  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 Earths 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.
