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Limited knowledge of local groundwater systems often results in the failure of boreholes to yield water of the required quantity and quality. This is particularly problematic in the developing world, where financial resources are often limited, and failed wells represent a significant financial burden. To enhance understanding of local hydrological systems, noninvasive geophysical methods can aid the understanding of hydrogeological structures and identification of groundwater sources needed to optimize siting of wells. Here, we highlight the utility of a relatively new towed-transient electromagnetic system, called tTEM. This system is a rapidly deployable mobile geophysical method well-suited to cost-efficient characterization of local-to-regional groundwater systems. Results from tTEM surveys conducted in two refugee camps and several host communities in western Tanzania demonstrate the capability of the method to characterize shallow aquifer systems with high lateral and vertical resolution, with data collection typically exceeding 15 to 20 line-kilometers (km) per day. This work focuses on tTEM's ability to provide semiquantitative insights into regional hydrogeological settings when supporting data required for more rigorous interpretation/modeling is lacking. The system provided useful data within communities with low density of electrification and near buildings with metal roofs and walls. tTEM-derived resistivity profiles were correlated with limited local borehole lithologic information to develop conceptual models of the local groundwater systems. These models were used to successfully guide the siting of a production well and to identify future drilling targets in the refugee camps and surrounding communities.

Surface nuclear magnetic resonance is a geophysical technique providing non-invasive aquifer characterization. Two approaches are commonly used to invert surface nuclear magnetic resonance data: (1) inversions involving many depth layers of fixed thickness and (2) few-layer inversions without predetermined layer thicknesses. The advantage of the many-layer approach is that it requires little a priori knowledge. However, the many-layer inversion is extremely ill-posed and regularisation must be used to produce a reliable result. For optimal performance, the selected regularisation scheme must reflect all available a priori information. The standard regularisation scheme for many-layer surface nuclear magnetic resonance inversions employs an L2 smoothness stabiliser, which results in subsurface models with smoothly varying parameters. Such a stabiliser struggles to reproduce sharp contrasts in subsurface properties, like those present in a layered subsurface (a common near-surface hydrogeological environment). To investigate if alternative stabilisers can be used to improve the performance of the many-layer inversion in layered environments, the performance of the standard smoothness stabiliser is compared against two alternative stabilisers: (1) a stabiliser employing the L1-norm and (2) a minimum gradient support stabiliser. Synthetic results are presented to compare the performance of the many-layer inversion for different stabiliser functions. The minimum gradient support stabiliser is observed to improve the performance of the many-layer inversion for a layered subsurface, being able to reproduce both smooth and sharp vertical variations of the model parameters. Implementation of the alternative stabilisers into existing surface nuclear magnetic resonance inversion software is straightforward and requires little modification to existing codes.

During the last century, many large irrigation projects were carried out in arid lands worldwide. Despite a tremendous increase in food production, a common problem when characterizing these zones is land degradation in the form of waterlogging. A clear example of this phenomenon is in the Nubariya depression in the Western Desert of Egypt. Following the reclamation of desert lands for agricultural production, an artificial brackish and contaminated pond started to develop in the late 1990s, which at present extends for about 2.5 km2. The available data provide evidence of a simultaneous general deterioration of the groundwater system. An extensive hydrogeophysical investigation was carried out in this challenging environment using magnetic resonance sounding (MRS) and ground-based time-domain electromagnetic (TEM) techniques with the following main objectives: (1) understanding the hydrological evolution of the area; (2) characterizing the hydrogeological setting; and (3) developing scenarios for artificial aquifer remediation and recharge. The integrated interpretation of the geophysical surveys provided a hydrogeological picture of the upper 100 m sedimentary setting in terms of both lithological distribution and groundwater quality. The information is then used to set up (1) a regional groundwater flow and (2) a local density-dependent flow and transport numerical model to reproduce the evolution of the aquifer system and develop a few scenarios for artificial aquifer recharge using the treated water provided by a nearby wastewater treatment plant. The research outcomes point to the hydrological challenges that emerge for the effective management of water resources in reclaimed desert areas, and they highlight the effectiveness of using advanced geophysical and modeling methodologies.

Geophysical methods are routinely applied for investigation of near surface in areas of infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. A new towed transient electromagnetic tool, called ‘tTEM’, provides rapid, efficient, high‐resolution imaging of subsurface hydrogeology, and can deliver densely spaced profiles of resistivity. These profiles can be used to map a three‐dimensional subsurface in high resolution. In this paper, we present three case studies where the towed transient electromagnetic system was used to map the subsurface at the hectare scale. In the first case, we used towed transient electromagnetic to map raw materials in the northern Jutland, Denmark. The survey was carried out to map possible sand and gravel deposits. In the towed transient electromagnetic models, the potential sand/gravel targets are identified as high resistive bodies in the top 30 m. These bodies can have significant lateral variation at scales much less than one hundred metres. In the second case study, towed transient electromagnetic was used to map the thickness of a protecting clay layer above an aquifer in Vildbjerg, a town in the central part of Denmark. Results show that the overlying clay layer has sufficient thickness (>15 m) to protect the underlying aquifer from pesticide pollution in the area. Finally, in the third case study, we used towed transient electromagnetic for mapping geology in the vicinity of a landfill. The inversion results reveal a hitherto unknown buried valley‐like feature within the top 30 m of the subsurface that was not identified by older, regional TEM surveys – a feature that can have significant impact on the water flow.

Paleochannels typically act as pathways for groundwater movement and provide a potential source of groundwater. Their presence can be helpful in identifying areas suitable for recharge and at times in mitigating contamination problems in afflicted regions. Thus, mapping of paleochannels is significant in the planning and management of groundwater resources. An airborne electromagnetic (AEM) system employing dual pulse moments has been used extensively for this purpose in India. This paper presents the results over paleochannels defined in three different terranes. In northwest India, a 100 m wide by 80 m deep paleochannel within alluvium overlaying a Proterozoic basement illustrates the impact of neotectonic disturbances in changing the river course. In northeast India's Ganga Plains, a paleochannel is mapped that provides insight into managing groundwater resources of areas polluted with arsenic. In south India, a paleochannel buried under similar to 100 m thick sequence of coastal sediments is imaged with implications on submarine groundwater discharge.

The hydrogeology below large surface water features such as rivers and estuaries is universally under-informed at the long reach to basin scales (tens of km+). This challenge inhibits the accurate modeling of fresh/saline groundwater interfaces and groundwater/surface water exchange patterns at management-relevant spatial extents. Here we introduce a towed, floating transient electromagnetic (TEM) system (i.e. FloaTEM) for rapid (up to 15 km/h) high resolution electrical mapping of the subsurface below large water bodies to depths often a factor of 10 greater than other towed instruments. The novel FloaTEM system is demonstrated at a range of diverse 4th through 6th-order riverine settings across the United States including 1) the Farmington River, near Hartford, Connecticut; 2) the Upper Delaware River near Barryville, New York; 3) the Tallahatchie River near Shellmound, Mississippi; and, 4) the Eel River estuary, on Cape Cod, near Falmouth, Massachusetts. Airborne frequency-domain electromagnetic and land-based towed TEM data are also compared at the Tallahatchie River site, and streambed geologic scenarios are explored with forward modeling. A range of geologic structures and pore water salinity interfaces were identified. Process-based interpretation of the case study data indicated FloaTEM can resolve varied sediment-water interface materials, such as the accumulation of fines at the bottom of a reservoir and permeable sand/gravel riverbed sediments that focus groundwater discharge. Bedrock layers were mapped at several sites, and aquifer confining units were defined at comparable resolution to airborne methods. Terrestrial fresh groundwater discharge with flowpaths extending hundreds of meters from shore was also imaged below the Eel River estuary, improving on previous hydrogeological characterizations of that nutrient-rich coastal exchange zone. In summary, the novel FloaTEM system fills a critical gap in our ability to characterize the hydrogeology below surface water features and will support more accurate prediction of groundwater/surface water exchange dynamics and fresh-saline groundwater interfaces.

Soil contamination from industrial activities is a large problem in urban areas worldwide. Understanding the spreading of contamination to underlying aquifers is crucial to make adequate risk assessments and for designing remediation actions. A large part of the northern hemisphere has quaternary deposits consisting of glacial clayey till. The till often has a complex hydrogeological structure consisting of networks of fractures, sand stringers and sand lenses that each contribute to a transport network for water, free phase and dissolved contaminants. Thus, to determine the possible flow-paths of contaminants, the geology must be described in great detail. Normally, multiple boreholes would be drilled in order to describe the geology, but boreholes alone do not provide the needed resolution to map such sand lenses and their connectivity. Cross-borehole full-decay time-domain induced polarization (TDIP) is a new tool that allows for quantitatively mapping not only contrasts in bulk resistivity, but also contrasts in spectral IP parameters. We present a feasibility study with synthetic tests and a field application on a clayey moraine environment with embedded sand lenses, with hitherto unseen ground-truth verification. Indeed, the investigated area was above the water table, which allowed for digging out the entire area after the investigation for an unprecedented description of the lens interconnectivity. The TDIP data were acquired with a full-waveform acquisition at high sampling rate, signal-processed by harmonic denoising, background removal, and de-spiking, and subsequently the full-waveform data were stacked in log-increasing tapered gates (with 7 gates per decade). The resulting TDIP decays, with usable time-gates as early as two milliseconds, were inverted in terms of a re-parameterization of the Cole-Cole model. The inverted models of the field data show a remarkable delineation of the sand lenses/layers at the site, with structure in both the resistivity and the IP parameters matching the results from the ground-truthing. The synthetic examples show that in models both below and above the groundwater table, sand-lenses with thicknesses comparable to the vertical electrode spacing can be well resolved. This suggests that full-decay cross-borehole TDIP is an ideal tool for high-resolution sand-lens imaging.

In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady sources for groundwater due to inadequate understanding of the fracture networks. A comprehensive knowledge of fracture distribution at the regional scale is necessary to delineate sustainable aquifers and manage them efficiently. The resistivity maps derived from the airborne electromagnetic (AEM) survey over the Ankasandra watershed in Karnataka, India, reveal sharp and deep zones of low formation resistivity, which indicate groundwater-bearing zones. It is found that some of these zones are hydrogeologically connected through fracture networks resulting in augmented yield. AEM results in combination with an in-depth understanding of the geological structures successfully map these groundwater-saturated fracture networks (or hydrogeological lineaments) that we term as ‘Hydrolins’. As groundwater occurrence is generally associated with lineaments, we analyzed the drilling and geophysical logs from 21 wells within a 380 sq.km area to study the relationships of various lineaments with ‘Hydrolins’, particularly in respect of their groundwater potential. AEM results, though calibrated and correlated with a limited number of well data, revealed a threshold groundwater horizon (TGWH), found to be at 80 m depth for Ankasandra watershed, beyond which a strong correlation exists between the depth of a well and its yield. While the TGWH may differ for different watersheds, the approach presented here can be readily adopted to map sustainable groundwater sources in hardrocks worldwide.

In the present study, we show how persistent management and collection of hydrological and geophysical data at a national scale can be combined with innovative analysis methods to generate decision support tools for groundwater and surface water managers. This is exemplified by setting up a regional scale groundwater model in an area with geophysical data of varying age, type, and quality. The structure for the regional model is derived from a newly developed resistivity clay-fraction cluster analysis. This modelling strategy can be used in combination with local detailed geological modelling thus utilizing the detailed expertise locally, while securing a cost-effective (price vs. performance) solution to the numerical simulations of the regional scale water balance. In this way we avoid unwanted boundary effects on the local model simulations due to the presence of artificial numerical boundaries located proximate to the areas of interest. In this application, it is particularly important that boundary conditions are remote, due to the presence of a dense network of buried valley structures. Simulated impacts of groundwater abstraction from two existing well-fields spread through the valley system far beyond the local focus areas of the study.

Summary The top 0–50 m of the subsurface are critical for infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. Yet, the tools for imaging this zone are limited to boreholes or geophysical methods such as electrical resistivity imagining (ERT) with a limited applicability for area-covering surveys larger than a few hectares. We present a new highly efficient towed transient electromagnetic system, tTEM, which bridges the gap in coverage and resolution between point or line measurements and airborne electromagnetics. The system yields images in full 3D with a lateral resolution down to 10 × 10 m. The system is towed by and All-Terrain Vehicle, uses a 2 × 4 m2 transmitter coil and has a z-component receiver 9 m offset from the transmitter. The first bias-free gate is as early as 4 µs from ramp beginning (1.4 µs after ramp-end). The turn-off time is 2.6 µs. Data are processed and inverted using methods directly adopted from airborne electromagnetics. We discuss the system design and present a case study where the system has been used to map complex subsurface geology in large detail to build 3D geological/hydrogeological models.

In this study we present the prediction of hydraulic permeability (k) from time-domain spectral induced polarization (IP) data, measured in boreholes and along 2D surface profiles. The borehole data were collected with the El-log technique, which provides undisturbed “while drilling” measurements of the direct current (DC) resistivity, time-domain IP data and gamma radiation. Data were measured on unconsolidated formations at a landfill site in South Denmark, in three boreholes and along 16 2D profiles. DC and full-decay IP data were inverted in terms of the BIC re-parameterization of the Cole-Cole model, which present smaller parameter correlations and disentangles bulk and surface conduction. Permeability values were computed from bulk conductivity and the maximum imaginary conductivity, using the empirically-derived formulae presented in a recent study without any calibration. The IP-derived k estimates were compared to those estimated using grain size analysis and slug tests, for a total of 157 comparisons. A good correlation, on average within one decade, was found between the k estimates over four orders of magnitude, with similar depth-trends. In conclusion, IP can be reliably used for estimating hydraulic permeability on unconsolidated formations at the field scale, using the relations found in the laboratory without any further calibration.

Surface nuclear magnetic resonance (NMR) data are sensitive to key hydrogeological parameters including water content and pore size. The measured data are modelled as a complex sinusoidal exponential decay where the phase is a function of the physics of the experiment and instrumental factors, parameters that are difficult to decouple. When inverting surface NMR data, practitioners typically account for these phases either by considering only the amplitudes of the complex signals, thus eliminating the influence of the phase, or by iteratively rotating the complex data during inversion so the data phase matches the theoretical phase generated during forward modelling. Each of these approaches assumes the user has an accurate forward model; if not, the data will be incorrectly rotated and forced to fit an erroneous forward model. Additionally, this rotation will artificially reduce the total data misfit, thus masking the effect of the erroneous forward model. We demonstrate the pitfalls of using inverse methods that correct for the phase by inverting synthetic data with three types of deliberate modelling errors that may occur during a surface NMR experiment: errors in the offset between the Larmor and transmit frequencies, errors in the subsurface resistivity model, and errors in the relative positioning of a separated transmitter-receiver coil pair. The inverted water content profiles show that the modelling errors can introduce inversion artifacts. However, the amplitude inversions and complex inversions with iterative phase correction frequently produce χ2 misfit values close to unity, showing that these inverse methods will fail to “raise the alarm” when an incorrect forward model is implemented.

Surface nuclear magnetic resonance is a technique capable of providing insight into subsurface aquifer properties. To produce estimates of aquifer properties (such as the spatial distribution of water content and parameters controlling the duration of the nuclear magnetic resonance signal), an inversion is required. Essential to the reliable interpretation of the estimated subsurface models is an understanding of the uncertainty and correlation between the parameters in the estimated models. To quantify parameter uncertainty and correlation in the surface nuclear magnetic resonance inversion, a Markov chain Monte Carlo approach is demonstrated. Markov chain Monte Carlo approaches have been previously employed to invert surface nuclear magnetic resonance data, but the primary focus has been on quantifying parameter uncertainty. The focus of this paper is to further investigate whether the parameters in the estimated models exhibit correlation with one another; equally important to building a reliable interpretation of the subsurface is an understanding of the parameter uncertainty. The utility of the Markov chain Monte Carlo approach is demonstrated through the investigation of three questions. The first question investigates whether the parameters describing the water content and thickness of a layer exhibit a strong correlation. This question stems from applying concepts known to electromagnetic surveys (that the layer thickness and layer resistivity parameters are strongly correlated) to the surface nuclear magnetic resonance inversion. A water content–layer thickness correlation in surface nuclear magnetic resonance would not have large effects for quantifying total water content but would affect the ability to identify layer boundaries. The second question examines whether the parameter controlling the duration of the nuclear magnetic resonance signal exhibits a correlation with the water content and layer thickness parameters. The resolution of surface nuclear magnetic resonance typically does not consider the duration of the signal and focuses primarily on the distribution of current amplitudes that form the suite of transmit pulses. It is common to treat regions with short-duration signal with greater uncertainty, but it is important to understand whether the signal duration controls resolution for medium to long duration signals as well. The third question explores if the parameter uncertainty produced by the Markov chain Monte Carlo approach is consistent with that produced by an alternative approach based upon the posterior covariance matrix (for the linearised inversion). The ability of the Markov chain Monte Carlo approach to more thoroughly explore the model space provides a means to improve the reliability of surface nuclear magnetic resonance aquifer characterisations by quantifying parameter uncertainty and correlation.

The increased application of airborne electromagnetic surveys to hydrogeological studies is driving a demand for data that can consistently be inverted for accurate subsurface resistivity structure from the near surface to depths of several hundred metres. We present an evaluation of three commercial airborne electromagnetic systems over two test blocks in western Nebraska, USA. The selected test blocks are representative of shallow and deep alluvial aquifer systems with low groundwater salinity and an electrically conductive base of aquifer. The aquifer units show significant lithologic heterogeneity and include both modern and ancient river systems. We compared the various data sets to one another and inverted resistivity models to borehole lithology and to ground geophysical models. We find distinct differences among the airborne electromagnetic systems as regards the spatial resolution of models, the depth of investigation, and the ability to recover near-surface resistivity variations. We further identify systematic biases in some data sets, which we attribute to incomplete or inexact calibration or compensation procedures.

Summary Knowledge of the sensitivity function of an electromagnetic (EM) system helps to assess the advantages and shortcomings of the system set-up. We have computed 3D sensitivity functions of three different EM systems – one airborne (SkyTEM) and two ground-based systems (tTEM and GCM) – in order to compare the sensitivity distribution of the different transmitter-receiver set-ups. The results show that the GCM has the highest sensitivity to near-surface structures, but also has a very limited footprint depth. The SkyTEM and tTEM systems have comparable sensitivities vertically in the ground with a 90% footprint depth around 5 m for early time measurements. However, the sensitivity of the SkyTEM system is distributed must wider horizontally than the tTEM.

Summary The Mekong Delta in southern Vietnam is struggling with intensive use of the groundwater due to intensive irrigation of multicycle rice cultivation and aquaculture in the coastal zone. This has led to groundwater overexploitation and salt water intrusion of the fresh groundwater resources from the coast inwards. However, the picture in this enormous delta is much more complex with many of the groundwater resources still containing residual saline groundwater from previous transgressions. Furthermore, the lowered water table has led to high subsidence rates since the early 1990’s. At present, the condition of the Mekong Delta groundwater system is poorly understood, mainly described by boreholes explaining about geology and salinity, with only few geophysical surveys to connect the geological observations. In this abstract we present the first results from a groundbased TEM survey. We have mapped a large, very conductive area, with depth of investigation reaching down to -200 m. The results can feed into geological and hydrogeological models of the area and provide a better understanding of the connection between groundwater resources.

Time-domain electromagnetic data are conveniently inverted by using smoothly varying 1D models with fixed vertical discretization. The vertical smoothness of the obtained models stems from the application of Occam-type regularization constraints, which are meant to address the ill-posedness of the problem. An important side effect of such regularization, however, is that horizontal layer boundaries can no longer be accurately reproduced as the model is required to be smooth. This issue can be overcome by inverting for fewer layers with variable thicknesses; nevertheless, to decide on a particular and constant number of layers for the parameterization of a large survey inversion can be equally problematic. Here, we present a focusing regularization technique to obtain the best of both methodologies. The new focusing approach allows for accurate reconstruction of resistivity distributions using a fixed vertical discretization while preserving the capability to reproduce horizontal boundaries. The formulation is flexible and can be coupled with traditional lateral/spatial smoothness constraints in order to resolve interfaces in stratified soils with no additional hypothesis about the number of layers. The method relies on minimizing the number of layers of non-vanishing resistivity gradient, instead of minimizing the norm of the model variation itself. This approach ensures that the results are consistent with the measured data while favouring, at the same time, the retrieval of horizontal abrupt changes. In addition, the focusing regularization can also be applied in the horizontal direction in order to promote the reconstruction of lateral boundaries such as faults. We present the theoretical framework of our regularization methodology and illustrate its capabilities by means of both synthetic and field data sets. We further demonstrate how the concept has been integrated in our existing spatially constrained inversion formalism and show its application to large-scale time-domain electromagnetic data inversions.

Summary The SkyTEM data provides a comprehensive three-dimensional picture of the subsurface. The low moment data ensured the high resolution mapping of the near surface and the high moment data to the deeper level. The dual moment AEM data provided high resolution mapping of subsurface from top to ∼ 250 m depths. The results revealed significant contrast between the B-G layers that has a resistivity range of 10–15 Ωm and generally has groundwater yields of 6–8 lps. It is also noted that the groundwater yield (> 6 lps) increases for deeper (

Time-domain induced polarisation and magnetic data were acquired to map and characterise the decommissioned and un-engineered municipal solid waste disposal site of the Kwame Nkrumah University of Science and Technology, located in the Kumasi Metropolis of Ghana. In this survey, 13 induced polarisation profiles 500–800 m long and 26 magnetic profiles 400–800 m long were acquired. In addition, two boreholes were drilled to help in the interpretation of the geophysical data. The study was carried out with the aim of determining the risk posed by the waste deposit to the quality of the soil and the ground water system, which is the main potable water supply for the Secondary School, the University Teaching Hospital and the Veterinary School, situated within the catchment area of the site. Full-decay 2-D time-domain induced polarisation inversions in terms of Cole–Cole parameters were used for interpreting the induced polarisation data. The chargeability, resistivity and normalised chargeability distributions, together with the magnetic results, aided in a full characterisation of the site geology, the waste and the associated pollution plume. In particular, clear contrasts in resistivity and the polarisation parameters were found between the saprolite layer and the granitic bedrock, which are the main lithological units of the area. Furthermore, it was found that the Kwame Nkrumah University of Science and Technology waste deposit is characterised by a low-chargeability and low-resistivity signature and that the low-resistivity area spreads out from the waste deposit into the permeable saprolite layer, indicating the presence of a leachate plume. A fracture zone in the granitic bedrock beneath the waste deposit, which is a potential conduit for leachate contamination of the ground water system, was also identified. The study thus provides the information needed for assessing the future impact of the waste on the water quality in the area and for designing risk-mitigation actions.

In the present study, we have used 3D ERT for detailed mapping of a polluted site at a challenging river site in the southern part of the Denmark. The survey layout involves several crossings of the meander of a river where a leachate plume from an old chemical factory site is expected to reach the river. Both resistivity and time domain induced polarization data were acquired, and full 3D inversion was performed for the resistivity data and 2D inversion of the IP data. Two sets of 3D data were acquired; one at a larger scale covering total area 410x90m2 and one at small scale covering 126x42m2, which enabled both details in the shallow part while covering a large area. 3D DC data were inverted using the inversion package BERT and 2D DC-IP data were inverted using AarhusInv. 3D DC inversion models show the presence of low resistivity anomaly at the northern bank of the stream interpreted as a contaminant plume. The contaminant plume is also seen in a deeper aquifer, which is separated by a low permeable clay layer to the upper aquifer. We could not discriminate the clay-lithology from the contamination using only resistivity model however, including the 2D inversions of the IP data we clearly separate the clay layer from the contaminated aquifer.

With the growing dependence on groundwater, escalating demand, and increasing depletion of aquifers, the Government of India has accorded special emphasis to management of groundwater resources through precise aquifer mapping. We have carried out hydrogeophysical surveys, including airborne electromagnetic (AEM), in six hydrogeologically divergent areas comprising sedimentary basins, basalts, weathered and fractured granite gneisses and schists, desert, and costal alluvium with the objective to establish suitability of AEM for a countrywide aquifer mapping in India. Efficacy of the dual-moment AEM surveys in mapping the shallow and deep aquifers is evaluated in conjunction with geologic, geophysical, and borehole data. It is found that the AEM surveys provide reliable images of the subsurface resistivity distribution defining the 3D geometrical and electrical attributes of aquifers in different areas. The surveys helped identify the suitable zones for managed artificial recharge (MAR), subsurface structures controlling the groundwater conditions, regional continuity of principal aquifers, palaeoriver channels, variations in lithologic character of aquifers, and the quality of water in terms of salinity, etc. In this paper, we present the efficacy of the aquifer-mapping approach illustrated by an example from an alluvium-covered hard-rock terrain (Dausa, Rajasthan), where a weathered-fractured aquifer system with compartmentalization under alluvium cover is mapped. The integrated data set, comprising AEM, aeromagnetic, ground geophysical, and borehole measurements, was utilized to derive lithologic and hydrogeologic maps useful in developing an aquifer-based groundwater-management plan.

For about three decades, airborne electromagnetic (AEM) systems have been used for groundwater exploration purposes. Airborne systems are appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralization of the water, electromagnetic systems are suitable for providing information about the aquifer structures and water quality, respectively. More helicopter than fixed-wing systems are used in airborne groundwater surveys. Helicopter-borne frequency-domain electromagnetic (HEM) systems use a towed rigid-boom. Helicopter-borne time-domain (HTEM) systems, which use a large transmitter loop and a small receiver within or above the transmitter, are generally designed for mineral exploration purposes but recent developments have made some of these systems usable for groundwater purposes as well. The quantity measured, the secondary magnetic field, depends on the subsurface conductivity distribution. Due to the skin-effect, the penetration depths of the AEM fields depend on the system characteristics used: high-frequency data/early-time channels describe the shallower parts of the conducting subsurface and the low-frequency data/late-time channels the deeper parts. Typical investigation depths range from some ten metres (conductive grounds) to several hundred metres (resistive grounds), where the HEM systems are appropriate for shallow to medium deep (about 1–100 m) and the HTEM systems for medium deep to deep (about 10–400 m) investigations. Generally, the secondary field values are inverted into resistivities and depths using homogeneous or layered half-space models. As the footprint of AEM systems is rather small, one-dimensional interpretation of AEM data is sufficient in most cases and single-site inversion procedures are widely used. Laterally constrained inversion of AEM data often improves the stability of the inversion models, particularly for noisy data. Higher dimensional inversion is still not possible for standard-size surveys. Based on typical field examples the advantages as well as the limitations of AEM surveys compared to long-established ground-based geophysical methods used in groundwater surveys are discussed. In a case history from a German island an airborne frequency-domain system is used to successfully locate freshwater lenses on top of saltwater. An example from Denmark shows how a time-domain system is used to locate large-scale buried structures forming ideal groundwater aquifers.

SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 × 40 m 2 system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 × 12.5 m 2 square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 µs; the high moment draws approximately 50 A, and has a turn-off time of about 80 µs. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset. In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.

The transient electromagnetic (TEM) method has been used extensively for hydrogeophysical exploration in Denmark for the past decade. Innovative instrumentation combined with multi-dimensional modelling and interpretational insights based on experience gained through numerous case studies have proven to be a successful strategy. In the case study reported here, the combination revealed an unknown and unexpected buried valley complex. Drill hole data were in good agreement with estimates of both the bearings and depths of valleys defined by the TEM surveys. The Pulled Array Transient Electromagnetic (PATEM) system was built to provide high data density for increased lateral resolution. A High moment Transient Electromagnetic (HiTEM) system was developed for delineation of aquifers to depths up to 300 m. Because both of these systems provide high data density, data quality can be assessed as part of the interpretational strategy. When acquiring TEM measurements in areas as densely populated as the Danish countryside, precautions must be taken to minimize coupling between the TEM system and man-made conductors. Modelling the slope of the flanks of buried valleys has challenged the adequacy of the one-dimensional (1-D) assumption for inversion of TEM data. The study shows that for a valley structure in a low-resistive layer, the 1-D assumption is sufficient to track the presence of rather steep slopes. For a valley structure in a high-resistive layer, however, the insensitivity of the TEM method to resistors makes it difficult to determine a slope with a 1-D inversion, and only the overall structure is defined.

Precision agriculture applications rely on highly detailed information on variations in soil properties. This demand has led to recent developments in EMI instrumentation, resulting in stable and high data quality. The reliability of the data enables real non-linear inversion providing true formation resistivity with depth. With the aim of mapping soil heterogeneity, an EMI survey has been carried out in the central part of Jutland, Denmark. The collected data were carefully processed prior to inversion in order to remove any couplings from man-made conductors such as buried electrical cables, which can cause errors in the lithological interpretation. The data were subsequently modelled with a 1D layered spatially constrained inversion which takes the entire EMI data geometry into account. The inversion results gives a detailed quasi tree-dimensional image of the survey area, revealing several small scale resistivity variations which corresponds well with existing geological knowledge.

One of the main limiting factors to the accuracy of large scale groundwater models is the scarcity of hydraulic data. High-resolution Airborne Electromagnetic Methods (AEM) are capable of mapping the electrical resistivity structure of the subsurface in great detail and covering large areas in short time and on a limited budget. As such, there is great potential in integrating AEM data in groundwater modeling as a supplementing source of an extensive amount of information. We have developed several novel techniques that in combination allows for bringing groundwater and AEM models much closer together, i.e.: (1) a novel, scalable inversion engine that allows the AEM inversion to handle arbitrarily large areas at a time; (2) the spatially-decoupled inversion approach, which decouples the inversion model from the acquisition points and can operate on the same grid/voxel cells as the groundwater model; (3) a custom regularization scheme that allows for producing geophysical models with sharp vertical/horizontal resistivity transitions. In this study we present the very first application of the sharp spatially-decoupled inversion on an AEM survey flown for improving the groundwater model in the Kasted area, in the north of Aarhus (Denmark).

Regional hydrological models are important tools in water resources management. Model prediction uncertainty is primarily due to structural (geological) non-uniqueness which makes sampling of the structural model space necessary to estimate prediction uncertainties. Geological structures and heterogeneity, which spatially scarce borehole lithology data may overlook, are well resolved in AEM surveys. This study presents a semi-automatic sequential hydrogeophysical inversion method for the integration of AEM and borehole data into regional groundwater models in sedimentary areas, where sand/clay distribution govern groundwater flow. The coupling between hydrological and geophysical parameters is managed using a translator function with spatially variable parameters followed by a 3D zonation. The translator function translates geophysical resistivities into clay fractions and is calibrated with observed lithological data. Principal components are computed for the translated clay fractions and geophysical resistivities. Zonation is carried out by k-means clustering on the principal components. The hydraulic parameters of the zones are determined in a hydrological model calibration using head and discharge observations. The method was applied to field data collected at a Danish field site. Our results show that a competitive hydrological model can be constructed from the AEM dataset using the automatic procedure outlined above.

We present a comparison between two different regularisation schemes for multi-layer inversion of the MRS data. The two regularizations consist of the norm (Occam’s inversion) and the support (focused inversion) of the variation of the water content and relaxation time. The focused regularization provides a sharp inversion as it minimizes the area (the support) where the variations of the model parameters occur, and not the (L2) norm of the variations. Clearly, when sharp vertical transitions of the inversion parameters are present, the focused inversion retrieves the interfaces with higher precision and reconstructs more effectively the physical property values as well. This is a quite common situation in hydrogeophysics, e.g. in correspondence with the water table and/or at the boundary between aquifers and aquitards.

We present a geophysical inversion algorithm working directly in a voxel grid disconnected from the actual measuring points, which allows for straightforward integration of different data types in joint inversion, for informing geological/hydrogeological models directly and for easier incorporation of prior information. Inversion of geophysical data usually refers to a model space being linked to the actual observation points. For airborne surveys the spatial discretization of the model space reflects the flight lines. Often airborne surveys are carried out in areas where other ground-based geophysical data are available. The model space of geophysical inversions is usually referred to the positions of the measurements, and ground-based model positions do not generally coincide with the airborne model positions. Consequently, a model space based on the measuring points is not well suited for jointly inverting airborne and ground-based geophysical data. Furthermore, geological and groundwater models most often refer to a regular voxel grid not correlated to the geophysical model space, and incorporating the geophysical data into the geological/hydrological modelling grids is problematic. We present a voxel grid inversion routine that overcomes these problems and we discuss in detail the algorithm implementation.

Many volcanic islands face freshwater stress and the situation may worsen with climate change and sea level rise. In this context, an optimum management of freshwater resources becomes crucial, but is often impeded by the lack of data. With the aim of investigating the hydrogeological settings of Southern San Cristóbal Island (Galapagos), we conducted an helicopter-borne, transient electromagnetic survey with the SkyTEM system. It provided unprecedented insights in the 3-D resistivity structure of this extinct basaltic shield. Combined with remote sensing and fieldwork, it allowed the definition of the first hydrogeological conceptual model of the island. Springs are fed by a series of perched aquifers overlying a regional basal aquifer subject to seawater intrusion. Dykes, evidenced by alignments of eruptive cones at the surface, correspond to sharp sub-vertical contrasts in resistivity in the subsurface, and impound groundwater in a summit channel. Combined with geomorphological observations, airborne electromagnetics is shown to be a useful tool for hydrogeological exploratory studies in complex, poorly known environments. It allows optimal development of land-based geophysical surveys and drilling campaigns.

In order to fulfil the requirements of the EU Water Framework Directive nitrate load from agricultural areas to surface water in Denmark needs to be reduced by about 40%. The regulations imposed until now have been uniform, i.e. the same restrictions for all areas independent of the subsurface conditions. Studies have shown that on a national basis about 2/3 of the nitrate leaching from the root zone is reduced naturally, through denitrification, in the subsurface before reaching the streams. Therefore, it is more cost-effective to identify robust areas, where nitrate leaching through the root zone is reduced in the saturated zone before reaching the streams, and vulnerable areas, where no subsurface reduction takes place, and then only impose regulations/restrictions on the vulnerable areas. Distributed hydrological models can make predictions at grid scale, i.e. at much smaller scale than the entire catchment. However, as distributed models often do not include local scale hydrogeological heterogeneities, they are typically not able to make accurate predictions at scales smaller than they are calibrated. We present a framework for assessing nitrate reduction in the subsurface and for assessing at which spatial scales modelling tools have predictive capabilities. A new instrument has been developed for airborne geophysical measurements, Mini-SkyTEM, dedicated to identifying geological structures and heterogeneities with horizontal and lateral resolutions of 30–50 m and 2 m, respectively, in the upper 30 m. The geological heterogeneity and uncertainty are further analysed by use of the geostatistical software TProGS by generating stochastic geological realisations that are soft conditioned against the geophysical data. Finally, the flow paths within the catchment are simulated by use of the MIKE SHE hydrological modelling system for each of the geological models generated by TProGS and the prediction uncertainty is characterised by the variance between the predictions of the different models.

Airborne electromagnetic (AEM) methods supply data over large areas in a cost-effective way. We used Artificial Neural Networks (ANN) to classify the geophysical signal into a meaningful geological parameter. By using examples of known relations between ground-based geophysical data (in this case electrical conductivity, EC, from electrical cone penetration tests) and geological parameters (presence of glacial till), we extracted learning rules that could be applied to map the presence of a glacial till using the EC profiles from the airborne EM data. The saline groundwater in the area was obscuring the EC signal from the till but by using ANN we were able to extract subtle and often non-linear, relations in EC that were representative of the presence of the till. The ANN results were interpreted as the probability of having till and showed a good agreement with drilling data. The glacial till is acting as a layer that inhibits groundwater flow, due to its high clay-content, and is therefore an important layer in hydrogeological modelling and for predicting the effects of climate change on groundwater quantity and quality.

The SkyTEM system is being developed for almost ten years in Denmark and has been specially designed for the groundwater mapping. Through the years more and more early gates are becoming interpretable, and for almost two years gates below 10 µs have been used generically. The electronics and processing developments end recently with the new Mini-SkyTEM system which provides a first gate at 5-6 µs. This new system can also be flown up to a speed of 140 km/h, allowing a same coverage at a lower cost compared to the larger systems. The first pilot survey of the new Mini-SkyTEM was flown in June 2011 inside the NiCA project where the purpose is to model the oxydo-reduction of the nitrates in the near-surface aquifers for pollution management. Despite its very low transmitter moment, the Mini-SkyTEM system provides a depth of investigation of 100 m considering a mean resistivity of 50 Ωm. But more importantly, a good correlation between the resistivity boundaries and the geological interfaces defined in the borehole logs is observed in the upper 20 m. This gives us confidence in the future use of this transient airborne EM system for the mapping of the very near-surface aquifers.

Airborne electromagnetics (AEM) is an efficient tool for mapping the subsurface. AEM delivers a very high data coverage compared to the costs, and the direct outputs are high-resolution resistivity images of the subsurface. In particular the time domain methods (TEM) are well suited for mapping of the salt-fresh water boundary in coastal zones, aquifers, paleo-channels, mineralisations and general structural geological mapping. The development of AEM systems and data processing systems has been intensive during the past 10– 15 years because of better electronics and faster and more flexible computers. While most AEM systems in the 1980s were limited to detection of mineralisations, modern system generations measures full spectra, yields absolutely calibrated data and accurate descriptions of the system transfer function. This, in combination with much enhanced inversion and forward algorithms, makes AEM a suitable candidate for mapping targets which so far have not be possible.

Accuracy and stability of data from airborne electromagnetic systems is an issue addressed in numerous publications and conference presentations. We have recently done a comprehensive and well documented comparison of data from ground based measurements and repeated SkyTEM lines from different altitudes. the ground based test line was made on the National Danish Test Site and consists in two perpendicular lines each of a length of 1.0 km. On each of these lines 25 soundings with a 40x40 m2 TEM system was made. the TEM system measures the first time gate in 8 micro sec from begin of ramp and was prior to the measurements calibrated at the test site. Comparing Inversion results form the SkyTEM and the groundbased data shows that the model repeatability of the SkyTEM system is excellent in both the same altitude and in different altitudes. Also, there are no directional problems, i.e. it does not matter in which direction the lines are flown. the agreement to the groundbased reference sections is also very good, showing that the SkyTEM-system yields data of same high quality as the groundbased system. A validation of the SkyTEM system at the data level was set up to examine the responses gate by gate, which, among other things, rules out the equivalent model issue. the different comparisons in the data space show that the SkyTEM system repeats the test lines equally good in different altitudes, and the agreement to the ground based responses is very good and in general within the data standard deviation of 5%. the validation serves as a quality assurance of the high-precision modeling of the SkyTEM system, together with the robust processing and Inversion scheme developed for the data. the validation of the SkyTEM system now set the standard for the data quality expected for any airborne system to qualify for the Danish national groundwater mapping effort.

In this abstract we show the results from a comparison of models from the SkyTEM-system with the ground based models at the extended test site. The comparison will also be carried out in data space by an upward continuation of the reference models to a SkyTEM-system response at nominal height. Furthermore, the repeatability of the SkyTEM-system at different heights is demonstrated. The national TEM test site was established to ensure that any TEM system used in the ongoing Danish groundwater mapping campaign can reproduce the reference models and match the upward continued responses of the reference model. The site was established in 2001. In 2009 the test site was extended from a point location with a known reference response and resistivity model to two crossing lines. This makes calibration and validation possible for airborne TEM-systems that cannot make hovering spot measurements. SkyTEM and VTEM have been on the new test lines so far. For the SkyTEM system we can conclude that it reproduces the ground-based TEM data and models very good and that SkyTEM data shows consistent results for different altitudes. In the data space we calculate an upward-continued dataset to perform the comparisons.

The Okavango Delta, Botswana, is one of the largest inland deltas on the planet and is unique in that surface water remains remarkably fresh despite evapotranspiration being the dominant water removal mechanism. To help understand the phenomena of salt sequestration, airborne transient electromagnetic TEM data with 2 km line spacing were acquired over the entire delta in 2007-2008. These data have been inverted using a quasi-2D least-squares linear optimization scheme to produce a 5-layer resistivity model to 250 m depth. Comparison of the model with available borehole data shows a strong correlation between resistivity and lithology and salinity. Further analysis should contribute significantly to understanding the hydrogeology of the delta.

Airborne ElectroMagnetic methods (AEM) have been used for geophysical surveys for several decades, in particular due to its ability to cover large areas on a limited budget and in relatively little time. Ever since the method was first deployed the tendency has been for the typical survey size to be growing at a very rapid rate, in particular during the last decade. AEM surveys of today are often in the order of tens of thousands of flight line kilometres, producing huge datasets that needs to be inverted. Performing a full non-linear inversion of such datasets is a tremendous computational burden and consequently, these datasets are very often inverted using approximate inversion schemes. Several full non-linear inversion codes for AEM data is reported in the literature, however, most of these codes operate on single soundings only and are not optimized for large datasets. We discuss how the existing constrained inversion code em1dinv has been optimized for modern parallel computing and show performance results. The parallel optimized code is capable of handling very large datasets and provides a significant reduction in the total inversion time. Furthermore, we show how the optimized code eliminates inversion artefacts inherent of the previous implementation.

The hydrology of volcanic islands is poorly characterized due to their complex internal structure and challenging access. The Galapagos Islands, an isolated basaltic environment with unique ecosystems and growing anthropogenic pressure, suffer from scarcity of freshwater resources and from the lack of fundamental knowledge on their hydrology. To overtake these constraints and provide fresh water to the population, a geophysical survey and a hydroclimatologic network at watershed scale have been performed over Santa Cruz. An innovative helicopter borne transient electromagnetic method permitted a quasi-3D (an inversion scheme allowing for a 3D model description but with a local 1D forward algorithm) resistivity mapping of the massif and the identification of its hydrogeological potential. The latter is composed of a perched aquifer on the windward mountainside and a basal aquifer. Our hydroclimatologic network provided useful data records and allowed us to characterize the hydrodynamic properties of the basal aquifer. The geophysical data allow us to precise its extension. The conceptual hydrogeological model proposed for Santa Cruz confirms one of the models proposed for Le Piton de la Fournaise (Reunion Island). It has now to be confirmed with boreholes and a detailed geochemical study including noble gas analysis.

A large amount of new geophysical and geological data has been collected in the central part of Vendsyssel, Denmark, as a part of a large groundwater investigation.

Formålet med denne abstrakte er at give en kort beskrivelse af de væsentligste ideer i den danske nationale strategi for omfattende kortlægning af grundvandet resources.Emphasis vil blive lagt på en beskrivelse af de fordele ved at kombinere Erhvervelse af rumligt tætte geofysiske data, der dækker store områder med oplysninger fra et optimalt antal nye undersøgelse boringer, eksisterende boringer, logbøger og vandprøver for at få en integreret og detaljeret beskrivelse af grundvandsressourcerne og deres vulnerability.Development mere tid effektiv og luftbårne geofysiske data erhvervelse platforme (f.eks SkyTEM) har foretaget omfattende kortlægning af attraktive og økonomisk overkommelige i planlægning og forvaltning af grundvandsressourcer. Håndtering og optimeret anvendelse af enorme mængder af geofysiske data, som dækker store områder har også krævet en omfattende database, hvor data kan nemt gemmes, dokumenteres, udvundet, fortolket, rekombineret og genbruges en gang efter den anden. The aim of this abstract is to give a short description of the essential ideas of the Danish national strategy for large scale mapping of the groundwater resources.Emphasis will be put on a description of the advantages obtained by combining acquirement of spatially dense geophysical data covering large areas with information from an optimum number of new investigation boreholes, existing boreholes, logs and water samples to get an integrated and detailed description of the groundwater resources and their vulnerability.Development of more time efficient and airborne geophysical data acquisition platforms (e.g. SkyTEM) have made large-scale mapping attractive and affordable in the planning and administration of groundwater resources. The handling and optimized use of huge amounts of geophysical data covering large areas has also required a comprehensive database, where data can easily be stored, documented, extracted, interpreted, recombined and reused one time after the other.

International audience; Due to the complexity of geological formations and limited subsurface data in volcanic islands, hydrogeological conceptual models can differ from one island to another and for the same island. The Galapagos Islands, like most inhabited volcanic islands, face important water resource problems which might have a major impact on their unique and pristine ecosystems, Endanger World Heritage list (June 2007). The scarcity of geological and hydrological data combined with the difficulty of access for field measurements lead to a Poor understanding of the island hydrogeology and unconvincing interpretation of traditional geophysical data. Here we present three dimensional (3-D) resistivity maps for the supposedly "waterless" Santa Cruz Island, obtained by using the SkyTEM device, a helicopter-borne transient electromagnetic method. The latter is non-invasive and measurements over inaccessible terrain were of vital importance. We show that even in high-relief terrain with extreme subsurface resistivity contrasts [1-6 000 ohm-m], the method is sensitive to low-resistivity layers of hydrogeological interest [50-200 ohm-m] to a depth of approximately 300 m. The unique spatial resolution and 3-D view of the subsurface resistivity structures allow identification of two zones of hydrogeological importance: a previously unknown extensive perched aquifer (50 km(2)) on the southern mountain side and the geometry of the salt-water wedge in the basal aquifer located with an accuracy of a few meters, This finding supports the existence of hidden perched aquifers on basaltic islands, until now only inferred from hydrogeological studies. It is seen to be affected by faulting. Beyond the vital implications for water resource and ecosystem management in the unique yet severely threatened Galapagos Islands, some conceptual flow models of volcanic islands have to be reconsidered. The presence of such structures in a volcanic edifice recognized as low strength layer may help to explain the origin of potentially catastrophic landslides. (C) 2008 Elsevier B.V. All rights reserved.

Geophysical and hydrogeological investigations have been carried out around Sawmills in Zimbabwe, Africa. The investigations are components of a larger investigation to assess the groundwater potential of the Karoo sedimentary basin with regards to supplying water to Bulawayo City. The Sawmills area was selected due to the availability of borehole logs indicating favourable stratigraphy for groundwater availability and due to the high yields from the aquifers measured from these boreholes. Data collected using two geophysical methods are presented here: transient electromagnetic (TEM) and continuous vertical electrical sounding (CVES) data. The data have also been processed using laterally constrained inversion (LCI). Because the CVES provides greater detail in the shallow subsurface, whereas TEM is more effective at depth, a more accurate image of the entire subsurface profile is provided based on using both methods. The results suggest that LCI of CVES and TEM data, in the subsurface at the required depths at Sawmills, is able to provide a substantially more accurate image of the subsurface than either method alone. The hydrogeological interpretation of the geophysical data is valuable for determining the depth to and thickness of the potential aquifer horizon(s) and for identifying the position of potential recharge zones.

We describe the challenges for the realization of a transient EM (TEM) system capable of measuring both the z- and x-components. This involves significant changes to instrumentation as well as to the processing and inversion algorithms. Geophysical mapping of groundwater resources using TEM involves both delineation of the aquifer structure and the aquifer protection. The demands for the instrumentation and the processing algorithms are quite different for the two tasks. Structural mapping requires large depth penetration and thereby a large transmitter moment while aquifer protection mapping requires early time measurements and extremely accurate determination of the system transfer function. Furthermore, the inversion software must be capable of modelling this system transfer function. For an airborne TEM system systematic analysis of the resolution capabilities of relatively thin low resistive layers in the upper 30 - 50 m of the subsurface has shown that the resolution is enhanced if not only the transient z-component but also the transient x-component of the induced fields in the Earth is measured and included in the inversion. The abstract will describe and discuss three major challenges related to the inclusion of the x-component in an airborne TEM system.

A033 Geostatistical estimation of structural vulnerability Summary 1 In this abstract we present a newly developed concept using geostatistical estimation and nonlinear inversion to optimize a function translating geophysical resistivity models to geophysical clay thickness. The algorithm uses information on reported clay thicknesses in drill holes. We call it geoStatistical estimation of Structural Vulnerability (SSV). The clay thickness in the upper part of the subsurface is an important factor for the water infiltration speed and thereby the vulnerability of underlying aquifers to pesticides nitrate etc. Drill hole information contains the most detailed information on clay thicknesses but most often drill

A033 INTEGRATED INVERSION OF CVES AND TEM DATA USING LATERAL CONSTRAINTS Summary 1 We present a mutually and laterally constrained inversion between transient electromagnetic (TEM) and geoelectric (DC) data sets. Although both methods measure in some sense the electrical resistivity or conductivity of the subsurface they sample different volumes and have different sensitivities. The different sampling volumes and different sensitivities are exploited by the mutually and laterally constrained inversion algorithm combining the coinciding profile data sets. The output model incorporates the information from both profile data sets to obtain the optimum layered 1D models fitting both data types. All data

In Denmark, 99% of the water supply is based on groundwater abstraction. Increasing problems with the groundwater quality, due to urban growth and agricultural contamination, have caused the Danish Minister of Environment to approve a plan for improving ground water protection.

By the integrated use of geophysical methods a ltydrogeological investigation with a very high r~~solution of the geology has heen carried out in Denmark. llsing newly developed methods such as auger Ellog drilling with levrl specific Water sampling and pulled array contitttlous electric profiling together along with more Ouniliar techniques such as transient electromagnetic sounding we have perfortned a general exantinatiott of the vulnerability, the volumetric extent and the chemical properties of the aquifer itt the area. The protection of the aquifer was less than expected. There was a rorrelation between the water quality and the lithology of the aquifer.