method and apparatus for optimizing the measurement of the characteristics of the decay of thermal neutrons.

摘要

The thermal neutron decay characteristics of an earth formation are measured by detecting indications of the thermal neutron concentration in the formation during a selected set of two measurement intervals following irradiation of the formation with a burst of fast neutrons. These measurement intervals may consist of a sequence of contiguous, discrete time gates which begins following a discrete time delay after the preceding neutron burst and extends over substantially the remainder of the interval between neutron bursts. The time gates are divided into a number of groups. The durations of the individual discrete time gates within the groups are equal, but the duration of the time gates progressively increases from group to group in the sequence. The duration of the neutron bursts, time delay between the burst and the start of the sequence and of the individual time gates may all be adjusted by a common, selected one of a finite number of scale factor values. The particular set of measurement intervals used is selected from among a number of possible sets as a function of a previously measured value of the decay characteristic. Each measurement interval set is used over only a specific range of decay characteristic values for which it has been determined, in accordance with a previously established relationship between the decay characteristic value and a function of the thermal neutron concentration measurements for the set, to afford enhanced statistical accuracy in the measured value of the decay characteristic. IMAGE