A geothermal aquifer in the dilation zones on the southern margin of the Dublin Basin

Abstract

SUMMARY We present modelling of the geophysical data from the Newcastle area, west of Dublin, Ireland within the framework of the IRETHERM project. IRETHERM's overarching objective was to facilitate a more thorough strategic understanding of Ireland's geothermal energy potential through integrated modelling of new and existing geophysical, geochemical and geological data. The Newcastle area, one of the target localities, is situated at the southern margin of the Dublin Basin, close to the largest conurbation on the island of Ireland in the City of Dublin and surrounds. As part of IRETHERM, magnetotelluric (MT) soundings were carried out in the highly urbanized Dublin suburb in 2011 and 2012, and a description of MT data acquisition, processing methods, multidimensional geoelectrical models and porosity modelling with other geophysical data are presented. The MT time-series were heavily noise-contaminated and distorted due to electromagnetic noise from nearby industry and Dublin City tram/railway systems. Time-series processing was performed using several modern robust codes to obtain reasonably reliable and interpretable MT impedance and geomagnetic transfer function ‘tipper’ estimates at most of the survey locations. The most ‘quiet’ 3-hr subsets of data during the night time, when the DC ‘LUAS’ tram system was not operating, were used in multisite and multivariate processing. The final 2-D models underwent examination using a stability technique, and the final two 2-D profiles, with reliability estimations expressed through conductance and resistivity, were derived. In the final stage of this study, 3-D modelling of all MT data in the Newcastle area was also undertaken. Comparison of the MT models and their interpretation with existing seismic profiles in the area reveals that the Blackrock–Newcastle Fault (BNF) zone is visible in the models as a conductive feature down to depths of 4 km. The investigated area below Newcastle can be divided into two domains of different depths, formed as depth zones. The first zone, from the surface down to 1–2 km, is dominated by NE–SW oriented conductors connected with shallow faults or folds probably filled with less saline waters. The conductors are also crossing the surface trace of the BNF. The second depth domain can be identified from depths of 2–4 km, where structures are oriented along the BNF and the observed conductivity is lower. The deeper conductive layers are interpreted as geothermal-fluid-bearing rocks. Porosity and permeability estimations from the lithological borehole logs indicate the geothermal potential of the bedrock, to deliver warm water to the surface. The fluid permeability estimation, based on Archie's law for porous structures and synthetic studies of fractured zones, suggests a permeability in the range 100 mD–100 D in the study area, which is prospective for geothermal energy exploitation.