three dimensional effects causing artifacts in two dimensional cross borehole electrical imaging summary cross borehole electrical resistance tomography ert experiments utilize downhole electrodes typically placed in a borehole filled with material of contrasting resistivity to the host rock the circular geometry of the boreholes is three dimensional and inversion routines are typically two dimensional because of the resistivity contrast artifacts in the form of borehole inversion effects develop in the resistivity images other 3d effects resulting in inversion artifacts are shadow effects caused by use of a 2d code to invert data from a 3d body located outside the image plane in both cases the inversion model misrepresents the spatial change in voltages as a corresponding spatial change in resistivity borehole inversion effects and shadow effects result as the forward solver attempts to resolve the discrepancy in the voltages for numerous electrode pairs into a meaningful resistivity distribution borehole inversion effects are shown to be related to the resistivity contrast between the borehole fill and the host medium and to borehole diameter borehole inversion effects do not materialize with small diameter boreholes e g 0 1 m when the fill resistivity contrast is one order of magnitude or less however a borehole fill resistivity contrast of two orders of magnitude causes artifacts in the form of sheaths near the boreholes and a conductive artifact between the boreholes larger diameter borehole e g 0 2 m induce significant borehole inversion effects with as little as one order of magnitude fill resistivity contrast the general resistivity patterns are similar for the borehole inversion effects however the artifacts are amplified as resistivity contrast and or borehole diameter increase these results are significant because borehole inversion effects may mask a target heterogeneity or an artifact may be confused for an anomaly resulting in improper actions for site characterizations or remediation strategies suggestions of best practices for experimental design to prevent or minimize borehole inversion effects include minimizing borehole diameter minimizing borehole fill resistivity contrast measuring and inputting the resistivity of the borehole fill water into the model and inverting changes in resistivity for time lapse ert data shadow effects are shown to be related directly to the distance of the target heterogeneity out of the image plane our results show that shadow effects become insignificant when the target is between 3 and 5 m outside the 10 m wide image plane because the size of the target and resistivity contrast of the target to the host rock are site specific variables the actual distance at which shadow effects become insignificant will vary from site to site to reduce misinterpretations by shadow effects reduce the dipole length of the four electrode measurements and supplement data from other well pairs and or other methods

coupled modeling of hydrogeochemical and electrical resistivity data for exploring the impact of recharge on subsurface contamination 1 the application of geophysical methods in particular electrical resistivity measurements may be useful for monitoring subsurface contamination however interpreting geophysical data without additional data and without considering the associated hydrogeochemical processes is challenging since the geophysical response is sensitive to not only heterogeneity in rock properties but also to the saturation and chemical composition of pore fluids we present an inverse modeling framework that incorporates the simulation of hydrogeochemical processes and time lapse electrical resistivity data and apply it to various borehole and cross borehole data sets collected in 2008 near the s 3 ponds at the u s department of energy s oak ridge integrated field research challenge site where efforts are underway to better understand freshwater recharge and associated contaminant dilution our goal is to show that the coupled hydrogeochemical geophysical modeling framework can be used to 1 develop a model that honors all the available data sets 2 help understand the response of the geophysical data to subsurface properties and processes at the site and 3 allow for the estimation of petrophysical parameters needed for interpreting the geophysical data we present a series of cases involving different data sets and increasingly complex models and find that the approach provides useful information about soil properties recharge related transport processes and the geophysical response spatial heterogeneity of the petrophysical model can be described sufficiently with two layers and its parameters can be estimated concurrently with the hydrogeochemical parameters for successful application of the approach the parameters of interest must be sensitive to the available data and the experimental conditions must be carefully modeled

electrical resistivity imaging of the architecture of substream sediments 1 the modeling of fluvial systems is constrained by a lack of spatial information about the continuity and structure of streambed sediments there are few methods for noninvasive characterization of streambeds invasive methods using wells and cores fail to provide detailed spatial information on the prevailing architecture and its continuity geophysical techniques play a pivotal role in providing spatial information on subsurface properties and processes across many other environments and we have applied the use of one of those techniques to streambeds we demonstrate through two examples how electrical resistivity imaging can be utilized for characterization of subchannel architecture in the first example electrodes installed in riparian boreholes and on the streambed are used for imaging under the river bed the thickness and continuity of a highly permeable alluvial gravel layer overlying chalk in the second example electrical resistivity images determined from data collected using electrodes installed on the river bed provide a constrained estimate of the sediment volume behind a log jam vital to modeling biogeochemical exchange which had eluded measurement using conventional drilling methods owing to the boulder content of the stream the two examples show that noninvasive electrical resistivity imaging is possible in complex stream environments and provides valuable information about the subsurface architecture beneath the stream channels

glacier shrinkage in the ebinur lake basin tien shan china during the past 40 years the ebinur lake basin tien shan china was described in the early 20th century as the green labyrinth this relatively productive area supports agriculture and stands in contrast to the regional aridity glacier melt runoff is an important source of water for this arid region and has played a significant role in its economic development however over the past 40 years the ecology and environment of the region have seriously degenerated due to human activity and climate change to investigate changes in glacier water supply measurements from two reference glaciers were performed and used to validate satellite data and historical data for glaciers in the water catchment variability in regional glacier total area and volume over the past 40 years was reconstructed from historical documents aerial photographs and remote sensing data our investigations of 446 glaciers showed that from 1964 to 2004 the total glacier area decreased by 14 7 0 4 a 1 corresponding to an estimated volume change of 20 5 during this period the most dramatic decrease in glacier area corresponded to a rapid rise in temperature this reduction in glacier area is accelerating and impacting the future sustainability of the region s water resources

pilot scale field validation of the long electrode electrical resistivity tomography method a validation experiment carried out in a scaled field setting was attempted for the long electrode electrical resistivity tomography method in order to demonstrate the performance of the technique in imaging a simple buried target the experiment was an approximately 1 17 scale mock up of a region encompassing a buried nuclear waste tank on the hanford site the target of focus was constructed by manually forming a simulated plume within the vadose zone using a tank waste simulant the long electrode results were compared to results from conventional point electrodes on the surface and buried within the survey domain using a pole pole array both point and long electrode imaging techniques identified the lateral extents of the pre formed plume with reasonable fidelity but the long electrode method was handicapped in reconstructing vertical boundaries the pole dipole and dipole dipole arrays were also tested with the long electrode method and were shown to have the least favourable target properties including the position of the reconstructed plume relativetotheknownplumeandtheintensityoffalsepositivetargets thepoorperformance of the pole dipole and dipole dipole arrays was attributed to an inexhaustive and non optimal coverage of data at key electrodes as well as an increased noise for electrode combinations with high geometric factors however when comparing the model resolution matrix among the different acquisition strategies the pole dipole and dipole dipole arrays using long electrodes were shown to have significantly higher average and maximum values within the matrix than any pole pole array the model resolution describes how well the inversion model resolves the subsurface given the model resolution performance of the pole dipole and dipole dipole arrays it may be worth investing in tools to understand the optimum subset of randomly distributed electrode pairs to produce maximum performance from the inversion model

saline tracer visualized with three dimensional electrical resistivity tomography field scale spatial moment analysis 1 cross well electrical resistivity tomography ert was used to monitor the migration of a saline tracer in a two well pumping injection experiment conducted at the massachusetts military reservation in cape cod massachusetts after injecting 2200 mg l of sodium chloride for 9 hours ert data sets were collected from four wells every 6 hours for 20 days more than 180 000 resistance measurements were collected during the tracer test each ert data set was inverted to produce a sequence of 3 d snapshot maps that track the plume in addition to the ert experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values using modified moment analysis of the electrical conductivity tomograms the mass center of mass and spatial variance of the imaged tracer plume were estimated although the tomograms provide valuable insights into field scale tracer migration behavior and aquifer heterogeneity standard tomographic inversion and application of archie s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass such underestimation is attributed to 1 reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and 2 spatial smoothing regularization from tomographic inversion the center of mass estimated from the ert inversions coincided with that given by migration of the tracer plume using 3 d advective dispersion simulation the 3 d plumes seen using ert exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well

electrical resistivity tomography of vadose water movement cross borehole electrical resistivity tomography ert was used to image the resistivity distribution before and during two infiltration experiments in both cases water was introduced into the vadose zone and the change in resistivity associated with the plume of wetted soil was imaged as a function of time the primary purpose of this work was to study the capabilities and limitations of ert to image underground structure and ground water movement in the vadose zone a secondary goal was to learn specifics of unsaturated flow in a complex geologic setting tomographs of electrical resistivity taken before infiltration image coarser well drained soils sands and gravels as more resistive zones whereas finer grained soils silts and clays which hold more water by capillarity are imaged as more conductive images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology in the ert images we see the effects of capillary barriers and infer differences between capillary driven flow through fine sediments and gravity driven flow through very permeable sediments images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs ert can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements

application of time lapse ert imaging to watershed characterization time lapse electrical resistivity tomography ert has many practical applications to the study of subsurface properties and processes when inverting time lapse ert data it is useful to proceed beyond straightforward inversion of data differences andtakeadvantageofthetime lapsenatureofthedata weassess various approaches for inverting and interpreting time lapse ertdataanddeterminethattwoapproachesworkwell thefirst approachismodelsubtractionafterseparateinversionofthedata from two time periods and the second approach is to use the inverted model from a base data set as the reference model or prior information for subsequent time periods we prefer this second approach data inversion methodology should be considered when designing data acquisition i e to utilize the second approach it is important to collect one or more data sets for which the bulk of the subsurface is in a background or relatively unperturbed state athird and commonly used approach to time lapse inversion invertingthedifferencebetweentwodatasets localizes the regions of the model in which change has occurred however varying noise levels between the two data sets can be problematic to further assess the various time lapse inversion approaches weacquiredfielddatafromacatchmentwithinthedry creek experimental watershed near boise idaho u s a we combined the complimentary information from individual static ertinversions time lapseertimages andavailablehydrologicdatainarobustinterpretationschemetoaidinquantifyingseasonalvariationsinsubsurfacemoisturecontent

a distribution based parametrization for improved tomographic imaging of solute plumes summary difference geophysical tomography e g radar resistivity and seismic is used increasingly for imaging fluid flow and mass transport associated with natural and engineered hydrologic phenomena including tracer experiments in situ remediation and aquifer storage and recovery tomographic data are collected over time inverted and differenced against a background image to produce snapshots revealing changes to the system these snapshots readily provide qualitative information on the location and morphology of plumes of injected tracer remedial amendment or stored water in principle geometric moments i e total mass centres of mass spread etc calculated from difference tomograms can provide further quantitative insight into the rates of advection dispersion and mass transfer however recent work has shown that moments calculated from tomograms are commonly biased as they are strongly affected by the subjective choice of regularization criteria conventional approaches to regularization tikhonov and parametrization image pixels result in tomograms which are subject to artefacts such as smearing or pixel estimates taking on the sign opposite to that expected for the plume under study here we demonstrate a novel parametrization for imaging plumes associated with hydrologic phenomena capitalizing on the mathematical analogy between moment based descriptors of plumes and the moment based parameters of probability distributions we design an inverse problem that 1 is overdetermined and computationally efficient because the image is described by only a few parameters 2 produces tomograms consistent with expected plume behaviour e g changes of one sign relative to the background image 3 yields parameter estimates that are readily interpreted for plume morphology and offer direct insight into hydrologic processes and 4 requires comparatively few data to achieve reasonable model estimates we demonstrate the approach in a series of numerical examples basedonstraight raydifference attenuationradarmonitoringofthetransportofanionictracer and show that the methodology outlined here is particularly effective when limited data are available

electrical characterization of non fickian transport in groundwater and hyporheic systems 1 recent work indicates that processes controlling solute mass transfer between mobile and less mobile domains in porous media may be quantified by combining electrical geophysical methods and electrically conductive tracers whereas direct geochemical measurements of solute preferentially sample the mobile domain electrical geophysical methods are sensitive to changes in bulk electrical conductivity bulk ec and therefore sample ec in both the mobile and immobile domains consequently the conductivity difference between direct geochemical samples and remotely sensed electrical geophysical measurements may provide an indication of mass transfer rates and mobile and immobile porosities in situ here we present 1 an overview of a theoretical framework for determining parameters controlling mass transfer with electrical resistivity in situ 2 a review of a case study estimating mass transfer processes in a pilot scale aquifer storage recovery test and 3 an example application of this method for estimating mass transfer in watershed settings between streams and the hyporheic corridor we demonstrate that numerical simulations of electrical resistivity studies of the stream hyporheic boundary can help constrain volumes and rates of mobile immobile mass transfer we conclude with directions for future research applying electrical geophysics to understand field scale transport in aquifer and fluvial systems subject to rate limited mass transfer

vadose zone flow model parameterisation using cross borehole radar and resistivity imaging cross borehole geoelectrical imaging in particular electrical resistivity tomography ert and transmission radar tomography can provide high resolution images of hydrogeological structures and in some cases detailed assessment of dynamic processes in the subsurface environment through appropriate petrophysical relationships these tools offer data suitable for parameterising and constraining models of groundwater flow this is demonstrated using cross borehole radar and resistivity measurements collected during a controlled vadose zone tracer test performed at a field site in the uk sherwood sandstone both methods show clearly the vertical migration of the tracer over a 200 h monitoring period by comparing first and second spatial moments of changes in moisture content predicted from a numerical simulation of vadose zone flow with equivalent statistics from two and three dimensional ert and cross borehole radar profiles the effective hydraulic conductivity is estimated to be approximately 0 4 m d such a value is comparable to field estimates from borehole hydraulic tests carried out in the saturated zone at the field site and provides valuable information that may be utilised to parameterise pollutant transport models of the site

borehole cylindrical noise during hole surface and hole hole resistivity measurements drilled boreholes generally are the only feasible means to access the subsurface for the emplacement of downhole electrodes for most hole hole and hole surface resistivity experiments however the very existence of the borehole itself creates the potential for significant noise due to the inevitable conductivity contrast that develops between the borehole walls and the formation borehole cylindrical noise develops whenever a current source is placed in a drilled borehole borehole geometries may range from nearly perfect cylinders to highly irregular rugose holes in consolidated rock to relatively minor collapsed disturbed zones in caving sediments boreholes in non caving formations generally are filled with artificial conductive materials to afford crucial electrical continuity between downhole electrodes and the borehole walls filled boreholes form cylindrically shaped heterogeneities that create significant noise due to preferential current flow up and down the conductive columns selected conditions are simulated with a finite difference model to illustrate the significance of borehole cylindrical noise on hole hole and hole surface mise a la masse electrical potentials near a current electrode mise a la masse electrical potentials measured during a field tracer experiment also are presented these measurements are used to illustrate significant errors may develop in the interpretation of apparent resistivity estimates out to a distance of several meters from the current source if borehole cylindrical noise is not recognized and accounted for in the analysis of electrical potential data

three dimensional stochastic estimation of porosity distribution benefits of using ground penetrating radar velocity tomograms in simulated annealing based or bayesian sequential simulation approaches 1 estimation of the three dimensional 3 d distribution of hydrologic properties and related uncertainty is a key for improved predictions of hydrologic processes in the subsurface however it is difficult to gain high quality and high density hydrologic information from the subsurface in this regard a promising strategy is to use highresolution geophysical data that are relatively sensitive to variations of a hydrologic parameter of interest to supplement direct hydrologic information from measurements in wells e g logs vertical profiles and then generate stochastic simulations of the distribution of the hydrologic property conditioned on the hydrologic and geophysical data in this study we develop and apply this strategy for a 3 d field experiment in the heterogeneous aquifer at the boise hydrogeophysical research site and we evaluate how much benefit the geophysical data provide we run high resolution 3 d conditional simulations of porosity with both simulated annealing based and bayesian sequential approaches using information from multiple intersecting crosshole gound penetrating radar gpr velocity tomograms and neutron porosity logs the benefit of using gpr data is assessed by investigating their ability when included in conditional simulation to predict porosity log data withheld from the simulation results show that the use of crosshole gpr data can significantly improve the estimation of porosity spatial distribution and reduce associated uncertainty compared to using only well log measurements for the estimation the amount of benefit depends primarily on the strength of the petrophysical relation between the gpr and porosity data the variability of this relation throughout the investigated site and lateral structural continuity at the site

large scale natural gradient tracer test in sand and gravel cape cod massachusetts 1 experimental design and observed tracer movement a large scale natural gradient tracer experiment was conducted on cape cod massachusetts to examine the transport and dispersion of solutes in a sand and gravel aquifer the nonreactive tracer bromide and the reactive tracers lithium and molybdate were injected as a pulse in july 1985 and monitored in three dimensions as they moved as far as 280 m down gradient through an array of multilevel samplers the bromide cloud moved horizontally at a rate of 0 42 m per day it also moved downward about 4 m because of density induced sinking early in the test and accretion of areal recharge from precipitation after 200 m of transport the bromide cloud had spread more than 80 m in the direction of flow but was only 14 m wide and 4 6 m thick the lithium and molybdate clouds followed the same path as the bromide cloud but their rates of movement were retarded about 50 relative to bromide movement because of sorption onto the sediments

time lapse seismic imaging using regularized full waveform inversion with a prior model which strategy full waveform inversion is an appealing technique for time lapse imaging especially when prior model information is included into the inversion workflow once the baseline reconstruction is achieved several strategies can be used to assess the physical parameter changes such as parallel difference two separate inversions of baseline and monitor data sets sequential difference inversion of the monitor data set starting from the recovered baseline model and double difference inversion of the difference data starting from the recovered baseline model strategies using syntheticmarmousi data sets we investigate which strategy should be adopted to obtain more robust and more accurate time lapse velocity changes in noise free and noisy environments this synthetic application demonstrates that the double difference strategy provides the more robust time lapse result in addition we propose a target oriented time lapse imaging using regularized full waveform inversion including a prior model and model weighting if the prior information exists on the location of expected variations this scheme applies strong prior model constraints outside of the expected areas of timelapse changes and relatively less prior constraints in the time lapse target zones in application of this process to the marmousi model data set the local resolution analysis performed with spike tests shows that the target oriented inversion prevents the occurrence of artefacts outside the target areas which could contaminate and compromise the reconstruction of the effective time lapse changes especially when using the sequential difference strategy in a strongly noisy case the target oriented prior model weighting ensures the same behaviour for both time lapse strategies the double difference and the sequential difference strategies and leads to a more robust reconstruction of the weak time lapse changes the double difference strategy can deliver more accurate time lapse variation since it can focus to invert the difference data however the double difference strategy requires a preprocessing step on data sets such as time lapse binning to have a similar source receiver location between two surveys while the sequential difference needs less this requirement if we have prior information about the area of changes the target oriented sequential difference strategy can be an alternative and can provide the same robust result as the doubledifference strategy

fully coupled hydrogeophysical inversion of a laboratory salt tracer experiment monitored by electrical resistivity tomography 1 we perform a salt tracer experiment monitored by time lapse electrical resistivity tomography in a quasi two dimensional sandbox with the aim of determining the hydraulic conductivity distribution in the domain we use sodium chloride as a tracer together with cochineal red for visual monitoring the time series of observed resistance for each electrode configuration is characterized by its temporal moments we invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by pollock and cirpka 2010 this is the first application of the approach to experimental data the results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling using this estimation a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox the latter is compared to pictures taken during the experiment these results show an even better agreement indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present we conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion

characterizing hydrological processes on loess slopes using electrical resistivity tomography a case study of the heifangtai terrace northwest china abstract from the perspective of engineering geology loess has long been considered as a homogeneous and porous material it is commonly believed that water penetrates loess via pores and in some cases causing mass movements however several researchers have expressed doubts about this mechanism as a cause of slope failures in loess and moreover the actual hydrological processes operating in loess deposits and their effect on slope failures have not been fully investigated here we present the results of an electrical resistivity survey of the heifangtai loess terrace in northwestern china designed to characterize the hydrological processes in loess slopes and their relationship with slope failures the heifangtai loess terrace is located on the fourth terrace of the yellow river and consists of 57 m thickness of aeolian loess 2d and 3d electrical resistivity tomography ert was used to monitor the movement of ground water before and after irrigation and rainfall events and the evolution of a sink hole in the toe of the landslide deposits our main findings are as follows 1 based on the 2d ert results the depth of infiltration into the thick unsaturated loess is not more than 5 m in the profile at the top of the landslide 2 electrical resistivity decreased as a result of water infiltration through sinkholes and this process can increase the soil water content and induce soil liquefaction which can eventually result in land sliding 3 landslide deposits block the groundwater drainage channels through the loess which results in the concentration of water in the toe of the landslide consequently groundwater together with rainfall triggers the failure of sinkholes or cracks which may induce a continuing process of new slope failures at the sites of past landslide

quantitative imaging of solute transport in an unsaturated and undisturbed soil monolith with 3 d ert and tdr electrical resistivity tomography ert has proved to be a valuable tool for imaging solute transport processes in the subsurface however a quantitative interpretation of corresponding ert results is constrained by a number of factors one such factor is the nonuniqueness of the ert inverse problem if no additional constraints are imposed in the vadose zone further problems arise from the general ambiguity of the imaged bulk electrical conductivity in terms of water content and solute concentration in this study we address these issues in detail for a solute tracer experiment conducted in an undisturbed unsaturated soil monolith where the tracer transport was monitored by means of 3 d smoothness constrained ert and time domain reflectometry tdr measurements the experimental design allowed the determination of solute tracer concentrations directly from imaged bulk electrical conductivity independent tdr data and effluent tracer concentrations provided a ground truth for the ert derived apparent convection dispersion equation transport parameters the apparent transport velocity calculated from the ert results was consistent with that based on tdr data and the sampled effluent independent of the degree of smoothness imposed in the ert inversion on the other hand the apparent dispersivity calculated from the ert results was larger than that estimated from tdr data but smaller than that estimated from the sampled effluent with the magnitude of deviations dependent on the degree of smoothing importantly no mass balance problems were observed in the ert results we believe that this is largely a consequence of the uniform application of the tracer as a front and of the configuration of the electrode array with respect to the main transport direction in conclusion the study demonstrates that ert can yield unprecedented quantitative information about local and column scale solute transport characteristics in natural soils

a salt tracer test monitored with surface ert to detect preferential flow and transport paths in fractured karstified limestones in hard rock aquifers fractured zones constitute adequate drinking water exploitation areas but also potential contamination paths one critical issue in hydrogeological research is to identify characterize and monitor such fractured zones at a representative scale a tracer test monitored with surface electrical resistivity tomography ert could help by delineating such preferential flow paths and estimating dynamic properties of the aquifer however multiple challenges exist including the lower resolution of surface ert compared with crosshole ert the finite time that is needed to complete an entire data acquisition and the strong dilution effects we conducted a natural gradient salt tracer test in fractured limestones to account for the high transport velocity we injected the salt tracer continuously for four hours at a depth of 18 m we monitored its propagation with two parallel ert profiles perpendicular to the groundwater flow direction concerning the data acquisition we always focused

evaluation of longitudinal dispersivity estimates from simulated forced and natural gradient tracer tests in heterogeneous aquifers 1 we simulate three types of forced gradient tracer tests converging radial flow unequal strength two well and equal strength two well and natural gradient tracer tests in multiple realizations of heterogeneous two dimensional aquifers with a hydraulic conductivity distribution characterized by a spherical variogram we determine longitudinal dispersivities l by analysis of forced gradient test breakthrough curves at the pumped well and by spatial moment analysis of tracer concentrations during the natural gradient tests results show that among the forced gradient tests a converging radial flow test tends to yield the smallest l an equal strength two well test tends to yield the largest l and an unequal strength two well test tends to yield an intermediate value this finding is qualitatively explained by considering the aquifer area sampled by a particular test a converging radial flow test samples a small area and thus the tracer undergoes a low degree of spreading and mixing an equal strength two well test samples a much larger area so the tracer is spread and mixed to a greater degree results also suggest that if the distance between the tracer source well and the pumped well is short relative to the lengths over which velocity is correlated then the l estimate can be highly dependent on local heterogeneities in the vicinity of the wells finally results indicate that l estimated from forced gradient tracer tests can significantly underestimate the l needed to characterize solute dispersion under natural gradient flow only a two well tracer test with a large well separation in an aquifer with a low degree of heterogeneity can yield a value of l that characterizes natural gradient tracer spreading this suggests that a two well test with a large well separation is the preferred forced gradient test for characterizing solute dispersion under natural gradient flow

structure coupled joint inversion of geophysical and hydrological data abstractin groundwater hydrology geophysical imaging holds considerable promise for improving parameter estimation due to the generally high resolution and spatial coverage of geophysical data however inversion of geophysical data alone cannot unveil the distribution of hydraulic conductivity jointly inverting geophysical and hydrological data allows us to benefit from the advantages of geophysical imaging and at the same time recover the hydrological parameters of interest we have applied a coupling strategy between geophysical and hydrological models that is based on structural similarity constraints model combinations for which the spatial gradients of the inferred parameter fields are not aligned in parallel are penalized in the inversion this structural coupling does not require introducing a potentially weak unknown and nonstationary petrophysical relation to link the models the method was first tested on synthetic data sets and then applied to two combinations of geophysical hydrolog

neutrally buoyant tracers in hydrogeophysics field demonstration in fractured rock electrical and electromagnetic methods are extensively used to map electrically conductive tracers within hydrogeologic systems often the tracers used consist of dissolved salt in water leading to a denser mixture than the ambient formation water density effects are often ignored and rarely modeled but can dramatically affect transport behavior and introduce dynamics that are unrepresentative of the response obtained with classical tracers e g uranine we introduce a neutrally buoyant tracer consisting of a mixture of salt water and ethanol and monitor its movement during push pull experiments in a fractured rock aquifer using ground penetrating radar our results indicate a largely reversible transport process and agree with uranine based push pull experiments at the site which is in contrast to results obtained using dense saline tracers we argue that a shift toward neutrally buoyant tracers in both porous and fractured media would advance hydrogeophysical research and enhance its utility in hydrogeology

stochastic inversion of tracer test and electrical geophysical data to estimate hydraulic conductivities 1 quantifying the spatial configuration of hydraulic conductivity k in heterogeneous geological environments is essential for accurate predictions of contaminant transport but is difficult because of the inherent limitations in resolution and coverage associated with traditional hydrological measurements to address this issue we consider crosshole and surface based electrical resistivity geophysical measurements collected in time during a saline tracer experiment we use a bayesian markov chain monte carlo mcmc methodology to jointly invert the dynamic resistivity data together with borehole tracer concentration data to generate multiple posterior realizations of k that are consistent with all available information we do this within a coupled inversion framework whereby the geophysical and hydrological forward models are linked through an uncertain relationship between electrical resistivity and concentration to minimize computational expense a facies based subsurface parameterization is developed the bayesian mcmc methodology allows us to explore the potential benefits of including the geophysical data into the inverse problem by examining their effect on our ability to identify fast flowpaths in the subsurface and their impact on hydrological prediction uncertainty using a complex geostatistically generated two dimensional numerical example representative of a fluvial environment we demonstrate that flow model calibration is improved and prediction error is decreased when the electrical resistivity data are included the worth of the geophysical data is found to be greatest for long spatial correlation lengths of subsurface heterogeneity with respect to wellbore separation where flow and transport are largely controlled by highly connected flowpaths

inversion of time lapse electrical resistivity imaging data for monitoring infiltration monitoring hydrologic processes in the vadose zone is of great importance in developing management strategies for groundwater resources flow and transport behavior in variably saturated media control the rate at which and path along which fluids or contaminants reach groundwater aquifers electrical resistivity imaging eri is sensitive to changes in the subsurface conductivity structure which is highly dependent on the water saturation distribution in the vadose zone time lapse eri therefore has been widely used for environmental and hydrologic monitoring experiments

contact metamorphism in middle ordovician arc rocks sw sardinia italy new paleogeographic constraints abstract in the early cambrian bithia formation in the variscan foreland of sardinia a middle ordovician granitic intrusion 478 457 ma is hosted by marly metasedimentary rocks that were affected by high temperature ht metamorphism a detailed structural petrographical transect was conducted through the granitic intrusion and its host rocks field data and relationships between ht low pressure lp mineral assemblages in the metasedimentary rocks grt wo ves in carbonate lenses and and in pelite demonstrate that the study area was affected by a polyphase ht overprint i t 520 620 c at xco2 0 1 p 0 2 0 4 gpa and ii t 600 670 c at xco2 0 1 p 0 2 0 4 gpa that pre dates the variscan tectonic metamorphic and igneous phases in the canigo or canigou massif eastern pyrenees the somail massif montagne noire and the ruitor massif internal massifs nw alps middle ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by variscan amphibolite to granulite facies metamorphism and subsequent alpine low grade metamorphism comparisons of associated igneous and metasedimentary rocks in the sardinia foreland with the high grade metamorphic complex in the variscan axial zone and the canigou massif indicate a convergent middle ordovician evolution that was overprinted by ht variscan metamorphism

imaging and characterisation of subsurface solute transport using electrical resistivity tomography ert and equivalent transport models abstract we assess the usefulness of electrical resistivity tomography ert in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers a field tracer experiment was conducted at the krauthausen test site germany the spatial and temporal evolution of the injected nabr solute plume was monitored in a 2d ert image plane located downstream of the injection well for 90 days since ert maps changes in bulk electrical conductivity the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation the concentration maps are then analysed using an equivalent convection dispersion model cdm which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity as demonstrated by associated synthetic model studies ert resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm since for heterogeneous media local concentrations within the plume deviate from those predicted by the equivalent cdm we also interpret the ert derived pixel breakthrough curves in terms of an equivalent stream tube model stm the stm represents transport in the aquifer by a set of 1d convection dispersion processes allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified we believe that the observed tracer plume is satisfactorily described by the equivalent cdm probably because the tracer plume was small relative to the heterogeneity scale of the aquifer even though application of the stm revealed some deviation from the ideal homogeneous case the equivalent dispersivity in the stm matches the longitudinal dispersivity of the cdm closely consistent with predominantly homogeneous mixing however the stm analysis illustrates how ert results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers

a multi method field experiment to determine local groundwater flow in a glacier forefield we implemented multiple independent field techniques to determine the direction and velocity of groundwater flow at a specific stream reach in a glacier forefield time lapse experiments were conducted using two electrical resistivity tomography ert lines installed in a cross pattern a circular array of groundwater tubes was also installed to monitor groundwater flow via discrete salt injections both inter borehole and ert results confirmed this stream section as a losing reach and enabled quantification of the flow direction both techniques yielded advection velocities varying between 5 7 and 21 8 m day estimates of groundwater flow direction and velocity indicated that groundwater infiltrates from the stream nearby and not from the adjacent lateral moraine groundwater age estimated from radon concentration measurements supported this hypothesis despite uncertainties inherent to each of the methods deployed the combination of multiple field techniques allowed drawing consistent conclusions about local groundwater flow we thus regard our multi method approach as a reliable way to characterize the two dimensional groundwater flow at sites where more invasive groundwater investigation techniques are difficult to carry out and local heterogeneities can make single measurements unreliable copyright 2014 john wiley sons ltd

fluid migration in the vadose zone from 3 d inversion of resistivity monitoring data the movement of a small plume of fresh water through the vadose zone was monitored using surface resistivity tomography and pole pole potential measurements sets of potential measurements on a square grid at several times throughout the experiment show gradual progressive and systematic development of low resistivity zones that are inferred to be loci of fluid concentration a procedure for inverting percentage changes in potentials is developed here and used to map maximum potential changes of 13 into resistivity decreases of up to 40 through 3 d resistivity inversion the resulting patterns are complex with both resistivity increases and decreases needed to match the observed data the resistivity reductions show a spatial connection to the plume s source and are suggestive of the fluid migration resistivity increases generally appear to form rims surrounding the decreases and may be artifacts of the inversion process however some isolated increases in the surface layer may be caused by evapora

a tracer test in a shallow heterogeneous aquifer monitored via time lapse surface electrical resistivity tomography we illustrate a case study of a saline tracer test in a shallow highly heterogeneous aquifer monitored by means of surface time lapse ert the test was aimed at identifying the system s hydraulic properties some of the expected limitations of the method particularly caused by the strong decrease in ert resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location the vast heterogeneity of the system ranging in lithology from clay to gravel at a scale of meters to tens of meters reflects itself in the tracer migration and distribution over time the tracer is trapped in the low permeability regions and from these it is slowly released over time high resolution surface ert proves effective at picturing this system behavior over time the extreme heterogeneity is also a challenge in the attempt to translate bulk electrical

hydrogeophysical data integration at larger scales application of bayesian sequential simulation for the characterization of heterogeneous alluvial aquifers knowledge of the spatial distribution of hydraulic conductivity k within an aquifer is critical for reliable predictions of solute transport and the development of effective groundwater management and or remediation strategies while core analyses and hydraulic logging can provide highly detailed information such information is inherently localized around boreholes that tend to be sparsely distributed throughout the aquifer volume conversely larger scale hydraulic experiments like pumping and tracer tests provide relatively low resolution estimates of k in the investigated subsurface region as a result traditional hydrogeological measurement techniques contain a gap in terms of spatial resolution and coverage and they are often alone inadequate for characterizing heterogeneous aquifers geophysical methods have the potential to bridge this gap the recent increased interest in the application of geophysical methods to hydrogeological problems is clearly evidenced by the formation and rapid growth

time lapse mise a la masse measurements and modelling for tracer test monitoring in a shallow aquifer abstract the main goal of this study is to evaluate the reliability of the mise a la masse malm technique associated with saline tracer tests for the characterization of groundwater flow direction and velocity the experimental site is located in the upper part of the alento river alluvial plain campania region southern italy in this paper we present the hydrogeological setting the experimental setup and the relevant field results subsequently we compare those data against the simulated results obtained with a 3d resistivity model of the test area coupled with a model describing the advection dispersion equation for continuous tracer injection in particular we calculate a series of 3d forward solutions starting from a reference model all derived from electrical tomography results but taking into consideration different values of mean flow velocity and directions each electrical resistivity 3d model is used to produce synthetic voltage maps for malm surveys finally the synthetic malm voltage maps are compared with the ones measured in the field in order to assess the information content of the malm dataset with respect to the groundwater field characteristics the results demonstrate that the information content of the malm data is sufficient to define important characteristics of the aquifer geometry and properties this work shows how a combination of three dimensional time lapse modeling of flow tracer transport and electrical current can substantially contribute towards a quantitative interpretation of malm measurements during a saline tracer test this approach can thus revive the use of malm as a practical low cost field technique for tracer test monitoring and aquifer hydrodynamic characterization

time lapse imaging of saline tracer transport in fractured rock using difference attenuation radar tomography 1 accurate characterization of fractured rock aquifer heterogeneity remains one of the most challenging and important problems in groundwater hydrology we demonstrate a promising strategy to identify preferential flow paths in fractured rock using a combination of geophysical monitoring and conventional hydrogeologic tests cross well differenceattenuation ground penetrating radar was used to monitor saline tracer migration in an experiment at the u s geological survey fractured rock hydrology research site in grafton county new hampshire radar data sets were collected every 10 min in three adjoining planes for 5 hours during each of 12 tracer tests an innovative inversion method accounts for data acquisition times and temporal changes in attenuation during data collection the inverse algorithm minimizes a combination of two functions the first is the sum of weighted squared data residuals second is a measure of solution complexity based on an a priori space time covariance function subject to constraints that limit radarattenuation changes to regions of the tomograms traversed by high difference attenuation ray paths the time series of tomograms indicate relative tracer concentrations and tracer arrival times in the image planes from these we infer the presence and location of a preferential flow path within a previously identified zone of transmissive fractures these results provide new insights into solute channeling and the nature of aquifer heterogeneity at the site index terms 0910 exploration geophysics data processing 0915 exploration geophysics downhole methods 1829 hydrology groundwater hydrology 1832 hydrology groundwater transport 1894 hydrology instruments and techniques keywords radar tomography fractured rock ground penetrating radar geophysics hydrogeophysics

fully coupled hydrogeophysical inversion of synthetic salt tracer experiments 1 we present a method for the determination of hydraulic conductivity from monitoring of salt tracer tests by electrical resistivity tomography ert to ensure that the underlying principles of flow transport and geoelectrics are obeyed in the inversion we perform a fully coupled hydrogeophysical analysis using temporal moments of electrical potential perturbations in the predictive mode we use moment generating equations with corresponding adjoint equations for the evaluation of sensitivities for inversion we apply the quasi linear geostatistical inversion approach the method is tested in a synthetic case study mimicking a laboratory scale quasi two dimensional sandbox in which 48 electrodes and 8 piezometers are used the hydraulic conductivity field is estimated from the mean arrival times of electrical potential perturbations and hydraulic heads the estimated hydraulic conductivity field reproduces most features with however a loss of variability even though only the temporal moments of the electrical signals are used for inversion the transient behavior is satisfactorily recovered also the spatial patterns of concentration arrival times in the true and estimated cases are in good agreement so that the propagation of the tracer plume can be followed fairly accurately we test the effects of large measurement errors and erroneous prior information on the performance of the inversion while prior statistical parameters are of minor importance in detecting the major pattern of hydraulic conductivity a large measurement error could have an important impact on the solution also the choice of electrode configurations appears to be important in particular strictly surface based geoelectrical surveys do not seem to be very suitable for identifying spatial patterns of hydraulic conductivity by ert monitoring of salt tracer tests within aquifers

cross borehole resistivity tomography electrical resistivity tomography ert is a method for determining the electrical resistivity distribution in a volume from discrete measurements of current and voltage made within the volume or on its surface we have developed an ert algorithm that is an iterative modified least squares inversion based on a finite element forward solution of laplace s equation we report the results of tests on this algorithm designed to determine how resistance measurements made from two boreholes may be used to image the resistivity distribution between them a number of simple but geophysically significant structures are modeled these include a single isolated block anomaly two layers a thin isolated continuous layer and a vertical band the main features of most resistivity models were identifiable in the reconstructions limited data accuracy and noise were simulated and found to cause a deterioration of the image however even with measurements of only one significant figure accuracy the algorithm converg

hydrologic connectivity as a framework for understanding biogeochemical flux through watersheds and along fluvial networks abstract hydrologic connections can link hillslopes to channel networks streams to lakes subsurface to surface land to atmosphere terrestrial to aquatic and upstream to downstream these connections can develop across vertical lateral and longitudinal dimensions and span spatial and temporal scales each of these dimensions and scales are interconnected creating a mosaic of nested hydrologic connections and associated processes in turn these interacting and nested processes influence the transport cycling and transformation of organic material and inorganic nutrients through watersheds and along fluvial networks although hydrologic connections span dimensions and spatiotemporal scales relationships between connectivity and carbon and nutrient dynamics are rarely evaluated within this framework the purpose of this paper is to provide a cross disciplinary view of hydrologic connectivity highlighting the various forms of hydrologic connectivity that control fluxes of organic material and nutrients and to help stimulate integration across scales and dimensions and collaboration among disciplines

field study of dispersion in a heterogeneous aquifer 1 overview and site description results are presented for a large scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near columbus mississippi the study was initiated with a 48 hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers pentaflourobenzoic acid o trifluoromethylbenzoic acid and 2 6 diflourobenzoic acid over a 20 month period seven comprehensive samplings of the tracer plume were performed at approximately 1 to 4 month intervals using an extensive three dimensional sampling well network the dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction this asymmetry was produced by accelerating groundwater flow along the plume travel path that in turn resulted from an approximate 2 order of magnitude increase in the mean hydraulic conductivity between the near field and far field regions of the site the columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer the large scale spatial variations in groundwater velocity and the extensive set of hydraulic conductivity measurements for the aquifer

imaging and characterization of solute transport during two tracer tests in a shallow aquifer using electrical resistivity tomography and multilevel groundwater samplers 1 the relevance of aquifer heterogeneity for flow and transport is recognized broadly however its characterization is hampered by the inaccessibility of the subsurface time lapse electrical resistivity tomography ert offers the possibility of imaging noninvasively subsurface transport we present results of two tracer tests that were carried out successively in a shallow aquifer at the krauthausen test site germany the breakthroughs of an electrically conductive and a resistive tracer were monitored with ert and local multilevel groundwater samplers mls along two cross sections perpendicular to the mean flow direction sinking of the conductive salt tracer due to density effects was observed with ert we applied a stream tube model to characterize the spatially variable transport ert derived stream tube parameters showed similar patterns for the two tracer experiments reflecting the effect of aquifer heterogeneity on transport mls data did not show similar spatial patterns which indicates that these measurements may be prone to subtle changes of the flow field in the small sampling volume and mixing within screened wells between 50 and 10 of the tracer was recovered in the ert derived breakthrough curves compared with transport simulations in a homogeneous aquifer ert derived time integrated changes in electrical conductivity were locally larger but focused in a smaller area mls data indicated that in this area ert did not underestimate the tracer recovery the relatively low tracer recovery was attributed to undetected tracer breakthrough in regions with low ert sensitivity and in regions where the length of the tracer plume and the electrical conductivity contrast were small

regional scale integration of multiresolution hydrological and geophysical data using a two step bayesian sequential simulation approach significant progress has been made with regard to the quantitative integration of geophysical and hydrological data at the local scale for the purpose of improving predictions of groundwater flow and solute transport however extending corresponding approaches to the regional scale still represents one of the major challenges in the domain of hydrogeophysics to address this problem we have developed a regional scale data integration methodology based on a two step bayesian sequential simulation approach our objective is to generate high resolution stochastic realizations of the regional scale hydraulic conductivity field in the common case where there exist spatially exhaustive but poorly resolved measurements of a related geophysical parameter as well as highly resolved but spatially sparse collocated measurements of this geophysical parameter and the hydraulic conductivity to integrate this multi scale multi parameter database we first link the low and high resolution geophysical data via a stochastic downscaling procedure this is followed by relating the downscaled geophysical data to the high resolution hydraulic conductivity distribution after outlining the general methodology of the approach we demonstrate its application to a realistic synthetic example where we consider as data high resolution measurements of the hydraulic and electrical conductivities at a small number of borehole locations as well as spatially exhaustive low resolution estimates of the electrical conductivity obtained from surface based electrical resistivity tomography the different stochastic realizations of the hydraulic conductivity field obtained using our procedure are validated by comparing their solute transport behaviour with that of the underlying true hydraulic conductivity field we find that even in the presence of strong subsurface heterogeneity our proposed procedure allows for the generation of faithful representations of the regional scale hydraulic conductivity structure and reliable predictions of solute transport over long regional scale distances

estimation of the hydraulic parameters of unsaturated samples by electrical resistivity tomography in situ and laboratory experiments have shown that electrical resistivity tomography ert is an effective tool to image transient phenomena in soils however its application in quantifying soil h