Traditional electromagnetic induction methods,
working
at frequencies low enough that displacement currents can be ignored,
have been widely used for hydrologic and environmental studies.
Because many environmental problems involve organic compounds
encountered within 5 meters of the surface, the idea of going to
higher frequencies has been investigated [ Stewart et
al., 1994]. The use of higher frequencies can provide greater
resolution and increased sensitivity to variations in dielectric
properties of the ground. The high-frequency sounder (HFS) described
by Stewart et al. is a frequency-domain, loop-loop
instrument operating in the frequency range of 300 kHz to 30 MHz with
an intercoil spacing of one to four meters. It was originally designed
for detection of organic pollutants, such as gasoline, floating on the
water table. Development of this instrument required new
numerical techniques for data interpretation [
Anderson, 1991, 1994]. The prototype instrument has shown great
promise, but has yet to be tried on an organic contaminant spill.
While the HFS method obtains information about conduction and dielectric properties in the depth range between GPR and EM methods, the original configuration of the USGS sounder required a great deal of time for data collection. One approach being tried to improve data collection speed is to go to a time-domain instrument [ Pellerin et al., 1994]. This instrument, called the very early time electromagnetic (VETEM) system, is the focus of a large research effort funded by the Department of Energy to develop numerical modeling techniques, physical modeling, instrumentation, and interpretation tools. The instrument will consist of a small diameter (30-100 cm) transmitter loop with a peak current of 1-10 A. Transmitter turn off time will be less than 10 ns with receiver windows in the 10-1000 ns time range.
A promising development is the adaptation of the nuclear magnetic resonance (NMR) method for ground-water exploration. This technique, which was developed in Russia, provides a means of directly detecting the presence of subsurface water. Lieblich et al. [1994] describe the use of this method at a site in Connecticut.