Gamma Ray Logging
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Gamma Ray Logging is a method of measurement used by oil industry well loggers to determine rock formation properties. The log works on the principle that different types of rock emit different amounts and different types of gamma ray radiation. In particular, shales emit more gamma rays than sands, which allows the two to be distinguished from one another.
The log works by detecting gamma ray emission from 3 elements: Potassium, Thorium and Uranium. These elements are used for two reasons, first they have a radioactive half life similar to the age of the earth, so even relatively old material (shale and clay) continue to emit radiation. Second, these three elements are more common in shales than in sand, which means an area with a lower gamma ray count is likely to be a sand.
The log is not infallible. Certain types of sands also contain shale like quantities of these elements. These include evaporites, with higher than expected Potassium concentration, and sands bearing organic materials, with higher than expected Uranium concentration. However, it is usually possible to identify these areas because only one of the three elements will have an increase in gamma ray activity, rather than all three as is the case with shales.
