Integrated geophysical imaging of the lithosphere beneath Britain: 3D magnetotelluric modelling, multi-physics inversion, and space weather applications

Abstract

Geophysical methods are one of the main tools to study the Earth’s interior. Through observations at the Earth’s surface, advanced numerical methods, and fundamental physical laws, geophysical inversion allows us to create models of the distribution of physical properties in the subsurface.

The magnetotelluric (MT) method uses simultaneous measurements of electric and magnetic fields at the Earth’s surface to probe the electrical properties at a broad range of frequencies and, therefore, depths. Given its ability to reveal electrical conductivity contrasts associated with geological variations and other physical parameters, the MT method is widely applied to resource exploration, geohazards monitoring, lithospheric studies, and, recently, space weather studies. During geomagnetic storms, the rapid variations in the geomagnetic field induce electrical currents in the subsurface. These so-called Geomagnetically Induced Currents (GICs) can damage ground-based infrastructure, such as electrical power networks and railways. To assess and forecast the risk of GICs, knowledge of electric fields at the Earth’s surface is necessary. The strength of the geoelectric field depends not only on the magnetic source but also on the ground electrical conductivity. Therefore, models that accurately reflect conductivity contrasts are required to improve the estimation of geoelectric fields, especially in areas of complex geology, such as the UK.

This thesis presents the first three-dimensional (3D) electrical resistivity model of the lithosphere beneath Britain (BERM-2024) and explores its geological implications and applications to space weather. BERM-2024 was derived from the v inversion of long-period magnetotelluric (LMT) data primarily acquired during a field campaign conducted by the British Geological Survey and the University of Edinburgh between 2021 and 2024, with additional legacy data incorporated to establish a nationwide dataset of 70 sites across Great Britain.

A systematic workflow for 3D MT inversion was developed through extensive testing of prior models, inversion strategies, and regularization parameters. The influence of bathymetry and marine sediments was assessed through a pilot study using legacy data from the Isle of Skye. Computational resources and data misfit criteria were also considered, resulting in an optimized methodology that can provide general guidance for 3D MT studies, and a robust and geologically meaningful model.

BERM-2024 reveals strong lateral and vertical resistivity variations that correlate with known geological and tectonic structures, and provides new insights into the lithosphere down to depths of ∼200 km. In the upper crust, high conductivity anomalies correlate to sedimentary basins in western Britain, such as the Cheshire and Welsh basins, while resistive features align with granite plutons in Scotland and Cornwall. Sharp resistivity contrasts identified at mid- to lower crustal depths in northern Scotland, the Southern Uplands, northern England, and Wales align closely with major terrane boundaries. Notably, a prominent conductor at 85-150 km depth beneath the West Midlands region is imaged for the first time, and is here termed the West Midlands Conductor (WMC). Integrated geophysical-thermochemical modelling suggests that high water contents (∼700-600 ppm) in the lithospheric mantle provide a plausible explanation for the resistivity response of the WMC.

Beyond its geological implications, BERM-2024 provides a new input for estimating geoelectric fields in the UK, a key element in modelling and forecasting geomagnetically induced currents in critical infrastructure during space weather events. Geoelectric fields modelled for major geomagnetic storms demonstrate a strong correlation with fields measured at geomagnetic observatories. Discrepancies in amplitude, however, highlight the need for denser data coverage and further research.

This thesis also pioneers the application of multi-physics joint inversion to nationwide geophysical datasets in Britain. Land gravity data and Rayleigh-wave group-velocity travel times from ambient noise seismology, not previously used to generate 3D models, were individually and jointly inverted along with the LMT dataset. Challenges related to data coverage and resolution, as well as specific parameters involved in the joint inversion methodology, are addressed and discussed. The resulting high-resolution crustal models of density, shear-wave velocity, and resistivity consistently recover robust structures such as sedimentary basins, intrusions, and terrane boundaries, and suggest potential unrecognised intrusions. The extensive parameter testing conducted provides guidance for future joint inversion studies.

Overall, this research delivers new insights into the lithospheric structure of Britain, and establishes a new baseline for regional and local geophysical research. The models and methodologies developed here can support future geological and geophysical studies, deepen our understanding of Britain’s lithosphere, and improve assessments of space weather hazards.