| 摘要 |
In the exemplary embodiments of the invention disclosed, thermal neutron decay characteristics of an earth formation, e.g. the thermal neutron decay time constant tau and the macroscopic capture cross section SIGMA , are measured by detecting indications of the thermal neutron concentration in the formation during a selected set of two different measurement intervals between neutron bursts and forming a ratio of such measurements, as accumulated for an appropriate length of time, from which the measured value of the decay characteristic is derived. 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 the ratio of the thermal neutron concentration measurements for the set, to afford enhanced statistical accuracy in the measured value of the decay characteristic. The measured value of the decay characteristic is then determined from the actual measured value of the ratio for the selected measurement interval set and the previously established relationship for that set. Each measurement interval preferably consists of a plurality of contiguous, discrete time gates whose durations and times of occurrence are adjustable in common by a finite number, e.g. four, of discrete scale factor values F. Each F value corresponds to a specific range of decay characteristic values, with the F value used for any given measurement being selected on the basis of a previously measured value of the decay characteristic. Regions of overlap are provided between the decay characteristic value ranges for adjacent F values so that F need not be changed merely as a result of statistical variations in the decay characteristic.
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