Total and Spectrometric Gamma-Ray Surveys from Helicopters and Vehicles

A single- channel gamma-ray spectrometer has been built and tested in surveys from the air and vehicles. Using a 5 in. x 5 in. sodium iodide (thalium-activated) scintillation crystal, thorium, uranium and potassium anomalies can be detected in the upper 10 to 25 cm of soil or rock. Thorium and uranium anomalies are of interest as pathfinder elements to locate and define placer deposits, changes in lithology, changes in sedimentary facies, and differences in soil type, e.g. tropical iron and aluminium ores. The 750 km of modern beach in the state of Texas were completely surveyed by helicopter in five flying days. Anomalies containing as little as 6 ppm thorium and 3 ppm uranium as determined by analysis of ground samples were readily detected against a background of beach sand containing 1 to 2 ppm thorium and 0.3 to 0.6 ppm uranium at ground speeds of 105 km/h (60 knots) and elevations of 17 ± 1 m. At such altitudes and ground speeds, a complete coverage of the surface can be obtained in the Texas uranium districts at costs well under $ 175/km2. The relative contributions of thorium or uranium to a particular total gamma-ray anomaly have been successfully measured by gamma -spectrometry from the air at the above altitudes and ground speeds. The instrumentation has application for the direct exploration for potassium, thorium and uranium, and for the pathfinder applications, e. g. bedded phosphates that contain over 10 ppm uranium and placer deposits that contain twice background thorium or uranium. Placers are prime exploration targets because they often contain multiple mineral values, and modern placers along beaches or rivers provide convenient ground control for navigation purposes. In evaluating placers, a combination of gamma-ray and aeromagnetic techniques is advantageous because many placer deposits contain enough ilmenite and magnetite to give a detectable aeromagnetic anomaly under the conditions of the gamma-ray survey. Furthermore, the aeromagnetic anomaly will arise from minerals at much greater depths than is the case with the gamma-ray anomaly. (author)