Gamma ray spectrometry is the study of the interaction between nuclear radiation and geological material. The emission of gamma rays occurs when an excited atomic nuclei undergoes an energy state change. The cause of the energy state changes is typical ‘natural’, which is to say the geological material contains atomic isotopes which have been present, or steadily decaying, since the formation of the material. In some instances an active source is used to incite the emission of Gamma rays through radioactive decay. The latter is highly regulated and typically only undertaken in easy to controlled environments, for instance liberators or boreholes.

Through recording the number of gamma ray photons in a given time frame and there energy it is possible to classify rock formations and infer how the rock was formed. Modern microprocessors enable portable gamma ray spectrometers to preform autonomous background compensation algorithms, and flash memory permits systems to store calibration spectra, meaning there is no longer the need to carry radioactive calibration samples. Advances in manufacturing have also improved the quality of NaI and BGO crystal detectors reducing the length of time required to perform a measurement.

Gamma ray spectrometry is well established for mineral exploration and in the oil and gas industry. More recently the impact of industrial practices is being considered and gamma ray spectrometry is being used to classify contaminated brownfield sites prior to redevelopment.