The technique of measurement of natural radiation in the earth’s surface is called “radiometric method” (also known as Gamma ray spectrometry). This is a process in which the unstable atom becomes stable in the process of its nucleus. Energy is released in the form of radiation, viz., alpha, beta, and gamma rays. Radioactive uranium and thorium minerals occur naturally in earth materials. Since they are radioactive, their presence or any anomalous concentration can be detected by radiometric surveys.
Although alpha, beta, and gamma particles may be emitted, only gamma particles will penetrate a sufficient distance to be useful in exploration. Gamma rays have the highest energy and will penetrate several hundred feet through atmosphere but only a few inches of earth are sufficient to attenuate natural radiation by one half. Therefore, the radioactive elements or their daughter products must occur in outcrop to be detected. The energy of gamma ray is characteristic of the radioactive element, it came from. Gamma ray can be measured in the ground or by a low flying aircraft.
The concentration, spatial distribution of radioactive elements such as uranium, thorium, and potassium from the upper layer (1060 cm) of earth’s surface having no vegetation, is measured by radiometric surveys. The gamma ray detection indicates the abundance of these elements. The alteration zones mapping can be done by knowing the variation in the element concentration in the host rock. The presence of uranium can be detected directly by radiometric surveys.
Greater cost of the radiometric surveying is compensated by the additional information gathered. Wherever the primary mineralization process is related to potash metasomatism (like tin, tungsten, and gold deposits), radiometrics is also used. Higher altitudes (above 300 m) may affect radiometric data negatively. Radiometrics cannot be used offshore or in perma-frost areas since water attenuates the radiometric signal.
In order to recognize the incidence of usual radioactive elements such as uranium, thorium, and potassium, scintillometry (gamma ray method) is used. Multichannel spectrometers are used to detect individual radioelement abundances. The method has application in exploration for uranium since it can detect directly. Among the three radioelements, “thorium” is the most immobile and resembles in behavior to that of zirconium. With increase in alkalinity in rocks (as in felsic rocks), thorium content increases like uranium content. Radiometric surveys can cover large areas and ideally suited for plane areas.
