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

Claim 1

A method for observing earth formation properties comprising the steps of: irradiating an earth formation with pulses of high energy neutrons; detecting over a wide energy band the gamma radiation produced by the inelastic scattering of said neutrons by nuclei of the earth formation elements during the time interval commencing after the initiation of each neutron pulse and terminating before thermal neutron absorption gamma radiation becomes predominant and generating signals in response thereto; providing a first output signal responsive to detector signals produced by gamma rays from said inelastic scattering over a selected, broad energy band; deriving from said first output signal a first indicating signal as a functional indication of carbon in said formation; deriving from said first output signal a second indicating signal as a functional indication of oxygen in said formation; deriving and recording a ratio of said first and second indicating signals as a functional representation of the hydrocarbon content of said formation; detecting gamma rays during a second observation interval occurring substantially subsequent to said neutron pulse and in which capture gamma radiation predominates; providing a second output signal responsive to detector signals produced by detected capture gamma radiation; deriving from said second output signal a third indicating signal as a functional indication of calcium in said formation; deriving from said second output signal a fourth indicating signal as a functional indication of silicon in said formation; and deriving and recording a ratio of said third and fourth indicating signals as a functional representation of the lithology of said formation.

Claim 2

A method for observing earth formation properties comprising the steps of: irradiating an earth formation with pulses of high energy neutrons; detecting over a wide energy band the gamma radiation produced by the inelastic scattering of said neutrons by nuclei of the earth formation elements during the time interval commencing after the initiation of each neutron pulse and terminating before thermal neutron absorption gamma radiation becomes predominant and generating signals in response thereto; providing a first output signal responsive to detector signals produced by gamma rays from said inelastic scattering over a selected, broad energy band; deriving from said first output signal a first indicating signal as a functional indication of carbon in said formation; deriving from said first output signal a second indicating signal as a functional indication of oxygen in said formation; deriving and recording a ratio of said first and second indicating signals as a functional representation of the hydrocarbon content of said formation; detecting gamma rays during a second observation interval occurring substantially subsequent to said neutron pulse and in which capture gamma radiation predominates; providing a second output signal responsive to detector signals produced by detected capture gamma radiation; deriving from said second output signal a third indicating signal as a functional indication of calcium in said formation; deriving from said second output signal a fourth indicating signal as a functional indication of silicon in said formation; and deriving and recording a ratio of said third and fourth indicating signals as a functional representation of the lithology of said formation.

Claim 3

A method of measuring selected characteristics of a subsurface earth formation, comprising: generating high energy neutrons during a discrete irradiation interval to produce inelastic scattering and thermal neutron capture reactions of said neutrons with nuclei of said formation, detecting gamma rays during a first discrete observation interval during said irradiation interval and terminating before thermal neutron capture gamma radiation becomes predominant, detecting gamma rays during a second discrete observation interval occurring substantially subsequent to said irradiation interval and in which capture gamma radiation predominates, deriving a first measurement signal from said gamma rays detected during said first observation interval, deriving a second measurement signal from said gamma rays detected during said second observation interval, and deriving and recording an indication of a selected formation characteristic as a function of at least one of said first and second signals.

Claim 4

A method of measuring selected characteristics of a subsurface earth formation, comprising: generating high energy neutrons during a discrete irradiation interval to produce inelastic scattering and thermal neutron capture reactions of said neutrons with nuclei of said formation, detecting gamma rays during a first discrete observation interval during said irradiation interval and terminating before thermal neutron capture gamma radiation becomes predominant, detecting gamma rays during a second discrete observation interval occurring substantially subsequent to said irradiation interval and in which capture gamma radiation predominates, deriving a first measurement signal from said gamma rays detected during said first observation interval, deriving a second measurement signal from said gamma rays detected during said second observation interval, and deriving and recording an indication of a selected formation characteristic as a function of at least one of said first and second signals.