Gamma ray logging is a borehole tool that can provide useful information for environmental and engineering investigations and water resource exploration. In practice, the tool is sensitive to clay content, and functions as a sand/clay log for unconsolidated sediments, or a shale log in rock sequences. The method measures natural gamma irradiation from radioactive isotopes in the sediment/rock minerals. Potassium is the most common radioisotope in surficial geologic materials in the Midwest.
It is particulary helpful for wells with poor or absent boring logs, and can be useful in lithologic correlations. The method ties well with observed lithologic profiles and can be used to help constrain earth resistivity surveys. Ned Bleuer has applied gamma logging to the analysis of glacigenic facies (Indiana Geological Survey Special Report 65).
Gamma ray logs can be obtained from in situ wells and cased boreholes. Because it detects natural gamma radiation, there is no radioactive source within the tooling, and no NRC permitting is required.