Spectral Gamma-Ray Logging II: Borehole Correction Factors

Experimentally determined factors are presented that correct spectral gamma-ray logs for the effects of varying borehole fluid and casing attenuation. These corrections are for gamma-ray spectra from the naturally radioactive isotopes of potassium (K), uranium (U), and thorium (Th). Measurements were performed on the U. S. Department of Energy (DOE) calibration models in Grand Junction, Colorado. A sodium iodide spectral gamma-ray probe was used to collect data on the 1.46 MeV aroma ray from 4%, the 1.76 MeV gamma ray from 214Bi (a decay product of 2?8u), and the 2.61 MeV gamma ray from 208Tl (a decay product of 232Th). Count rates for these gamma rays were measured for various borehole fluid and casing conditions. Borehole fluid corrections are presented for both centralized and sidewalled probe geometries for hole diameters from 3 to 12 inches. For a typical hole diameter of 4.5 inches and a probe diameter of 2.0 inches, the water correction is 23 percent for the potassium signal, and 15 percent for the uranium and thorium signals. For the smaller borehole diameters, the water correction is nearly independent of probe placement. As the borehole diameter increases to 12 inches, the correction for the uranium and thorium signals increases to 50 percent for the sidewalled geometry and to 100 percent for the centralized geometry. Gamma-ray transport calculations were performed at the Los Alamos Scientific Laboratory to predict the effects of borehole water for the centralized probe geometry, and the resulting water factor corrections are somewhat lower than the measured values. The difference is attributed to the fact that detector response has not yet been included in the calculations. It is expected that future calculations, which will include detector response and sidewalled probe geometries, will provide accurate estimates of borehole correction factors. Casing corrections are presented for dry boreholes with thicknesses of steel casing from 1/16 inch to 1/2 inch. The data are fit reasonably well with exponential curves having effective linear attenuation coefficients of 1.26 inch^-l for uranium and 1.03 inch^-l for thorium.