Epithermal die-away porosity logging

In radioactive well logging where a source of fast neutrons includes a means for producing neutron pulses to irradiate earth formations penetrated by a borehole and where detector means detects resultant epithermal neutrons during successive intervals between said pulses, the method which comprises:

• (a) counting the output of said detector during each of a plurality of time windows following each of said pulses,
• (b) establishing a plurality of ratio functions from the number of neutrons counted during each time window and with the total number of neutrons counted during the preceding pair of time windows,
• (c) comparing said ratio functions with a predetermined reference level, and
• (d) establishing a porosity function in response to one of said ratio functions bearing a predetermined relation to said reference level.

In a radioactivity well logging system where a source of fast neutrons includes means for impulsing said source to irradiate earth formations and where detector means detects substantially only epithermal neutrons during the successive intervals between said pulses, the improvement which comprises:

• (a) means for counting the output of said detector during a plurality of time windows following each of said pulses,
• (b) means for establishing a ratio function from the number of neutrons counted during each individual time window and the total number of neutrons counted during the immediately preceding time windows having combined time lengths equal to that of said individual time window, said ratio function being proportional to the slope of the epithermal neutron die-away for the formation surrounding the borehole,
• (c) means for comparing said ratio function with a predetermined reference level, and
• (d) means for establishing a porosity function in response to each said ratio function which bears a predetermined relation to said reference level.

In porosity measurements wherein a source of high energy neutron pulses repeatedly is actuated to irradiate a medium and some of the neutrons emanating from said medium are detected by an epithermal neutron detector, the combination which comprises:

• (a) first means cyclically operable beginning with each said pulse for generating a plurality of control pulses of successively equal and longer time intervals the first of which is of a few microseconds in length,
• (b) a second means connected to said detector and to said first means for sequentially counting representative numbers of said epithermal neutrons during respective ones of said time intervals.
• (c) a third means for generating ratio functions representative of the ratios of said representative numbers for successively later equal length combinations of said time intervals, said ratio functions being proportional to the slope of the epithermal neutron die-away for the formation surrounding the borehole,
• (d) means for comparing said ratio functions successively with a predetermined reference level, and
• (e) means for establishing a porosity function in dependence upon the ratio function which earliest in time with reference to said pulses bears a predetermined relation to said reference level.

In porosity measurements wherein a source of high energy neutron pulses repeatedly is actuated to irradiate a medium and some of the neturons emanating from said medium are detected by an epithermal neutron detector, the combination which comprises:

• (a) time delay units sequentially operable beginning a predetermined time after each said pulse to generate output control states of the series .DELTA., .DELTA., 2.DELTA., 3.DELTA., 5.DELTA., 8.DELTA. where .DELTA. is a few microseconds in length,
• (b) a set of counters connected to said detector and to said set of delay units for counting representative numbers of said epithermal neutrons during each of said states,
• (c) means for generating ratio functions representative of the ratios of said representative numbers for successively later equal length combinations of said control states,
• (d) means for comparing said functions successively with a predetermined threshold level, and
• (e) means for establishing a porosity function in response to the ratio function which earliest in time with reference to said pulses bears a predetermined relation to said threshold level.

In porosity measurements wherein a source of high energy neutron pulses irradiates a medium and some of the neutrons emanating from said medium are detected by an epithermal neutron detector, the combination which comprises:

• (a) clock actuated means having a fixed pulse rate of frequency between about 5 kHz to 10 kHz repeatedly to actuate said source for periods of .DELTA. microseconds,
• (b) timing units operable beginning .DELTA. microseconds after each said pulse to generate output control states for each of time intervals .DELTA., .DELTA., 2.DELTA., 3.DELTA., 5.DELTA., and 8.DELTA. where .DELTA. is a few microseconds in length,
• (c) a set of counters connected to said detector and to said timing units for sequentially counting representative numbers of said epithermal neutrons during each of said time intervals,
• (d) first means for generating ratio functions representative of the ratios of said representative numbers for the combinations of said time intervals .DELTA./.DELTA., (.DELTA.+.DELTA.)/2.DELTA., (.DELTA.+2.DELTA.)/3.DELTA., (2.DELTA.+3.DELTA.)/5.DELTA. and (3.DELTA.+5.DELTA.)/8.DELTA.,
• (e) seconds means to compare said ratio functions successively with a predetermined threshold level at a sample rate low compared to said fixed pulse rate, and
• (f) third means to establish a porosity function in response to a selected ratio function which earliest in time following one of said pulses bears a predetermined relation to said threshold level.