| Abstract |
This paper summarizes results from CO2 flux studies conducted in a variety of geothermal systems world-wide. Results show general relationships between the intensity of soil CO2 flux, and characteristics of the underlying reservoir. In general, the magnitude of soil diffuse CO2 flux at the surface can be related to topography and reservoir temperature. This general relationship can be explained by the following sequence of processes. Firstly, surface topography directs surface and subsurface recharge; all else being equal, geothermal systems located in lower elevation areas receive a greater lateral inflow of recharge. Conversely, higher elevation systems receive less recharge, and this raises reservoir temperature; higher elevation systems are generally hotter for the same reason engines run hotter when they are low on coolant. Accordingly, topography influences reservoir temperature. Secondly, temperature controls the concentration of CO2 in the reservoir via temperature-dependent mineral-water equilibrium. Thirdly, reservoir CO2 content controls the magnitude of soil diffuse CO2 flux at the surface. The relationship between reservoir CO2 content and temperature is the basis for a new CO2 flux-based geothermometer for geothermal exploration. These findings have implications for the development of hydrothermal electricity, currently slowed by the economic risks of exploration. We emphasize recharge as an important factor in the science of geothermal exploration, and the utility of the CO2 flux survey technique for geothermal resource evaluation. |