Simulations of spectral gamma-ray logging tool response to formation and borehole wall source distributions

We have developed simulation models for the large-detector spectral gamma-ray (SGR) logging tool in use at the Nevada Test Site. Simulations show that the shape and magnitude of gamma-ray spectra from sources distributed on the borehole wall, such as potassium-bearing mudcakes or from plate-out of radon daughters, depend on detection position within the air-filled borehole as well as on hole diameter. No such dependence is observed for sources uniformly distributed in the formation. In addition, sources on the borehole wall produce anisotropic angular fluxes for certain energies. These differences in borehole effects and in angular flux are important to the process of correcting SGR logs and they suggest a technique for distinguishing between spectral contributions from formation sources and sources on the borehole wall. These results indicate the existence of a standoff effect not present for spectra measured in air-filled boreholes from formation sources. (Author)