| 摘要 |
<p>In the exemplary embodiments of the invention disclosed, the decay of the thermal neutron concentration in an earth formation following irradiation thereof with a burst of fast neutrons is measured by detecting indications of the thermal neutron concentration during a sequence of contiguous, discrete time gates between neutron bursts. The time gate sequence begins following a discrete time delay after the commencement of the preceding neutron burst and extends over the entire, or substantially the entire, remainder of the interval between neutron bursts. The time gates are grouped into a number of gate groups, e.g. four, each of which includes a number, e.g. four, of discrete time gates. Both the time gates within each group and the groups themselves are contiguous in time. The durations of the individual discrete time gates within the separate 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, the duration of time delay between the end of the bursts and the beginning of the gating sequence, and the durations of the individual time gates may all be adjusted by a common, selected one of a finite number, e.g. four, of scale factor values. Selection of the particular scale factor value used in the measurement of a given formation is made in a manner to optimize the statistical precision of the measurement of thermal neutron decay constants, e.g.? and .SIGMA., of the formation.</p> |
