Underground imaging by frequency-domain electromagnetic migration

A new method of the resistivity imaging based on frequency-domain electromagnetic migration is developed. Electromagnetic (EM) migration involves downward diffusion of observed EM fields whose time flow has been reversed. Unlike downward analytical continuation, migration is a stable procedure that accurately restores the phase of the upgoing field inside the Earth. This method is indented for the processing and interpretation of EM data collected for both TE and TM modes of plane-wave excitation. Until recently, the method could be applied only for determining the position of anomalous structures and for finding interfaces between layers of different conductivity. There were no well developed approaches to the resistivity imaging, which is the key problem in the inversion of EM data. We provide a novel approach to determining not only the position of anomalous structures but their resistivity as well. The main difficulty in the practical realization of this approach is determining the background resistivity distribution for migration. We discuss the method of the solution of this problem based on differential transformation of apparent resistivity curves. The final goal of migration is to provide a first order interpretation using a computational effort equivalent to a forward modeling calculation.