The proportional relationship between the grade-thickness product and the area under a gross gamma-ray log is well known and generally accepted as correct. A set of conditions for its correctness is derived.It is shown that the proportionality factor (K factor) is independent of the dip angle of the bed, because both the thickness of the bed (measured along the borehole) and the area under the gross gamma-ray log increase in the ratio of the secant of the dip angle. Experimental data supporting this conclusion are presented.It is also shown that the dip angle of a relatively thick bed can be estimated by deconvolving the log near the edge of the bed.Finally, the comparison parameters between a formation and a calibration model are shown to be grade, gamma-ray mass attenuation coefficient, moisture content, and an integral function of the build-up factor and the gamma-ray linear absorption coefficient.

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