TIME-DOMAIN ELECTROMAGNETIC MIGRATION IN THE SOLUTION OF INVERSE PROBLEMS

Time-domain electromagnetic (TDEM) migration is based on downward extr apolation of the observed field in reverse time. In fact, the migrated EM field is the solution of the boundary-value problem for the adjoin t Maxwell's equations. The important question is how this imaging tech nique can be related to the solution of the geoelectrical inverse prob lem. In this paper we introduce a new formulation of the inverse probl em, based on the minimization of the residual-field energy flow throug h the surface or profile of observations. We demonstrate that TDEM mig ration can be interpreted as the first step in the solution of this sp ecially formulated TDEM inverse problem. However, in many practical si tuations this first step produces a very efficient approximation to th e geoelectrical model, which makes electromagnetic migration so attrac tive for practical applications. We demonstrate the effectiveness of t his approach in inverting synthetic and practical TDEM data.