Radiological well logging methods and apparatus for reducing the effect of activation from the detector crystal
阅读：274发布：2021-06-05
专利汇可以提供Radiological well logging methods and apparatus for reducing the effect of activation from the detector crystal专利检索，专利查询，专利分析的服务。并且Improved radioactivity well logging methods and apparatus are provided for determining the macroscopic thermal neutron capture cross section of subsurface earth materials, wherein a correction is made to reduce or eliminate the effects of the portion of the measurement which is due to activation of the crystal in the detector. The high energy neutron source is pulsed at a preselected rate during the first portion of the operating cycle, and the resulting capture-induced radiation is measured within the first portion of the operating cycle during two preselected time intervals following the termination of each neutron burst. Thereafter, the source is inactivated during the balance of the operating cycle, and a measurement is made of the radiations registering in the activated crystal. This latter measurement is then utilized to correct the measurements obtained during the first portion of the operating cycle to a predetermined relationship.，下面是Radiological well logging methods and apparatus for reducing the effect of activation from the detector crystal专利的具体信息内容。
1. A method of radioactivity well logging to investigate subsurface earth formations traversed by a borehole, comprising passing a logging instrument through said borehole bombarding said formations with a plurality of discrete bursts of high energy neutrons emitted from said instrument during a preselected irradiation time interval occurring during a first portion of the operating cycle to produce a succession of discrete thermal neutron populations in said borehole and formations, detecting radiation at said instrument resulting from capture of said thermal neutron populations during preselected portions of said first portion of the operating cycle, after termination of the first portion of the operating cycle including said irradiation interval and after said thermal neutron populations in said formations have declined to negligible proportions, detecting during a preselected subsequent detection time interval other radiations resulting from unstable isotopes produced primarily within a component of said logging instrument as a result of said high energy neutrons, deriving a first electrical indication generally related to the occurrence of said resulting radiations detected during said first portion of the operating cycle, deriving a second electrical indication generally related to the occurrence of said radiations detected during said subsequent detection interval, and deriving from said first and second electrical indications a corrected representation of a characteristic of the earth material bombarded by said high energy neutrons.
2. The method described in claim 1, wherein said method further includes generating said bursts of neutrons at a predetermined frequency such that each of said thermal neutron populations declines to negligible proportions before the occurrence of the next succeeding of said bursts of neutron generated during said irradiation time interval, during said irradiation time interval, detecting resulting radiations occurring during the intervals between each of said bursts of neutrons, and during said detection time interval detecting radiations resulting from said thermal neutron populations substantially exclusively of radiations resulting from thermal neutron capture reactions occurring in said earth formations and in said borehole.
3. The method described in claim 2, further comprising deriving electrical pulses functionally related to said detected radiations, establishing a first electrical signal composed of those radiations detected during a first predetermined portion of said irradiation time interval, establishing a second electrical signal composed of those radiations detected during a second predetermined portion of said irradiation time interval, and establishing a third electrical signal composed of those radiations detected during said detection interval and following the termination of said irradiation time interval.
4. The method described in claim 3, wherein said first and second electrical signals are substantially composed of pulses attributable both to radiations emanating from unstable isotopes and to radiations resulting from neutron capture reactions occurring in said formations, and wherein said third electrical signal is substantially composed of pulses attributable only to radiations emanating from unstable isotopes.
5. The method described in claim 4, further comprising deriving from said first and third electrical signals a fourth electrical signal functionally representative of substantially only radiations resultIng from neutron capture reactions occurring in said formations during a first preselected portion of said irradiation time interval and following the occurrence of each of selected ones of said discrete bursts of high energy neutrons, deriving from said second and third electrical signals a fifth electrical signal functionally representative of substantially only radiations resulting from neutron capture reactions occurring in said formations during a second different preselected portion of said irradiation time interval equal in duration to said first portion and following the occurrence of each of said selected ones of said discrete bursts of high energy neutrons, and deriving a sixth electrical signal functionally representative of the ratio of said fourth and fifth signals.
6. The method described in claim 5, further comprising deriving from said first electrical signal a first count rate signal functionally related to the rate of occurrence of radiations detected during said first preselected portion of said irradiation interval, deriving from said second electrical signal a second count rate signal functionally related to the rate of occurrence of radiations detected during said second preselected portion of said irradiation interval, deriving from said third electrical signal a third count rate signal functionally related to the rate of occurrence of radiations detected during said preselected detection interval following the termination of said irradiation interval, deriving a first electrical function signal representative of the duration of said first preselected portion of said irradiation interval, deriving a second electrical function signal representative of the duration of said preselected detection interval which follows the termination of said irradiation interval, deriving a third electrical function signal corresponding to a time dependent function of said irradiation interval and the number of bursts of high energy neutrons occurring during said irradiation interval, deriving said fourth electrical signal as a preselected function of said first and third count rate signals and said first, second and third function signals, and deriving said fifth electrical signal as a preselected function of said second and third count rate signals and said first, second and third function signals.
7. The method described in claim 6, further comprising deriving the ratio of said first and second function signals, deriving as a fourth function the product of said third count rate signal and said ratio of said first and second function signals, deriving as a fifth function the product of said third function signal and said fourth function, and deriving said fourth electrical signal as the difference between said first count rate signal and said fifth function.
8. The method described in claim 7, further comprising deriving said fourth electrical signal together with a correlative indication of borehole depth.
9. The method described in claim 6, further comprising deriving the ratio of said first and second function signals, deriving as a fourth function the product of said third count rate signal and said ratio of said first and second function signals, deriving as a fifth function the product of said third function signal and said fourth function, and deriving said fifth electrical signal as the difference between said second count rate signal and said fifth function.
10. The method described in claim 9, further comprising deriving said fifth electrical signal together with a correlative indication of borehole depth.
11. A radioactivity well logging system comprising a subsurface logging instrument adapted to be passed through a borehole, a source of high energy neutrons arranged in said instrument for irradiating subsurface earth materials surrounding said borehole, a detector arranged in said instrument for detecting radiatiOn from earth formations adjacent the borehole as a result of irradiation thereof by neutrons from said source, said detector comprising as a component thereof material susceptible of being activated into an unstable radioisotope due to radiation emitted by said source, whereby radiation is emitted from said activated material of the detector, pulsing means for intermittently actuating said source to produce said neutrons in discrete bursts during a first portion of an operating cycle including a predetermined irradiation time interval, and means including said detector for selectively detecting radiation resulting from said irradiation of said earth formations at preselected time intervals during said first portion of said operating cycle and for selectively detecting radiation resulting primarily from activation of component material of said detector during a predetermined detection time interval occurring after substantial decay of said irradiating bursts of neutrons in said earth material and following the termination of said first portion of said operating cycle including said irradiation time interval.
12. The well logging system described in claim 11, including signalling means interconnected with said detector for establishing first and second electrical signals functionally related to radiations detected during said irradiation time interval and a third electrical signal functionally related to radiations detected during said detection time interval and following the termination of said irradiation time interval.
13. The well logging system described in claim 12, wherein said third electrical signal is composed of pulses functionally relates substantially only to radiations emanating from a predetermined unstable isotope, and wherein said first and second signals are each composed of pulses functionally related to radiations emanating from said unstable isotope as well as from capture reactions engaged in between nuclei of said earth materials and neutrons from said source.
14. The well logging system described in claim 13, wherein said radiation detector comprises an inorganic crystal adapted to scintillate in response to incident radiations and formed of a stable isotope capable of being transformed into said unstable isotope under bombardment by said high energy neutrons from said source, a photomultiplier tube optically coupled to said crystal for producing electrical pulses functionally related to radiations incident on said crystal and causing said crystal to scintillate, and signalling means for forming said electrical pulses produced by said photomultiplier tube into time dependent groups of pulses constituting said first, second and third electrical signals.
15. The well logging system described in claim 14, further including adjustment means responsive to said third electrical signal for deriving from said first and second signals, respectively, fifth and sixth electrical signals functionally related substantially only to the rate of occurrence of said radiations sensed by said crystal and attributable to capture reactions engaged in between nuclei of said earth materials and neutrons from said source.
16. The well logging system described in claim 15, wherein said adjustment means comprises first count rate means for determining the rate of occurrence of said pulses in said first electrical signal, second count rate means for determining the rate of occurrence of said pulses in said second electrical signal, third count rate means for determining the rate of occurrence of said pulses in said third electrical signal, first function means for determining a function of the ratio of the durations of said first and third electrical signals, second function means interconnected with said third count rate means and said first function means to determine a function of the product of said ratio and a function of said third electrical signal, and a third function means interconnectEd with one of said first and second count rate means to determine a function of the product of the corresponding one of said first and second electrical signals and said product determined by said second function means.
17. The well logging system described in claim 16, wherein said first function means comprises a first signal generator for producing a first electrical indication of the time interval during which pulses occur in the first electrical signal, a second signal generator for producing a second electrical indication of the time interval during which pulses occur in the third electrical signal, and dividing means responsive to said first and second signal generators for providing a ratio signal functionally related to the ratio of said first electrical indication to said second electrical indication.
18. The well logging system described in claim 17, wherein said second function means comprises first multiplying means responsive to said dividing means for deriving a first product signal functionally related to the product of said ratio signal and the rate of occurrence of said pulses in said third electrical signal.
19. The well logging system described in claim 18, wherein said third function means comprises a third function generator for producing a third electrical indication of a time dependent function of the actuation of said source of high energy neutrons, and second multiplying means responsive to said first multiplying means for deriving a second product signal functionally related to the product of said third electrical indication and said first product signal.
20. The well logging system described in claim 19, wherein said third function means further comprises difference means responsive to said second product signal and said first electrical signal for providing an electrical indication of the rate of occurrence of radiations resulting substantially only from neutron capture reactions occurring in said subsurface earth materials.
21. A method of radioactivity well logging to investigate subsurface earth formations traversed by a borehole, comprising: passing a logging instrument through said borehole, bombarding said formations with a discrete burst of high energy neutrons emitted from said instrument during a first portion of the operating cycle including a preselected irradiation time interval to produce a discrete thermal neutron population in said borehole and formations, during said first portion of the operating cycle detecting radiation at said instrument resulting from capture of at least said thermal neutron population in said formations during preselected time intervals following said burst of high energy neutrons, after termination of said first portion of the operating cycle including said irradiation interval and after said thermal neutron populations have declined to negligible proportions, detecting during a preselected detection time interval other radiations resulting from unstable isotopes produced primarily within a component of said logging instrument as a result of said high energy neutrons, deriving a first electrical indication generally related to the occurrence of said resulting radiations detected during said first portion of said operating cycle, and deriving a second electrical indication generally related to the occurrence of said resulting radiations detected during the second portion of said operating cycle, and, deriving from said first and second electrical indications a corrected representation of a characteristic of the material of the formations bombarded by said high energy neutrons.
22. A method as defined in claim 1 wherein said other radiations resulting from unstable isotopes produced primarily within a component of said logging instrument are emitted from material comprising a component of the radiation detector.
23. The method of claim 22 wherein said other radiations resulting from unstable isotopes produced primarily within a compOnent of said logging instrument as a result of said high energy neutrons are emitted by iodine-128 within the detector of the logging instrument.
24. The method of claim 23 wherein both the radiation detected during said first portion of the operating cycle and the radiation detected during the subsequent preselected detection time interval include radiation at least as low in energy as about 2 MEV.
25. Apparatus as defined in claim 11 wherein the detector is a scintillation detector and the material comprising a component of said detector susceptible of activation is a component of the luminophor of said detector.
26. Apparatus as defined in claim 25 wherein said detector is a scintillation detector including a luminophor having iodine as a component material thereof susceptible of activation.
27. Apparatus as defined in claim 26 wherein the means for detecting radiation resulting from said irradiation of the earth formations and the means for selectively detecting radiation resulting from activation of a component material of said detector are both biased to include the detection of relatively low energy radiations having a lower limit of at least as low as about 2 MEV.
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