[1] Geomagnetically Induced Currents (GIC) can be damaging to high-voltage power transmission systems. GIC are driven by rapid changes in the strength of the magnetic field external to the Earth's surface. Electric fields are produced in the ground by the interaction between this changing magnetic field, the sea and the conductivity structure of the Earth. Using a technique known as the "thin-sheet approximation," we can determine the electric field at the Earth's surface, which in turn allows the calculation of GIC in the earthing connections of high-voltage transformers within a power grid. We describe two new developments in the modeling of GIC in the UK, though the results are applicable to GIC-related research in other regions. Firstly, we have created an updated model of the UK surface conductivity by combining a spatial database of the UK geological properties (i.e., rock type) with an estimate of the conductivity for specific formations. Secondly, we have developed and implemented a sophisticated and up-to-date model for the 400 kV and 275 kV electrical networks across the whole of Great Britain and, in addition, the 132 kV network in Scotland. We can thus deduce the expected GIC at each transformer node in the system based on the network topology from an input surface electric field. We apply these developments to study the theoretical response of the UK high-voltage power grid to modeled extreme 100 year and 200 year space weather scenarios and to a scaled version of the

Capacitive resistivity (CR) is an emerging geophysical technique designed to extend the scope of the conventional methodology of DC resistivity to environments where galvanic coupling is notoriously difficult to achieve, for example across engineered structures (roads, pavements), hard rock, dry soil or frozen ground. Conceptually, CR is based on a four-point array capacitively coupled to the ground. Under A parametric study of the complex quasi-static transfer impedance reveals the existence of a restricted range of practical parameters that permits successful operation of CR instruments at low induction numbers. Theory predicts that emulation of the DC measurement is compromised if low-induction-number operation is not maintained throughout a survey area, for example in a zone of high conductivity.

The geomagnetically induced current (GIC) risk to the power transmission grid in the United Kingdom is discussed with reference to an example of a geomagnetic storm during which GICs were suspected of causing abnormal transformer behaviour. A simple measure of the power of the magnetic field variation, the hourly standard deviation (HSD) in the north or east horizontal component, is used to determine the general risk to the UK power grid from rapid magnetic variations, according to season and local time.Monitoring and forecasting of HSD may be a useful means of gauging the likely risk to high-cost power engineering equipment. A simplified but representative threedimensional geological model of the UK landmass and surrounding seas is used to provide an indication of the surface electric field for various amplitudes and orientations of external magnetic field variations. It is found that the resistivity contrast between seawater and the onshore geology, particularly around the Scottish metamorphic terranes, produces enhanced electric fields at coastal sites. These are as much as 4 V/km for a 1 A/m (or 1257 nT) external field with 10 minute period.

Naturally occurring electric potentials at the Earth’s surface are traditionally studied using self‐potential geophysics. Recent theoretical and experimental work has reinvestigated the manner in which the measurement can be made dynamically using a pressure source. The methodology, often referred to as seismoelectric, relies on electrokinetic coupling at interfaces in the streaming potential coefficient. The ultimate aim of the developing methodologies lies in the detection of zones of high fluid mobility (permeability) and fluid geochemical contrasts within the subsurface. As yet there are no standard methods of recording and interpretation: the technique remains experimental. Field measurements are made using a seismic source and by recording electric voltage across arrays of surface dipoles. This study presents observational characteristics of electrokinetic coupling based on experiments carried out in a wide range of environments. Theory concerning the coupled elastic and electromagnetic wave equations in a saturated porous medium is discussed. It is predicted that coupling will produce electromagnetic radiation patterns from vertical electric dipoles generated at interfaces. Surface‐ and body‐wave coupling mechanisms should provide different time–distance patterns. Vertical electric dipole radiation sources are modelled and their spatial characteristics presented. A variety of experimental configurations have been used, and geometries that exploit phase asymmetry to enhance the separation of signal and noise are emphasized. The main experimental results presented are detailed observations in the immediate vicinity of the source. Simultaneous arrivals across arrays of surface dipoles are not common. The majority of such experiments have indicated that shot‐symmetric voltages which display low‐velocity moveout are the dominant received waveforms.

As frequency domain airborne electromagnetic studies (AEM) move towards more detailed assessments of the near surface, the behaviour of system footprints, and hence the spatial averages involved in the measurement, becomes important.Published estimates suffer from two main limitations; firstly they are based on perfectly conducting, thin sheet models and, secondly, they are system specific. The present study is a revision of footprint estimates based on (i) a finitely conducting half-space and (ii) an at-surface scale estimate that uses the spatial equivalent of the conventional electromagnetic skin depth.In order to remove the system dependence, a transmitter footprint is defined in terms of electromagnetic skin distance. Only the limiting cases of vertical and horizontal magnetic dipole sources then require analysis. Electromagnetic skin distances, two for each of the coil orientations, are defined. The revised definition makes it possible to investigate the footprint behaviour of both towed-bird and fixed-wing AEM systems over an altitude range from 20 to 100 m. The footprint/altitude ratio has a primary dependence on altitude and a secondary dependence on both resistivity and frequency. The analysis covers a frequency range from 1 to 100 kHz and results are presented for two specific resistivity values that represent conductive (10 m) and resistive (1000 m) environments.The revised footprint parameters display a quasi-linear behaviour with altitude, particularly for mid-range frequencies. This behaviour enables the coefficients of linear, least-squares relationships to be obtained thus assisting with the prediction of footprint estimates for survey planning and interpretation. A comparison of the new estimates with published values suggests that existing footprint values for a vertical magnetic dipole should be revised downward.3