small variations in the bulk conductivity of the terrain can often be detected. A further advantage is that most electromagnetic techniques allow measurements to be made rapidly, and survey costs are generally less than those associated with conventional DC resistivity surveys or, conversely, larger areas can be surveyed in greater detaul for comparable cost. A disadvantage of electromagnetic instrumentation is that although the shallower units cost about the same as resistivity equipment, the deeper penetration systems are relatively expensive. In general, electromagnetic systems are most effective in looking for the better conductors and are ineffective in searching for resistive material. In all cases some knowledge of electromagnetic theory is desirable for a successful interpretation. In this paper we present several case history selected from the literature in which a variety of electromagnetic systems (horizontal loop EM, ground conductivity meters and VLF) are used either alone or in conjunction with conventional resistivity to explore for groundwater.