Apparatus and methods for determining gas saturation and porosity of a formation penetrated by a gas filled or liquid filled borehole

A method of measuring at least one parameter of earth formation penetrated by a borehole, the method comprising:

• (a) irradiating said earth formation with high energy neutrons produced by a neutron generator within said borehole:
• (b) within said borehole, axially spacing a gamma ray detector at a first spacing from said neutron generator;
• (c) measuring prompt gamma radiation at said first spacing with said gamma ray detector;
• (d) within said borehole, axially spacing a second detector at a second spacing from said neutron generator;
• (e) measuring said prompt gamma radiation at said second spacing with said second detector;
• (f) measuring fast neutrons at a high energy level and a low energy level at said second spacing with said second detector;
• (g) combining said prompt gamma radiation measurement at said first spacing and said prompt gamma ray measurement at said second spacing to minimize effects of said borehole upon said determination of said parameter of said earth formation;
• (h) combining said measure of fast neutrons and said measure of prompt gamma radiation to determine said parameter of said earth formation;
• and
• (i) combining said fast neutron high energy measurement and said fast neutron low energy measurement to minimize effects of said borehole upon said determination of said parameter of said earth formation.

A method for measuring at least one property of material penetrated by a borehole, comprising:

• (a) providing a logging tool comprising
• (i) a pulsed source of high energy neutrons;
• (ii) a first detector spaced axially at a near spacing from said neutron source and sensitive to gamma radiation;
• and
• (iii) a second detector spaced axially at a far spacing from said neutron source and sensitive to gamma radiation and neutron radiation;
• (b) with said first detector, measuring a first gamma ray measurement during pulsing of said neutron source;
• (c) with said second detector, measuring a second gamma ray measurement during said pulsing of said neutron source;
• (d) with said second detector, measuring neutron radiation during said pulsing of said neutron source;
• (e) correcting said first gamma ray measurement for capture gamma radiation thereby obtaining a first measure of prompt gamma radiation, and correcting said second gamma ray measurement thereby obtaining a second prompt gamma ray measurement;
• (f) correcting said neutron measurement for effects of gamma radiation thereby obtaining a fast neutron scattering measurement;
• and
• (g) combining said first prompt gamma ray measurement, said second prompt gamma ray measurement, said fast neutron scatter measurement to obtain a measure of said property in which the effects of said borehole are minimized.

A logging system for determining at least one property of a formation penetrated by a borehole comprising:

• (a) a pressure housing containing
• (i) a pulsed neutron generator;
• (ii) a first detector sensitive to gamma radiation;
• (iii) a second detector sensitive to fast neutrons and to gamma radiation;
• and
• (iv) circuitry means for delineating the response of said second detector to scattered fast neutrons and prompt gamma radiation;
• (b) a computer;
• and
• (c) a predetermined relationship for combining, within said computer, prompt gamma radiation measured with said first detector, prompt gamma radiation measured with said second detector, and fast neutron radiation measured with said second detector thereby obtaining a measure of said at least one property in which adverse effects of said borehole are minimized.