Scouting for Permeable Structures in Geothermal Systems Using Soil Radon Gas Signatures

Jigo W. Mismanos and Atoz A. Vasquez

Energy Development Corporation
38/F One Corporate Center, J. Vargas cor. Meralco Ave.,
Ortigas Center, Pasig City, Philippines



Radon-222 is a naturally-occurring radioactive gas belonging to the uranium-238 decay chain. It is produced from the decay of radium-226 and has a half-life of 3.8 days. 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 and seismic monitoring and forecasting, geothermal, mineral, and petroleum exploration, as well as radiowaste containment.

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.

In geothermal systems, radon signatures can be used fromthe exploration to the exploitation of a geothermal resource. Soil radon gas geochemistry surveys may help delineate permeable segments of a fault which can be targeted for drilling. Together with detailed structural analysis and advanced geophysical data, spatial soil radon gas data has the potential to accurately scout for highlypermeable structures in the exploitation of a geothermal resource.

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