NEAR-SURFACE CO2 MONITORING AND ANALYSIS TO DETECT HIDDEN GEOTHERMAL SYSTEMS

摘要

"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. We investigate the potential for CO_(2) detection and monitoring below and above ground in the near-surface environment as an approach to exploration targeting hidden geothermal systems. We focus on CO_(2) because it is the dominant noncondensible gas species in most geothermal systems and has moderate solubility in water. We carried out numerical simulations of a CO_(2) migration scenario to calculate the magnitude of expected fluxes and concentrations. Our results show that CO_(2) concentrations can reach high levels in the shallow subsurface even for relatively low geothermal source CO_(2) fluxes. However, once CO_(2) seeps out of the ground into the atmospheric surface layer, winds are effective at dispersing CO_(2) seepage. In natural ecological systems in the absence of geothermal gas emissions, near-surface CO_(2) fluxes and concentrations are predominantly controlled by CO_(2) uptake by photosynthesis, production by root respiration, microbial decomposition of soil/subsoil organic matter, groundwater degassing, and exchange with the atmosphere. Available technologies for monitoring CO_(2) in the near-surface environment include the infrared gas analyzer, the accumulation chamber method, the eddy covariance method, hyperspectral imaging, and light detection and ranging. To meet the challenge of detecting potentially small-magnitude geothermal CO_(2) emissions within the natural background variability of CO_(2), we propose an approach that integrates available detection and monitoring techniques with statistical analysis and modeling strategies. The proposed monitoring plan initially focuses on rapid, economical, reliable measurements of CO_(2) subsurface concentrations and surface fluxes and statistical analysis of the collected data. Based on this analysis, areas with a high probability of containing geothermal CO_(2) anomalies can be further sampled and analyzed using more expensive chemical and isotopic methods. Integrated analysis of all measurements will determine definitively if CO_(2) 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 CO_(2) monitoring program.