[1] Carbon dioxide (CO 2 ) diffuse degassing structures (DDS) at Furnas volcano (São Miguel Island, Azores) are mostly associated with the main fumarolic fields, evidence that CO 2 soil degassing is the surface expression of rising steam from the hydrothermal system. Locations with anomalous CO 2 flux are mainly controlled by tectonic structures oriented WNW-ESE and NW-SE and by the geomorphology of the volcano, as evidenced by several DDS located in depressed areas associated with crater margins. Hydrothermal soil CO 2 emissions in Furnas volcano are estimated to be ∼968 t d . Discrimination between biogenic and hydrothermal CO 2 was determined using a statistical approach and the carbon isotope composition of the CO 2 efflux. Different sampling densities were used to evaluate uncertainty in the estimation of the total CO 2 flux and showed that a low density of points may not be adequate to quantify soil emanations from a relatively small DDS. Thermal energy release associated with diffuse degassing at Furnas caldera is about 118 MW (from an area of ∼4.8 km 2 ) based on the H 2 O/CO 2 ratio in fumarolic gas. The DDS also affect Furnas and Ribeira Quente villages, which are located inside the caldera and in the south flank of the volcano, respectively. At these sites, 58% and 98% of the houses are built over hydrothermal CO 2 emanations, and the populations are at risk due to potential high concentrations of CO 2 accumulating inside the dwellings.

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[1] Diffuse degassing through the soil is commonly observed in volcanic areas and monitoring of carbon dioxide flux at the surface can provide a safe and effective way to infer the state of activity of the volcanic system. Continuous measurement stations are often installed on active volcanoes such as Furnas (Azores archipelago), which features low temperature fumaroles, hot and cold CO 2 rich springs, and several diffuse degassing areas. As in other volcanoes, fluxes measured at Furnas are often correlated with environmental variables, such as air temperature or barometric pressure, with daily and seasonal cycles that become more evident when gas emission is low. In this work, we study how changes in air temperature and barometric pressure may affect the gas emission through the soil. The TOUGH2 geothermal simulator was used to simulate the gas propagation through the soil as a function of fluctuating atmospheric conditions. Then, a dual parameters study was performed to assess how the rock permeability and the gas source properties affect the resulting fluxes. Numerical results are in good agreement with the observed data at Furnas, and show that atmospheric variables may cause the observed daily cycles in CO 2 fluxes. The observed changes depend on soil permeability and on the pressure driving the upward flux.

Recent geophysical evidence for large-scale regional crustal inflation and localized crustal magma intrusion has made Lastarria volcano (northern Chile) the target of numerous geological, geophysical, and geochemical studies. The chemical composition of volcanic gases sampled during discrete campaigns from Lastarria volcano indicated a well-developed hydrothermal system from direct fumarole samples in A.D. 2006, 2008, and 2009, and shallow magma degassing using measurements from in situ plume sampling techniques in 2012. It is unclear if the differences in measured gas compositions and resulting interpretations were due to artifacts of the different sampling methods employed, short-term excursions from baseline due to localized changes in stress, or a systematic change in Lastarria's magmatic-hydrothermal system between 2009 and 2012. Integrated results from a two-day volcanic gas sampling and measurement campaign during the 2014 International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) Commission on the Chemistry of Volcanic Gases (CCVG) 12 th Gas Workshop are used here to compare and evaluate current gas sampling and measurement techniques, refine the existing subsurface models for Lastarria volcano, and provide new constraints on its magmatic-hydrothermal system and total degassing budget. While compositional differences among sampling methods are present, distinct compositional changes are observed, which if representative of longterm trends, indicate a change in Lastarria's overall magmatic-hydrothermal system. The composition of volcanic gases measured in 2014 contained high proportions of relatively magma-and water-soluble gases consistent with degassing of shallow magma, and in agreement with the 2012 gas composition. When compared with gas compositions measured in 2006-2009, higher relative H 2 O/CO 2 ratios combined with lower relative CO 2 /S t and H 2 O/S t and stable HCl/St ratios (where S t is total S [SO 2 + H 2 S]) are observed in 2012 and 2014. These compositional changes suggest variations in the magmatichydrothermal system between 2009 and 2012, with possible scenarios to explain these trends including: (1) decompression-induced degassing due to magma ascent within the shallow crust; (2) crystallization-induced degassing of a stalled magma body; (3) depletion of the hydrothermal system GEOSPHERE

Electrical conductivity tomography is a well-established galvanometric method for imaging the subsurface electrical conductivity distribution. We characterize the conductivity distribution of a set of volcanic structures that are different in terms of activity and morphology. For that purpose, we developed a large-scale inversion code named ECT-3D aimed at handling complex topographical effects like those encountered in volcanic areas. In addition, ECT-3D offers the possibility of using as input data the two components of the electrical field recorded at independent stations. Without prior information, a Gauss-Newton method with roughness constraints is used to solve the inverse problem. The roughening operator used to impose constraints is computed on unstructured tetrahedral elements to map complex geometries. We first benchmark ECT-3D on two synthetic tests. A first test using the topography of Mt. St Helens volcano (Washington, USA) demonstrates that we can successfully reconstruct the electrical conductivity field of an edifice marked by a strong topography and strong variations in the resistivity distribution. A second case study is used to demonstrate the versatility of the code in using the two components of the electrical field recorded on independent stations along the ground surface. Then, we apply our code to real data sets recorded at (i) a thermally active area of Yellowstone caldera (Wyoming, USA), (ii) a monogenetic dome on Furnas volcano (the Azores, Portugal), and (iii) the upper portion of the caldera of Kīlauea (Hawai'i, USA). The tomographies reveal some of the major structures of these volcanoes as well as identifying alteration associated with high surface conductivities. We also review the petrophysics underlying the interpretation of the electrical conductivity of fresh and altered volcanic rocks and molten rocks to show that electrical conductivity tomography cannot be used as a stand-alone technique due to the non-uniqueness in interpreting electrical conductivity tomograms. That said, new experimental data provide evidence regarding the strong role of alteration in the vicinity of preferential fluid flow paths including magmatic conduits and hydrothermal vents.

Time series of soil CO 2 efflux recorded in the Azores archipelago volcanic-hydrothermal areas feature daily and seasonal variations. The recorded CO 2 efflux values were lower during summer than in the winter season. The diurnal CO 2 efflux values were higher at dawn and lower in the early afternoon, contrary to that observed in biogenic environments. CO 2 efflux cycles correlated well with the environmental variables, such as air temperature, wind speed, and barometric pressure, which also showed low-and high-frequency periodicities. Several simulations were performed here using the Transport of Unsaturated Groundwater and Heat 2 (TOUGH2) geothermal simulator to complement the study of Rinaldi et al. (2012). The effects of the water table depth, air temperature perturbation amplitude, and soil thermal gradient contributed to an explanation of the contrasts observed in the diurnal (S 1 ) and semidiurnal (S 2 ) soil CO 2 efflux peaks for the different monitoring sites and seasons. Filtering techniques (multivariate regression analysis and fast Fourier transform filters) were also applied to the recorded time series to remove effects of external variables on the soil CO 2 efflux. The resulting time series (the residuals) correspond to the best approach to the deep-seated (volcanic/hydrothermal) CO 2 emissions and thus should be used in seismovolcanic monitoring programs. Even if no evident correlation can be established yet between the soil CO 2 residuals and seismicity over the monitored time, a seismic swarm that occurred around the end of 2008 might have triggered some deviations from the observed daily cycles.