Rock and Soil Discrimination by Low Altitude Airborne Gamma-Ray Spectrometry in Payne County, Oklahoma

Abstract

The ability to identify and discriminate rock and soil types from the air using gammaray spectrometry was investigated in Payne County, Oklahoma. The data, which were reduced to concentration values for K, U, and Th, were obtained from a helicopter at an average altitude of 75 feet above the ground. The area investigated was underlain by a variety of sedimentary rocks which encompassed a transitional sequence from continental deltaic deposits through marine sediments.Because of the shallow depths sensed aeroradiometrically (about one foot), the data were first related to the pedology of the area considering the regolith independent of underlying bedrock. However, it was soon discovered that in situ soils could only be distinguished to the extent that their parent lithologies could be discriminated. Concentrations of K, U, and Th determined from the air suggest that the signatures of these elements in underlying bedrock are largely preserved in in situ soils allowing the identification of lithologies and the discrimination of contacts from the air. Transported soils were generally distinguishable from in situ soils and hence, common lithologies of the area.A sequence of sedimentary formations was established on the basis of shale content. Quantitative estimates of percent clay were made using thorium contents as an indicator of clay (shale content) and an attempt was made to relate clay content to soil engineering properties.In an effort to find new improved means of aeroradiometric data presentation, a multivariate technique, cluster analysis, was used. This technique merged the K, U, and Th data and produced lithologically significant groupings of rock formations. Such an approach might be used as a step toward generating a single lithologic map directly as opposed to the traditional individual element, multi-map technique currently used.The study emphasizes that spectral data converted to concentration units (ppm or %) are superior to qualitative count rates (cps) or total radioactivity for the discrimination and identification of lithologies. Ratioed spectral data in the study area were relatively insensitive to changes and suggest that anomalies may be enhanced by such a ratioing procedure.