Simultaneous gamma ray measurement of formation bulk density and casing thickness

Claim 1

A method for logging a cased well borehole to determine simultaneously the bulk density of earth formations behind the wall casing and the thickness of the well casing, comprising the steps of: emitting low energy gamma radiation from a source of such radiation in a well borehole into the casing and earth formations surrounding a cased well borehole; detecting at a first, longitudinally spaced distance from said source, gamma radiation scattered back into the borehole by the casing and earth formations and generating first count rate signals representative thereof; detecting at a second, differently longitudinally spaced distance from said source, gamma radiation scattered back into the borehole by the casing and earth formations and generating secound count rate signals representative thereof; combining said first and second count rate signals according to a predetermined relationship to derive a third signal representative of the earth formation bulk density compensated for detector standoff from the earth formation caused by the casing and a fourth correction signal representing the correction required to compensate said third signal for the standoff; combining said third and fourth signals according to a second, different predetermined relationship, to derive a fifth signal representative of the thickness of the casing in the vicinity of said detectors; and recording at least said third and fifth signals.

Claim 2

A method for logging a cased well borehole to determine simultaneously the bulk density of earth formations behind the wall casing and the thickness of the well casing, comprising the steps of: emitting low energy gamma radiation from a source of such radiation in a well borehole into the casing and earth formations surrounding a cased well borehole; detecting at a first, longitudinally spaced distance from said source, gamma radiation scattered back into the borehole by the casing and earth formations and generating first count rate signals representative thereof; detecting at a second, differently longitudinally spaced distance from said source, gamma radiation scattered back into the borehole by the casing and earth formations and generating secound count rate signals representative thereof; combining said first and second count rate signals according to a predetermined relationship to derive a third signal representative of the earth formation bulk density compensated for detector standoff from the earth formation caused by the casing and a fourth correction signal representing the correction required to compensate said third signal for the standoff; combining said third and fourth signals according to a second, different predetermined relationship, to derive a fifth signal representative of the thickness of the casing in the vicinity of said detectors; and recording at least said third and fifth signals.

Claim 10

A method for simultaneously measuring formation bulk density and casing thickness in a cased well borehole comprising the steps of: irradiating earth formations in the vicinity of a cased well borehole with gamma radiation from a continuous gamma ray source disposed in the borehole; detecting at first and second longitudinally spaced distances in the borehole from said source, gamma rays scattered back into the borehole by earth formations and casing and generating first and second detector signals representative thereof; combining said detector signals according to a predetermined empirical relationship to derive a corrected formation bulk density signal and a standoff correction signal; and combining said detector signals according to a second predetermined empirical relationship to derive a signal indicative of the thickness of the casing in the cased well borehole.

Claim 11

A method for simultaneously measuring formation bulk density and casing thickness in a cased well borehole comprising the steps of: irradiating earth formations in the vicinity of a cased well borehole with gamma radiation from a continuous gamma ray source disposed in the borehole; detecting at first and second longitudinally spaced distances in the borehole from said source, gamma rays scattered back into the borehole by earth formations and casing and generating first and second detector signals representative thereof; combining said detector signals according to a predetermined empirical relationship to derive a corrected formation bulk density signal and a standoff correction signal; and combining said detector signals according to a second predetermined empirical relationship to derive a signal indicative of the thickness of the casing in the cased well borehole.