Spectral Gamma-Ray Logging III : Formation And Thin Bed Effects

The effect of borehole formation parameters on spectral gamma-ray probe response has been calculated from radiation theory. Results are presented for the effect of formation bulk density and composition on the gamma-ray spectra from potassium, uranium, and thorium. Experimentally determined response functions are presented for spectral gamma-ray probes logging through thin horizontal beds of potassium, uranium, and thorium. Potassium and thorium thin bed results were obtained by numerical differentiation of the probe response across the interface between a thick zone containing potassium or thorium and a thick barren zone. Uranium results were obtained both from differentiation of thick zone interface measurements and directly using thin bed uranium models. All measurements were performed at the U. S. Department of Energy (DOE) calibration facility in Grand Junction, Colorado, using sodium iodide detectors of various sizes. Detector size and shape was found to have a larger effect on the thin bed response function than did the formation parameters which can affect the transport properties of the gamma rays. Results have been used to deconvolve observed borehole logs and obtain concentration with depth for thin ore zones.