HIGH-RESOLUTION ELECTROMAGNETIC IMAGING OF THE CONDUCTIVE EARTH INTERIOR

Ohmic dissipation in conductive media considerably limits the penetrat ive power of high-frequency electromagnetic imaging methods and implie s that deep regions can be probed only with low-frequency fields. Unfo rtunately, these low-frequency fields are governed by a diffusive equa tion which prevents direct high-resolution imaging as in seismic and g eoradar imaging. However, a clue for high-resolution imaging in the di ffusive approximation is given by a Fredholm integral equation of the first kind which links diffusive fields to their propagative duals. If these duals could be recovered by inverting this integral equation, t he seismic imaging toolbox might be used, at least from a theoretical point of view, to produce fine electromagnetic images. Spectral decomp osition of the integral operator shows that the inverse problem is num erically ill-posed for both noisy and/or incomplete data. High-resolut ion can be achieved only by adding sparsity constraints upon the sough t solution to the information content of the data. This type of a prio ri information also strongly regularizes the inversion but implies tha t the inverse problem must be treated as non-linear. A numerical algor ithm, designed to work in a continuous parameter space, couples both t he stimulated annealing and the simplex to recover the propagative fie ld. Numerical applications for pseudo-data with additive noise reveal that reflective interfaces can be imaged even within the poorly-favour able magnetotelluric setup.