Two Fortran computer programs described in this paper offer solutions to the "forward" and "inverse" problems of resistivity sounding. The "forward" problem consists of computing an apparent resistivity curve for some theoretical model. This is accomplished by program RESIST using linear filter theory (Ghosh, 1971a,b). The "inverse" problem consists of finding a theoretical model whose apparent resistivity curve matches a set of field data to reasonable accuracy. Program INVERSE accomplishes this by applying Marquardt's algorithm (Marquardt, 1963) to an initial model, modifying this model iteratively until it produces a match with the field curve. Both programs were written in Minnesota FORTRAN (MNF) for a Control Data Corporation Cyber 74 computer. The basic model used in both programs is an infinite half-space divided into a total of E horizontal layers, each electrically homogeneous and isotropic. The model parameters include the resistivity (R) and the thickness (H) of each layer. The bottom layer is assumed infinite in extent, so there are a total of 2E-1 parameters. Real field situations will not normally match these ideal conditions. Deviations from the model will produce errors in the field data, although if these errors are relatively small, a layered model interpretation is still possible. This paper does not address interpretation problems such as equivalence or suppression. For a discussion of these, see Davis (1979) or Mooney (1979) and references therein.
Davis, Philip A..
Information Circular 17. Interpretation of Resistivity Sounding Data: Computer Programs for Solutions to the Forward and Inverse Problems.
Minnesota Geological Survey.
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