[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

This study considers gamma ray attenuation in relation to the soils and bedrock of Northern Ireland using simple theory and data from a high resolution airborne survey. The bedrock is considered as a source of radiogenic material acting as parent to the soil. Attenuation in the near-surface is then controlled by water content in conjunction with the porosity and density of the soil cover. The Total Count radiometric data together with 1:250k mapping of the soils and bedrock of Northern Ireland are used to perform statistical analyses emphasising the nature of the low count behaviour. Estimations of the bedrock response characteristics are improved by excluding areas covered by low count soils (organic/humic). Equally, estimations of soil response characteristics are improved by excluding areas underlain by low count bedrock (basalt). When the spatial characteristics of the soilclassified data are examined in detail, the low values form spatially-coherent zones (natural clusters) that can potentially be interpreted as areas of increased water content for each soil type. As predicted by theory, the highest attenuation factors are associated with the three organic soil types studied here. Peat, in particular, is remarkably skewed to low count behaviour in its radiometric response. Two detailed studies of blanket bogs reveal the extent to which peat may be mapped by its radiometric response while the intra-peat variations in the observed response may indicate areas of thin cover together with areas of increased water content.

The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Geochemical maps provide invaluable evidence to guide decisions on issues of mineral exploration, 9 agriculture, and environmental health. However, the high cost of chemical analysis means that the 10 ground sampling density will always be limited. Traditionally, geochemical maps have been 11 produced through the interpolation of measured element concentrations between sample sites 12 using models based on the spatial autocorrelation of data (e.g semivariogram models for ordinary 13 kriging). In their simplest form such models fail to consider potentially useful auxiliary information 14 about the region and the accuracy of the maps may suffer as a result. In contrast, this study uses 15 quantile regression forests (an elaboration of random forest) to investigate the potential of high 16 resolution auxiliary information alone to support the generation of accurate and interpretable 17 geochemical maps. This paper presents a summary of the performance of quantile regression forests 18 in predicting element concentrations, loss on ignition and pH in the soils of south west England using 19 high resolution remote sensing and geophysical survey data. 20Through stratified 10-fold cross validation we find the accuracy of quantile regression forests in 21 predicting soil geochemistry in south west England to be a general improvement over that offered 22 by ordinary kriging. Concentrations of immobile elements whose distributions are most tightly 23 controlled by bedrock lithology are predicted with the greatest accuracy (e.g. Al with a 24 cross-validated R 2 of 0.79), while concentrations of more mobile elements prove harder to predict. 25In addition to providing a high level of prediction accuracy, models built on high resolution auxiliary 26 variables allow for informative, process based, interpretations to be made. In conclusion, this study 27 has highlighted the ability to map and understand the surface environment with greater accuracy 28 2 and detail than previously possible by combining information from multiple datasets. As the quality 29 and coverage of remote sensing and geophysical surveys continue to improve, machine learning 30 methods will provide a means to interpret the otherwise-uninterpretable. 31 32 33

Ground conductivity meters, comprising a variety of coil-coil configurations, are intended to operate within the limits provided by a low induction number (LIN), electromagnetic condition. They are now routinely used across a wide range of application areas and the measured apparent conductivity data may be spatially assembled and examined/correlated alongside information obtained from many other earth science, environmental, soil and land use assessments. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (<12 mS/m) conductivities. Beyond this limit, non-linear departures from the apparent conductivity that would be associated with a LIN condition occur and are a function of the coil configuration, the instrument height and the prevailing conductivity. Using both theory and experimental data, it is demonstrated that this has the potential to provide biased and spatially distorted measurements. A simple correction procedure that can be applied to the measured data obtained from any of the LIN instruments is developed. The correction procedure would, in the limit of a uniform subsurface, return the same (correct) conductivity, irrespective of the ground conductivity meter used, the prevailing conductivity or the measurement height.