Scouting for Permeable Structures in Geothermal Systems using Soil Gas Radon

Abstract

Radon-222 is a naturally-occurring radioactive gas belonging to the uranium-238 decay chain. Radon gas has been recognized as an important natural tracer in geologic systems because of 1) the ubiquity of its parent nuclides, 2) its enhanced mobility as a gas, 3) its inert chemical nature, 4) its short half-life, and 5) the ease of detecting it electronically even at low levels. Several studies harnessing these properties of radon dealt with volcanic/seismic monitoring & forecasting and mineral & petroleum exploration. Fluid convection or the presence of a carrier gas enables the migration of radon to the near-surface, through pathways with enhanced permeability (e.g. faults, fractures). Because of a faster velocity of transport, more radon is carried and there is less time for decay; hence, radon concentrations may become enriched in the soil. To investigate the applicability of soil radon signatures in determining potentially permeable fluid pathways in geothermal systems, a soil radon survey campaign was set out at Leyte Geothermal Field, Philippines. Gridded soil radon measurements were conducted using an alpha spectroscopic soil gas probe to investigate the correlation of soil radon concentration at structural traces with permeability reported from drilled wells. In general, most of the high soil radon anomalies were found at fault intersections and permeable structural segments. Together with detailed structural analysis and advanced geophysical data, spatial soil radon gas data has the potential to accurately scout for highly permeable structures in the exploration and development of a geothermal resource.