Spectral gamma ray logging-while-drilling system

A gamma ray logging-while-drilling system comprising:

• (a) at least one gamma ray detector that measures a gamma ray energy spectrum;
• (b) a calibration source disposed near or within said at least one detector and emitting calibration radiation;
• and
• (c) dual gain circuitry comprising a standard amplification circuit and a high gain circuit;
• wherein the system is configured to:
• (i) branch and input said spectrum into said dual gain circuitry to produce a standard gain spectrum and an amplified gain spectrum;
• (ii) in said amplified gain spectrum, compare the observed position of a calibration peak from said calibration radiation with a predetermined standard position for said calibration peak;
• and
• (iii) correct said standard gain spectrum to a standard detector gain using results of said comparison.

A method for measuring gamma radiation while drilling a borehole, the method comprising the steps of:

• (a) measuring a gamma ray energy spectrum with least one gamma ray detector;
• (b) disposing a calibration source near or within said at least one detector;
• (c) measuring within said gamma ray energy spectrum calibration radiation emitted by said calibration source;
• (d) providing a dual gain circuitry comprising a standard amplification circuit and a high gain circuit;
• (e) branching said spectrum into said dual gain circuitry thereby producing a standard gain spectrum and an amplified gain spectrum;
• (f) in said amplified gain spectrum, observing a position of a calibration peak resulting from said calibration radiation;
• (g) comparing said observed position with a predetermined standard position for said calibration peak;
• and
• (h) using said comparison to correct said standard gain spectrum to a standard detector gain.

A gamma radiation logging-while-drilling system for measuring elemental concentration of at least one naturally occurring radioactive element in a formation penetrated by a borehole, the system comprising:

• (a) at least one gamma ray detector;
• and
• (b) a calibration source in the vicinity of said at least one gamma ray detector;
• wherein said gamma ray detector measures a gamma ray spectrum comprising a first component from said at least one naturally occurring radioactive element and a second component from said calibration source;
• and
• wherein said system is configured to:
• determine a first detector gain correction from features of said first component;
• determine a second detector gain correction from said second component;
• and
• combine said first and said second gain corrections to correct for gain shifts in said gamma ray detector.

A method for measuring, while drilling a borehole, elemental concentration of at least one naturally occurring radioactive element contained in formation penetrated by said borehole, the method comprising the steps of:

• (a) conveying at least one gamma ray detector within said borehole;
• (b) disposing a calibration source in the vicinity of said at least one gamma ray detector;
• (c) measuring, with said at least one gamma ray detector, a gamma ray spectrum comprising a first component from said at least one naturally occurring radioactive element and a second component from said calibration source;
• (d) determining a first detector gain correction from features of said first component;
• (e) determining a second detector gain correction from said second component;
• and
• (f) combining said first and said second detector gain corrections to correct for gain shifts in said at least one gamma ray detector.

A gamma ray logging-while-drilling system comprising:

• (a) at least one gamma ray detector;
• and
• (b) a processor operationally connected to said at least one detector;
• wherein said gamma ray detector cooperates with said processor to yield a spectrum encompassing a range of about 3 MeV and comprising gamma ray count rate recorded as a function of energy channel.

A method for measuring gamma radiation while drilling a borehole, the method comprising the steps of:

• (a) disposing at least one gamma ray detector within said borehole;
• (b) operationally connecting a processor to said at least one detector;
• and
• (c) with said processor cooperating with said at least one gamma ray detector, measuring an energy spectrum of said gamma radiation encompassing a range of about 3 MeV and comprising gamma ray count rate recorded as a function of energy channel.