The goal of this study is to show that physical modeling can provide important support for three-dimensional (3D) interpretation of electromagnetic geophysical data for environmental problems. This is specially true when high-frequency electromagnetic methods are used, which are difficult to model with existing 3D forward modeling programs. Existing electromagnetic geophysical systems usually operate in the frequency range of a few hertz to several hundred hertz. For environmental problems, such as characterization of waste sites, systems with higher frequencies are desirable. This is because at lower frequencies, the depth of investigation is too deep for environmental characterizations. This leads to subsurface images, which don`t have enough resolution to map small shallow objects. Electromagnetic 3D modeling programs which solve the full wave equation are still not widely available, even though 3D modeling has improved remarkably during the last few years (Oristaglio and Spies, 1995). Since such a program was not available for this study, we used a specialized 3D program EM1DSH (Zhou, 1989). With this program, we can model layered-earth cases, taking dielectric effects into account over the whole frequency range of interest. Stewart et al. (1994) published ellipticity curves for similar system configurations and frequency ranges that indicate that dielectric effects can not be neglected for model calculations using frequencies above several 100 kHz. EM1DSH can also model thin conductive sheets in a two-layer earth but neglecting dielectric effects. Therefore we are only able to model and compare our field data with 3D forward modeling results for the lower frequencies. One way of bridging the gap between the interpretation needs and limitations of existing 3D forward modeling programs is to conduct physical modeling experiments. 6 refs., 2 figs
