Carbon dioxide is the most abundant, non-condensable gas in volcanic systems, released into the atmosphere through either diffuse or advective fluid flow. The emission of substantial amounts of CO2 at Earth’s surface is not only controlled by volcanic plumes during periods of eruptive activity or fumaroles, but also by soil degassing along permeable structures in the subsurface. Monitoring of these processes is of utmost importance for volcanic hazard analyses, and is also relevant for managing geothermal resources. Fluid-bearing faults are key elements of economic value for geothermal power generation. Here, we describe for the first time how sensitively and quickly natural gas emissions react to changes within a deep hydrothermal system due to geothermal fluid reinjection.
For this purpose, we deployed an automated, multi-chamber CO2 flux monitoring system within the damage zone of a deep-rooted major normal fault in the Los Humeros Volcanic Complex (LHVC) in Mexico and recorded data over a period of five months. After removing the atmospheric effects on variations in CO2 flux, we calculated correlation coefficients between residual CO2 emissions and reinjection rates, identifying an inverse correlation of ρ = -0.51 to -0.66. Our results indicate that gas emissions respond to changes in reinjection rates within 24 hours, proving an active hydraulic communication between the hydrothermal system and Earth’s surface. This finding is a promising indication not only for geothermal reservoir monitoring but also for advanced long-term volcanic risk analysis. Response times allow for estimation of fluid migration velocities, which is a key constraint for conceptual and numerical modelling of fluid flow in fracture-dominated systems.
Magmatic volatiles can be considered as the surface fingerprint of active volcanic systems, both during periods of quiescent and eruptive volcanic activity. The spatial variability of gas emissions at Earth’s surface is a proxy for structural discontinuities in the subsurface of volcanic systems. We conducted extensive and regular spaced soil gas surveys within the Los Humeros geothermal field to improve the understanding of the structural control on fluid flow.Surveys at different scales were performed with the aim to identify areas of increased gas emissions on reservoir scale, their relation to unknown/knows volcano-tectonic structures on fault scale favoring fluid flow, and determine the origin of gas emissions. Herein, we show results from a carbon dioxide efflux scouting survey, which was performed across the main geothermal production zone together with soil temperature measurements. We identified five areas with increased carbon dioxide emissions, where further sampling was performed with denser sampling grids to understand the fault zone architecture and local variations in gas emissions.We show that a systematic sampling approach on reservoir scale is necessary for the identification and assessment of major permeable fault segments. The combined processing of CO2 efflux and carbon/helium isotopes facilitated the detection of permeable structural segments with a connection to the deep, high-temperature geothermal reservoir, also in areas with low to intermediate carbon dioxide emissions. The results of this study complement existing geophysical datasets and define further promising areas for future exploration activities in the north- and southwestern sector of the production field.The data are presented as one zip folder with 4 data tables (tab delimited text format) according to the measurement variable. The columns are defined in each data file.
The proposed project aims (1) to quantify the contribution of root-rhizosphere respiration to total soil respiration and to net ecosystem CO2 fluxes and (2) to assess the environmental and biological driving factors. It contributes to WP 2 of COST E38 (Fine root dynamics).