In terms of conducting geophysical surveys, gamma spectrometry or airborne radiometric surveys can be employed as a means of detecting gamma rays or natural radioactive emanations. This detectable radiation comes from the decaying of some of the natural elements occurring in the Earth. For geological mapping, three elements are most diagnostically useful: potassium, uranium and thorium.
A radiometric survey is capable of detecting the presence of U, Th, and K at the Earths surface level. The data attained during a geophysical survey is mainly used for the following purposes:
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Detecting the presence of uranium
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Locating other mineral deposits
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Fluctuations in concentration levels of U, Th. and K – data to be applied directly to changes in lithology
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Tool for reconnaissance geological mapping
There are several variables that can influence the measurements and data acquired during a radiometric geophysical survey. Successfull data acquisition of gamma spectrometric survey are dependent upon the following factors:
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Influence of cosmic environment (planets)
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Certain meteorological conditions (precipitation in the previous 24 hours period, or accumulation of snow)
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Speed of the aircraft used in an airborne survey
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Volume of crystal packs on board the aircraft (minimal acceptable volume is considered to be two packs, ~42 litres or 2,560 cubic inches)
- Topography of the area to be surveyed
An airborne gamma-ray spectrometer can be calibrated to selectively record "windows" corresponding to the energy levels of interest. The on-board radiometric packs are designed to acquire precise data by measuring tiny, high frequency, high energy bursts that are emitted by the isotopes of the elements in question such as uranium. Gamma rays are measured by their electron volts so the data acquisition techniques used cannot be the same as those used in a magnetic survey.
A geophysical company utilizes data acquisition systems that have the following characteristics and capabilities:
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An automatic spectrum stabilization on each PMT/crystal and gain control
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Zero dead time or pulse pile up
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Real time spectra non-linearity corrections
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User defined detector volume of up to 18 crystals
Gamma spectrometry is a method of geophysical exploration that can offer highly relevant data in mapping lithology and mineral prospecting. Along with aeromagnetic surveys and electromagnetic surveys, gamma spectrometry is a common method used in geophysical exploration.
(Source; AGSO Journal of Australian Geology and Geophysics)