Radiometric surveys detect and map natural radioactive emanations, called gamma rays, from rocks and soils. All detectable gamma radiation from earth materials come from the natural decay products of only three elements, i.e. uranium (U), thorium (Th), and potassium (K). In parallel with the magnetic method, that is capable of detecting and mapping only magnetite (and occasionally pyrrhotite) in soils and rocks, the radiometric method is capable of detecting the presence of U, Th, and K at the surface of the ground.
The basic purpose of radiometric surveys is to determine either the absolute or relative amounts of U, Th, and K in the surface rocks and soils. No other geophysical method, and probably no other remote sensing method, requires consideration of so many variables in order to reduce the observational data to a form that is useful for geological interpretation. Meteorological conditions, the topography of the survey area, the influence of the planets cosmic environment, the height of the sensor above ground and the speed of the aircraft are just a few of the variables which affect radiometric measurements, and which can bias an analysis unless dealt with very thoroughly.
A few of the benefits that could be expected from the interpretation of radiometric surveys include:
- Changes in the concentration of the three radioelements U, Th, and K accompany most major changes in lithology, hence the method can be used as a reconnaissance geologic mapping tool in many areas.
- Variations in radioelement concentrations may indicate primary geological processes such as the action of mineralizing solutions or metamorphic processes.
- Variations also characterize secondary geological processes like supergene alteration and leaching.
- Radiometric surveys are capable of directly detecting the presence of uranium.
- This data can also assist in locating some intrusive related mineral deposits.
In appropriate areas, when used as a reconnaissance technique for mapping geology and for prospecting, the cost/benefit ratio for airborne radiometric surveying is nearly as good as that for airborne magnetometer surveying.