IRETHERM: Geophysical modeling of the southern margin of the Dublin Basin

Abstract:

Multi-dimensional magnetotelluric (MT) modelling of data collected in the Newcastle area are presented in the frame of the IRETHERM project. The Newcastle area, situated on the southern margin of the Dublin Basin, exhibits elevated geothermal gradient (>30 °C/km) in the exploratory boreholes drilled by GT Energy.

The MT soundings were carried out in the highly urbanized Dublin suburb and are heavily noise-contaminated and distorted due to EM noise from nearby industry and the DC tram system. Processing the “quietest” 4-hour night time subsets of data using several robust codes and the ELICIT method we obtained reliable and interpretable MT impedance and geomagnetic transfer functions at most sites.

Tensor decomposition was applied at each site to ascertain if the data are suitable for 2D modelling and to determine the appropriate geoelectric strike direction. The final 2-D models underwent examination using a new stability technique, and the final two 2-D profiles with reliability estimations, expressed through conductance and resistivity, were derived.

3-D models of all MT data in the Newcastle area have also been determined. The 3-D models exhibit higher conductive structures in comparison to the 2-D models, with similarly resistive background rocks. The shallow conductive structures, to the depth of 1 km, have north-south elongation correlated with surface traces of faults, which are perpendicular to the regional Blackrock to Newcastle Fault (BNF). Deeper structures become more oriented to regional geoelectric strike similar to 2-D regional strike.

The 2-D and 3-D modeling reveal that the BNF is imaged as a conductive zone to depths of 4 km and is likely highly fractured. Generally, the area south of the BNF is more resistive and compact with a horizontal conductive layer at approximately 1 km depth and with a very thin surficial sedimentary layer. In contrast, the structures north of the BNF are more heterogeneous, with deeper conductive layers (2-3 km depth) and thicker (several hundred meters) sedimentary layers above.

The deeper conductive layers are interpreted as water bearing or geothermal fluids and estimated porosity and permeability indicates potential to deliver warm water to the surface.