Spectral Gamma-Ray (Kut) Borehole Logging

Abstract

Natural gamma-ray borehole logging is a recognized technique for evaluating uranium deposits and providing lithographic data. Spectral gammaray (KUT) borehole logging is a refinement of the technique which catagorizes gamma-rays, according to their energies, as comin9 from Potassium (K), uranium (U), and thorium (T). This spectral information enhances the usefulness of the natural gamma-ray log by providing additional information on the composition of the formation producing the gamma-rays.KUT logging is particularly applicable to evaluation of low-grade uranium deposits, and to cases where thorium anomalies are suspected. The ability to identify potassium and thorium can also aid in the classification of many rock strata.Calibration of a spectral gamma-ray probe for proper sensitivity to gamma-ray energy is of particular concern in KUT logging. A means of continuously controlling the sensitivity of a KUT probe, using a computer and a manganese-54 reference source, is described.Evaluation of elemental concentrations of potassium, equivalent uranium, and equivalent thorium, involves applying a stripping matrix of nine factors to the raw count rate data from the probe. Both the raw count rate data and the concentration data can be plotted versus depth. This evaluation procedure and the calibration method which yields the stripping matrix are described. Accuracy of the data produced, and the effect of logging rate on accuracy, are also discussed.