Radon Soil-Gas Surveys with Diffusion-Model Corrections in Geothermal Exploration

Abstract

A diffusion model approach to interpretation of radon soil gas surveys takes advantage of radon's ability to act as a natural tracer. A correction, based on pure diffusion in a homogeneous soil, is applied to field measurements and accounts for sampling depth variations in a region of high concentration gradients near the surface. With the depth correction, the field measurement is adjusted downward in depth through the concentration gradient to a secular equilibrium value where radon production is balanced by decay. In addition, a sample of soil from the depth of the field measurement is sealed inside a jar with an alpha track detector (ATD). The jar measurement is corrected for dilution and for concentration growth up to secular equilibrium. The difference between the depth corrected field value and the emanation corrected jar value is a measure of deviation from the diffusion model. Deviation can result from adjective processed, emanation heterogeneity, and in variations of actual soil porosity and diffusion coefficients from model values.