Inversion of horizontal loop electromagnetic frequency soundings

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Abstract
A procedure has been developed to perform automatic inversion of HLEM frequency soundings by the method of Backus and Gilbert (BG). Using a linearized iterative scheme, a layered conductivity is constructed satisfying the available soundings to an accuracy consistent with the observational uncertainties. Subsequently, in order to investigate the resolving power of the data and hence (hopefully) to determine the broad scale features which are common to all conductivity models which satisfy the data, BG average conductivities are computed for the constructed model at a number of depths. In examples with both synthetic data and field data these averages proved to be identical (except for random error) to the corresponding average conductivities computed for a number of different models satisfying the same data. The additional models for this analysis were generated by maximizing or minimizing the conductivities of individual layers in the original constructed model using linear programming techniques. On the basis of these empirical results, in conjunction with the uniqueness theorem for HLEM soundings, it is concluded that BG average conductivities computed for a particular constructed model will often provide a valid characterization of the true ground conductivity over a depth range governed by survey parameters.

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