The most common sources of natural gamma rays are potassium, thorium, and uranium. These elements are found in feldspars (i.e. granites, feldspathic), volcanic and igneous rocks, sands containing volcanic ash, and clays.
Gamma-ray measurement has the following applications:
* Well to well correlation: gamma-ray log fluctuates with changes in formation mineralogy. As such, gamma-ray logs from different wells within the same field or region can be very useful for correlation purposes, because similar formations show similar features.
* Logging runs correlation: Gamma-ray tools is typically run in every logging tools runs in a well. Being a common measurement, logging data can be put on depth with each other by correlating the gamma-ray feature of each run. 
* Quantitative evaluation of shaliness: Since natural radioactive elements tend to have greater concentration in shales than in other sedimentary lithologies, the total gamma ray measurement is frequently used to derive a shale volume (Ellis-1987, Rider-1996). This method however, is only likely to be used in a simple sandstone-shale formation, and is subject to error when radioactive elements are present in the sand.