Natural gamma ray spectroscopy applied to borehole measurements

Current techniques for performing natural gamma-ray spectroscopy measurements in borehole environments are described. 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 13 selected energy ranges (windows) using the method of weighted-least-squares, which incorporates real-time compensation for borehole effects. 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. The low-energy data are processed into ratios of Compton and photoelectric-window count rates. Two examples demonstrate the presentation of the logs and the quality of the results.<>