Near-Surface Moisture and Biomass Influences on the Reliability of Aerial Radiometric Surveys as a Measure of Natural Radioelement Concentrations

摘要

Significant differences in aerial radiometric system calibration constants in humid-forested (Portland quadrangle) and desert (Lake Mead test strip) regions indicated the probability of substantial gamma-ray absorption by soil moisture in humid areas. A set of test quadrangles was selected to investigate relationships among a group of environmental variables and ratios of whole-quadrangle mean (X) stream sediment uranium (U) and thorium (Th) contents to corresponding mean aerial radiometric uranium and thorium measurements. It was assumed that stream sediment values might be sufficiently representative of the average surface radioelement concentrations to be used in ground-calibrating the aerial systems. Preliminary testing established strong statistical correlations between the radioelement ratios and moisture and biomass variables. However, it is recognized that moisture and biomass relationships represent a variety of variables affecting the aerial gamma-ray measurements, such as soil formation, including weathering effects. Also, the data suggested that in certain very humid areas placering effects caused the mean thorium and uranium content of stream sediments to be much greater than actual concentrations in the average surface rocks. These effects appeared to be minimal in the arid regions, where the stream sediment data were more representative of the surface materials in general. These observations prompted a follow-up study using surface-to-aerial ratios based on analyses of surface samples in humid regions and including the stream sediment-to-aerial data ratios only for the relatively arid areas. Multiple linear regression analyses of the data indicated two-variable models would provide reasonable correction factors for aerial data obtained in humid environments. (ERA citation 05:030232)