THREE-DIMENSIONAL INVERSION OF MULTITRANSMITTER ELECTROMAGNETIC DATA BASED ON THE LOCALIZED QUASI-LINEAR APPROXIMATION

Abstract

Three-dimensional (3-D) electromagnetic (EM) inversion is increasingly important for the correct interpretation of EM data sets in complex environments. To this end, several approximate solutions have been developed that allow the construction of relatively fast inversion schemes. We have developed a localized quasi-linear (LQL) approximation that is source-independent and, therefore, appropriate for multisource array-type surveys, typical in many geophysical applications, such as airborne EM, cross-well tomography, and well logging. This method is based on the assumption that the anomalous electric field within an inhomogeneous domain is linearly proportional to the background electric field through an electrical reflectivity tensor λ^ . This tensor is determined by solving a source-independent minimization problem based on the integral equation for the scattering currents. We have also developed a new, fast 3-D EM inversion method, based on this new approximation, and applied it to synthetic and real helicopter-borne EM data. The results demonstrate the stability and efficiency of the method and show that the LQL approximation can be a practical solution to the problem of 3-D inversion of multitransmitter frequency-domain EM data.