Neutron-inelastic gamma radiation well logging method and apparatus with background accounted for

Claim 5

The method described in claim 2, wherein said step of deriving said ratio further includes the steps of deriving as an indication of the magnitude of the carbon nuclei in said formation the difference between said first energy-dependent selection and a function of said third energy-dependent selection, deriving as an indication of the magnitude of the oxygen nuclei in said formation the difference between said second energy-dependent selection and a function of said fourth energy-dependent selection, and deriving as an indication of the ratio of carbon and oxygen in said formation the ratio of said difference between first and third energy-dependent selections to the difference between said second and fourth energy-dependent selections.

Claim 6

The method described in claim 3, further including deriving a function of the summation of said first, second, third and fourth energy-dependent selections, and recording said function of said summation to indicate porosity in conjunction with said indication of said ratio of carbon and oxygen together with a correlative indication of borehole depth.

Claim 7

The method described in claim 4, further including making a second time-dependent selection of those of said pulses corresponding to those selected gamma rays resulting from capture of said neutrons by said bombarded nuclei, and recording said second time-dependent selection in correlation with the record of said ratio and summation to obtain a measurement of the porosity of said formation.

Claim 8

The method of claim 5 and further including the steps of making, during said second time-dependent selection, a further energy-dependent selection of those pulses corresponding to gamma rays attributable to capture of said neutrons by at lEast two selected elements in the earth formation surrounding the borehole; and recording signals representative of said further energy-dependent selection made during said second time-dependent selection as an indication of formation lithology.

Claim 9

The method of claim 6 wherein the elements selected during said second time-dependent selection and said further energy-dependent selection are silicon and calcium, which are selected by using energy windows in the 2.5 to 3.2 Mev. range for silicon and 5.2 to 6.25 Mev. range for calcium, respectively.

Claim 12

The method described in claim 8, further comprising separately deriving an additional ratio of a function of said carbon background signal to a function of said oxygen background signal, and recording said ratio of said adjusted carbon and oxygen indication signals in correlation with said additional ratio of said carbon and oxygen background signals to indicate lithology.

Claim 13

The method described in claim 9, further including deriving a function of the sum of said indication and said background signals to indicate porosity and, recording said function of said sum in correlation with said recording of said ratios.

Claim 16

The method of claim 11 and further including the steps of establishing, during said later discrete detection interval, a further energy dependent selection signal composed of pulses corresponding to gamma rays attributable to neutron capture by at least two selected elements in the earth formations surrounding the borehole; and recording said further signal in correlation with said measurement signal as an indication of formation lithology.

Claim 17

The method described in claim 11, further including deriving from said detection signal a third indicating signal functionally representative of the rate of occurrence of those pulses having amplitudes attributable to carbon background radiation arising from fast neutron processes during the earlier of said detection intervals, deriving from said detection signal a fourth indicating signal functionally representative of the rate of occurrence of those pulses having amplitudes attributable to oxygen background radiation arising from fast neutron processes during the earlier of said detection intervals, deriving from the difference between said first and thrid indicating signals a functional indication of the carbon in said formation, deriving from the difference between said second and fourth indicating signals a functional indication of the oxygen in said formation, and deriving said measurement signal as the ratio of said functional carbon indication to said functional oxygen indication.

Claim 18

The method described in claim 13, wherein said first indicating signal is composed of those electrical pulses having amplitudes greater than substantially 3.0 Mev. and not substantially greater than 4.7 Mev., wherein said second indicating signal is composed of those electrical pulses having amplitudes greater than substantially 5.0 Mev. and not substantially greater than 6.5 Mev., wherein said third indicating signal is composed of those electrical pulses having amplitudes greater than substantially 4.7 Mev. and not substantially greater than 5.0 Mev., and wherein said fourth indicating signal is composed of those electrical pulses having amplitudes greater than substantially 6.5 Mev., and not substantially greater than 7.5 Mev.

Claim 19

A radiological well logging system for investigating a subsurface earth formation traversed by a borehole, comprising a source of high energy neutrons for irradiating said formation, a radiation detector for sensing gamma rays emanating from said irradiated formation, signalling means for generating electrical pulses coincident with and having amplitudes functionally related to the terminal energies of gamma rays sensed by said detector, analyzing means responsive to said signalling means for establishing a first group of said pulses having amplitudes corresponding to gamma rays resulting from fast neutron reactions with carbon nuclei aNd a second group of said pulses having amplitudes corresponding to gamma rays resulting from fast neutron reactions with oxygen nuclei, said analyzing means further establishing a third group of said pulses having a preselected range of amplitudes corresponding to carbon background radiation and a fourth group of said pulses having a preselected range of amplitudes corresponding to oxygen background radiation, differential means for establishing a carbon signal indicative of the difference between a function of said first group of pulses and said function of said third group of pulses, said differential means further establishing an oxygen signal indicative of the difference between a function of said second group of pulses and said function of said fourth group of pulses, and ratio means for deriving a recordable indication of the ratio of said carbon and oxygen signals.

Claim 20

A radiological well logging system for investigating a subsurface earth formation traversed by a borehole, comprising a source of high energy neutrons for irradiating said formation, a radiation detector for sensing gamma rays emanating from said irradiated formation, signalling means for generating electrical pulses coincident with and having amplitudes functionally related to the terminal energies of gamma rays sensed by said detector, analyzing means responsive to said signalling means for establishing a first group of said pulses having amplitudes corresponding to gamma rays resulting from fast neutron reactions with carbon nuclei aNd a second group of said pulses having amplitudes corresponding to gamma rays resulting from fast neutron reactions with oxygen nuclei, said analyzing means further establishing a third group of said pulses having a preselected range of amplitudes corresponding to carbon background radiation and a fourth group of said pulses having a preselected range of amplitudes corresponding to oxygen background radiation, differential means for establishing a carbon signal indicative of the difference between a function of said first group of pulses and said function of said third group of pulses, said differential means further establishing an oxygen signal indicative of the difference between a function of said second group of pulses and said function of said fourth group of pulses, and ratio means for deriving a recordable indication of the ratio of said carbon and oxygen signals.

Claim 21

The system described in claim 15, wherein said analyzing means further includes a first window means for deriving said carbon signal from those pulses generated by said signalling means having amplitudes within a first preselected energy range, a second window means for deriving said oxygen signal from those pulses generated by said signalling means having amplitudes within a second preselected energy range greater than said first range, a third window means for deriving said carbon background signal from those pulses generated by said signalling means having amplitudes within a third preselected energy range greater than said first range and less than said second range, and a fourth window means for deriving said oxygen background signal from those pulses generated by said signalling means having amplitudes within a fourth preselected energy range greater than said second range.

Claim 22

The system described in claim 16 wherein said first energy range encompasses predetermined energy resonances attributable to the energies of gamma rays produced by inelastic scattering of said neutrons by carbon nuclei in said irradiated formation, and wherein said second energy range encompasses predetermined energy resonances attributable to the energies of gamma rays produced by inelastic scattering of said neutrons by oxygen nuclei in said irradiated formation.

Claim 23

The system described in claim 17, further comprising summing means responsive to said analyzing means for deriving a separately recordable indication of a function of the sum of said carbon, carbon background, oxygen and oxygen background signals.

Claim 24

The system described in claim 17, further comprising pulsing means actuating said neutron source for a preselected discrete irradiation time interval during which said high energy neutrons engage in inelastic scattering reactions with carbon and oxygen nuclei in said irradiated formation, and gating means interconnected to select pulses generated by signalling means during said irradiation interval and to form said selected pulses into a time-dependent detection signal for transmission to said analyzing means.

Claim 25

The system described in claim 19, wherein said analyzing means forms said groups of pulses from said time-dependent detection signal.

Claim 26

The system described in claim 20, wherein said window means derives said carbon and oxygen signals in time-dependent relationship to said irradiation time interval.

Claim 27

The system described in claim 21, wherein said gating means further includes timing means interconnected to actuate said gating means in time-dependent relationship to actuation of said neutron source.

Claim 28

A radiological well logging system comprising a source of high energy neutrons adapted to be passed through a borehole, pulsing means for cyclically energizing said source of neutrons during a discrete irradiation interval, radiation sensing means responsive to gamma radiation and adapted to be passed through said borehole in spaced relationship to said source of neutrons, signalling means respOnsive to said sensing means for producing electrical pulses having amplitudes corresponding to the terminal energies of detected gamma rays, selection means interconnected with said pulsing and signalling means for selecting pulses corresponding to gamma rays incident on said sensing means during a discrete detection interval, segregating means interconnected with said selection means for establishing a first energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei and a second energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, first counting means for deriving from said segregating means a first measurement signal functionally related to the rate of occurrence of said pulses in said first energy dependent group, second counting means for deriving from said segregating means a second measurement signal functionally related to the rate of occurrence of said pulses in said second energy dependent group, ratio means interconnected with said first and second counting means for deriving a recordable indication of the ratio of said carbon nuclei to said oxygen nuclei, means for establishing a third energy dependent group of said selected pulses attributable to carbon background radiation arising from fast neutron processes and a fourth energy dependent group of said selected pulses attributable to oxygen background radiation arising from fast neutron processes, and further including third counting means for deriving a third measurement signal functionally related to the rate of occurrence of said pulses in said third energy dependent group, fourth counting means for deriving a fourth measurement signal functionally related to the rate of occurrence of said pulses in said fourth energy dependent group, first difference means interconnected to provide said ratio means with an indication of said carbon nuclei functionally related to the difference between said first and third measurement signals, and second difference means interconnected to provide said ratio means with an indication of said oxygen nuclei functionally related to the difference between said second and fourth measurement signals.

Claim 29

A radiological well logging system comprising a source of high energy neutrons adapted to be passed through a borehole, pulsing means for cyclically energizing said source of neutrons during a discrete irradiation interval, radiation sensing means responsive to gamma radiation and adapted to be passed through said borehole in spaced relationship to said source of neutrons, signalling means respOnsive to said sensing means for producing electrical pulses having amplitudes corresponding to the terminal energies of detected gamma rays, selection means interconnected with said pulsing and signalling means for selecting pulses corresponding to gamma rays incident on said sensing means during a discrete detection interval, segregating means interconnected with said selection means for establishing a first energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei and a second energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, first counting means for deriving from said segregating means a first measurement signal functionally related to the rate of occurrence of said pulses in said first energy dependent group, second counting means for deriving from said segregating means a second measurement signal functionally related to the rate of occurrence of said pulses in said second energy dependent group, ratio means interconnected with said first and second counting means for deriving a recordable indication of the ratio of said carbon nuclei to said oxygen nuclei, means for establishing a third energy dependent group of said selected pulses attributable to carbon background radiation arising from fast neutron processes and a fourth energy dependent group of said selected pulses attributable to oxygen background radiation arising from fast neutron processes, and further including third counting means for deriving a third measurement signal functionally related to the rate of occurrence of said pulses in said third energy dependent group, fourth counting means for deriving a fourth measurement signal functionally related to the rate of occurrence of said pulses in said fourth energy dependent group, first difference means interconnected to provide said ratio means with an indication of said carbon nuclei functionally related to the difference between said first and third measurement signals, and second difference means interconnected to provide said ratio means with an indication of said oxygen nuclei functionally related to the difference between said second and fourth measurement signals.

Claim 30

The system described in claim 23, wherein said ratio means is further interconnected with said difference means to derive said ratio of carbon and oxygen nuclei as a function of the ratio of said difference between said first and third signals to the difference between said second and fourth signals.

Claim 31

The system described in claim 24, further including summation means for deriving a recordable indication of a function of the sum of said measurement signals derived by said counting means said sum being functionally related to porosity , and recording means interconnected with said summation means and said ratio means to record said indication of said sum in correlative relation to a separate indication of said ratio.

Claim 32

The system described in claim 25, wherein said segregating means includes a multi-channel pulse height analyzer having a carbon window adapted to pass pulses from said selection means having amplitudes greater than substantially 3.0 Mev. and not substantially greater than 4.7 Mev., a carbon background window adapted to pass pulses from said selection means having amplitudes greater than substantially 4.7 Mev. and not substantially greater than 5.0 Mev., an oxygen window adapted to pass pulses from said selection means having amplitudes greater than substantially 5.0 Mev. and not substantially greater than 6.5 Mev., and an oxygen background window adapted to pass pulses from said selection means having amplitudes greater than substantially 6.5 Mev. and not substantially greater than 7.5 Mev.

Claim 33

The system of claim 25 and further including seCond selection and segregating means for establishing a second time-dependent and energy-dependent group of selected pulses attributable to silicon and calcium gamma rays resulting from the capture of thermal neutrons by nuclei in formations surrounding the wellbore.

Claim 34

The system of claim 27 wherein said energy segregating means includes a multi-channel pulse height analyzer having a silicon window adapted to pass pulses from said second selection means having amplitudes greater than 2.5 Mev. and not substantially greater than 3.2 Mev., and a calcium window adapted to pass pulses from said second selection means having amplitudes greater than 5.2 Mev. and not substantially greater than 6.25 Mev.

Claim 37

The method described in claim 29, further including deriving a function of the difference between said first and third signals, deriving a function of the difference between said second and fourth signals, and deriving said function of the ratio of carbon to oxygen as a function of the ratio of said difference between said first and third signals to said difference between said second and fourth signals.

Claim 38

The method described in claim 30, further including deriving a function of the sum of said first, second, third and fourth signals in functional correlation with said derivation of said ratio functions as an indication of porosity.

Claim 39

The method described in claim 31, wherein said first signal is derived from pulses having amplitudes greater than about 3.0 Mev. and not greater than about 4.7 Mev., wherein said second signal is derived from pulses having amplitudes greater than about 5.0 Mev. and not greater than about 6.5 Mev., wherein said third signal is derived from pulses having amplitudes greater than about 4.7 Mev. and not greater than about 5.0 Mev., and wherein said fourth signal is derived from pulses having amplitudes greater than about 6.5 Mev.

Claim 40

Apparatus for investigating the character of a subsurface earth formation traversed by a borehole, comprising means for irradiating said formation with high energy neutrons, sensing means for detecting gamma rays emanating into said borehole from said irradiated formation, signalling means coupled to said sensing means for generating electrical pulses functionally related in occurrence and amplitude to the detection and energy of said detected gamma rays, first pulse analysis means for deriving a first electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of detected gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei, second pulse analysis means for deriving a second electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of detected gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, third pulse analysis means for deriving a third electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of carbon background radiation resulting from fast neutron processes, fourth pulse analysis means for deriving a fourth electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of oxygen background radiation resulting from fast neturon processes, first ratio means for deriving a function of the ratio of said first and second signals as an indication of the ratio of carbon to oxygen in said formation, and second ratio means for deriving a function of the ratio of said third and fourth signals as an indication of the lithology of said formation.

Claim 41

Apparatus for investigating the character of a subsurface earth formation traversed by a borehole, comprising means for irradiating said formation with high energy neutrons, sensing means for detecting gamma rays emanating into said borehole from said irradiated formation, signalling means coupled to said sensing means for generating electrical pulses functionally related in occurrence and amplitude to the detection and energy of said detected gamma rays, first pulse analysis means for deriving a first electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of detected gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei, second pulse analysis means for deriving a second electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of detected gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, third pulse analysis means for deriving a third electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of carbon background radiation resulting from fast neutron processes, fourth pulse analysis means for deriving a fourth electrical signal composed of those pulses having amplitudes attributable to the energy characteristics of oxygen background radiation resulting from fast neturon processes, first ratio means for deriving a function of the ratio of said first and second signals as an indication of the ratio of carbon to oxygen in said formation, and second ratio means for deriving a function of the ratio of said third and fourth signals as an indication of the lithology of said formation.

Claim 42

The apparatus described in claim 33, further including first difference means interconnected with said first and third analysis means for deriving a function of the difference between said first and third signals, and second difference means interconnected with said second and fourth analysis means for deriving a function of the difference between said second and fourth signals, said first ratio means being interconnected to derive said function of the ratio of said difference between said first and third signals to said difference between second and fourth signals.

Claim 43

The apparatus described in claim 34, further including summation means for deriving a function of the sum of said first, second, third and fourth signals as a porosity indication, in functional correlation with said derivation of said ratio functions.

Claim 44

The apparatus described in claim 35, wherein said first analysis means is adapted to derive said first signal from pulses having amplitudes greater than about 3.0 Mev. and not greater than about 4.7 Mev., wherein said second analysis means is adapted to derive said second signal from pulses having amplitudes greater than about 5.0 Mev. and not greater than about 6.5 Mev., wherein said third analysis means is adapted to derive said third signal from pulses having amplitudes greater than about 4.7 Mev. and not greater than about 5.0 Mev., and wherein said fourth analysis means is adapted to derive said fourth signal from pulses having amplitudes greater than about 6.5 Mev.

Claim 45

A radiological well logging system comprising a source of high energy neutrons adapted to be passed through a borehole; pulsing means for cyclically energizing said source of neutrons during a discrete radiation interval; radiation sensing means responsive to gamma radiation and adapted to be passed through said borehold in spaced relationship to said source of neutrons; signalling means responsive to said sensing means for producing electrical pulses having amplitudes corresponding to the terminal energies of detected gamma rays; selection means interconnected with said pulsing and signalling means for selecting pulses corresponding to gamma rays incident on said sensing means during a discrete detection interval, segregating means interconnected with said selection means for establishing a first energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei, a second energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, a third energy dependent group of said selected pulses attributable to carbon background rAdiation arising from fast neutron processes and a fourth energy dependent group of said selected pulses attributable to oxygen background radiation arising from fast neutron processes, first counting means for deriving from said segregating means a first measurement signal functionally related to the rate of occurrence of said pulses in said first energy dependent group; second counting means for deriving from said segregating means a second measurement signal functionally related to the rate of the occurrence of said pulses in said second dependent group; third counting means for deriving from said segregating means a third measurement signal functionally related to the rate of occurrence of said pulses in said third energy dependent group; fourth counting means for deriving a fourth measurement signal functionally related to the rate of occurrence of said pulses in said fourth energy dependent group; first difference means connected to the first and third counting means for providing a signal corresponding to said carbon nuclei in accordance with the difference between said first and third measurement signals; second difference means connected to the second and fourth counting means for providing a signal corresponding to said oxygen nuclei in accordance with the difference between said second and fourth measurement signals; and ratio means interconnected with said first and second difference means for deriving a recordable indication of the ratio of said carbon nuclei to said oxygen nuclei.

Claim 46

A radiological well logging system comprising a source of high energy neutrons adapted to be passed through a borehole; pulsing means for cyclically energizing said source of neutrons during a discrete radiation interval; radiation sensing means responsive to gamma radiation and adapted to be passed through said borehold in spaced relationship to said source of neutrons; signalling means responsive to said sensing means for producing electrical pulses having amplitudes corresponding to the terminal energies of detected gamma rays; selection means interconnected with said pulsing and signalling means for selecting pulses corresponding to gamma rays incident on said sensing means during a discrete detection interval, segregating means interconnected with said selection means for establishing a first energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by carbon nuclei, a second energy dependent group of said selected pulses attributable to gamma rays resulting from inelastic scattering of said neutrons by oxygen nuclei, a third energy dependent group of said selected pulses attributable to carbon background rAdiation arising from fast neutron processes and a fourth energy dependent group of said selected pulses attributable to oxygen background radiation arising from fast neutron processes, first counting means for deriving from said segregating means a first measurement signal functionally related to the rate of occurrence of said pulses in said first energy dependent group; second counting means for deriving from said segregating means a second measurement signal functionally related to the rate of the occurrence of said pulses in said second dependent group; third counting means for deriving from said segregating means a third measurement signal functionally related to the rate of occurrence of said pulses in said third energy dependent group; fourth counting means for deriving a fourth measurement signal functionally related to the rate of occurrence of said pulses in said fourth energy dependent group; first difference means connected to the first and third counting means for providing a signal corresponding to said carbon nuclei in accordance with the difference between said first and third measurement signals; second difference means connected to the second and fourth counting means for providing a signal corresponding to said oxygen nuclei in accordance with the difference between said second and fourth measurement signals; and ratio means interconnected with said first and second difference means for deriving a recordable indication of the ratio of said carbon nuclei to said oxygen nuclei.

Claim 47

Apparatus for investigating the charter of a subsurface formation traversed by a borehole, comprising means for bombarding the earth formations surrounding the borehole with repetitive pulses of fast neutrons, means for generating electrical pulses functionally related to gamma radiation emanating from said formation, selection means for making a time-dependent selection of those pulses attributable by amplitude to gamma rays occurring from inelastic scattering of neutrons by carbon and oxygen nuclei and excluding pulses attributable by amplitude to carbon and oxygen background radiation occurring from fast neutron processes, ratio means for deriving the ratio of carbon to oxygen in said formation as a function of those selected pulses relating to carbon nuclei to a function of those selected pulses relating to said oxygen nuclei, means for deriving a carbon indication signal from said selected pulses relating to carbon nuclei, means for deriving an oxygen indication signal from said selected pulses relating to oxygen nuclei, said ratio means deriving said carbon to oxygen ratio as the ratio of a function of said carbon indication signal to a function of said oxygen indication signal, means for deriving a carbon background signal from those of said excluded pulses attributable by amplitude to said carbon background radiation occurring from fast neutron processes, means for deriving an oxygen background signal from those of said excluded pulses attributable by amplitude to said oxygen background radiation occurring from fast neutron processes, means for deriving an adjusted carbon indication signal from a function of the difference between said carbon indication signal and said carbon background signal, and means for deriving an adjusted oxygen indication signal from a function of the difference between said oxygen indication signal and said oxygen background signal, said ratio means further deriving said carbon to oxygen ratio as the ratio of a function of said adjusted carbon indication signal to a function of said adjusted oxygen indication signal.

Claim 48

Apparatus for investigating the charter of a subsurface formation traversed by a borehole, comprising means for bombarding the earth formations surrounding the borehole with repetitive pulses of fast neutrons, means for generating electrical pulses functionally related to gamma radiation emanating from said formation, selection means for making a time-dependent selection of those pulses attributable by amplitude to gamma rays occurring from inelastic scattering of neutrons by carbon and oxygen nuclei and excluding pulses attributable by amplitude to carbon and oxygen background radiation occurring from fast neutron processes, ratio means for deriving the ratio of carbon to oxygen in said formation as a function of those selected pulses relating to carbon nuclei to a function of those selected pulses relating to said oxygen nuclei, means for deriving a carbon indication signal from said selected pulses relating to carbon nuclei, means for deriving an oxygen indication signal from said selected pulses relating to oxygen nuclei, said ratio means deriving said carbon to oxygen ratio as the ratio of a function of said carbon indication signal to a function of said oxygen indication signal, means for deriving a carbon background signal from those of said excluded pulses attributable by amplitude to said carbon background radiation occurring from fast neutron processes, means for deriving an oxygen background signal from those of said excluded pulses attributable by amplitude to said oxygen background radiation occurring from fast neutron processes, means for deriving an adjusted carbon indication signal from a function of the difference between said carbon indication signal and said carbon background signal, and means for deriving an adjusted oxygen indication signal from a function of the difference between said oxygen indication signal and said oxygen background signal, said ratio means further deriving said carbon to oxygen ratio as the ratio of a function of said adjusted carbon indication signal to a function of said adjusted oxygen indication signal.

Claim 49

The apparatus described in claim 38, further comprising additional ratio means for separately deriving an indication of the lithology of said formation as the ratio of a function of said carbon background signal to a function of said oxygen background signal, and means for recording said ratio of said adjusted carbon and oxygen indication signals in cOrrelation with said ratio of said carbon and oxygen background signals.

Claim 50

The apparatus described in claim 39, further including summation means for deriving a function of the sum of said indication and said background signals to indicate porosity, said recording means also responsive to said summation means for recording said function of said sum in correlation with the cording of said recording of said ratios.

Claim 53

The method of claim 41 and further including the steps of making, during said second time-dependent selection, a further energy-dependent selection of those pulses corresponding to gamma rays attributable to capture of said neutrons by at least two selected elements in the earth formation surrounding the borehole; and recording signals representative of said further energy-dependent selection made during said second time-dependent selection as an indication of formation lithology.

Claim 54

The method of claim 42 wherein the elements selected during said second time-dependent selection and said further energy-dependent selection are silicon and calcium, which are selected by using energy windows in the 2.5 to 3.2 Mev. range for silicon and 5.2 to 6.25 Mev. range for calcium, respectively.