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

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 inelastic scattering of said neutrons by nuclei of 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;
• and
• converting said inelastic gamma radiation signals to an output signal functionally related to inelastic gamma rays over a selected, broad energy band to provide an indication of hydrogenous matter content of earth formations under observation.

A system for observing earth formation properties comprising:

• means for irradiating the earth formation with a pulse of high energy neutrons;
• means for detecting over a broad energy band gamma radiation produced by the inelastic scattering of said neutrons by nuclei of the formation elements during an interval of 10.mu. sec or less in the early part of said neutron pulse and for producing signals in response thereto;
• and
• circuit means responsive to detector signals produced by inelastic gamma rays over a selected, broad energy band, for providing an output signal indicative of a characteristic of the formation under observation.

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;
• and
• providing an output signal indicative of a characteristic of the formation under observation responsive to detector signals produced by gamma rays over a selected, broad energy band.

A system for observing earth formation properties comprising:

• means for irradiating the earth formation with a pulse of high energy neutrons;
• means for detecting gamma radiation produced by the inelastic scattering of said neutrons by nuclei in the earth formation during the time interval commencing after the initiation of each neutron pulse and terminating before thermal neutron absorption gamma radiation becomes predominant and generating a signal in response thereto;
• circuit means for deriving from said inelastic gamma radiation signal, a first indicating signal functionally related to carbon in the formation as indicated by gamma radiation detected over an energy band including 3 to 4.7 M.E.V.;
• circuit means for deriving from said inelastic gamma radiation signal, a second indicating signal functionally related to oxygen in the formation as indicated by gamma radiation detected over an energy band including 5 to 6.5 M.E.V.;
• and
• circuit means for deriving as an output signal a ratio of said first and second indicating signals as an indication of a characteristic of the formation under observation.

A method for observing earth formation properties comprising the steps of:

• (a) irradiating an earth formation with pulses of high energy neutrons;
• (b) 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;
• (c) detecting the gamma radiation produced by thermal neutron absorption by nuclei of the earth formation elements during another time interval during which said thermal neutron absorption gamma radiation predominates over said gamma radiation produced by the inelastic scattering of said neutrons and generating signals in response thereto;
• (d) providing an output signal indicative of a characteristic of the formation under observation responsive to detector signals produced by gamma rays over a selected, broad energy band by converting detector signals generated in response to said gamma radiation produced by inelastic scattering to a signal functionally related to the ratio of carbon to oxygen in the formation;
• and
• (e) providing another output signal indicative of a characteristic of the formation under observation by converting detector signals generated in response to said gamma radiation produced by thermal neutron capture to a signal functionally related to the ratio of silicon to calcium in the formation.