Magnetotelluric imaging of a low-enthalpy geothermal system in the Lough Neagh Basin, Northern Ireland

Sedimentary basins in Northern Ireland are increasingly recognised as promising targets for low- to medium-temperature geothermal energy, yet their subsurface architecture and reservoir properties remain poorly constrained. The Lough Neagh Basin exhibits elevated geothermal gradients relative to other regions of Ireland and represents a low-enthalpy, sedimentary-basin geothermal system. The basin is known to contain sedimentary rocks ranging from at least Permian to Upper Cretaceous in age, overlain by thick Paleogene basalts. Within this system, the electrically conductive Triassic Sherwood Sandstone Group is considered the principal geothermal reservoir, although deeper Permian sandstone units may also have reservoir potential.

In this study, we assess the geothermal potential of the Lough Neagh Basin using a broadband magnetotelluric (MT) dataset comprising more than 250 MT stations, including 118 newly acquired stations as part of the GEMINI (Geothermal Energy Momentum on the Island of Ireland) project. MT data were processed to derive impedance tensors and vertical magnetic transfer functions, followed by phase tensor and induction arrow analyses to characterise dimensionality and lateral resistivity variations. Three-dimensional (3-D) MT inversion was then applied to recover the subsurface resistivity structure.

The resulting 3-D resistivity structure reveals laterally extensive low-resistivity zones with a spatially variable upper boundary, typically initiating at depths of ~0.7–1.0 km and extending to ~2–2.5 km beneath a high-resistivity basalt sequence. These zones are interpreted as the Triassic Mercia Mudstone Group–Sherwood Sandstone Group succession with variable thickness, consistent with borehole constraints and regional geological understanding. While MT primarily constrains the geometry of this conductive package, integration with petrophysical measurements from selected rock samples and other geophysical datasets (i.e., gravity and passive seismics) aids discrimination between mudstone- and sandstone-dominated intervals, enabling first-order estimates of reservoir geometry and associated heat capacity within the Lough Neagh Basin geothermal system.