Natural gamma radiation borehole logging system

In a system for determining the amounts of natural gamma radiation from potassium, uranium, and thorium in the earth formations and having a gamma-ray detector which provides an output representative of the total natural gamma radiation and three energy band selectors which discriminate the output of the gamma-ray detector into potassium, uranium, and thorium energy band signals centered, respectively, about the energy levels at which potassium, uranium, and thorium exhibit peak gamma radiation intensities, the improvement comprising:

• a. first means for adjusting the amplitude of said uranium energy band signal by the fraction of the thorium energy band signal that would reduce said uranium energy band signal to zero when the gamma-ray detector is exposed to a source of thorium gamma radiation, and
• b. second means for adjusting the amplitude of said potassium energy band signal by the fractions of the thorium and uranium energy band signals that would reduce said potassium energy band signal to zero when the gamma-ray detector is exposed separately to a source of thorium gamma radiation and to a source of uranium gamma radiation.

A method of logging for the natural gamma radiation from potassium, uranium, and thorium in the earth formations surrounding a borehole by means of a borehole logging tool having a gamma-ray detector and three energy band selectors which discriminate the count rate output of the gamma-ray detector into potassium, uranium, and thorium energy band signals centered, respectively, about the energy levels at which potassium, uranium, and thorium exhibit peak gamma radiation intensities, comprising the steps of:

• a. lowering the logging tool into the borehole to provide potassium, uranium, and thorium energy band signals representative of the natural gamma radiation of the earth formations surrounding the borehole,
• b. compensating the count rates in the potassium and uranium energy band signals from step (a) by the fraction of the count rate of the thorium energy band signal that represents the contribution of thorium gamma radiation on the count rates in the potassium and uranium energy band signal, and
• c. compensating the count rate in the potassium energy band signal from step (a) by the fraction of the count rate of the uranium energy band signal that represents the contribution of uranium gamma radiation on the count rate in the potassium energy band signal.