Well logging method for determining formation characteristics using pulsed neutron capture measurements

Abstract

A method for determining thermal capture cross-sections of formations surrounding an earth borehole, including the following steps: providing a logging device that is moveable through the borehole; transmitting, from the logging device, bursts of neutrons into the formations; detecting, at the logging device, resultant gamma ray counts, and deriving a measurement spectrum from the gamma ray counts; deriving a forward model comprising a combination of model exponential components having respective model decay times and model amplitudes; deriving an error function that depends on comparison between the forward model and the measurement spectrum; and determining, by regularized inversion, optimized exponential components of the model that substantially minimize the error function; the optimized exponential components being indicative of the thermal capture cross-sections of the formations.