Formation sigma measurement from thermal neutron detection

In a logging system including a tool for traversing a borehole in an earth formation, means carried by the tool for irradiating the formation and generating detector signals indicative of the response of the borehole environment in and around the tool to the radiation, and data processing means for computing desired characteristics of the borehole environment from the detector signals, said characteristics including the macroscopic thermal absorption cross section of the formation (formation sigma), the logging method comprising the steps of:

• (a) irradiating the formation with a pulsed source of high energy neutrons from the tool as the tool is displaced at a controlled speed through the borehole, whereby the neutrons generated at each pulse thermalize as a result of scattering interactions and are absorbed as a result of thermal neutron capture interactions in the borehole environment, said capture interactions including the production of capture gamma rays;
• (b) detecting the time-dependent intensity of capture gamma rays at a particular elevation in the borehole during a first period of time between successive source pulses, and generating a first detector signal commensurate therewith;
• (c) detecting the time-dependent population of thermal neutrons at said particular elevation in the borehole during a second period of time between successive source pulses and generating a second detector signal commensurate therewith;
• (d) from said first detector signal, computing in said data processing means, and recording, a first estimate of formation sigma at said particular elevation;
• (e) from said second detector signal, computing in said data processing means, and recording, a second estimate of formation sigma at said particular elevation;
• (f) repeating steps (a)-(e) for a multiplicity of other elevations;
• (g) comparing either the first detector signal with the second detector signal, or the first estimate with the second estimate, for a plurality of said multiplicity of elevations;
• and
• (h) in response to said comparing, for at least said given elevation, recording one of the first and second estimates of formation sigma as the measured formation sigma.

In a logging system including a sonde for traversing a borehole in an earth formation, means carried by the sonde for irradiating the formation and generating detector signals indicative of the response of the borehole environment in and around the sonde to the radiation, and data processing means for computing desired characteristics of the borehole environment from the detector signals, the logging method comprising the steps of:

• (a) irradiating the formation with a pulsed source of high energy neutrons from the sonde as the sonde is displaced at a controlled speed through the borehole, whereby the neutrons generated at each pulse interact with the borehole environment to produce a neutron population having a space, time and energy distribution including epithermal and thermal energies;
• (b) detecting the population of epithermal neutrons at two positions in the borehole spaced from the source and generating two epithermal detectors signals commensurate therewith;
• (c) from the epithermal detector signals, computing an estimate of the characteristic of formation porosity at a given elevation in the borehole;
• (d) detecting the time-dependent population of thermal neutrons at an azimuthally limited portion of the exterior surface of the sonde at said elevation in the borehole and generating a thermal neutron detector signal commensurate therewith;
• and
• (e) in response to said thermal neutron detector signal, adjusting said estimate of the formation porosity to obtain a measured value of the porosity at said elevation.

A logging tool for traversing a borehole in an earth formation which has a spatial variation in macroscopic thermal absorption cross-section (formation sigma), comprising:

• a generally cylindrical sonde having a longitudinal axis of travel and an exterior surface;
• source means mounted in the sonde, for periodically generating a series of pulses of high energy neutrons;
• detector means mounted in the sonde at a location axially spaced from the source means, the detector means having a sensitive volume for sensing the time-dependent thermal neutron population at said location following pulses of the source means and means for generating a detector signal commensurate with said time-dependent neutron population;
• focus means associated with the detector means, for defining an azimuthally limited detection surface on the exterior surface of the sonde, such that substantially all thermal neutrons that enter the sensitive volume at said location have first passed through the detection surface on the sonde;
• and
• signal processing means for fitting the detector signal to a mathematical model of thermal neutron population time dependence at said location when the sonde is operatively situated in the borehole.