Accurate description of transport pathways on the gross scale, the location of contamination, and characterization of heterogeneity within the vadose zone, are now realized as vital for proper treatment, confinement and stabilization of subsurface contamination at Department of Energy (DOE) waste sites. Electromagnetic (EM) methods are ideal for these tasks since they are directly sensitive to the amount of fluid present in porous media, as well as fluid composition. At many DOE sites it is necessary to employ lower frequency (<1 MHz) or diffusive electromagnetic fields because of the inability of ground penetrating radar (GPR) to penetrate to sufficient depths. The high frequency impedance method, which operated in the diffusive frequency range (10 Hz to 1 MHz), as well as the low end of the spectrum employed by GPR (1MHz-10 MHz), is an ideal technique to delineate and map the aforementioned targets. Unfortunately, one does not precisely know when one is in the far-field of the transmitter, because this depends on the geology we wish to image. The second limiting factor is the scarcity of complete 2D and 3D inversion schemes necessary to properly invert the data. While approximate 2D schemes are now emerging, rigorous 2D and 3D inversion codes are needed to bound the range of applicability of the approximate methods. We propose to address these problems in the following manner: (1) implement full non-linear 2D/3D inverse solutions that incorporate source coordinates and polarization characteristics, (2) use these solutions to study improvements in image resolution that can be obtained by making measurements in the near- and mid-field regimes using multiple source fields, (3) collect data at the Hanford Reservation with recently developed earth impedance measurement systems, and (4) interpret the field data with the newly developed inversion capability, as well as with additional and independent information such as well logs from boreholes.
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