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EGU General Assembly 2020 Online, 4-8 May 2020, B asal slip is an important mechanism by which glaciers and ice-sheets flow, and is a major source of uncertainty in simulations of ice-mass loss and sea level rise from the Greenland Ice Sheet (GrIS). Sub-ice geology is a dominant control on ice flow velocity with fast flow often coinciding with the presence of deformable subglacial till eroded from underlying sedimentary rocks. The subglacial geology of Greenland has received relatively little attention thus far and its control on ice flow is poorly understood. Seismic studies of the crust beneath the GrIS have been limited due to a lack of seismic stations and the reliance on earthquake event data. However, in the past decade, there has been a rapid increase in the number of both permanent and temporary seismic stations deployed in Greenland as well developments in ambient noise methods, allowing for improved spatial resolution of crustal geology. Ellipticity measurements, the ratio of the horizontal to vertical component of a Rayleigh wave, have been shown to be particularly sensitive to the geological structure directly beneath the station. Ambient noise H/V measurements have been used for decades in geotechnical and civil engineering for site characterisation, making them a well-suited technique to determine the subglacial geology of the GrIS. Using all available broadband stations deployed on Greenland from 2012 to 2018 we extract Rayleigh wave ellipticity measurement from ambient noise data using the degree-of-polarization (DOP) method where meaningful signals are defined as a waveform with an arbitrary polarization which remains stable for a given time window. We invert these ellipticity measurements in the period range of 4 ¿ 9 s to generate Vs profiles of the first 5 km beneath each station. Our inversions indicate that: (1) off-ice stations along the margins of the GrIS produce a good agreement with the litho1.0 model to within error and (2) an additional subglacial layer 1.0 - 2.0km thick with a Vs < 3.0km is necessary to match the data recorded at several of the on-ice stations. We attribute these observations to the widespread presence of sedimentary rocks beneath the GrIS, potentially capable of sustaining extensive subglacial till layers that can support enhanced basal slip.

We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light-emitting diode-illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding-based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density-luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV-derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E-W gradient in fohn-driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice.

Numerical models of contemporary as well as paleo-ice sheets suggest that groundwater and heat exchanges between subglacial sedimentary basins and the ice sheet above, can be substantial and influence the flow of ice above. So far, an approach for the measurement and assessment of such heat fluxes has not been available. Here, we summarise existing evidence for groundwater and heat exchanges between contemporary and paleo ice sheets and the substrate below. We then explain the utility of electromagnetic geophysical measurements in elucidating such exchanges, and present magnetotelluric synthetic models of the deep sedimentary basin beneath the Institute Ice Stream in West Antarctica by way of illustration. Finally, we propose a simple empirical model by which heat exchanges between subglacial sedimentary basins and the overlying ice sheet can be estimated to first-order from electromagnetic data.

The mining and processing of metal ores in the UK has left a legacy of environmental degradation, and abandoned metal mines still pose a significant threat to terrestrial and fluvial environments. Flow gauging, water quality and geophysics were combined in an integrated assessment of surface and subsurface hydrological contamination at Esgair Mwyn, an abandoned mine in Ceredigion, Wales. Heavy metals discharged from the site are polluting downstream watercourses, leading to widespread Environmental Quality Standards (EQS) compliance failures. Through salt water dilution gauging and water quality sampling, a daily efflux of 876 g of heavy metals was calculated, with contaminant mobilisation occurring mainly in two primary surface streams draining an exposed tailings heap. Electrical resistivity tomography subsurface imaging found a seepage plane within the tailings lagoon wall, whilst the main tailings heap became increasingly saturated with depth. A large adjacent field also had a high potential to convey pollutants in solution, yet its morphological characteristics have limited transmission, as the area acts as a passive treatment type system. With remediation of already polluted water both difficult and expensive, this approach provides a cost-effective way to identify the origins and pathways of contaminants, informing mitigation strategies focussed on containment. Esgair Mwyn is not an isolated case, as abandoned metal mines release at least 860 t of heavy metals annually into UK water bodies. These techniques could reduce or prevent abandoned metal mine hydrological pollution for decades to come, and enable associated UK water bodies to comply with future water quality standards.

[1] Antarctic ice shelves are fed primarily by the glaciers flowing into them. Downstream of promontories separating these glaciers, supercooled water can rise and freeze into suture zones, leading to the accretion of marine ice. Marine ice bodies have been found in several Antarctic ice shelves, but little is known about their detailed geometry, rate of accretion, or influence on ice dynamics. In this study, we investigate marine ice in a suture zone downstream of the Joerg Peninsula in the southern part of the Larsen C Ice Shelf, Antarctic Peninsula. We present ground penetrating radar data from which we infer the base of the meteoric ice and, in combination with GPS data and assuming hydrostatic equilibrium, estimate marine ice thickness within a suture zone. We show that the Joerg Peninsula suture zone contains marine ice layer, which is increasing in thickness along flow from ~140 m to 180 m over 20 km, implying an average basal accretion rate of ~0.5 m a−1 in our study area. We examined the impact of this inferred marine ice on ice shelf dynamics by modeling the suture zone within an ice flow model. The results, which replicate observed surface velocities and strain rates, show that the warmer and thus softer ice of the suture zone serves to channel shear deformation. This enables decoupling of neighboring flow units with different flow velocities, while maintaining the structural integrity of the ice shelf.

Despite significant advances over the past decades, our understanding of drumlin formation and associated ice-bed processes is still incomplete. In this paper, we present the integrated use of geomorphological, sedimentological and geophysical techniques as a powerful means to force a breakthrough towards solving the drumlin enigma. We report on investigations of the anatomy of the Pigeon Point drumlin, Clew Bay, Ireland.We found that the bulk of the landform, which displays a classical drumlin shape, consists of silty-clayey diamicton showing evidence of deformation, hydrofracturing and comminution. The unit is interpreted as a sub-glacial traction till/comminution till. The thin unit overlying this basal till consists of silty-sandy diamicton, and is interpreted as a para-glacially modified melt-out till. The partly cemented third unit consists of stratified, massive to graded sands and gravels. Its contact with the sub-glacial traction till consists of a series of concave shapes, which suggests that it was deposited in meltwater channels that flowed in sub-glacial cavities and that cut laterally into the drumlin. We propose that highs in the undulating rockhead relief, as shown in the seismic profile, have provided nuclei which initiated drumlin formation. This idea is supported by the observation of local detached bedrock slabs that grade upwards into a comminution till. In the long profile, very high normalized induced polarization (IP) values form a wedge-shape, which is interpreted as a set of conjugate thrusts, or a ‘pop-up’ structure. The structure is positioned directly above one of the undulations in the bedrock, suggesting a direct relationship. The high values are thought to reflect the presence of pre-existing clays, which were sheared into the till, thus forming linings in the thrust features. It is concluded that glacitectonic processes, notably differential bedrock weathering and thrusting, have played a key role in the formation of this drumlin.

SUMMARY We propose a new approach for the inversion of anisotropic P-wave data based on Monte Carlo methods combined with a multigrid approach. Simulated annealing facilitates objective minimization of the functional characterizing the misfit between observed and predicted traveltimes, as controlled by the Thomsen anisotropy parameters (e, δ). Cycling between finer and coarser grids enhances the computational efficiency of the inversion process, thus accelerating the convergence of the solution while acting as a regularization technique of the inverse problem. Multigrid perturbation samples the probability density function without the requirements for the user to adjust tuning parameters. This increases the probability that the preferred global, rather than a poor local, minimum is attained. Undertaking multigrid refinement and Monte Carlo search in parallel produces more robust convergence than does the initially more intuitive approach of completing them sequentially. We demonstrate the usefulness of the new multigrid Monte Carlo (MGMC) scheme by applying it to (a) synthetic, noise-contaminated data reflecting an isotropic subsurface of constant slowness, horizontally layered geologic media and discrete subsurface anomalies; and (b) a crosshole seismic data set acquired by previous authors at the Reskajeage test site in Cornwall, UK. Inverted distributions of slowness (s) and the Thomson anisotropy parameters (e, δ) compare favourably with those obtained previously using a popular matrix-based method. Reconstruction of the Thomsen e parameter is particularly robust compared to that of slowness and the Thomsen δ parameter, even in the face of complex subsurface anomalies. The Thomsen e and δ parameters have enhanced sensitivities to bulk-fabric and fracture-based anisotropies in the TI medium at Reskajeage. Because reconstruction of slowness (s) is intimately linked to that e and δ in the MGMC scheme, inverted images of phase velocity reflect the integrated effects of these two modes of anisotropy. The new MGMC technique thus promises to facilitate rapid inversion of crosshole P-wave data for seismic slownesses and the Thomsen anisotropy parameters, with minimal user input in the inversion process.

Strategies available to evaluate the performance of in situ permeable reactive barriers are currently not well developed and often rely on fluid and media sampling directly from the permeable reactive barrier (PRB). Here, we investigate the utility of the self-potential (SP) method as a technique to monitor in situ PRB performance. Our field study was conducted at in situ biological PRB in Portadown, Northern Ireland, UK, which was emplaced to assist in the remediation of groundwater contamination (e.g., hydro-carbons, ammonia) that resulted from the operations and waste disposal practices of a former gasworks. Borehole SP measurements were collected during the injection of contaminant groundwater slugs in an attempt to monitor/detect the response of the microbial activity associated with the breakdown of the added contaminants into the PRB. In addition, an uncontaminated groundwater slug was injected into a different portion of the PRB as a ‘control’ and SP measurements were collected for comparison to the SP response of the contaminant slugs. The results of the SP signals due to the contaminant injections show that the magnitude of the response was relatively small ( < 10 mV) yet showed a consistent decrease during both contaminant injections. The net decrease in SP recorded during the contaminant injections slowly rebounded to near background values through ~44 hours post-injection. The SP response during the uncontaminated injection showed a slight, albeit negligible (within the margin of error), 1 mV increase in the measured SP signals, in contrast to the contaminant injections. The results of the SP signals recorded from the uncontaminated groundwater injection also persisted through a period of ~47 hours after injection but show a net increase in SP relative to pre-injection values. Based on the difference in SP response between the contaminated and uncontaminated injections, we suggest that the responses are likely to be the result of differences in the chemistry of the injection types (contaminated versus uncontaminated) and in situ groundwater. We argue that the SP signals associated with the contaminated injections are dominated by diffusion (electrochemical) potential, possibly enhanced by a microbial effect. While the results of our investigation show a consistent SP response associated with the contaminant injections that is dominated by diffusional effects, further studies are required in order to better understand the effect of microbial activity on SP signals and the potential utility for the SP method to detect/monitor changes that may be indicative of biological PRB performance.

Ground-penetrating radar (GPR) is a rapid geophysical technique that we have used to assess four illegally buried waste locations in Northern Ireland. GPR allowed informed positioning of the less-rapid, if more accurate use of electrical resistivity imaging (ERI). In conductive waste, GPR signal loss can be used to map the areal extent of waste, allowing ERI survey lines to be positioned. In less conductive waste the geometry of the burial can be ascertained from GPR alone, allowing rapid assessment. In both circumstances, the conjunctive use of GPR and ERI is considered best practice for cross-validation of results and enhancing data interpretation.

Frozen regions of the earth are known as the cryosphere. The arctic, Antarctica, permafrost, ice sheets, and glaciers are some of the most challenging places to measure subsurface parameters, but they can also be some of the most important places to science and engineering research due to their susceptibility to environmental change. Ground-based, airborne, and space-borne geophysical methods are deployed to observe targets below the ground or in ice that may be difficult or impossible to measure using conventional direct observations and measurements. The papers in this special section address recent advances in instrumentation development and deployment and computational capabilities that have advanced cryosphere geophysical sciences. As such, many of these papers discuss the science that the methodology has helped reveal. A wide range of cryosphere science questions are being addressed using geophysical data, and most are highly relevant to climate change. For example: How does liquid water affect snow, glacier, and permafrost dynamics; what controls snowpack distribution and water content; what controls water movement in the active layer above permafrost; and how are sensitive special features like Antarctic lakes and ice shelves evolving? In all cases, geophysical measurements provide parameters vital to understanding the system function, often in 2D or 3D space or through time. We present 21 papers that include topics ranging from long-established direct-current electrical, seismic, and ground-penetrating radar (GPR) methods, to emerging measurements such as surface nuclear magnetic resonance (NMR), all with novel examples. Holbrook et al. image snow stratigraphy and estimate snow water equivalent (SWE) from a GPR system mounted on a snowmobile. The authors measure snow thickness, snow density, …

I review current understanding of the low-frequency electrical properties of glaciers and ice sheets, and identify future research directions that challenge near-surface geophysicists and glaciologists. In cold ice electrical conduction occurs principally via [a] movement of protonic point defects in the lattice in low-impurity ice; [b] networks of impurities at grain boundaries in ice of moderate impurity content; and [c] triple junctions and grain boundaries in ice of high impurity content. I infer that in temperate ice Archie-type conduction is likely dominant. Arrhenius and Looyenga type models, respectively, describe well the increase in bulk resistivity with decreasing ice temperature and density. The activation energy in cold ice and firn is constant at [Formula: see text] but poorly constrained in temperate ice and snow. The bulk resistivity of cold ice ranges from [Formula: see text] at [Formula: see text] at [Formula: see text], and is much higher in temperate ice (up to [Formula: see text]). The effects of impurity characteristics, temperature, and density on complex conductivity are poorly understood, although selected real conductivity components apparently increase with frequency, impurity concentration, or temperature. Future research should exploit more rigorously low-frequency electrical techniques in the field and laboratory, and develop or adapt from other areas of electrical geophysics novel mathematical and statistical concepts for joint data inversion and integration, for glaciological purposes such as ice core logging and investigations of glacier dynamics, ice fracturing, and glacier hydrology. I conclude that low-frequency electrical techniques have unduly been neglected in glaciology as compared with higher-frequency radar techniques over the past few decades, suggesting opportunities for concerted research efforts into these techniques.

Blast-iron-furnace plantations were widespread throughout colonial and postcolonial America and therefore represent sites of specific archaeological interest. Because such plantations often were a really extensive, historical reconstruction of a site is challenging using conventional archaeological field techniques alone. Therefore, we appraise the usefulness of integrating magnetic gradiometer, electrical-resistivity tomography (ERT), and electromagnetic (EM31) data in detecting and delineating buried structures related to former operation of the Shade furnace, a typical industrial plantation in 19th-century Pennsylvania. The geophysical results were ground-truthed in seven locations by archaeological excavation. Geophysical results demonstrated that (1) the distribution of remaining original soils could be mapped; (2) waste materials (e.g., slag, used brick, burned charcoal, iron ore, and broken-up sandstone or limestone) within or above the original soils could be detected and delineated, whether the material occurred in pits or lenses or was dispersed across the ground surface; and (3) the location and spatial extent of many former structures related to furnace operation could be identified, such as building or bridge foundations and casting-related structures (e.g., the tapping channel or the casting floor, and the former courses of a water canal (raceway) and a supply road). We conclude that geophysical techniques can play a key role in reconstructing man-made ground at former industrial furnace sites in North America.

The value of geophysical methods in investigating iron furnaces in North America (of which several hundred exist) has so far been poorly evaluated. We report on the magnetic gradiometer and electrical resistivity surveys conducted at the Shade iron furnace, Pennsylvania, USA, together with results from ground-truthing excavations. We find that (i) numerous useful data sets could be recorded despite challenging field conditions; (ii) reconnaissance gradiometer and resistivity data yield complementary information allowing large-scale site reconstruction; (iii) high-resolution resistivity data reflect archaeologically valuable site lithologies with decimetre-scale accuracy; and (iv) extreme absolute background values (several hundred nT) of, and spatial contrasts (several hundred to > 1000 nT) in, the gradiometer data are common owing to the presence of much burnt material and iron artefacts. We conclude that geophysical investigations of iron furnaces in North America may yield spatially extensive subsurface information of substantial archaeological value, which is impossible to obtain using traditional archaeological survey methods alone. Copyright © 2004 John Wiley & Sons, Ltd.

The aim of this study was to assess, in the laboratory, the efficiency of a barrier of oxygen release compound (ORC) to block and divert a diesel plume migration in a scaled aquifer model using miniature electrical resistivity tomography (ERT) as the monitoring system. Two plumes of contaminant (diesel) were injected in a soil model made of local sand and clay. The diesel plumes migration was imaged and monitored using a miniature resistivity array system that has proved to be accurate in soil resistivity variations in small-scaled models of soil. ERT results reflected the lateral spreading and diversion of the diesel plumes in the unsaturated zone. One of the contaminant plumes was partially blocked by the ORC barrier and a diversion and reorganisation of the diesel in the soil matrix was observed. The technique of time-lapse ERT imaging showed that a dense non-aqueous phase liquid (DNAPL) contaminant like diesel can be monitored through a bioremediation barrier and the technique is well suited to monitor the efficiency of the barrier. Therefore, miniature ERT as a small-scale modelling tool could complement conventional techniques, which require more expensive and intrusive site investigation prior to remediation.

We have used geophysics, microbiology, and geochemistry to link large-scale (30+ m) geophysical self-potential (SP) responses at a groundwater contaminant plume with its chemistry and microbial ecology of groundwater and soil from in and around it. We have found that microbially mediated transformation of ammonia to nitrite, nitrate, and nitrogen gas was likely to have promoted a well-defined electrochemical gradient at the edge of the plume, which dominated the SP response. Phylogenetic analysis demonstrated that the plume fringe or anode of the geobattery was dominated by electrogens and biodegradative microorganisms including Proteobacteria alongside Geobacteraceae, Desulfobulbaceae, and Nitrosomonadaceae. The uncultivated candidate phylum OD1 dominated uncontaminated areas of the site. We defined the redox boundary at the plume edge using the calculated and observed electric SP geophysical measurements. Conductive soils and waste acted as an electronic conductor, which was dominated by abiotic iron cycling processes that sequester electrons generated at the plume fringe. We have suggested that such geoelectric phenomena can act as indicators of natural attenuation processes that control groundwater plumes. Further work is required to monitor electron transfer across the geoelectric dipole to fully define this phenomenon as a geobattery. This approach can be used as a novel way of monitoring microbial activity around the degradation of contaminated groundwater plumes or to monitor in situ bioelectric systems designed to manage groundwater plumes.

Measurements of electrical resistivity and magnetic susceptibility have been made in order to determine the geophysical signature of a number of fulachta fia (burnt mounds of probable Bronze Age) on Clare Island, County Mayo, Ireland. Visible remains of fulachta fia on Clare Island frequently show a characteristic horseshoe-shaped structure at the surface, covered by a very thin layer of topsoil. Magnetic susceptibility and electrical resistivity surveys over two intact fulachta fia were carried out in order to map their lateral and vertical extent. The magnetic susceptibility surveys provided horseshoe-shaped signatures, which immediately identify the extent and orientation of both the surveyed monuments. The success of the resistivity method was found to be dependant upon the proximity of unwanted sources of resistivity variation not related to these archaeological targets, and possibly climatological and method-related influences. These results illustrate the feasibility of using geophysical methods to define a characteristic signature related to fulachta fia. The reproduction of this signature over suspected archaeological sites can provide confirmation of the existence of a monument without the need for an intrusive excavation. Resistivity surveys over two damaged fulachta fia partially exposed in the bank of a drainage channel have revealed useful information on the degree of preservation of these monuments. Resistivity anomalies were identified coincident with the location of fractured rock exposed in the channel bank, despite the terrain effect caused by the proximity to the channel edge.

International audience; The seismoelectric method is based on the interpretation of the electrical field associated with the conversion of mechanical to electromagnetic energy during the propagation of seismic waves in heterogeneous porous media. We propose an extension of a poroacoustic model that takes into account fluid flow and the effect of saturation. This model is coupled with an electrokinetic model accounting for the effect of saturation and in agreement with available experimental data in sands and carbonate rocks. We also developed new scaling laws for the permeability, the streaming potential coupling coefficient and the capillary entry pressure of porous media. The theory is developed for frequencies much below the critical frequency at which inertial effects starts to dominate in the Navier–Stokes equation (>10 kHz). The equations used to compute the propagation of the P waves and the seismoelectric effect in unsaturated condition are solved with finite elements using triangular meshing. We demonstrate the usefulness of a recently developed technique, seismoelectric beamforming, to localize saturation fronts by focusing seismic waves and looking at the resulting seismoelectric conversions. This method is applied to a cross-hole problem showing how a saturation front characterized by a drop in the electrical conductivity and compressibility is responsible for seismoelectric conversions. These conversions can be used, in turn, to determine the position of the front over time.

Recent work has suggested that the electrical self-potential (SP) geophysical technique may be used to non-invasively map redox conditions associated with contaminant plumes or bioremediation schemes. The proposed mechanism linking SP response and redox involves the generation of a current source and sink in the subsurface whereby electrons are transferred between anoxic and oxic environments via a conductive biofilm and/or biominerals, creating a biogeobattery. Here, we seek direct experimental evidence for the biogeobattery hypothesis, which has so far remained elusive, using a flow-through column experimental set-up specifically designed for this purpose. We successfully created contrasting redox zones, with an oxic section containing clean sand transitioning into an Fe(III)-reducing section. In experiments, Fe(III)-reduction was mediated both by a natural microbial community and by a pure culture of the model organism Shewanella oneidensis MR-1 in two separate column experiments. Visual observations and electron microscopy showed that ferrihydrite was sequentially transformed to goethite and magnetite; despite this, no SP signal was generated in either column. Electron microscopy suggested that in the pure culture column, S. oneidensis MR-1 cells did not form a continuous, interconnected biofilm but rather interacted with the iron (oxyhydr)oxide surfaces as individual cells. We thus conclude that (i) there is still no unambiguous direct proof of the biogeobattery hypothesis and (ii) development of Fe(III)-reducing conditions does not necessarily lead to SP signal generation. Overall, this study shows that key redox variations are not necessarily accompanied by a large SP response, suggesting that SP cannot be used in isolation to monitor subsurface biogeochemical conditions.

Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7-8 cm (CATS2008a and TPXO7.2) and 11-12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4-8 cm) and S2 (4-5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.

There is a growing interest in the use of geophysical methods to aid investigation and monitoring of complex biogeochemical environments, for example delineation of contaminants and microbial activity related to land contamination. We combined geophysical monitoring with chemical and microbiological analysis to create a conceptual biogeochemical model of processes around a contaminant plume within a manufactured gas plant site. Self-potential, induced polarization and electrical resistivity techniques were used to monitor the plume. We propose that an exceptionally strong (>800 mV peak to peak) dipolar SP anomaly represents a microbial fuel cell operating in the subsurface. The electromagnetic and electrical geophysical data delineated a shallow aerobic perched water body containing conductive gasworks waste which acts as the abiotic cathode of microbial fuel cell. This is separated from the plume below by a thin clay layer across the site. Microbiological evidence suggests that degradation of organic contaminants in the plume is dominated by the presence of ammonium and its subsequent degradation. We propose that the degradation of contaminants by microbial communities at the edge of the plume provides a source of electrons and acts as the anode of the fuel cell. We hypothesize that ions and electrons are transferred through the clay layer that was punctured during the trial pitting phase of the investigation. This is inferred to act as an electronic conductor connecting the biologically mediated anode to the abiotic cathode. Integrated electrical geophysical techniques appear well suited to act as rapid, low cost sustainable tools to monitor biodegradation.

International audience; Although recent research suggests that contaminant plumes behave as geobatteries that produce an electrical current in the ground, no associated model exists that honors both geophysical and biogeochemical constraints. Here, we develop such a model to explain the two main electrochemical contributions to self-potential signals in contaminated areas. Both contributions are associated with the gradient of the activity of two types of charge carriers, ions and electrons. In the case of electrons, bacteria act as catalysts for reducing the activation energy needed to exchange the electrons between electron donors and electron acceptors. Possible mechanisms that facilitate electron migration include iron oxides, clays, and conductive biological materials, such as bacterial conductive pili or other conductive extracellular polymeric substances. Because we explicitly consider the role of biotic processes in the geobattery model, we coined the term "biogeobattery." After theoretical development of the biogeobattery model, we compare model predictions with self-potential responses associated with laboratory and field scale investigations conducted in contaminated environments. We demonstrate that the amplitude and polarity of large (>100 mV) self-potential signatures requires the presence of an electronic conductor to serve as a bridge between electron donors and acceptors. Small self-potential anomalies imply that electron donors and electron acceptors are not directly interconnected, but instead result simply from the gradient of the activity of the ionic species that are present in the system.

Kulessa, B., A.D. Booth, A. Hobbs, and A.L. Hubbard. (2008). Automated monitoring of subglacial hydrological processes with ground-penetrating radar (GPR) at high temporal resolution: scope and potential pitfalls. Geophysical Research Letters 35 (24), L24502.

[1] Repeatable and strong seismoelectric signals were recorded on Glacier de Tsanfleuron, Switzerland, using a vertical sounding geometry. Electromagnetic waves are inferred to be generated by electrokinetic conversion of seismic energy within the snow pack and near the dry-wet ice and ice-bed interfaces. A simple gradient-based scheme allows such electrokinetic interface responses (EIRs) to be isolated from noise. EIRs depend sensitively on the azimuthal orientation of the receiving array of electrical dipoles. Seismoelectric techniques promise to allow mapping even of thin water-bearing strata within or beneath glaciers or frozen ground, estimation of hydraulic or fluid properties of such strata, as well as monitoring of ice fracturing or basal properties and processes at improved spatial resolution.

[1] This study aims to demonstrate that self-potential measurements can yield unique information about subglacial drainage conditions, which has so far been doubtful. At the bed of warm-based glaciers arborescent and distributed drainage systems interact, and streaming and electrochemical potentials coincide. A key secondary aim of this study is, therefore, the development of a mathematical framework for the analysis of self-potentials generated by such interacting flow systems. We consider cross coupling between hydraulic, electrical, and chemical driving forces, and apply the resulting model solution to self-potential data recorded beneath Haut Glacier d'Arolla, Switzerland. We find that (1) the streaming and electrochemical contributions to the total self-potential signal can be separated mathematically, (2) streaming potentials directly reflect the direction of water flow at the glacier bed, (3) combination of streaming potentials with hydraulic conductivity estimates from slug tests allows estimation of in situ subglacial water flow velocities, (4) the subglacial zeta potential, calculated from estimated values of the streaming coupling coefficient, is small (∼-0.022 V) due to systematic flushing of clay minerals from the channel area during the melt season, and (5) uncertainties in exact electrode location in the borehole are unlikely to produce self-potential noise. Self-potential measurements may therefore yield unique information about subglacial drainage conditions, and especially the direction and velocity of in situ subglacial water flow are difficult to obtain by other methods. We further anticipate that our mathematical framework will be applicable to other areas of subsurface hydrology, especially where spatially distributed seepage feeds preferential flow pathways.

In support of the development of the magneto-electrical resistivity imaging technique (MERIT), a framework is presented for forward and inverse modeling of near-surface magnetometric resistivity (MMR) data, and results are reported from initial laboratory tests. Application of MMR involves injection of a low-frequency current into the ground, and measurement of the resulting magnetic field using state-of-the-art lock-in technology to suppress magnetic noise around the centre frequency. Forward and inverse calculations are both two-step procedures, involving intermittent computation of current density. The validity of this framework is demonstrated using synthetic numerical tests. The laboratory experiment reported considers a saturated sand sample placed in a non-conducting container furnished with electrodes. Migration of a conductive salt tracer was monitored over three days using a fluxgate magnetometer mounted on a device that allowed repeat scans of the sample. Data inversion uses any two of the three measured magnetic field components, and is unique in two spatial dimensions. Combination of inverted data for different electrode arrangements allows visualization of temporal and spatial tracer migration, consistent with preferential tracer movement along pathways located near the container walls.

This Special Issue of the Journal of Environmental and Engineering Geophysics arises from a conference on Geophysics and Glacial Materials, hosted by the Environmental and Industrial Geophysics Group, The Geological Society, UK (http:∕∕www.geolsoc.org.uk∕template.cfm?name=geogroup12). This conference was held at the School of Applied Sciences, Northumbria University, UK, in conjunction with the Annual Meeting of the British Branch of the International Glaciological Society from 14–16 September 2005.