ASYMPTOTIC THEORY FOR DIFFUSIVE ELECTROMAGNETIC IMAGING

We propose an asymptotic theory for diffusive electromagnetic imaging. Three steps are required to perform this imaging. (1) A high-frequenc y solution is first constructed which mimics the one usually found in wave-propagation phenomena. (2) This solution, valid for a smooth cont inuous description of the resistivity in the medium, is used in a firs t-order Born approximation leading to a linear relation between the re sistivity perturbation of the subsurface and the perturbation of the e lectric signal obtained at the free surface. (3) This linear relation is asymptotically inverted by using an iterative quasi-Newtonian inver sion based on a least-squares criterion developed by Jin et al. (1992) . Although the extension to smooth heterogeneous reference medium is p ossible, we have only tested the inversion scheme for homogeneous refe rence media as Zhdanov & Frenkel (1983) previously did with another me thod.