"Hidden" geothermal systems are systems devoid of obvious surface hydrothermal manifestations. Emissions of moderate to low solubility gases may be one of the primary near-surface signals from these systems. A tool to discover new geothermal systems may therefore be the detection of these gas emissions. We investigate the potential for CO2 detection and monitoring below and above ground in the near-surface environment to serve as a means to discern hidden geothermal systems. We focus the investigation on CO2 because (1) it is the dominant noncondensible gas species in most geothermal systems, (2) it has moderate solubility in water, and (3) a wide range of technologies are available to monitor CO2 in the near-surface environment. However, monitoring for CO2 derived from hidden geothermal reservoirs is complicated by the large variation in CO2 fluxes and concentrations arising from natural biological and hydrologic processes.

In the near-surface environment, the flow and transport of CO2 at high concentrations will be controlled by its high density, low viscosity, and high solubility in water relative to air. Numerical simulations of CO2 migration show that concentrations can reach very high levels in the shallow subsurface even for relatively low geothermal source CO2 fluxes. However, once CO2 seeps out of the ground into the atmospheric surface layer, winds are effective at dispersing CO2 seepage.

In natural ecological systems in the absence of geothermal gas emissions, near-surface CO2 fluxes and concentrations are predominantly controlled by CO2 uptake by photosynthesis, production by root respiration, and microbial decomposition of soil/subsoil organic matter, groundwater degassing, and exchange with the atmosphere. Available technologies for monitoring CO2 in the near-surface environment include (1) the infrared gas analyzer (IRGA) for measurement of concentrations at point locations, (2) the accumulation chamber (AC) method for measuring soil CO2 fluxes at point locations, (3) the eddy covariance (EC) method for measuring net CO2 flux over a given area, (4) hyperspectral imaging of vegetative stress resulting from elevated CO2 concentrations, and (5) light detection and ranging (LIDAR) that can measure CO2 concentrations over an integrated path.

To meet the challenge of detecting potentially small-magnitude geothermal CO2 emissions within the natural background variability of CO2, we propose an approach that integrates available detection and monitoring techniques with statistical analysis and modeling strategies. Within the area targeted for geothermal exploration, point measurements of soil CO2 fluxes and concentrations should be made along grids using the AC method and a portable IRGA, respectively, accompanied by measurements of net surface flux using EC.

Particular attention should be paid to characterizing gas flow along faults/fractures. Also, the natural spatial and temporal variability of soil CO2 fluxes concentrations should be quantified within a background area with similar geologic, climatic, and ecosystem characteristics to the area targeted for geothermal exploration. Statistical analyses of data collected from both areas should be used to guide sampling strategy, discern spatial patterns that may be indicative of geothermal CO2 emissions, and assess the presence of geothermal CO2 within the natural background variability with a desired confidence level.

Once measured CO2 concentrations and fluxes have been determined with high confidence to be of geothermal origin, more expensive sampling of gas profiles with depth and chemical and isotopic analyses can be undertaken. Integrated analysis of all measurements will determine definitively if CO2 derived from a deep geothermal source is present, and if so, the spatial extent of the anomaly. The suitability of further geophysical measurements, installation of deep wells, and geochemical analyses of deep fluids can then be determined based on the results of the near surface CO2 monitoring program.

Acknowledgement: This work was completed at Lawrence Berkeley National Laboratory, under U.S. Department of Energy Contract No. DE-AC03765F00098.

Press the Back button in your browser, or search again.

Copyright 2005, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.