Inversion of airborne electromagnetic data

Abstract

Airborne electromagnetic geophysics is based on analysis of the interaction of an electromagnetic field with the geoelectric properties of the earth. Inversion, or inverse modelling, of airborne electromagnetic (AEM) data refers to a particular mathematical methodology for solving the AEM inverse problem, that is, deducing the earth's geoelectric properties from observed electromagnetic interactions. This is a difficult problem for several reasons. First, like most geophysical inverse problems, the AEM inverse problem with a finite number of noisy data is ill-posed, and consequently, the geoelectric properties of the earth cannot be uniquely determined. To generate a unique solution a priori information must be added to the inverse problem: a procedure referred to as regularisation. Second, since the geoelectric properties of the earth and the observed AEM data are not linearly related the inverse problem is nonlinear and requires solution by an iterative method. Third, the forward problem of calculating the response from a given geoelectric earth model, which is an essential part of the inverse problem, is itself a difficult and time consuming problem for 2.5D or 3D models. Fourth, AEM geophysics is characterised by enormous quantities of data. These difficulties and how they can be addressed are the focus of this paper. Particular emphasis is placed on the non-uniqueness of the AEM inverse problem and how it can be resolved through regularisation using a priori information. The applicability of 1D inversion in multi-dimensional environments and the advantages of multi-dimensional inversion are demonstrated, as is the potential value of joint inversion of AEM data and other geophysical data.