device for winning a neutronenabklingzeitcharakteristik in erdformationen

摘要

Apparatus and methods for verifying the validity of data derived at least in part by neutron logging of earth formations, and, where indicated, for affording neutron diffusion-corrected values of such data, are disclosed. In the illustrative embodiments described, an earth formation is irradiated with discrete pulses of high-energy neutrons, and a neutron characteristic of the formation is measured at a first location spaced from the neutron source by observing the neutron population decay at that location during a time period between pulses that is a function of a like earlier measurement of the neutron characteristic. The neutron population decay at a second location in the formation is also measured during a time period that is functionally related to a measured value of the neutron characteristic, conveniently the earlier measured value at the first location. An indication is then derived in response to the population decay measurement at the second location of the validity, specifically, of either the earlier or current value of the neutron characteristic measured at the first location as a measure of the intrinsic value of the neutron characteristic and, generally, of the validity of other neutron-logging derived data as well. Advantageously, the neutron characteristic measured is a thermal neutron absorption characteristic of the formation, such as the thermal decay time constant. In effect, apparent values of the neutron characteristic are observed at both measurement locations and a comparison of these observations made to determine the extent of correlation therebetween. Close agreement of the apparent values indicates an accurate measurement of the intrinsic formation characteristic. Substantial disagreement betweenthe apparent values evidences that perturbational effects have significantly influenced the neutron characteristic measurement and that appropriate correction of the measurement should be made.