INTRODUCTION

Although the basic physics underlying nuclear well logging with gamma rays has long been understood,(Davidson and Evans, 1952) the inherent complexity of multiple scattering in a non-uniform medium has prevented an analytical treatment of the photon transport. Monte Carlo techniques have proven to be more successful, (MIDAS, 1982) but have also proven to require large amounts of computer time. This would make the study of tool response over a wide range of formations and mud cake conditions very expensive (MIDAS, 1982). If, in addition, one wished to check a number of proposed tool configurations, the cost would become prohibitive. In order to overcome the difficulties inherent in analytical and Monte Carlo models, a new model utilizing phenomenological techniques has been developed. This model combines analytical solutions, results of Monte Carlo analysis, and empirical fitting of free parameters into one model. It does this by breaking the problem of modeling the spectrum seen by the detector into a number of parts, each one of which is solved by the most appropriate method. The results of the model are then compared to data, first to fit the free parameters, and then to check the accuracy of the model.

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