Inversion of airborne electromagnetic data

Abstract

A BSTR A C T A irborne electrom agnetic geophysics is based on analysis o f the interaction o f an electrom agnetic field w ith the geoelectric properties o f the earth. Inversion, or inverse m odelling, o f airborne electrom agnetic (AEM ) data refers to a particular m athem atical m ethodology for solving the A E M inverse problem , that is. ded u cin g the e a rth ’s geoelectric p ro p erties fro m o b served electro m ag n etic interactions. This is a difficult problem for several reasons. First, like m ost geophysical inverse problem s, the A EM inverse problem w ith a finite num ber o f noisy data is illposed, and consequently, the geoelectric properties o f the earth cannot be uniquely determ ined. To generate a unique solution a priori inform ation m ust be added to the inverse problem : a procedure referred to as regularisation. Second, since the geoelectric properties of the earth and the observed A EM data are not linearly related the inverse problem is nonlinear and requires solution by an iterative m ethod. Third, the forw ard problem o f calculating the response from a given geoelectric earth m odel, w hich is an essential part o f the inverse problem , is itself a difficult and tim e consum ing problem for 2.5D or 3D models. Fourth, A E M geophysics is characterised by enorm ous q u an titie s o f data. T hese difficulties and how they can be addressed are the focus o f this paper. Particular em phasis is placed on the non­ uniqueness o f the A E M inverse problem and how it can be resolved through regularisation using a priori inform ation. The applicability o f ID inversion in m ulti-dim ensional environm ents and the advantages o f m ulti-dim ensional inversion are dem onstrated, as is the potential value o f joint inversion o f A EM data and other geophysical data. IN T R O D U C T IO N