Pulsed neutron well logging.

摘要

A method of measuring high vertical resolution earth formation thermal neutron capture cross sections of earth formations in the vicinity of a well borehole, comprising the steps of: (a) repetitively emitting in a well borehole relatively short duration bursts of fast neutrons which are rapidly moderated by interaction with nuclei of materials in the borehole and surrounding earth formations and slowed to thermal energy, creating repetitive time dependent distributions of the thermal neutron population in the borehole and surrounding earth formations; (b) detecting, as a function of depth, in the borehole during time intervals between said repetitive bursts of fast neutrons, radiations representative of the thermal neutron population in the borehole and surrounding earth formations in at least two time subintervals subsequent to each burst of fast neutrons and generating at least two count rate signals representative of said thermal neutron populations during said at least two time subintervals; (c) filtering said count rate signals to reduce statistical fluctuations in subsequent computations; (d) combining said at least two filtered count rate signals according to a predetermined relationship to obtain a neutron die-away based measurement of the formation thermal neutron capture cross section SIGMA F; (e) selecting at least one of said at least two filtered gate count rate signals, or a sum of selected filtered count rate signals, to obtain a combined gate count rate signal G and using a predetermined functional relationship f(G) between G and the previously determined filtered formation capture cross section SIGMA F to obtain as a function of depth a normalization parameter Co; (f) obtaining a combined count rate signal G min from said at least one of said at least two count rate signals in step (b), prior to filtering in step (c), in a manner analogous to obtaining filtered count rate signal G; (g) determining from f(G min ) and the normalization parameter Co in step (e) a high resolution thermal neutron capture cross section SIGMA HR with the approximate vertical resolution of the count rate signals in step (b); and (h) recording SIGMA HR as a function of borehole depth to produce an enhanced depth resolution measurement of thermal neutron capture cross section, relative to the SIGMA F measurement in step (d). f