High-frequency electromagnetic impedance method for subsurface imaging

This paper presents a high-frequency electromagnetic (EM) impedance method that extends the utility of conventional controlled-source audio-frequency magnetotelluric (CSAMT) method to the frequency range from 100 kHz to 100 MHz. In this frequency range diffusion and wave propagation must be considered together. In principle, both the electrical conductivity and the permittivity of the shallow subsurface can be imaged using impedance data gathered on the surface of the earth. The impedance approach has a distinct advantage in that coupling with the source is unnecessary, provided that the source can be positioned far enough away to yield plane waves at the receiver positions. At high frequencies the EM impedance is a function of the angle of incidence or the horizontal wavenumber, so the electrical properties cannot be readily extracted without eliminating the effect of horizontal wavenumber on the impedance. For this purpose, this paper considers two independent methods for accurately determining the horizontal wavenumber, which in turn is used to correct the impedance data. The apparent electrical properties derived from the corrected impedance data correlate poorly to the real structure, especially for the model with a resistive overburden. However, the impedance data along with the incidence angles thus estimated can be successfully inverted to yield an accurate subsurface layering through the simulated annealing inversion scheme.