Geophysics

Geochemical analyses of fluids from exploratory geothermal wells on Montserrat indicate low levels of metals consistent with the external fluid source being primarily seawater introduced down faults and heated by magma at depth. To optimise geothermal exploration for both metals and heat will require further characterisation of the sub-surface to identify lenses of hot metal-rich magmatic brines that resemble more closely those analysed as fluid inclusions drillcore samples. Our preliminary analysis suggest that the current wells lie to the west of the optimal target area for geothermal metals.

Geophysical methods are ideally suited to imaging hot, saline geofluids. Seismic imaging gives information about their location and compressibility; electrical conductivity imaging yields constraints on fluid salinity and interconnectivity (permeability); gravity surveys provide evidence of density anomalies associated with fluid-rich reservoirs. Together, these surveys help constrain the nature and distribution of fluids at depth. In WP1 we will concentrate on seismic imaging, as conductivity and gravity experiments have already been conducted during the geothermal scoping studies. We will use Oxford’s own next-generation seismic nodes (broadband autonomous sensors) to image the sub-surface. Through collaboration with the Government of Montserrat (GoM) and the Montserrat Volcano Observatory (MVO), we will deploy over 100 seismic sensors across the island, including regions in the current exclusion zone. A ten-fold increase in the density of seismometer coverage will reduce seismicity detection thresholds substantially, allowing us to detect deep seismicity at the brittle/ductile transition, a critical parameter in defining crustal fluid reservoirs. Tomographic imaging will be used to map subsurface structure and, working with outputs from WP2, help to constrain the types of fluids at depth. This information will be integrated with the available electrical conductivity images and well-logs to create a detailed image of the sub-surface that can be used to identify new locations with brine lenses suitable for metals and heat co-recovery.