Electromagnetic (EM) instruments have been used for the past 20 years to map areas of elevated subsoil conductivity, but only very recently have there been any changes to the design of the instrumentation or to the techniques of its use. The biggest change to EM instrumentation occurred in September 1997 with the commercial introduction of a multi-frequency EM unit that can be programmed from 300 to 20,000 Hz. This opens up a huge range of potential uses, because now one person operating the equipment can detect changes in EM responses from near surface to 40 m in depth. When combined with a real-time global positioning system (GPS), the labor component is removed because the GPS creates a virtual grid, thus eliminating the need for a physical grid. This helps to reduce costs, while maintaining sub-meter accuracy of the survey. Automated software processing can enable data reduction and mapping in the field, thereby avoiding expensive delays waiting for the EM survey map. The advantages of mapping a site with EM prior to sampling are better delineation of target areas for drilling and reliable estimates of horizontal plume migration. With the advent of the multi-frequency EM, a much better estimate of depth of contamination can be obtained.

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