Time-integrated radon soil-gas surveys in geothermal exploration in the Southern Rio Grande Rift, New Mexico. Volume 1, Final report.

摘要

This report evaluates time-integrated radon soil-gas surveys for convective low- to intermediate-temperature geothermal resource exploration in areas with variable soil development and arid climate. A practical method for conducting and interpreting alpha-track detector (ATD) radon soil-gas surveys is outlined. Initial surveying was performed over a well characterized geothermal system around Tortugas Mountain, with subsequent exploratory surveying near Radium Springs and Rincom. Data filtering and interpretation is facilitated by application of corrections to ATD measurements that are based on a model of pure diffusion. This approach yields two important results: most radon highs; not associated with advective or geothermal processes are filtered out; and anomalously high positive radon residuals have identified faults and areas with higher temperature gradients. This approach shows excellent pathfinder potential for targetting shallow upflow zones. Conclusions drawn from this study include: field ATD measurements have lognormal distributions. Lab ATD radon emanation measurements of regional soil samples also follow lognormal distributions. While results of field ATD surveys are not quantitatively respectable, they are qualitatively repeatable. A component of advective transport is probably observed in most surveyed soils during ATD exposure. The Rincon study showed a correlation between radon emanation and soil moisture. Windblown sandy soils generally have higher radon emanation. When bedrock is shallow, or highly developed caliche horizons exist, consistently higher negative radon residuals are frequently observed. Geological findings of this survey are listed, as are recommended parameters for ATD radon soil-gas surveys. (MHB) 119 refs., 103 figs., 7 tabs.