Patent Document ID: 5536938
Application ID: 08391852
Patent Flag: 1

Claim One:
1. A method for converting signals representing the die-away of nuclear radiation in a subsurface formation surrounding a borehole into a log of certain intrinsic formation properties versus depth within the formation, comprising the steps of: a) lowering a logging tool having a pulsed source of fast neutrons and first and second spaced-apart secondary radiation detectors into a borehole traversing a subsurface formation whose intrinsic properties are to be determined, b) irradiating said subsurface formation at an initial depth position within said borehole with fast neutrons from said neutron source, c) recording intensity signals with said first and second spaced-apart secondary radiation detectors representing the die-away of nuclear radiation after said fast neutrons from said neutron source have passed through and been moderated by said subsurface formation, d) generating a model of the die-away of nuclear radiation in a subsurface formation, such model consisting of exponentially varying terms containing amplitudes and decay constants responding to borehole and formation effects, e) producing weighted moments of said model and of said intensity signals from said first and second detectors, f) equating said weighted model moments with said weighted intensity moments for each of said first and second detectors, g) solving simultaneously said equated weighted model moments and weighted intensity moments to obtain values for a borehole decay constant, a formation decay constant and a formation-to-borehole amplitude ratio for each of said first and second detectors, h) producing intrinsic values of a formation macroscopic thermal neutron absorption cross section, a formation porosity and a borehole fluid cross section from said borehole decay constant, said formation decay constant and said formation-to-borehole amplitude ratio for each of said first and second detectors using a trained neural network, i) traversing said logging tool along said borehole from said initial depth position, j) repeating steps b) to h) as said logging tool traverses along said borehole and k) generating a log of said intrinsic values of said formation macroscopic thermal neutron absorption cross section, said formation porosity and said borehole fluid cross section versus depth within said subsurface formation.