Method and apparatus for subsurface formation evaluation

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the flow of drilling fluid therethrough;
• b. a gamma ray source positioned within said tubular body against its interior side and adapted to permit the downward flow of drilling fluid on one side thereof;
• c. first gamma ray detecting means positioned at a first spaced location from said gamma ray source against the interior side of said tubular body for detecting gamma rays and for generating a signal indicative thereof, said first gamma ray detecting means comprising a first gamma ray scintillator coupled to a photomultiplier;
• d. second gamma ray detecting means positioned at a second spaced location from said gamma ray source against the interior side of said tubular body for detecting gamma rays and for generating a signal indicative thereof, said second gamma ray detecting means comprising a second gamma ray scintillator coupled to a photomultiplier, said first and second gamma ray detecting means being adapted to permit the downward flow of drilling fluid on one side thereof;
• and
• e. means for combining said signals for generating a signal indicative of a formation characteristic selected from the group comprising formation density and formation photoelectric absorption coefficient.

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the downward flow of drilling fluid therethrough;
• b. a neutron source positioned within said tubular body on its cylindrical axis and adapted to permit the downward flow of drilling fluid therearound for bombarding the formation with high energy neutrons and for producing secondary radiation as a result of neutron-formation interaction;
• c. first secondary radiation detector means positioned within said tubular body at a first spaced location from said neutron source for detecting secondary radiation and for generating a first signal indicative thereof;
• d. second secondary radiation detector means positioned within said tubular body at a second spaced location from said neutron source for detecting secondary radiation and for generating a second signal indicative thereof;
• e. a gamma ray source positioned against the interior side of said tubular body at an axial position different from the position of said neutron source and permitting the downward flow of drilling fluid on one side thereof through the interior of said tubular body;
• f. first gamma ray detecting means positioned at a first spaced location from said gamma ray source against the interior side of said tubular body for detecting gamma rays and for generating a third signal indicative thereof, said first gamma ray detecting means comprising a first gamma ray scintillator coupled to a photomultiplier;
• g. second gamma ray detecting means positioned at a second spaced location from said gamma ray source against the interior side of said tubular body for detecting gamma rays and for generating a fourth signal indicative thereof, said second gamma ray detecting means comprising a second gamma ray scintillator coupled to a photomultiplier, said first and second gamma ray detecting means being adapted to permit the downward flow of drilling fluid on one side thereof through the interior of said tubular body;
• and
• h. means for combining said first and second signals and said third and fourth signals to respectively generate signals indicative of formation porosity and a formation characteristic selected from the group comprising formation density and formation photoelectric absorption coefficient.

A method of investigating characteristics of earth formations surrounding a borehole by means of an instrument coupled to the drill string, including the steps of:

• a. irradiating the formations with neutrons from a source positioned in said borehole;
• b. detecting neutrons returning to the borehole from said formations and generating signals indicative thereof;
• c. detecting capture gamma rays returning to the borehole from said formations and generating signals indicative thereof;
• and
• d. in response to said signals, generating a signal indicative of chlorine.

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the flow of drilling fluid therethrough;
• b. a neutron source carried by said tubular body for bombarding the formation with high energy neutrons and for producing secondary radiation as a result of neutron-formation interactions;
• c. detector means carried by said tubular body at a spaced location from said neutron source for detecting secondary radiation and for generating a signal indicative thereof, said detector means comprising at least two different types of secondary radiation detectors having different detection characteristics;
• and
• d. means responsive to said signal indicative of secondary radiation for generating a signal indicative of porosity.

A method of investigating characteristics of earth formations surrounding a borehole by means of an instrument coupled to the drill string, including the steps of:

• a. irradiating the formations with gamma rays from a source positioned in said borehole;
• b. detecting gamma rays returning to the borehole from said formations at first and second locations spaced from said source;
• c. detecting gamma rays passing through material in said borehole and generating signals indicative of density and photoelectric absorption coefficient therefrom;
• and
• d. in response to said signals, generating a signal indicative of the density and photoelectric absorption coefficient of said formation corrected for the effects of density and photoelectric absorption of the materials in the borehole.

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the flow of drilling fluid therethrough;
• b. a gamma ray source carried by said tubular body;
• c. a gamma ray detector positioned within said tubular body for detecting gamma rays after they have traversed the interior of said tubular body and for generating signals indicative of the density and photoelectric absorption of the drilling fluid;
• and
• d. means responsive to said signals for generating a signal indicative of a formation characteristic selected from the group comprising formation density and formation photoelectric absorption coefficient.

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the flow of drilling fluid therethrough;
• b. a gamma ray source carried by said tubular body;
• c. detector means positioned at a spaced location from said gamma ray source for detecting gamma rays and for generating a signal indicative thereof, said detector means comprising a gamma ray scintillator coupled to a photomultiplier;
• d. means for detecting gamma ray background and for making background corrections to said signals indicative of gamma rays;
• e. means responsive to said signals for generating a signal indicative of formation density.

An apparatus for investigating properties of earth formations surrounding a borehole comprising:

• a. a tubular body adapted to be coupled to a drill string and adapted to permit the flow of drilling fluid therethrough;
• b. a neutron source carried by said tubular body for bombarding the formation with high energy neutrons and for producing secondary radiation as a result of neutron-formation interactions;
• c. secondary radiation detecting means carried by said tubular body for detecting secondary radiation and for generating a signal indicative thereof;
• d. means responsive to said signal indicative of secondary radiation for generating a signal indicative of secondary radiation substantially free of the effects of microphonic noise produced by said secondary radiation detecting means.

A method of investigating characteristics of earth formations surrounding a borehole by means of an instrument adapted to be moved through said borehole, including the steps of:

• a. irradiating the formations with neutrons from a source positioned in said borehole;
• b. detecting thermal neutrons returning to the borehole from said formations and generating a signal indicative of thermal neutrons;
• c. detecting gamma rays in the borehole originating from interactions between said neutrons and elemental nuclei and generating a signal indicative of gamma rays, at least a fraction of said detected gamma rays originating from the activation of the instrument;
• and
• d. generating a signal indicative of formation porosity from said signal indicative of gamma corrected for activation background by said thermal neutron signal.