Abstract: Methods and apparatus are disclosed for producing an indication of the partition between a borehole and a formation of the constituents identified by means of a nuclear investigation. A borehole logging sonde containing a pulsed neutron source and a gamma radiation detector traverses the borehole and resulting capture gamma rays are detected and counted according to energy in each of two time gates. The resulting energy spectra are analyzed to determine the type and relative gamma ray yield of each constituent of the borehole and formation. A characteristic neutron capture decay time constant for each constituent is derived from the yields and total gamma ray counts in the two time gates, and time constants for the borehole and formation overall are set equal to the derived time constants for constituents, such as iron and silicon, occurring predominantly in the borehole and formation respectively.

Abstract: An on-line environmentally-corrected measurement of the thermal neutron decay time (.tau.) of an earth formation traversed by a borehole is provided in a two-detector, pulsed neutron logging tool, by measuring .tau. at each detector and combining the two .tau. measurements in accordance with a previously established empirical relationship of the general form:.tau.=.tau..sub.F +A(.tau..sub.F +.tau..sub.N B)+C,where .tau..sub.F and .tau..sub.N are the .tau. measurements at the far-spaced and near-spaced detectors, respectively, A is a correction coefficient for borehole capture cross section effects, B is a correction coefficient for neutron diffusion effects, and C is a constant related to parameters of the logging tool. Preferred numerical value of A, B and C are disclosed, and a relationship for more accurately approximating the A term to specific borehole conditions.