Borehole Geophysics - Gamma Ray (GR)

The objective of this Activity is to provide a basic framework and criteria for the use of natural gamma (GR) logs. An exercise will be used to as a teaching tool for this Activity.

Gamma ray (GR) tools measure naturally occurring gamma ray radiation emitted spontaneously from the formation by uranium, thorium, and the potassium 40 isotope. Natural gamma logs are powerful tools in lithologic identification and correlation, identification of potential migration pathways, and evaluation of water quality with respect to radionuclides, such as uranium salts. GR logs usually show the clay content in sedimentary rocks, because heavy radioactive elements (potassium, thorium and uranium-radium) tend to concentrate in clays. While clays and clayey sands are higher in radioactivity, clean sands (i.e., with little to no clay content) and carbonates usually exhibit low levels of radioactivity. The GR curve can differentiate between sands, clays, and the gradation between the two. Consequently, gamma ray logs are directly applicable to hydrogeologic assessments involving the identification of clean sands and clay beds and as a correlation tool.