| 摘要 |
A borehole logging tool with a pulsed source of fast neutrons is lowered into a borehole traversing a subsurface formation, and a neutron detector measures the die-away of nuclear radiation in the formation. A model of the die-away is produced using exponential terms varying as the sum of borehole, formation and thermal neutron background components. Exponentially weighted moments of both the die-away measurements and a model are determined and equated. The equated moments are solved for the ratio of the borehole to formation amplitude components of the measurements. The formation decay constant is determined from the formation and thermal neutron background. An epithermal neutron lifetime is determined from the formation decay constant and is used with the amplitude ratio by a trained neural network to determine a lifetime correction. A standoff corrected lifetime is determined from the epithermal neutron lifetime and the lifetime correction. A porosity log is produced which is corrected for detector standoff from the borehole wall as a function of the standoff corrected epithermal neutron lifetime calibrated in borehole models of known porosities.
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