Gamma‐ray logging

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Abstract
The response of a gamma‐ray sonde to formation radioactivity is derived as a function of the density of the formation, the borehole diameter, and the density of the borehole fluid. Also the response to radioactive material in the borehole, as affected by the borehole diameter and fluid density, is examined. Two important geometrical parameters of a logging sonde are its radial depth of investigation and its vertical resolution. Values are obtained for typical operating conditions, and it is shown how they vary with source and detection energies, with formation density, and with borehole geometry. The influence of high atomic number materials, either in the formation or the borehole, is elucidated, both for the total gamma‐ray counting rate and for specific detector energy intervals.
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