UNDERGROUND IMAGING BY FREQUENCY-DOMAIN ELECTROMAGNETIC MIGRATION

A new method of the resistivity imaging based on frequency-domain elec tromagnetic migration is developed. Electromagnetic (EM) migration inv olves downward diffusion of observed EM fields whose time flow has bee n reversed. Unlike downward analytical continuation, migration is a st able procedure that accurately restores the phase of the upgoing field inside the Earth. This method is indented for the processing and inte rpretation of EM data collected for both TE and TM modes of plane-wave excitation. Until recently, the method could be applied only for dete rmining the position of anomalous structures and for finding interface s between layers of different conductivity. There were no well develop ed approaches to the resistivity imaging, which is the key problem in the inversion of EM data. We provide a novel approach to determining n ot only the position of anomalous structures but their resistivity as well. The main difficulty in the practical realization of this approac h is determining the background resistivity distribution for migration . We discuss the method of the solution of this problem based on diffe rential transformation of apparent resistivity curves. The final goal of migration is to provide a first order interpretation using a comput ational effort equivalent to a forward modeling calculation.