Inverting AEM data using a damped eigenparameter method

Abstract

Because of the non-uniqueness of the EM inversion problem, one must constrain the inversion procedure in some way to achieve a geologically valid model. All constraints are regarded as subjective except when they agree with the prejudices of the person using the inversion. Conventionally, constraints have included: inverting to models with a small number of parameters; forcing conformation with a priori geological information; or requiring smooth or minimal spatial derivatives with respect to conductivity (Occam inversion). We prefer to use the constraint philosophy of the damped eigenparameter method (also called the Jupp-Vozoff algorithm). After incorporating a priori geological knowledge in the starting model, this process constrains change to individual model parameters on the basis of the strength of their effect on the data when grouped into eigenparameters. The initial model can thus be regarded as a soft constraint. In this paper, we extend this inversion philosophy to the case where the number of unknown parameters exceeds the number of data points; i.e., an under-determined problem. Inverting to both undetermined and over-determined layered earth models, we have tested this method on two sets of synthetic AEM data: one generated from a 1D model, the other from a 2.5D model.