A Generalized Approach to the Interpretation Of Controlled Source Electromagnetic Data Collected in Sedimentary Basins

Abstract

Electrical and electromagnetic (EM) methods are widely used to explore layered geological structures such as sedimentary basins. Current methods are well developed but have rather low resolution and are easily disturbed if the earth is not well layered. To improve the resolution and accuracy of sounding methods requires that a very large set of field measurements be taken over a wide range of spatial scales and frequencies. In practice, such measurements are usually made about a spatially extended EM source. However, the mathematical complexities of modeling extended source surveys tends to obscure the simple physics behind the method which, in turn, makes interpretation difficult. I have devised a new method to aid the interpretation of survey data acquired about an extended source on a quasi-stratified terrain. The method overcomes the deleterious effects of a complicated source/receiver geometry and makes it possible to account for moderate lateral structure in the earth. It is shown that EM fields can always be factored into "geometrical" operators and "electrical" kernels. The electrical kernels represent the geometrical expansion of the EM field in the earth and are independent of the survey geometry. Conversely, the geometrical operator is independent of the earth model and represents the source/receiver configuration. Field data are transformed to estimate the "average" electric kernels of the horizontal layering. The layered earth data is then interpreted using standard inversion techniques to obtain the best fitting one dimensional model. The EM fields scattered from lateral heterogeneity are defined as the residual data which cannot be interpreted in terms of layered earth models. I develop some simple linear methods of interpreting the scattered fields in terms of anomalous lateral structure in the 1D earth. To test the interpretation concepts, a field study was carried out in the sedimentary sequence of southern Ontario using a grounded bipole source. The survey site is mainly layered, but there is also strong evidence of scattering in the data. The data were successfully separated into layered earth electric kernels and the EM fields scattered from the lateral structure, and subsequently interpreted. The results of the interpretation are consistent with the known geology indicating the utility of the methods developed in this thesis.