Natural gamma ray spectroscopy applied to borehole measurements

This paper describes current techniques for performing natural gamma ray spectroscopy measurements in borehole environments. The data were obtained using the system known as the Compensated Spectral Natural Gamma tool which uses the complete energy spectrum to provide information on the borehole environment in addition to concentrations of potassium (K), uranium (U) and thorium (Th) in the formation. Elemental concentrations are estimated from count rates in thirteen selected energy ranges (windows) using the method of weighted-least-squares which incorporated real-time compensation for borehole effects. This is accomplished by using an attenuation model which accounts for count rate reduction and spectral shape changes due to borehole absorption and scattering. The use of a low atomic number toolcase allows gamma rays in the low energy, photoelectrically sensitive part of the spectrum to be transmitted to the detector. The system gain stabilizer operates in a coincidence mode so that formation gammas incident on the detector can be separated from those which originate in the /sup 241/Am stabilization source. This low energy data is processed into ratios of Compton and photoelectric window count rates. The ratio in uncased holes is optimized to provide qualitative indications of lithology and changes in lithology. A similar ratio in cased wells gives diagnostic information regarding the condition of the steel tubulars in the borehole. Two examples demonstrate the presentation of the logs and the quality of the results