Background subtraction system for pulsed neutron logging of earth boreholes

Claim 3

The method of claim 1 wherein the step of correcting said inelastic gamma ray representative information is performed by subtracting said background representative information from said representative information detected during said neutron pulses.

Claim 4

The method of claim 1 wherein the steps are performed repetitively while moving a well tool through the borehole and the compensated inelastic gamma ray representative information are logged as a function of borehole depth of said tool.

Claim 5

The method of claim 1 wherein the step of detecting, immediately following each high energy neutron pulse, the background gamma radiation, is performed by detecting gamma radiation occurring during a relatively short duration interval, said background gamma radiation detecting interval being of comparable duration with said inelastic gamma ray detection interval duration.

Claim 6

The method of claim 4 wherein said relatively short duration detection interval comprises approximately an eighteen microsecond duration interval.

Claim 7

The method of claim 1 wherein said representative information comprises counts related to the detection gamma rays.

Claim 8

Apparatus for determining the characteristics of earth formations surrounding a well borehole comprising: means for repetitively irradiating the earth formations surrounding the well bore with relatively short duration pulses of high energy neutrons; means for detecting during each such pulse of high energy neutrons, gamma radiation due to the inelastic scattering of neutrons by materials comprising the earth formations surrounding the borehole and for storing gamma ray counts representative thereof; means for detecting immediately following each such pulse of high energy neutrons, background gamma radiation due to thermal neutron capture and for storing counts representative thereof; and means for correcting said inelastic gamma ray counts to compensate for said background counts.

Claim 9

Apparatus for determining the characteristics of earth formations surrounding a well borehole comprising: means for repetitively irradiating the earth formations surrounding the well bore with relatively short duration pulses of high energy neutrons; means for detecting during each such pulse of high energy neutrons, gamma radiation due to the inelastic scattering of neutrons by materials comprising the earth formations surrounding the borehole and for storing gamma ray counts representative thereof; means for detecting immediately following each such pulse of high energy neutrons, background gamma radiation due to thermal neutron capture and for storing counts representative thereof; and means for correcting said inelastic gamma ray counts to compensate for said background counts.

Claim 10

The apparatus of claim 7 wherein said correcting means includes means for subtracting said background counts from said counts occurring during said neutron pulses.

Claim 11

The apparatus of claim 7 further comprising: means for moving said apparatus through the borehole; and means for logging the compensated inelastic gamma ray counts as a function of borehole depth of said apparatus.

Claim 12

The apparatus of claim 7 wherein the respective detection intervals of said background detecting means and said inelastic gamma ray detecting means are comparable in duration.

Claim 13

The apparatus of claim 10 wherein the duration of the respective detection intervals is approximately eighteen microseconds.

Claim 14

The apparatus of claim 7 wherein said irradiating means is operated at a repetition rate of about 4,000 pulses per second or greater.

Claim 19

Apparatus for investigating the composition of an earth formation traversed by a well bore, comprising: (a) means for repetitively irradiating the formation with time-spaced bursts of fast neutrons; (b) means for detecting gamma rays resulting from interactions between the incident neutrons and constituents of the formation and for generating signals representative of the energies of the detected gamma rays; (c) means responsive to the signals from the detector means for segregating them into a plurality of channels according to the energies of the corresponding gamma rays, thereby to generate an energy spectrum of said corresponding rays; (d) means for controlling the operation of the spectrum generating means so as (1) to generate a first energy spectrum of gamma rays detected during first time periods related to the occurrence of the neutron bursts such that the gamma rays detected therein result predominately from inelastic scattering neutron interactions and (2) to generate a second energy spectrum of background gamma rays detected during second time periods which closely follow said first time periods during which the detected gamma rays result predominately from neutron interactions other than inelastic scattering interactions; and (e) means for utilizing signals representing said second energy spectrum to moddify signals representing said first energy spectrum.

Claim 20

Apparatus for investigating the composition of an earth formation traversed by a well bore, comprising: (a) means for repetitively irradiating the formation with time-spaced bursts of fast neutrons; (b) means for detecting gamma rays resulting from interactions between the incident neutrons and constituents of the formation and for generating signals representative of the energies of the detected gamma rays; (c) means responsive to the signals from the detector means for segregating them into a plurality of channels according to the energies of the corresponding gamma rays, thereby to generate an energy spectrum of said corresponding rays; (d) means for controlling the operation of the spectrum generating means so as (1) to generate a first energy spectrum of gamma rays detected during first time periods related to the occurrence of the neutron bursts such that the gamma rays detected therein result predominately from inelastic scattering neutron interactions and (2) to generate a second energy spectrum of background gamma rays detected during second time periods which closely follow said first time periods during which the detected gamma rays result predominately from neutron interactions other than inelastic scattering interactions; and (e) means for utilizing signals representing said second energy spectrum to moddify signals representing said first energy spectrum.