A new subsurface temperature model for Ireland from joint geophysical–petrological inversion of seismic, surface heat flow and petrophysical data

Emma Louise Chambers , Javier Fullea, Duygu Kiyan, Sergei Lebedev, Christopher J. Bean, Pat Meere, Stephen Daly, Nicola Willmot Noller, Robert Raine, Sarah Blake, Brian M. O'Reilly

High-quality maps of subsurface temperature and the geothermal gradient are useful when assessing the geothermal potential of a region. However, determining geothermal potential is a challenge when direct measurements of in-situ temperature and thermal property information are sparse and indirect geophysical methods are sensitive to a range of parameters, not just temperature. Here, we produce subsurface temperature maps of Ireland using a joint geophysical–petrological inversion, where seismic and other geophysical and petrophysical data are inverted directly for temperature in 1-D columns and are collated into a pseudo 3-D temperature volume. Additionally, the inversion produces new models for Moho and LAB depth and for the average crustal radiogenic heat production.

To assess the robustness of the resulting temperature model, an uncertainty analysis has been performed by inverting all of the 1-D columns for a range of reasonable input parameters applicable to the Irish crust (rather than the ‘best’ input parameters). The resulting uncertainty model suggests temperature estimates at 2 km depth in our model could vary by ± 2 to 5 °C with an average of 3.5 °C in most locations. The uncertainty model can be used to assess confidence in different regions of the temperature model. In addition, 3-D forward modelling was performed to assess the lateral heat flow variations when compared to the purely 1-D inversion. The upper-crustal geothermal gradient ranges from 20 to 40 °C km−1 indicating a higher geothermal gradient for Ireland than previously reported with subsurface temperatures at 2 km depth > 60 °C everywhere, sufficient for residential and industrial heating purposes. The temperature gradient is typically higher in areas with thinner lithosphere. However, in some locations, the observed geotherms are elevated further due to high radiogenic heat production in granitic rocks. In Northern Ireland, a thin lithosphere, coupled with a weakly conductive basalt layer overlying warm crust, results in elevated temperatures. These are the first temperature maps for Ireland that include uncertainty estimates, providing ranges for the subsurface temperature values, and demonstrate that the maps are comparable to direct independent borehole temperature measurements, which are observed to fall within the model uncertainty. Our new methodology provides workflows for determining the geothermal potential in areas with limited direct temperature measurements. The final temperature model with uncertainty provides useful constraints for geothermal exploration and utilization on the island of Ireland.