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Inversion and SVD analysis of frequency-domain EM data


, : Inversion and SVD analysis of frequency-domain EM data. SEG Abstracts 1985(1): 251-254

The traditional application of frequency-domain electromagnetic (EM) surveying has been the location and definition of discrete conductors in the search for sulfide mineralization. Increasingly, however, both airborne EM (AEM) methods and ground EM (GEM) methods are being used for mapping the variation in ground conductivity, vertically as well as laterally. Two parallel methods have been developed for the automated inversion of AEM and GEM data to a layered earth model. There are two principal differences in the two techniques: (1) different methods of integration are used to evaluate the kernel functions contained in the forward solutions to the layered earth model; and (2) the models themselves differ. The AEM method utilizes a two-layer model with the uppermost layer defined by depth-to-top (beneath the EM sensor), apparent resistivity and thickness parameters, and the second layer by an apparent half-space resistivity. The GEM method utilizes an n-layer model, each layer defined by apparent resistivity, magnetic permeability and thickness parameters (the nth layer by an apparent half-space resistivity and magnetic permeability). A half-space model is also available for processing AEM data. Geologic or geophysical knowledge may be incorporated by holding some of the layer parameters fixed during inversion. It is not only important to obtain values for desired parameters through an inversion of geophysical data, but also to know the reliability of each fitted parameter and its contribution to the overall model. Analysis of the fitted parameters by singular value decomposition (SVD) is an integral part of the inversion technique. This allows the inherent ambiguities of the model-fitting procedure to be described in a more physical way than by a simple statement of the errors in the model parameters. The inversion technique has been tested and implemented on a wide variety of frequency-domain AEM and GEM configurations. It has proven to be a significant advance for the mapping of the earth's conductivity using the EM method.

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