Characterising thermal water circulation in fractured bedrock using a multidisciplinary approach: a case study of St. Gorman’s Well, Ireland

A hydrogeological conceptual model of the source, circulation pathways and temporal variation of a low-enthalpy thermal spring in a fractured limestone setting is derived from a multidisciplinary approach. St. Gorman’s Well is a thermal spring in east-central Ireland with a complex and variable temperature profile (maximum of 21.8 °C). Geophysical data from a three-dimensional(3D)audio-magnetotelluric(AMT) survey are combined with time-lapse hydrogeological data and information from a previously published hydrochemical analysis to investigate the operation of this intriguing hydrothermal system. Hydrochemical analysis and time-lapse measurements suggest that the thermal waters flow within the fractured limestones of the Carboniferous Dublin Basin at all times but display variability in discharge and temperature. The 3D electrical resistivity model of the subsurface revealed two prominent structures: (1) a NW-aligned faulted contact between two limestone lithologies; and (2) a dissolutionally enhanced, N-aligned, fault of probable Cenozoic age. The intersection of these two structures, which has allowed for karstification of the limestone bedrock, has created conduits facilitating the operation of relatively deep hydrothermal circulation (likely estimated depths between 240 and 1,000 m) within the limestone succession of the Dublin Basin. The results of this study support a hypothesis that the maximum temperature and simultaneous increased discharge observed at St. Gorman’s Well each winter is the result of rapid infiltration, heating and recirculation of meteoric waters within a structurally controlled hydrothermal circulation system.

An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My and is just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.

Supplemental Material: Miocene to modern hydrothermal circulation and high topography during synconvergent extension in the Cordillera Blanca, Peru

Methods details; Table S1 (sample isotopic, depth, and location information); Table S2 (deformation temperatures); Table S3 (paleoelevation model results); and Table S4 (recharge elevation model results from modern water samples).<br>

Supplemental Material: Miocene to modern hydrothermal circulation and high topography during synconvergent extension in the Cordillera Blanca, Peru

Methods details; Table S1 (sample isotopic, depth, and location information); Table S2 (deformation temperatures); Table S3 (paleoelevation model results); and Table S4 (recharge elevation model results from modern water samples).<br>

CDGP, a data center of EPOS TCS Anthropogenic Hazards, to help analysis of geothermal anthropogenic seismicity

&lt;p&gt;The Data Centre for Deep Geothermal Energy (CDGP &amp;#8211; Centre de Donn&amp;#233;es de G&amp;#233;othermie Profonde, https://cdgp.u-strasbg.fr) was launched in 2016 by the LabEx G-Eau-Thermie Profonde - now ITI G&amp;#233;osciences pour la transition &amp;#233;nerg&amp;#233;tique GeoT, https://iti-geot.unistra.fr/ - to preserve, archive and distribute data acquired on geothermal sites in Alsace. At the moment, it archives and gives access to data from Soultz-sous-For&amp;#234;ts (1988-2010), Rittershoffen (2012-2014) and Vendenheim (2016-2021).&lt;/p&gt;&lt;p&gt;Access to patrimonial data like those from Soultz-sous-For&amp;#234;ts (SSF, 1993, 2000) or from Rittershoffen allows reprocessing of data, validation of new ideas. Cauchie et al. (2020) reinvestigated earthquakes during SSF 1993 stimulation and discussed implications for detecting the transition between events related to pre-existing faults and the onset of fresh fractures. Vallier et al. (2019) used a simplified 2D thermo-hydro-mechanical model of SSF reservoir to infer that the sediments&amp;#8211;granite interface has a weak influence on the hydrothermal circulation, or that the brine viscosity has a huge impact on the hydrothermal circulation. Koepke et al. (2020) applied pseudo-probabilistic fracture network method to the seismicity induced during the SSF 2000 stimulation to confirm the existence of a large prominent fault. Drif et al. (2020) used data from Vendenheim area to determine the seismic moment, the source size, the average stress drop and the focal mechanism associated to the M3 event in November 2019.&lt;/p&gt;&lt;p&gt;Some of the CDGP data are also available on the EPOS Thematic Core Service Anthropogenic Hazards platform (https://tcs.ah-epos.eu/, Orlecka-Sikora et al., 2020), with other geothermal episodes, and with applications to process and analyse the data. This platform is a functional e-research infrastructure that allows free experimentations in a virtual laboratory, promoting interdisciplinary collaborations between stakeholders (the scientific community, industrial partners and society).&lt;/p&gt;&lt;p&gt;Cauchie, L., Lenglin&amp;#233;, O. &amp; Schmittbuhl, J., 2020 - Seismic asperity size evolution during fluid injection: case study of the 1993 Soultz-sous-For&amp;#234;ts injection. Geophysical Journal International 221, 968&amp;#8211;980.&lt;br&gt;Drif, K., Lengline, O., Lambotte, S., Kinscher, J. &amp; Schmittbuhl, J., 2020 - Source parameters of the Ml3.0 StrasbourgEarthquake (12th November 2019). Communication at EGW2020, http://labex-geothermie.unistra.fr/wp-content/uploads/2020/12/abstracts-egw2020-en.pdf#page=68.&lt;br&gt;Koepke, R., Gaucher, E. &amp; Kohl, T., 2020 - Pseudo-probabilistic identification of fracture network in seismic clouds driven by source parameters. Geophys J Int 223, 2066&amp;#8211;2084.&lt;br&gt;Orlecka-Sikora B., Lasocki S., Kocot J., Szepieniec T., Grasso J-R., Garcia-Aristizabal A., Schaming M., Urban P., Jones G., Stimpson, I., Dineva S., Sa&amp;#322;ek P., Leptokaropoulos K., Lizurek G., Olszewska D., Schmittbuhl J., Kwiatek G., Blanke A., Saccorotti G., Chodzi&amp;#324;ska K., Rudzi&amp;#324;ski &amp;#321;., Dobrzycka I., Mutke G., Bara&amp;#324;ski A., Pierzyna A., Kozlovskaya E., Nevalainen J.,&amp;#160; Kinscher J., Sileny J., Sterzel M., Cielesta, S., Fischer T., 2020 -An open data infrastructure for the study of anthropogenic hazards linked to georesource exploitation. Scientific Data 7, 89. doi:10.1038/s41597-020-0429-3.&lt;br&gt;Vallier, B., Magnenet, V., Schmittbuhl, J. &amp; Fond, C, 2019 - Large scale hydro-thermal circulation in the deep geothermal reservoir of Soultz-sous-For&amp;#234;ts (France). Geothermics &lt;strong&gt;78&lt;/strong&gt;, 154&amp;#8211;169.&lt;/p&gt;

Tectonic constraints on submarine hydrothermal activity, degassing, and subseafloor gas storage (Milos Shallow Water Hydrothermal System, Greece)

&lt;p&gt;The Milos hydrothermal field is one of the largest known shallow water hydrothermal systems, and shows both fluid and gas outflow through the seafloor. Recent studies based on imagery acquired by both aerial and submarine drones (Puzenat et al., submitted) reveal several types of fluid outflow associated with bacterial mats along the SE coast of the island (Paleochori, Spathi, and Agia Kyriaki bays). From these observations? include: a) zones of polygonal hydrothermal outflow and associated bacterial mats, b) extended white (bacterial) patches, and c) isolated ones. Subseafloor hydrothermal circulation is hosted in sediments with subseafloor temperatures &gt;50&amp;#176;C, and there is a clear association between hydrothermal circulation and active degassing.&lt;/p&gt;&lt;p&gt;To understand the controls on and relationships between fluid and gas outflow in the area, we need to characterise: a) the nature of the subseafloor (sediment thickness, composition &amp; permeability); b) the distribution of gas and subseafloor fluids, and c) the distribution of gas flares emanating from the seafloor. In November 2020, we conducted a short pilot geophysical study at Paleochori Bay, deploying a towed catamaran with a multibeam echo sounder (iXblue Seapix) to obtain seafloor bathymetry, acoustic backscatter and water column detection of gas flares. We also deployed a sub-bottom profiler (iXblue Echoes 3500 T1) to image sediment architecture and gas/fluid diffusion within the sediment. Our survey focused on Paleochori Bay, surveing areas from ~5 m (nearshore) to ~100 m waterdepth (offshore).&lt;/p&gt;&lt;p&gt;Preliminary results of this geophysical survey suggest that subseafloor gas accumulations play a major role on the nature and structure of hydrothermal activity at Milos. These gas accumulations within the sediments develop along an onshore/offshore fault system, and likely control the shallow subseafloor thermal structure, establishing a thin thermal conductive layer between the roof of gas pockets and the seafloor.[GJ1]&amp;#160;[je2]&amp;#160;&amp;#160; We will report on the link between the distribution and geometry (extent, depth, acoustic nature of the accumulations) of gas pockets, fluid outflows, and gas outflows, all of which will be characterised from both seafloor imagery and subsurface geophysical surveys. We will also discuss how gas pocket geometry may be linked to both fluid flow and subseafloor temperature structure. [HA3]&amp;#160;&lt;/p&gt;&lt;div&gt; &lt;div&gt; &lt;div&gt;&amp;#160;&lt;/div&gt; &lt;/div&gt; &lt;div&gt; &lt;div&gt;&amp;#160;&lt;/div&gt; &lt;/div&gt; &lt;div&gt; &lt;div&gt;&amp;#160;&lt;/div&gt; &lt;/div&gt; &lt;/div&gt;