Method for determining thermal neutron capture cross-section of earth formations using measurements from multiple capture gamma ray detectors

摘要

A method for determining thermal neutron capture cross-section of an earth formation penetrated by a wellbore from counts of capture gamma rays detected at spaced apart locations from a source irradiating the formations with bursts of high energy neutrons. An apparent capture cross-section is determined from counts detected at one of the spaced apart locations. The apparent capture cross-section is adjusted by a correction factor calculated from counts detected at each of the spaced apart locations. The correction factor is calculated from an empirical relationship of known values of formation capture cross-section, wellbore diameter and wellbore capture cross section, with respect to counts detected at each of the spaced apart locations. Coefficients of the empirical relationship are determined by simulating counts which would be detected in response to the known values of formation capture cross-section, wellbore diameter and capture cross section. The coefficients are adjusted until differences between capture cross-section calculated by the relationship from the simulated counts and the known values of formation capture cross-section are minimized. In the preferred embodiment, coefficients are calculated for each one of at least two time regions into which the counts between successive bursts are segregated.