Methods for utilizing radiation decay curves and for determining thermal neutron decay-time constants

摘要

The transition point is determined between a portion of a curve comprising a statistically single exponential function and another portion of the curve comprising a composite of said exponential function with another function. This method is useful in the oil and gas industry in the rapid and accurate determination of the thermal neutron decay time constant of the formation surrounding a borehole. The measured decay function following radiation of the formation with a burst of fast neutrons is integrated in a plurality of contiguous time gates between the end of the neutron burst and the decay to background radiation levels, the decay function is integrated over each of two subsets of contiguous gates whose leading edges are offset by a known time width, a succession of ratios is constructed for a succession of test intervals each defined by two such subsets, ratios for different test intervals each successively further in time from the end of the radiation burst are statistically compared, statistical compatibility of two successive ratios implies that the decay function is a statistically single exponential over the time spanned by the two test intervals, and once the portion of the decay curve represented by a statistically single exponential function representative of the surrounding formation is determined, the ■formation decay constant is easily and accurately determined.