Neutron characteristic and spectroscopy logging methods and apparatus

Apparatus for investigating an earth formation traversed by a well bore, comprising:

• means for irradiating the earth formation with at least two time-spaced pulses of fast neutrons;
• means responsive to representations of neutron interactions with nuclei of the formation following the first neutron pulse for measuring a selected neutron characteristic of the formation;
• detector means for detecting gamma rays resulting from neutron interactions with nuclei of the formation and for generating corresponding signals representative of the energies of the detected gamma rays;
• means for analyzing at least a portion of the energy spectrum of the gamma rays detected during a first detection period following the second neutron pulse;
• and
• control means for controlling the operation of the analyzing means as a function of the value of the selected neutron characteristic measured following the first neutron pulse such that the gamma rays detected during the first detection period result predominantly from thermal neutron capture interactions with formation nuclei.

Apparatus for investigating an earth formation traversed by a well bore, comprising:

• means for irradiating the earth formation with a plurality of time-spaced pulses of fast neutrons during each of a succession of time intervals;
• detector means for detecting gamma rays resulting from thermal neutron capture interactions with nuclei of the formation following the neutron pulses and for generating corresponding time distributions of signals representative of the energies of the detected gamma rays;
• means responsive to signals generated by the detector means following one or more of the neutron pulses for analyzing at least a portion of the energy spectrum of the corresponding gamma rays;
• means responsive to signals generated by the detector means following one or more selected neutron pulses for measuring a thermal neutron characteristic of the formation;
• and
• control means for controlling the operation of the analyzing means as a function of the measured neutron characteristic.

A method of investigating an earth formation traversed by a well bore, comprising:

• obtaining representations of neutron interactions with nuclei of the formation following irradiation of the formation with a first pulse of fast neutrons;
• deriving a measurement of a neutron characteristic of the formation from the first neutron pulse-related representations;
• obtaining representations of the energies of gamma rays produced by thermal neutron capture interactions with nuclei of the formation during a first detection period following irradiation of the formation with a second pulse of fast neutrons;
• analyzing at least a portion of the energy spectrum of the gamma rays corresponding to said second neutron pulse-related representations;
• and
• controlling the timing of the first detection period as a function of the derived measurement of the neutron characteristic of the formation.

A method of investigating earth formations traversed by a well bore, comprising:

• successively irradiating the formations with time-spaced pulses of fast neutrons;
• deriving a measurement of a neutron characteristic of the formations following each of selected non-adjacent pulses in the succession of pulses;
• analyzing at least a portion of the energy spectrum of the gamma rays produced by thermal neutron capture interactions with nuclei of the formations during a first detection period following each of one or more neutron pulses intermediate in the succession of pulses to the selected pulses;
• and
• controlling the timing of each first detection period as a function of the measurement of the neutron characteristic derived following the selected neutron pulse next preceding said first detection period in the succession of pulses.

A method for investigating an earth formation traversed by a well bore, comprising:

• irradiating the earth formation with a plurality of time-spaced pulses of fast neutrons during each of a succession of time intervals;
• detecting gamma rays resulting from thermal neutron capture interactions with nuclei of the formation following the neutron pulses and generating corresponding signals representative of the energies of the detected gamma rays;
• analyzing at least a portion of the energy spectrum of the gamma rays corresponding to signals generated during a first detection period following one or more of the neutron pulses of each time interval;
• deriving a measurement of the thermal neutron decay time of the formation in response to signals generated during a second detection period following a neutron pulse in a first time interval in the succession of intervals;
• and
• controlling the time of occurrence of each first detection period of a second time interval subsequent in the succession of intervals to the first time interval as a function of the neutron decay time measured in response to signals generated during the second detection period of the first time interval.

A method for investigating earth formations traversed by a well bore, comprising:

• moving a well tool through the well bore;
• irradiating the formation and other well bore media surrounding the well tool with at least two time-spaced pulses of fast neutrons;
• detecting at least in part the time distribution of gamma rays resulting from neutron interactions with nuclei of the irradiated formation and other well bore media following each neutron pulse and generating a corresponding time distribution of signals representative of the energies of the detected gamma rays;
• selecting, in response to signals generated following the first of said neutron pulses, a portion of the time distribution of signals following the second of said neutron pulses which corresponds to gamma rays resulting predominantly from thermal neutron capture interactions with nuclei of the irradiated formation;
• and
• analyzing at least a portion of the energy spectrum of the detected capture gamma rays corresponding to the signals within the selected portion of the time distribution of signals following said second neutron pulse.