A magnetotelluric experiment around St. Austell, Cornwall

Huebert, J. ORCID: 2023 A magnetotelluric experiment around St. Austell, Cornwall. Edinburgh, UK, British Geological Survey, 48pp. (OR/23/031) (Unpublished)

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In this BGS Open Report we describe the results of a pilot study assessing the potential of the magnetotelluric (MT) method to characterize geothermal reservoirs at depth when the target site is in a semi-urban environment. During an MT survey, the natural variations of the electromagnetic field of the Earth are measured at the field site for time windows of a few hours up to a couple of days. As a passive geophysical technique, the data quality is dependent on the in-situ electromagnetic noise levels. The noise come from technological infrastructure that dominates the landscape of modern Britain such as power lines, high voltage transformers, electric fences, radio towers, electrified railways and gas pipeline protection systems. In the absence of artificial noise, the MT method can sample the shallow and deep subsurface of the Earth down to the lower crust and mantle depending on the frequency of the measured signal. MT images the bulk electrical conductivity of the rocks and can therefore indicate the presence of fluids at depth. For deep geothermal exploration, MT has been identified as one of the few geophysical techniques able to help characterise geothermal host rocks and zones of enhanced permeability and porosity which are necessary for the fluid circulation of an enhanced geothermal system. Deep geothermal exploration in the UK has, so far, been limited to the granites of Cornwall which are known to have relatively high heat flow. Along with a strong legacy knowledge of the geology of the subsurface these have encouraged two deep geothermal drill projects. At the Eden Geothermal site close to the Eden Project in St Austell, a 5,277 m deep borehole was drilled into the St Austell granite to intersect with a known geologic fault zone, the NNW-SSE striking Great Cross-Course (GXC) Fault, where the circulation of geothermal fluids in a closed loop system was deemed possible. The drilling was partly sponsored by the European Regional Development Fund and designed as both a research facility and to provide green energy to the Eden Project and adjacent businesses. The position and extent of the fault zone at depth was not known and previous geophysical surveys had been limited to very shallow depths. Seismic monitoring has been an ongoing component of the drilling and ensuing pump testing. During a 10-day field campaign in March 2023 a team from BGS were able to collect broadband MT data at 30 sites in the St Austell area and one remote site in Bodmin Moor to help improve data quality through remote referencing time-series processing techniques. Data collected very close to the drill site and at some locations close to town centres have low quality, but overall data quality was sufficient to allow modelling of the electrical properties at depth. The derived MT impedance tensors were used to construct a 3D model of the area which has low spatial resolution but clearly shows that MT data can image the local geology at depth. The St Austell granite is characterized by high resistivities, whereas the onshore sediments and metasediments surrounding the intrusion have lower resistivities. Some vertical structures of lower resistivity within the granite were identified and are carefully investigated. Overall, the use of the MT method for the characterisation of geothermal reservoirs in granitic bodies is confirmed in this study. Further data collection and modelling is recommended to improve the spatial resolution at depth.

Item Type: Publication - Report (Project Report)
Funders/Sponsors: Eden Geothermal Ltd, British Geological Survey, BGS Innovative Flexible Fund
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
NORA Subject Terms: Earth Sciences
Date made live: 14 Dec 2023 10:57 +0 (UTC)

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