- The Gamma Ray Tool measures the naturally occuring gamma radiation in the formations.
- Most naturally occurring radiation comes from potassium which is contained in clay minerals. Thus the gamma ray log is useful for distinguishing shales from non-shales.
- Some gamma radiation comes from uranium (which is most often found in formations through which water once flowed) or thorium (which is found in various clay minerals).
- The spectral gamma ray device differentiates these three sources of gamma radiation based on the energy distribution of the gamma rays striking the detector.
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