Background subtraction system for pulsed neutron logging of earth boreholes
AssigneeSchlumberger Technology Corporation
InventorRussel C. Hertzog
Life of this patent
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Abstract
The composition of an earth formation is investigated by repetitively irradiating the formation with bursts of fast neutrons and generating an inelastic energy spectrum from gamma ray counts detected during first time intervals which are contemporaneous with the bursts. A second energy spectrum is generated from gamma ray counts detected during second time intervals which immediately follow respective first time intervals. Gamma ray counts comprising the second spectrum are thereafter subtracted from the gamma ray counts comprising the inelastic spectrum to provide an inelastic energy spectrum with reduced background.
Claims15
A method for determining the characteristics of earth formations surrounding a well borehole comprising the steps of:
- repetitively irradiating the earth formations surrounding the well bore with relatively short duration pulses of high energy neutrons;
- 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 providing information representative thereof;
- detecting immediately following each such pulse of high energy neutrons, background gamma radiation due to thermal neutron capture and providing information representative thereof;
- and
- correcting said inelastic gamma representative information to compensate for said background representative information.
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.
A method for investigating the composition of an earth formation traversed by a well bore, comprising the steps of:
- (a) repetitively irradiating the formation with time spaced bursts of fast neutrons;
- (b) detecting gamma rays during first time periods related to the occurrence of the neutron bursts such that the detected gamma rays result predominately from inelastic scattering interactions between the incident neutrons and constituents of the formation;
- (c) detecting background gamma rays during second time periods following the occurrence of neutron bursts during which the detected gamma rays result predominately from interactions between incident neutrons and constituents of the formation other than inelastic scattering interactions;
- (d) generating an energy spectrum of the predominately inelastic scattering gamma rays detected in step (b);
- (e) generating an energy spectrum of the background gamma rays detected in step (c);
- and
- (f) subtracting from said inelastic scattering gamma ray spectrum.
A method of investigating the composition of an earth formation, comprising the steps of:
- (a) providing an energy spectrum of gamma rays detected during first time periods related to a succession of time-spaced bursts of fast neutrons such that the detected gamma ray result predominately from inelastic scattering interactions between the incident neutrons and constituents of the formation;
- (b) providing an energy spectrum of background gamma rays detected during second time periods which closely follow said neutron bursts during which the detected gamma rays result predominately from interactions between the incident neutrons and constituents of the formation other than inelastic scattering interactions;
- and
- (c) subtracting the background gamma ray spectrum from said inelastic scattering gamma ray spectrum.
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.
Dependencies are inferred from the claim text.