The radiometric, or gamma-ray spectrometric method is a geophysical process used to estimate concentrations of the radioelements potassium, uranium and thorium by measuring the gamma-rays which the radioactive isotopes of these elements emit during radioactive decay. Airborne gamma-ray spectrometric surveys estimate the concentrations of the radioelements at the Earth's surface by measuring the gamma radiation above the ground from low-flying aircraft or helicopters.
All rocks and soils contain radioactive isotopes, and almost all the gamma-rays detected near the Earth's surface are the result of the natural radioactive decay of potassium, uranium and thorium. The gamma-rays are packets of electromagnetic radiation characterised by their high frequency and energy. They are quite penetrating, and can travel about 35 centimetres through rock and several hundred metres through the air. Each gamma ray has a characteristic energy, and measurement of this energy allows the specific potassium, uranium and thorium radiation to be diagnosed.
The gamma-ray spectrometric method has many applications but is used primarily as a geological mapping tool. Changes in lithology, or soil type, are often accompanied by changes in the concentrations of the radioelements. The method is capable of directly detecting mineral deposits. Potassium alteration, which is often associated with hydrothermal ore deposits, can be detected using the gamma-ray spectrometric method. It is also used for uranium and thorium exploration, heat flow studies and environmental mapping.
Geoscience Australia and the State and Northern Territory Geological Surveys have systematically surveyed most of Australia over the past 40 years using airborne gamma-ray spectrometry. These surveys have been combined into a Radiometric Map of Australia, consisting of potassium, uranium and thorium grids of the continent at 100 metre resolution. The data are available for free download through the Geophysical Archive Data Delivery System (GADDS).