Influence of Borehole Construction on the Results of Spectral Gamma-Logging

In quantitative interpretation of gamma-logging results the absorption of gamma-radiation in drilling fluid, casing tubes, cement or in the mud cake must be taken into account. This absorption is generally described as the so-called transmission factor (T) which is the ratio of the radiation passing through the absorbers in the borehole in relation to the incident beam of radiation from the rock. Transmission factor T considered for a given photon energy E is called the spectral transmission factor T(E). If the intensity of gamma-radiation above some energy level Ep is measured, the integral transmission factor T(Ep) intervenes in quantitative interpretation. Up to now the spectral transmission factor T(E) was given theoretically using the absorption of primary radiation in the borehole absorbers, which was equal to the consideration of the function exp [-μ(E)r]r-2 as an elementary function of absorption of the narrow beam of gamma-radiation (r is the distance between the considered elementary radioactive volume αV and the observation point, μ(E), the attenuation coefficient of the considered medium for photons having energy E). The results for the transmission factor T(E) for this elementary function were too low in comparison with the experimental ones. To obtain good agreement between the theory and the experiment one must take into account some build-up of radiation from the forward scattered gamma-photons. It is the aim here to derive the formulas for the T(E) factors for the cylindric and plane absorbers. Comparison of the theoretical values of T(E) with the experimental ones, obtained in the uranium ore model, for the iron and aluminium absorbents of different thicknesses, gave an excellent agreement between the theory and experiment in the whole range of the photon energy. Special nomograms for the rapid calculation of the T(E) values are given. (author)