Spectral gamma ray logging - a cost effective method for uranium exploration

The most useful technique in uranium exploration programme is undoubtedly radiometric surveys. This is due to the fact that uranium emits gamma rays ranging from as low as 47 kev to 2.22 Mev, which can be detected and quantified using suitable radiation detector. Combination of aerial radiometric surveys, ground examination of the detected anomalies, followed by drilling and gamma ray logging of drilled boreholes has resulted in the discovery of large uranium resources. Borehole logging provides the most important sub surface information required for exploration. An area known to contain only uranium, computed gamma ray logging with a Geiger Muller (G.M.) Detector rapidly gives the required sub-surface radioactivity information. Whereas, in a heterogeneously mineralized area of uranium with thorium, logging data using G.M detector may mislead to wrong interpretation. Under such condition scintillation detector based spectral gamma ray logging is carried out. Identifying uranium in the presence of thorium is a complex process and this paper deals with a case study of spectral gamma ray logging carried out to locate the subsurface uraniferous zone in Pakkanadu area, Salem district of Tamilnadu, where the surface anomaly indicated the presence of high thorium content. The various limitations like small detector size, large sample volume, high correction factor required for quantifying the individual elements and advantage in optimization of data acquisition time has been discussed. (author)