Airborne Electromagnetic Measurements Research Articles

Bayesian detectability of induced polarization in airborne electromagnetic data

SUMMARY Detection of induced polarization (IP) effects in airborne electromagnetic measurements does not yet have an established methodology. This work contributes a Bayesian approach to the IP detectability problem using decoupled transdimensional layered models and applies an approach novel to geophysics whereby transdimensional proposals are used within the embarrassingly parallelizable and robust sequential Monte Carlo class of algorithms for the simultaneous inference of parameters and models. This algorithm allows for adaptivity considerations for multiple models and proposal types. Methodological contributions to solid Earth geophysics include the decoupled layered model approach and proposal of a statistic that uses posterior model odds for IP detectability. A case study is included investigating the detectability of IP effects in airborne electromagnetic data at a broad scale.

Instantaneous Inversion of Airborne Electromagnetic Data Based on Deep Learning

AbstractThe airborne electromagnetic (AEM) method is becoming an effective means for subsurface electrical property reconstruction with the merits of terrain adaptability and acquisition efficiency. However, instantaneous inversion of time‐domain AEM data is still a challenge, owing to the huge amount of data. Inspired by Google's neural machine translation system, we develop a fast inversion operator guided by deep learning to translate time‐domain AEM measurements directly into subsurface resistivity structures. Trained by synthetic data, our system shows impressive adaptability to field observations and strong robustness against noise disturbance. Applied to the AEM data set acquired by the U.S. Geological Survey in Leach Lake Basin, CA, USA, our system successfully delivers results in seconds for a common PC from more than 740,000 AEM soundings. The inverted structures clearly delineate the geometries of the lake, surrounding mountains and faults. The inversion operator can support instantaneous subsurface resistivity reconstruction for AEM observations.

New regional stratigraphic insights from a 3D geological model of the Nasia sub-basin, Ghana, developed for hydrogeological purposes and based on reprocessed B-field data originally collected for mineral exploration

Abstract. Reprocessing of regional-scale airborne electromagnetic data is used to build a 3D geological model of the Nasia sub-basin, northern Ghana. The resulting 3D geological model consistently integrates all the prior pieces of information brought by electromagnetic data, lithologic logs, ground-based geophysical surveys, and geological knowledge of the terrain. The geo-modeling process is aimed at defining the lithostratigraphy of the area, chiefly to improve the stratigraphic definition of the area, and for hydrogeological purposes. The airborne electromagnetic measurements, consisting of GEOTEM B-field data, were originally collected for mineral exploration purposes. Thus, those B-field data had to be (re)processed and properly inverted as the original survey and data handling were designed for the detection of potential mineral targets and not for detailed geological mapping. These new geophysical inversion results, compared with the original conductivity–depth images, provided a significantly different picture of the subsurface. The new geophysical model led to new interpretations of the geological settings and to the construction of a comprehensive 3D geo-model of the basin. In this respect, the evidence of a hitherto unexposed system of paleovalleys could be inferred from the airborne data. The stratigraphic position of these paleovalleys suggests a distinctly different glaciation history from the known Marinoan events, commonly associated with the Kodjari formation of the Voltaian sedimentary basin. Indeed, the presence of the paleovalleys within the Panabako may be correlated with mountain glaciation within the Sturtian age, though no unequivocal glaciogenic strata have yet been identified. Pre-Marinoan glaciation is recorded in rocks of the Wassangara group of the Taoudéni Basin. The combination of the Marinoan and, possibly, Sturtian glaciation episodes, both of the Cryogenian period, can be an indication of a Neoproterozoic Snowball Earth. Hence, the occurrence of those geological features not only has important socioeconomic consequences – as the paleovalleys can act as reservoirs for groundwater – but also from a scientific point of view, they could be extremely relevant as their presence would require a revision of the present stratigraphy of the area.

Anisotropic complex electrical conductivity of black shale and mudstone from the Moffat Shale Group (Ireland)

ABSTRACTThe geological setting in the north of Ireland, especially concerning the origin of the Moffat Shale Group, has long been under discussion. Within the Tellus Programme of the Geological Survey Ireland, airborne electromagnetic measurements revealed high‐conductivity anomalies that have been interpreted as the response of a black shale. In order to petrophysically characterize the Moffat Shale, a laboratory study using material from two shallow boreholes was carried out. The study focuses on spectral induced polarization measurements on 23 oriented samples in the frequency range from 10−4 to 105 Hz.The sample material can be categorized into two groups. A mudstone‐like rock type shows weakly frequency‐dependent, porosity‐driven conductivities with a strong anisotropy. On the other hand, black shale samples are characterized by moderately anisotropic but strong polarization effects especially at low frequencies and a strong conductivity increase towards higher frequencies. The polarization in the black shale is controlled by the texture and volume fraction of the polarizable components. The spectral induced polarization data are processed by means of a Debye decomposition approach. The anisotropy of the complex electrical conductivity is determined by utilizing the foliation dip angle and assuming tilted transverse isotropic conditions. The relevance of the laboratory findings for airborne electromagnetic surveys is addressed with a synthetic one‐dimensional modelling study.

Correcting for static shift of magnetotelluric data with airborne electromagnetic measurements: a case study from Rathlin Basin, Northern Ireland

Abstract. Galvanic distortions of magnetotelluric (MT) data, such as the static-shift effect, are a known problem that can lead to incorrect estimation of resistivities and erroneous modelling of geometries with resulting misinterpretation of subsurface electrical resistivity structure. A wide variety of approaches have been proposed to account for these galvanic distortions, some depending on the target area, with varying degrees of success. The natural laboratory for our study is a hydraulically permeable volume of conductive sediment at depth, the internal resistivity structure of which can be used to estimate reservoir viability for geothermal purposes; however, static-shift correction is required in order to ensure robust and precise modelling accuracy.We present here a possible method to employ frequency–domain electromagnetic data in order to correct static-shift effects, illustrated by a case study from Northern Ireland. In our survey area, airborne frequency domain electromagnetic (FDEM) data are regionally available with high spatial density. The spatial distributions of the derived static-shift corrections are analysed and applied to the uncorrected MT data prior to inversion. Two comparative inversion models are derived, one with and one without static-shift corrections, with instructive results. As expected from the one-dimensional analogy of static-shift correction, at shallow model depths, where the structure is controlled by a single local MT site, the correction of static-shift effects leads to vertical scaling of resistivity–thickness products in the model, with the corrected model showing improved correlation to existing borehole wireline resistivity data. In turn, as these vertical scalings are effectively independent of adjacent sites, lateral resistivity distributions are also affected, with up to half a decade of resistivity variation between the models estimated at depths down to 2000 m. Simple estimation of differences in bulk porosity, derived using Archie's Law, between the two models reinforces our conclusion that the suborder of magnitude resistivity contrasts induced by the correction of static shifts correspond to similar contrasts in estimated porosities, and hence, for purposes of reservoir investigation or similar cases requiring accurate absolute resistivity estimates, galvanic distortion correction, especially static-shift correction, is essential.

3D aquifer mapping employing airborne geophysics to meet India's water future

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.

Estimation and scaling of hydrostratigraphic units: application of unsupervised machine learning and multivariate statistical techniques to hydrogeophysical data

Numerical models provide a way to evaluate groundwater systems, but determining the hydrostratigraphic units (HSUs) used in constructing these models remains subjective, nonunique, and uncertain. A three-step machine-learning approach is proposed in which fusion, estimation, and clustering operations are performed on different data sets to arrive at HSUs at different scales. In step one, data fusion is performed by training a self-organizing map (SOM) with sparse borehole hydrogeologic (lithology, hydraulic conductivity, aqueous field parameters, dissolved constituents) and geophysical (gamma, spontaneous potential, and resistivity) measurements. Estimation is handled by iterative least-squares minimization of the SOM quantization and topographical errors. Application of the Davies-Bouldin criteria to k-means clustering of SOM nodes is used to determine the number and location of discontinuous borehole HSUs with low lateral density (based on borehole spacing at 100 s m) and high vertical density (based on cm-scale logging). In step two, a scaling network is trained using the estimated borehole HSUs, airborne electromagnetic measurements, and numerically inverted resistivity profiles. In step three, independent airborne electromagnetic measurements are applied to the scaling network, and the estimation performed to arrive at a set of continuous HSUs with high lateral density (based on sounding locations at meter (m) spacing) and medium vertical density (based on m-layer modeled structure). Performance metrics are used to evaluate each step of the approach. Efficacy of the proposed approach is demonstrated to map local-to-regional scale HSUs using hydrogeophysical data collected at a heterogeneous surficial aquifer in northwestern Nebraska, USA.

Sea-ice surface roughness estimates from airborne laser scanner and laser altimeter observations in Fram Strait and north of Svalbard

AbstractWe present sea-ice surface roughness estimates, i.e. the standard deviation of relative surface elevation, in the Arctic regions of Fram Strait and the Nansen Basin north of Svalbard acquired by an airborne laser scanner and a single-beam laser altimeter in 2010. We compare the scanner to the altimeter and compare the differences between the two survey regions. We estimate and correct sensor roll from the scanner data using the hyperbolic response of the scanner over a flat surface. Measurement surveys had to be longer than 5 km north of Svalbard and longer than 15 km in Fram Strait before the statistical distribution in surface roughness from the scanner and altimeter became similar. The shape of the surface roughness probability distributions agrees with those of airborne electromagnetic induction measurements of ice thickness. The ice in Fram Strait had a greater mean surface roughness, 0.16 m vs 0.09 m, and a wider distribution in roughness values than the ice in the Nansen Basin. An increase in surface roughness with increasing ice thickness was observed over fast ice found in Fram Strait near the coast of Greenland but not for the drift ice.

Radiomagnetotelluric mapping of marlstone and limestone in the Silurian bedrock of Gotland

The Silurian of Gotland is composed of a series of stacked carbonate platforms. Airborne electromagnetic measurements show areas with high electrical resistivity, dominated by dense limestone, and areas with lower resistivity, dominated by marlstone. On Gotland, three main regional, up to 80 m thick, limestone-dominated bedrock packages overlie marlstone-dominated strata. The thickness of the limestone and the subsurface relationship between limestone and marlstone was investigated in the area of Ale–Ardre on central Gotland by use of the radiomagnetotelluric (RMT) method. A significant difference in resistivity between the limestone (1000–10 000 Ωm) and the marlstone ( < 100 Ωm) renders favorable RMT conditions. Four profiles, up to 4.3 km long, were measured across the upper part of the Klinteberg Formation and lower part of the Hemse Group (late Wenlock–early Ludlow). The results show a distinct change in resistivity at 50–80 m depth, correlating with the transition between biostromal–biohermal limestone and shelf marlstone–wackestone lithofacies, as observed in boreholes. The measurements were used to create resistivity models that make it possible to map the subsurface lateral relationship between limestone- and marlstone-dominated sequences. The results also provide important information on distribution and bedding, which give supporting information regarding the sequence stratigraphic model for the Klinteberg Fm–Hemse Group in the Ala–Ardre–Sigsarve area. The RMT method utilized in this study proved to be a very useful method for mapping subsurface bedrock in carbonate-dominated geological settings such as those on Gotland.

Modelling climate change effects on a Dutch coastal groundwater system using airborne electromagnetic measurements

Abstract. The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to the sea and dikes, but nowadays large efforts are still being done to find out the best techniques to describe complex fresh-brackish-saline groundwater dynamic systems. In this paper, we describe a methodology consisting of high-resolution airborne electromagnetic (EM) measurements used in a 3-D variable-density transient groundwater model for a coastal area in the Netherlands. We used the airborne EM measurements in combination with borehole-logging data, electrical conductivity cone penetration tests and groundwater samples to create a 3-D fresh-brackish-saline groundwater distribution of the study area. The EM measurements proved to be an improvement compared to older techniques and provided quality input for the model. With the help of the built 3-D variable-density groundwater model, we removed the remaining inaccuracies of the 3-D chloride field and predicted the effects of three climate scenarios on the groundwater and surface water system. Results showed significant changes in the groundwater system, and gave direction for future water policy. Future research should provide more insight in the improvement of data collection for fresh-brackish-saline groundwater systems as it is of high importance to further improve the quality of the model.

Regional-scale airborne electromagnetic surveying of the Yucatan karst aquifer (Mexico): geological and hydrogeological interpretation

Geometry and connectivity of high-permeability zones determine groundwater flow in karst aquifers. Efficient management of karst aquifers requires regional mapping of preferential flow paths. Remote-sensing technology provides tools to efficiently map the subsurface at such scales. Multi-spectral remote sensing imagery, shuttle radar topography data and frequency-domain airborne electromagnetic (AEM) survey data were used to map karst-aquifer structure on the Yucatan Peninsula, Mexico. Anomalous AEM responses correlated with topographic features and anomalous spectral reflectance of the terrain. One known preferential flow path, the Holbox fracture zone, showed lower bulk electrical resistivity than its surroundings in the AEM surveys. Anomalous structures delineated inland were sealed above by a low-resistivity layer (resistivity: 1–5 Ωm, thickness: 5–6 m). This layer was interpreted as ejecta from the Chicxulub impact (Cretaceous/Paleogene boundary), based on similar resistivity signatures found in borehole logs. Due to limited sensitivity of the AEM survey, the subsurface configuration beneath the low-resistivity layer could not be unambiguously determined. AEM measurements combined with remote-sensing data analysis provide a potentially powerful multi-scale methodology for structural mapping in karst aquifers on the Yucatan Peninsula and beyond.

Modelação baseada em dados para exploração de água subterrânea em terrenos cristalinos fraturados, nordeste do Brasil

It is not possible, using numerical methods, to model groundwater flow and transport in the fractured crystalline rock of northeastern Brazil. As an alternative, the usefulness of self-organizing map (SOM), k-means clustering, and Davies-Bouldin techniques to conceptualize the hydrogeology was evaluated. Also estimated was the well yield and groundwater quality across the Jua region. This process relies on relations in the underlying multivariate density function associated with a sparse local set of hydrogeologic (electrical conductivity, geology, temperature, and well yield) and a complete regional set of airborne geophysical (electromagnetic, magnetic, and radiometric) and satellite spectrometric measurements. Resampling of the regional well yield and electrical conductivity estimates provides sufficient resolution to construct variograms for stochastic modeling of the hydrogeologic variables. The combination of these stochastic maps provides a way to identify potential drilling targets for future groundwater development. The data-driven estimation approach, when applied to available airborne electromagnetic and water-well hydrogeologic measurements, provides a low-cost alternative to numerical groundwater flow modeling. In addition to fractured rock environments, the alternative modeling framework can provide spatial parameter estimates and associated variograms for constraints to improve the traditional calibration of equivalent groundwater-porous-media models.

Evaluation of Arctic sea ice thickness simulated by Arctic Ocean Model Intercomparison Project models

Six Arctic Ocean Model Intercomparison Project model simulations are compared with estimates of sea ice thickness derived from pan‐Arctic satellite freeboard measurements (2004–2008); airborne electromagnetic measurements (2001–2009); ice draft data from moored instruments in Fram Strait, the Greenland Sea, and the Beaufort Sea (1992–2008) and from submarines (1975–2000); and drill hole data from the Arctic basin, Laptev, and East Siberian marginal seas (1982–1986) and coastal stations (1998–2009). Despite an assessment of six models that differ in numerical methods, resolution, domain, forcing, and boundary conditions, the models generally overestimate the thickness of measured ice thinner than ∼2 m and underestimate the thickness of ice measured thicker than about ∼2 m. In the regions of flat immobile landfast ice (shallow Siberian Seas with depths less than 25–30 m), the models generally overestimate both the total observed sea ice thickness and rates of September and October ice growth from observations by more than 4 times and more than one standard deviation, respectively. The models do not reproduce conditions of fast ice formation and growth. Instead, the modeled fast ice is replaced with pack ice which drifts, generating ridges of increasing ice thickness, in addition to thermodynamic ice growth. Considering all observational data sets, the better correlations and smaller differences from observations are from the Estimating the Circulation and Climate of the Ocean, Phase II and Pan‐Arctic Ice Ocean Modeling and Assimilation System models.

Characterizing oil field salinization using airborne, surface, and borehole geophysics: An example from the Upper Colorado River Basin, Texas

Multidisciplinary investigations of natural and oil field salinization along the upper Colorado River, Texas, present an opportunity to integrate results from a stream-axis airborne geophysical survey, ground and borehole geophysical surveys, and well drilling and sampling. Airborne electromagnetic (EM) induction measurements along 437 km (272 mi) of river and tributary stream axes identified discrete salinized streambed segments, including several near oil fields. Identification of these salinized streambed segments allowed more intensive and invasive investigations to be focused on the most significant near-river sources of salinity. One of these streambed segments lies adjacent to an oil field, where production began in the 1950s before discharge of coproduced brine into surface pits was prohibited in Texas. Monitor wells drilled after the airborne survey verified groundwater salinization in the oil field but did not adequately delineate salinization nor identify specific salinity source areas. Subsequent ground and borehole geophysical surveys complemented airborne EM induction and well data by establishing lateral and vertical salinization bounds in the oil field, discovering possible salinity source areas, and determining optimal locations for additional wells.

Airborne electromagnetic measurements as basis for a 3D geological model of an Elsterian incision

Airborne electromagnetic measurements as basis for a 3D geological model of an Elsterian incision

On the recognition of areas with problematic sulphidic sediments using multi-disciplinary data

Increased concentrations of several trace elements have been observed in water draining areas with sulphidic sediments along the Baltic Sea coast of Sweden and Finland. Some of these elements are harmful to the water environment. The increased leaching of trace elements is often caused by antrophogenic activities, such as ditching, which cause oxidation of the sulphides and thereby acid soil conditions. It is therefore important to recognise areas with sulphidic sediments and, if possible, avoid land use, which causes sulphide oxidation. The aim of this study was to find time effective methods, which can be used to delineate areas with sulphidic sediments. Three areas in Sweden with known occurrences of sulphidic sediments were studied. Data obtained from the mapping program at the Geological Survey of Sweden (SGU) were first studied. Selected sites were thereafter visited and geoelectrical measurements were carried out by the use of several instruments. The results imply that areas with sulphidic sediments are reflected as areas with high trace element contents on SGUs biogeochemical maps, which are based on analyses of trace elements in aquatic plants. The same deposits are often shown as gyttja clay on the maps of Quaternary deposits. Furthermore models derived from airborne electromagnetic measurements show that areas with sulphidic sediments often have a lower electric resistivity than surrounding sediments. Also the results from ground geoelectrical measurements suggest that the sulphidic sediments have a lower resistivity than surrounding fine-grained deposits. That interpretation was verified after stratigraphical studies and analyses of the sulphur contents of the sediments. It is therefore suggested that data from ground geophysical measurements can be used for a detailed delineation of sulphidic sediments, whereas data obtained from airborne geophysical and geological maps give an overview over potential areas with these sediments. The absolute resistivity of the sulphidic sediments varies, however, between the three investigated areas. Complementary corings and stratigraphical descriptions are therefore always necessary to verify the occurrence of these sediments.

The JAC airborne EM system: AEM-05

This paper describes the airborne electromagnetic (AEM) system operated by the Joint Airborne geoscience Capability (JAC), a partnership between the Finnish and British Geological Surveys. The system is a component of a 3-in-1, fixed-wing facility acquiring magnetic gradiometer and full spectrum radiometric data alongside the wing-tip, frequency-domain AEM measurements. The AEM system has recently (2005) been upgraded from 2 to 4 frequencies and now provides a bandwidth from 900 Hz to 25 kHz. The fixed-wing configuration of 4 dual vertical coplanar coils, offers a high signal/noise by virtue of the wingspan separation of the sensors. This unique configuration allows 3-in-1 surveys to be successfully performed at a variety of survey elevations when regulatory conditions are imposed. Its deployment on a twin-engine aircraft also permits low altitude surveying in countries, such as the UK, where this is a requirement. The development of the new AEM-05 system has been incremental and its history can be traced back over five decades. The AEM data acquired in the Finnish National Mapping project, and across northern Europe, have been used extensively in mineral exploration. More recent projects have investigated the application of the data to environmental, hydrogeological and land quality issues. These studies have been enhanced by reducing the flight line separation from 200 m (the national high-resolution scale) to 50 m. Our surveys also increasingly involve the application of AEM across populated areas often with extensive infrastructure. Additional secondary instrumentation has been introduced to provide an increased understanding of the data and the AEM responses observed. The secondary systems include an accurate, high sampling rate laser altimeter, a downward-looking digital camera to record the flight path, a 50/60 Hz power line monitor and a GPS gyroscope. The paper is intended as an overview and provides descriptions of the new AEM system, the secondary systems now employed and some of the software used to provide accurate and levelled AEM data. Recent applications of the system are reviewed and the challenging nature of the new subsurface information being revealed is demonstrated.

Estimating subsurface porosity and salt loads using airborne geophysical data

ABSTRACTWe aim to map dissolved salts in aquifers which, with rising water tables, are a threat to fresh water resources and ecosystem health. Interpolated groundwater electrical conductivity (EC) maps from borehole measurements are combined with spatially detailed bulk electrical conductivities estimated from airborne electromagnetics to provide, through Archie’s law, a detailed prediction of subsurface porosity and total dissolved salt load of any near‐surface aquifer. This technique quantitatively integrates airborne electromagnetic and groundwater measurements of EC to provide the subsurface detail necessary for hydraulic properties. The process has been applied in Australia’s Murray River Basin, where the presence of salt is a threat to agriculture and fresh water supplies. Porosity predictions range between 1% and 50% and are spatially consistent with known geology, dissolved salt loads are estimated to be between 100 and 4000 tonnes per hectare. The predictions are yet to be compared to in‐situ measurements but if proven, will be a useful tool when in‐situ porosity is under‐sampled or is not available.

Filling in the gaps - validation and integration of airborne EM data with surface and subsurface observations for catchment management - an example from Bendigo, Victoria, Australia

An airborne electromagnetic (AEM) survey was flown with the TEMPEST System over the Kamarooka study area north of Bendigo.Conductivity values derived from AEM measurements were compared with borehole conductivity values obtained with an EM39 induction probe, indicating an approximately 1:1 relationship between the two quantities when taking into account the vertical (several metres) and horizontal (100 m) resolution of the AEM values.The thickness of Tertiary and Quaternary cover overlying Paleozoic basement was mapped as the transition from moderately conductive cover to resistive basement. Palaeodrainage lines were mapped in the lower parts of the survey area using the topography of this interface, and in the upper portions of these buried valleys by tracing low amplitude magnetic trends associated with the valley infill material.The lateral extent and depth to a shallow (0 to 15 m), highly conductive layer was mapped. This layer was interpreted to correspond to high salt concentrations at the groundwater table. Areas where this layer intersects the land surface correlate with mapped areas of saline discharge.The conductivity values derived from TEMPEST AEM data provided a regularly sampled 3D framework into which surface observations and sparsely distributed subsurface observations were incorporated as part of an integrated hydrogeological investigation.

Filling in the Gaps – Validation and Integration of Airborne EM Data with Surface and Subsurface Observations for Catchment Management – an Example from Bendigo, Victoria, Australia

An airborne electromagnetic (AEM) survey was flown with the TEMPEST AEM System over the Kamarooka study area north of Bendigo.Conductivity values derived from AEM measurements were compared with borehole conductivity values obtained with an EM39 induction probe, indicating an approximately 1:1 relationship between the two quantities when taking into account the vertical (several metres) and horizontal (100 m) resolution of the AEM values.The thickness of Tertiary and Quaternary cover overlying Paleozoic basement was mapped by resolving the transition from moderately conductive cover to resistive basement. Palaeodrainage lines were mapped in the lower parts of the survey area using the topography of this interface, and in the upper portions of these buried valleys by tracing low amplitude magnetic trends associated with the valley infill material.The lateral extent and depth to a shallow (0 to 15 m), highly conductive layer was mapped. This layer was interpreted to correspond to high salt concentrations at the groundwater table. Areas where this layer intersects the land surface correlate with mapped areas of saline discharge.The conductivity values derived from TEMPEST AEM data provided a regularly sampled 3D framework into which surface observations and sparsely distributed sub-surface observations were incorporated as part of an ongoing integrated hydrogeological investigation.