Electromagnetic (EM) geophysical exploration methods, both time and frequency domain, have historically operated in two disparate frequency bands. Methods such as magnetotellurics (MT), CSAMT, loop-loop, and TEM work at frequencies of a millihertz to tens of kilohertz, investigating depths from several tens of meters to tens of kilometers. Wave propagation methods such as Ground Penetrating Radar (GPR) operate at frequencies above a few tens of megahertz, with exploration depths of a few meters. The intervening gap in exploration frequencies is critical to much near-surface geophysics as practiced by the environmental and hydrologic geophysics communities. The transition between the diffusion and wave modes of electromagnetic propagation occurs in a small spectral interval of about one decade of frequency. Below the transition, displacement currents are negligible and conventional diffusive electromagnetic theory provides an adequate description, while above the transition, conduction currents are small and wave propagation theory applies. The rock properties conductivity and dielectric permittivity are frequency dependent, i.e., best described as complex quantities over portions of the spectrum with conductivity increasing with frequency while permittivity decreases. At issue for any work in the transition portion of the spectrum is how the dispersive behavior of the properties may influence the observed data in electromagnetic surveys for delineation of targets associated with the shallow subsurface.


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