In this paper, we use synthetic data sets from a profile to demonstrate the importance of aligning the 3-D mesh and data coordinate system with the dominant geo-electrical strike direction in 3-D inverse modelling. The resistivity model investigated consists of a regional, elongated 2-D conductive structure at 45° to the profile. We compare the results of full impedance tensor inversion with the results from inversion of only off-diagonal components of the magnetotelluric impedance tensor. The 3-D inversion result obtained with the complete tensor elements yields the subsurface model closest to the original model, whereas the result of inverting only off-diagonal components is the poor imaging of the continuity of the conductive 2-D body. However, the conductor can be correctly recovered using only the off-diagonal components if the model mesh and the data are aligned with quasi-2-D geo-electrical strike.

Water-covered areas may lead to gaps in surface electromagnetic surveys, causing reduced resolution and, as a consequence, increased uncertainty in derived subsurface models. We have evaluated a boat-towed floating central loop time-domain electromagnetic technique that mitigates this problem. It facilitates obtaining data with a spatial sampling density, which is rarely possible with standard instrumentation on land, and it only requires moderate logistical effort. A unique field study on a shallow volcanic lake demonstrated that this method is feasible with only a minor loss of accuracy when compared to anchored and land soundings. We found that the noise sources arising from the moving instrument and the boat engine can be neglected. The field survey was performed on the Lagoa das Furnas (São Miguel, Azores Islands, Portugal), which is located within an active volcanic area and is characterized by fumarolic fields and CO2 degassing. Thus, the associated hydrothermal system is expected to extend below the lake. However, the character, geometry, and extent of this system are unknown because of the lack of boreholes and geophysical studies. In total, 600 soundings, combining towed profiles with anchored and land-based soundings, were acquired with an aim of imaging the hydrothermal system beneath the lake down to 200 m. The results from all three types of measurements compare well and thus led to consistent 1D inversion models. The inversion results delineate a highly conductive, smectite-rich cap layer dipping below the lake away from the main fumarole zone. Near this zone, the extent of the conductor agrees well with an area of intense dispersed CO2 degassing, which appears to be controlled by at least two electrically distinctive fault zones in which the conductor is found at greater depths.

The geological setting in the north of Ireland, especially concerning the origin of the Moffat Shale Group, has long been under discussion. Within the Tellus Programme of the Geological Survey Ireland, airborne electromagnetic measurements revealed high‐conductivity anomalies that have been interpreted as the response of a black shale. In order to petrophysically characterize the Moffat Shale, a laboratory study using material from two shallow boreholes was carried out. The study focuses on spectral induced polarization measurements on 23 oriented samples in the frequency range from 10⁻⁴ to 10⁵ Hz.The sample material can be categorized into two groups. A mudstone‐like rock type shows weakly frequency‐dependent, porosity‐driven conductivities with a strong anisotropy. On the other hand, black shale samples are characterized by moderately anisotropic but strong polarization effects especially at low frequencies and a strong conductivity increase towards higher frequencies. The polarization in the black shale is controlled by the texture and volume fraction of the polarizable components. The spectral induced polarization data are processed by means of a Debye decomposition approach. The anisotropy of the complex electrical conductivity is determined by utilizing the foliation dip angle and assuming tilted transverse isotropic conditions. The relevance of the laboratory findings for airborne electromagnetic surveys is addressed with a synthetic one‐dimensional modelling study.

SUMMARY: 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.

Ground-based impacts of solar activity through the interaction of the Earth’s magnetic field with the solar wind have been classified as a major natural hazard in many mid-high latitude countries. Effects on the ground are closely coupled to local geology due to varying electrical conductivity of different rock types. The conductivity distribution can be estimated from magnetotelluric (MT) measurements, which then is used for the large-scale modelling of the time-varying ground electric field. This geoelectric field model serves as the input for the impact analysis of Space Weather on ground-based systems such as the high voltage power grid, gas pipelines and railways. Previous efforts in the UK had been based on a thin-sheet approach, but MT data better captures the true conductivity variations in the Earth’s crust and mantle that drive geoelectric fields during space weather activity. Within the UK-funded SWIMMR programme, we collected long-period MT data at 50 sites with site spacing of 50-70 km in Britain during a field campaign in 2021-23, adding to a few existing legacy data sets. Using a frequency-domain approach and geomagnetic field observations from the UK geomagnetic observatories and variometer sites we can derive models of the geoelectric field during geomagnetic active times. Using the new MT data, we revisit GIC estimates for both historic and modern geomagnetic storms, perform extreme value analysis of both geoelectric fields and GICs, and implement a fast now- and forecasting of ground effects within the SWIMMR framework.

One aim of the DIG Project (De-risking Ireland’s Geothermal Potential: Chambers et al., this conference) is to evaluate the geothermal energy potential of the Upper Devonian Munster Basin within the Variscides of southern Ireland. One of our primary targets is the Mallow Warm Springs Area (MWSA) which is sited along the Killarney-Mallow Fault Zone (KMFZ). The fault zone represents the major basin-bounding normal fault system to the basin and regionally controls sediment facies and thickness distribution. The KMFZ was re-activated as a compressional reverse fault in late Carboniferous-early Permian time, during the Variscan Orogeny. Tectonic strain variation is well understood from structural/geological analysis and increases towards Killarney, where the highest strain is associated with a large Bouguer gravity low in the (Avalonian) Caledonian basement. The basin-fill sediments are dominated by siliciclastic sediments, deposited in thick alluvial fan systems, that are succeeded by lower Carboniferous carbonate-prone marine limestones.Primary porosity in sediments is obliterated by sub-green-schist metamorphism and Variscan deformation fabrics. Fluid flow within the crust around the KMFZ is likely related to the Cenozoic tectonic reactivation of faults and thermally driven uplift as revealed by recent thermo-chronological results in the vicinity of the fault. There was also significant fluid flow during the Munster basin extension and Variscan basin inversion. The focussed part of the DIG study uses potential field (gravity/magnetic) and legacy wide-angle seismic data from the Munster Basin to develop a “new” geological model for crustal structure, with direct application to geothermal research. Critical properties such as thermal conductivity and heat production measurements will also encompass the island-wide aspect of the DIG Project. The constraints gathered by the magnetotelluric and passive seismic data within the KMFZ will be integrated with rock physics and geochemical data. This substantial body of work will also include a fluid chemistry program to understand the fluid rock interactions within the KMFZ and their impact on physical properties (electrical conductivity and velocity). Collectively, using this expertise, the study evaluates the geothermal and economic potential of the region and more specifically the MWSA. This local focus on the MWSA aims to directly image fault conduits and fluid aquifer sources at depth, within a convective/conductive region associated with the known occurrence of warm thermal springs. This will determine the scale of the geothermal anomaly, its correlation with our gathered data and will so evaluate the potential for both local and industrial-scale space heating in the survey locality.The 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 Ireland.