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Phreatic and phreatomagmatic eruptions represent some of the greatest hazards occurring on volcanoes. They result from complex interactions at a depth between rock, water, and magmatic fluids. Understanding and assessing such processes remain a challenging task, notably because a large‐scale characterization of volcanic edifices is often lacking. Here we focused on Miyakejima Island, an inhabited 8‐km‐wide stratovolcano with regular phreatomagmatic activity. We imaged its plumbing system through a combination of four geophysical techniques: magnetotellurics, seismicity, self‐potential, and thermal image. We thus propose the first comprehensive interpretation of the volcanic island in terms of rock properties, temperature, fluid content, and fluid flow. We identify a shallow aquifer lying above a clay cap (<1 km depth) and reveal its relation with magmatic‐tectonic features and past eruptive activity. At greater depths (2–4.5 km), we infer a seismogenic resistive region interpreted as a magmatic gas‐rich reservoir (≥370°C). From this reservoir, gases rise through a fractured conduit before being released in the fumarolic area at ∼180°C. During their ascent, these hot fluids cross a ∼1.2‐km‐long liquid‐dominated zone causing local steam explosions. Such magmatic‐hydrothermal interaction elucidates (i) the origin of the long‐period seismic events and (ii) the mixing mechanism between magmatic and hydrothermal fluids, which was previously observed in the geochemical signature of fumaroles. Our results demonstrate that combining multidisciplinary large‐scale methods is a relevant approach to better understand volcanic systems, with implications for monitoring strategies. Key Points: An elongated aquifer (<1 km depth) lies between a resistive unsaturated layer and a conductive clay capA gas‐rich reservoir (2–4.5 km depth) feeds the main fumarolic areaMagmatic gases interact during their ascent with a 1.2‐km‐long water‐rich region creating local steam explosions [ABSTRACT FROM AUTHOR]

Through a large number of magnetotelluric (MT) observations conducted in a study area, one can obtain regional one-dimensional (1-D) features of the subsurface electrical conductivity structure simply by taking the geometric average of determinant invariants of observed impedances. This method was proposed by Berdichevsky and coworkers, which is based on the expectation that distortion effects due to near-surface electrical heterogeneities will be statistically smoothed out. A good estimation of a regional mean 1-D model is useful, especially in recent years, to be used as a priori (or a starting) model in 3-D inversion. However, the original theory was derived before the establishment of the present knowledge on galvanic distortion. This paper, therefore, reexamines the meaning of the Berdichevsky average by using the conventional formulation of galvanic distortion. A simple derivation shows that the determinant invariant of distorted impedance and its Berdichevsky average is always downward biased by the distortion parameters of shear and splitting. This means that the regional mean 1-D model obtained from the Berdichevsky average tends to be more conductive. As an alternative rotational invariant, the sum of the squared elements (ssq) invariant is found to be less affected by bias from distortion parameters; thus, we conclude that its geometric average would be more suitable for estimating the regional structure. We find that the combination of determinant and ssq invariants provides parameters useful in dealing with a set of distorted MT impedances. [ABSTRACT FROM AUTHOR]

There are many potential sites for geothermal power plants in Thailand. After many years of geological and geophysical surveys, a pilot magnetotelluric (MT) survey was made to assess the reservoir of the Mae Chan geothermal area, northern Thailand, which is one of the key areas for geothermal development. Seven MT sites were deployed in a 3 km × 4 km area around the Mae Chan district covering the Mae Chan hot springs. The MT data were acquired at low and high frequency ranges and were inverted using a 3-D MT inversion to yield the 3-D resistivity structure of the area. The results show that there are two conductive zones near the surface associated with the hot fluid of the Mae Chan hydrothermal system. The hot fluid reservoir mostly resides at less than 500 m below the surface in weathered and fractured granite and in the overlying sedimentary deposits. Its source rock is imaged as a resistive zone corresponding to the hot granite batholith below it. The hot fluid rises up along the Mae Chan fault. The fault is clearly observed as a resistivity contrast extending from the surface to depth. It dips at a moderate angle. From the measured temperature of the fluid from a drill hole and the estimated temperature of the granite rock from the resistivity structure we conclude that the Mae Chan geothermal area is likely to be suitable for immediate development of a small-scale geothermal power plant. [ABSTRACT FROM AUTHOR]

We use 2-D magnetotelluric (MT) problems as a feasibility study to demonstrate that 3-D MT problems can be solved with a direct solver, even on a standard single processor PC. The scheme used is a hierarchical domain decomposition (HDD) method in which a global computational domain is uniformly split into many smaller non-overlapping subdomains. To make it more efficient, two modifications are made to the standard HDD method. Instead of three levels as in the standard HDD method, we classify the unknowns into four classes: the interiors, the horizontal interfaces, the vertical interfaces and the intersections. Four sets of smaller systems of equations are successively solved with a direct method (an LU factorization). The separation significantly reduces the large memory requirements of a direct solver. It also reduces the CPU time to almost half that of the standard HDD method although it is still slower than the conventional finite difference (FD) method. To further enhance the speed of the code, a red-black ordering is applied to solve the horizontal and vertical interface reduced systems. Numerical experiments on a 2-D MT problem of a given size running on a single processor machine shows that CPU time and memory used are almost constant for any resistivity models, frequencies and modes. This is a clear advantage of our algorithm and is of particular importance if the method is applied to 3-D problems. We show that our new method results in reductions in both memory usage and CPU time for large enough domains when compared to the standard FD and HDD schemes. In addition, we also introduce a 'memory minimization map', a graphical tool we can use instead of trial-and-error to pre-select the optimal size of subdomains, which yield the best performance in both CPU time and memory even running on a serial machine. [ABSTRACT FROM AUTHOR]

After early exploration during the 1980s and 1990s, the 0.3 MW Fang geothermal power plant was built as a demonstration to supply electricity to the local community. The shallow well (100 m) drilling program produced about 22 l/s of 125 °C water, and two wells to 500 m produced about 10 l/s. Due to the lack of detailed information on the geothermal system, the plan to expand to a larger power plant was halted to avoid the drilling missing the hot fluid. The plan was resumed in the last ten years starting with the magnetotelluric (MT) survey. Thirty three MT sites were deployed on the southern part of the Fang geothermal area. A remote site was installed about 600 km south of the study area for better data quality. After data processing, the data was inverted with WSINV3DMT to produce the 3-D resistivity model which clearly matches the near-surface geology and is also in agreement with the conceptual geology of the Fang geothermal system. The high resistivity zone is interpreted as the crystalline granitic rock, while the intermediate resistivity zone is associated with the Fang sedimentary basin. The resistivity contrast between the higher and lower resistivities helps reveal the orientations of the major Mae Chan Fault (MCF) and the two local faults of the area. The two main conductors of interests, C1 and C2, are directly linked to the hot fluid found at the surface. C1 is shallow (<50 m), and found only beneath the Fang hot spring, and so it is interpreted as the fracture reservoir. C2, which was not discovered in previous studies, extends from near the surface to a depth of 500 m, and at a depth of 200 m, it is about 1 km wide. It is about 1 km south of the Fang hot spring where the warm water was found to have seeped to the surface through the MCF. Two possible interpretations are proposed for the C2 conductor. The first is that there is an impermeable clay zone trapping a relatively high resistivity geothermal fluid reservoir beneath, like the caprock of a magmatic geothermal play type. This would require a deeper well to extract the hot fluid. As with the C1 conductor, the other interpretation is that the C2 conductor is a fracture geothermal reservoir where hot fluid from the deep resides within the pores of the sedimentary rock and fractures of the altered granite. This would require a shallower well. Both interpretations suggest that the C2 anomaly is of value. Since it has never been explored, a drilling over the C2 anomaly is recommended to probe its characteristic and also to extract more hot fluid for the future expansion of the geothermal power plant. [ABSTRACT FROM AUTHOR]

• Shallow reservoirs can be precisely explored with a dense MT survey. • Hot fluid reservoir in fracture zones is imaged as low resistivity structure. • New drilling results match the resistivity structure perfectly. • Confirmation that Mae Chan hot spring has a high potential for a power plant. In a non-volcanic geothermal system, like Mae Chan hot spring and many other hot springs in Thailand, hot water is heated deep underground and seeps to the surface through fractures and faults. Some of the hot water may aggregate in a hydrothermal alteration zone along the fracture zones of granite rocks to form shallow "hot water reservoirs". These networks lower the bulk resistivity of the granitic rock to form a low resistivity zone associated with the hot water reservoir, which can then be imaged via a magnetotelluric (MT) survey. A series of magnetotelluric surveys from 2013 to 2018 was conducted in order to assess the location, size and depth of the shallow geothermal reservoir of the Mae Chan hot spring. All data were combined to produce the final 3-D resistivity structure. The final MT survey had a high density of MT sites across the zone of interest which allowed us to precisely image the shallow reservoir for drilling purposes. Using the final MT results, five new boreholes with a maximum depth of 200 m were drilled. Hot water was found at various depths from each borehole with perfect agreement with the final resistivity structure derived from the MT data. This 3-D resistivity outline will be useful in developing the field with future production and re-injection wells. [ABSTRACT FROM AUTHOR]

Large earthquakes sometimes trigger local seismicity that is distal to their rupture zones. Various mechanisms for this triggered seismicity have been proposed, based on either the static stress change or ground shaking from seismic waves, but local geological structure is rarely studied to discern why this seismicity is remotely induced. We present the results of a joint three-dimensional resistivity and isotopic analysis of the groundwater system surrounding Mount Fuji, Japan, where increased seismicity was observed following the A.D. 2011 Tohoku- Oki megathrust earthquake. An electrically conductive zone and high concentrations of magmatic gases (He and CO2) correspond to the zone of triggered seismicity. In contrast, a contribution of magmatic water is not suggested from ²H (deuterium, D) and 18O isotope ratios. These results suggest that the earthquakes were triggered within a fractured zone through which magmatic gases preferentially migrated. We hypothesize that the upwelling of gas-rich hydrous fluids and/or gas bubbles occurred along this fracture pathway, causing an increase in the pore pressure and triggering the resultant earthquake sequence. [ABSTRACT FROM AUTHOR]