Developing Joint Geophysical and Petrological Inversion to Determine Temperature and Image the Lithosphere and Asthenosphere: De-risking Ireland's Geothermal Potential (DIG)

High-quality maps of the geothermal gradient and temperature are essential when assessing the geothermal potential of a region. However, determining geothermal potential is a challenge as direct measurements of in situ temperature are sparse and individual geophysical methods are sensitive to a range of parameters, not solely temperature. Here, we develop a novel approach to determine the geothermal gradient using a joint geophysical-petrological inversion which requires thermal property data, seismic and additional geophysical datasets. The seismic data provide new constraints on lithospheric boundaries which influence crustal geotherms. We utilise large seismic datasets and extract Rayleigh- and Love-wave phase velocity dispersion curves, measured for pairs of stations. The measurements were performed using two methods with complementary period ranges; cross-correlation of teleseismic earthquakes and waveform inversion, yielding measurements in a broad period range (4-500 s).

The joint analysis of Rayleigh and Love measurements constrains the isotropic-average shear-wave velocity, relatable to temperature and composition providing essential constraints on the thermal structure of a region's lithosphere. We demonstrate this by inverting the data using an integrated joint geophysical-petrological thermodynamically self-consistent approach (Fullea et al., GJI 2021), where seismic velocities, electrical conductivity, and density are dependent on mineralogy, temperature, composition, water content, and the presence of melt. The multi-parameter models produced by the integrated inversions fit the surface-wave and other data and reveal the temperatures and geothermal gradients within the crust and mantle which will be used for future geothermal exploration and utilisation.

We use Ireland as a case study (part of the De-risking Ireland's Geothermal Potential project - DIG) and find that our new methodology produces results comparable to past temperature and geophysical measures, and enhances resolution. Lithospheric and crustal thickness play a key control on the temperature gradient with areas of thinner lithosphere resulting in elevated geotherms. In some locations we observe geotherms elevated beyond expectations which result from high radiogenic heat production from granitic rocks. This new methodology provides a robust workflow for determining the geothermal potential in areas with limited direct measurements.

The DIG project is funded by the Sustainable Energy Authority of Ireland under the SEAI Research, Development & Demonstration Funding Programme 2019 (grant number 19/RDD/522) and by the Geological Survey of Ireland.