The gamma ray logging tool is used to measure the natural gamma rays emitted by the formation surrounding the wellbore. The instrument probe consists of a 4-inch sodium iodide crystal, a photomultiplier tube, and a 1.6 kV high-voltage module. The series of gamma ray logging tools are designed to be compact and are primarily used for production logging in cased wells, offering high sensitivity and resolution. When combined with other instruments, the tool can also be used for tracer flow logging. By adding another gamma source, it can be applied in gravel pack completion studies.