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We study the magnetoelectric effect in LiFePO4 using the pyrocurrent method to determine the magnetic-field-induced electric polarization as a function of temperature for various applied magnetic field strengths. We discover a diagonal magnetoelectric tensor element not previously reported. This new coupling reflects the emergence of an electric polarization along a induced by a magnetic field along the same direction and is consistent with the symmetry-imposed requirements for the small canting components of the magnetic ground state reported for this compound. Our observation is thus further evidence for a ground state which is not supported by the space group Pnma as originally assigned to LiFePO4.

The physical parameter derived from the inversion of electromagnetic surveys, the distribution of subsurface conductivity, is interesting in itself only in very few instances. In most cases, the conductivity distribution will have to be interpreted in terms of the target properties of the survey, for example: a geological interpretation of lithology; a hydrogeological interpretation of hydraulic conductivity; a biohazard/geotechnical interpretation of polluted/not‐polluted ground; and/or an archaeological interpretation of manmade/natural finds. The parameters of interest in these categories are often called derived products, indicating that the parameter of interest is not the same as the parameter whose distribution is found in the inversion process of the geophysical data. The interpretation process can be done in a wide variety of ways; from a predominantly cognitive approach based on professional experience, to an application of rigorous quantitative relations found from scientific endeavours. In most practical situations, the number of locations with independently measured information on the derived product is considerably smaller than the number of geophysics locations. It is precisely this sparsity of primary information on the derived product that encourages the use of geophysical inversion results as a sort of qualified interpolator through a formulation of a correlation between a geophysical parameter and the parameter characterising the derived product. In this paper, a general, quantitative approach to deriving the parameter of interest is presented using statistical analytic measures and an advanced use of an interpolation method that takes uncertainties into account. The approach is demonstrated in a field example from Olgod, Denmark, where the cumulated clay thickness in the upper 30 m is estimated using a combination of borehole drilling records and an airborne transient survey.

The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around TN observed for bulk systems. Our finding of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale.

The development of Northern Australia has been identified as a national priority, with water availability being fundamental to economic development. Surface water options are limited hence identification of new groundwater resources and water banking options is essential. Over the past four years, Geoscience Australia, in concert with State and Territory partners, has been involved in focussed groundwater investigations in 10 geographic areas as well as broader regional investigations (Figure 1). New data acquisition has included airborne electromagnetics (AEM); drilling (sonic, rotary mud, air core and diamond); slug and bore testing; ground geophysics (surface nuclear magnetic resonance, microgravity, passive seismic, seismic reflection and ground penetrating radar); borehole geophysics (induction, spectral gamma, nuclear magnetic resonance); hydrochemical and hydrodynamic analysis; age dating of water and landscape materials; and mapping (geomorphic, morphotectonic, regolith, geological and hydrological). These investigations inform hydrogeological assessments and water allocation planning.Overall, this multi-physics, inter-disciplinary approach has been critical in enabling the rapid identification and assessment of significant new potential fresh groundwater resources within tectonically inverted Palaeozoic sedimentary basins in the Fitzroy Basin (WA), Bonaparte Basin (WA-NT), Wiso Basin (NT), and Southern Georgina Basin (NT), and helped define the extents of a new groundwater resource for the town of Alice Springs (NT). Potential brackish and saline groundwater resources have also been identified in Cenozoic paleovalleys and Tertiary and Paleozoic Basins.

Interpretation of a hydrogeophysical survey is a complex and comprehensive process. In addition to an areal coverage with AEM data, most often an interpretation involves additional data that are time consuming to collect and complicated to integrate into an overall model, e.g. borehole logs, borehole core samples, water chemistry, surface vegetation, satellite imagery plus the generally accepted geological background knowledge. Compared with the complexities of the interpretation process, the acquisition, QC and inversion of AEM survey data are a more straightforward affair and considerably less time consuming. Interpretation basically has to do with identifying categories and finding boundaries between them so that depths, thicknesses, lithologies and a whole range of other model attributes can be estimated, qualitatively and quantitatively. To supplement the traditional product delivered by the inverter to the interpreter: inversion models displaying the distribution of subsurface electrical conductivity, I present two methods based on the Continuous Wavelet Transform that can deliver more focused attributes to assist in the interpretation. In the first method, layer boundaries in the smooth multi-layer models that are most often used in the inversion of large data sets are found. In the second method, the spatial distribution of the natural categories of the model parameter is found. Both methods are based on the inversion models and, evidently, they are useful to the extent that the variations in conductivity reflect geological / hydrogeological boundaries and categories - which is for the interpreter to decide.

Electromagnetic induction (EMI) sensors using sufficiently low-frequency harmonic sources and sufficiently small loop separations operate in the low-induction-number (LIN) domain for a relatively wide range of background conductivity. These systems are used in diverse near-surface investigations including applications from soil sciences, hydrology, and archaeology. The special case of portable multiconfiguration EMI sensors operating at frequencies [Formula: see text] offers the possibility of using a fast linear deconvolution method to interpret multichannel data sets in three dimensions. Here, we have developed a fast 3D inversion/deconvolution method regularized with 3D smoothness constraints and formulated in the hybrid spectral-spatial domain. Compared with other linear approaches, the spectral-spatial domain formulation significantly reduces the computational cost of the processing and opens the door for real-time 3D interpretation of large data sets consisting of more than 100,000 data points. First, we test our proposed algorithm on synthetic data sets computed with the full Maxwell theory. Then, we apply our method to a real four-configuration EMI data set acquired to map the thickness of peat layers embedded in a sandy environment. For the synthetic and the field example, we compared our result with the result obtained using a standard point-by-point 1D nonlinear inversion approach. This comparison demonstrates that the proposed methodology provides superior lateral resolution compared with the 1D nonlinear inversion, at the same time significantly reducing the computational cost of the processing.

The inverse modelling technique seeks to improve the existing estimates of natural recharge in hard rocks by coupling multiple hydrogeophysical parameters that jointly affect natural hydrogeological processes. This approach involves coupling of an initial set of multiple hydrogeophysical (soil resistivity, bedrock depth and rainfall) parameters in the form of exponents assigned to each parameter and a multiplication coefficient to obtain natural recharge. These model parameters (i.e., exponents and coefficients) are then quantified using linear least squares inversion against the known recharge values. To reduce the effect of geomorphic heterogeneity, viz., hills on natural recharge, laterally constrained inversion has been employed to integrate data sets (e.g., recharge measured at various points and logical expectation over exposed hills in an area) and constrained interpolation is then carried out along the grid lines for increased data density. Finally, Kriging interpolation over dense data obtained through data integration and constrained interpolation is used to significantly minimise the risks of overshooting the observations. Thus, the present approach provides a realistic spatial distribution of natural recharge values in a highly heterogeneous hard rock terrain.

Over the past decade, advances in new satellite and airborne sensor technologies provide an opportunity for rapid multi-scale mapping, measurement and monitoring of the physical state of the crust, including resolution of key elements of surface and sub-surface hydrological systems. These advances have been mirrored by the development in advanced computational research infrastructure which is now giving the groundwater research community access to high-resolution (spatial and temporal) biophysical datasets (e.g. climate, ecology, geoscience and geospatial) relevant to broader hydrological systems understanding. This infrastructure facilitates integration of multiple datasets and rapid and improved signal processing, inversion, and sophisticated analysis. These datasets provide a catalyst for collaboration, with inter-disciplinary approaches enabling new discovery science in a 'big data' environment, and enabling the qualitative and quantitative analysis and modelling of landscape and hydrological system processes. In Australian landscapes, airborne electromagnetics (AEM) is widely used in near-surface (

Over the past decade, a relatively rich record of neotectonics has been revealed in continental Australia, however very few investigations into the hydrogeological implications have been undertaken. While the most active intra-plate deformation zones are readily identified by seismicity monitoring and satellite and airborne terrain mapping, advances in airborne electromagnetic (AEM) technology and data optimization have made it possible to map numerous, more subtle ‘blind’ intra-plate fault systems concealed in near-surface floodplain landscapes. To date, fault geometries, displacements, and fault zone properties remain ambiguous due to the combination of AEM footprint resolution, the non-uniqueness of the conductivity models and derived hydrostratigraphy and fault geometry solutions produced by AEM equivalent inversion models, and the inherent uncertainty of stitched 1D AEM inversion models. The resultant uncertainty in fault zone characterisation inhibits investigations into the permeability heterogeneity and anisotropy introduced by these faults, making it difficult to resolve the significance of these structures for groundwater processes. In this study, a novel, inter-disciplinary approach has been developed that helps characterise the hydrogeology of one such intra-plate fault zone in unconsolidated, near-surface floodplain sediments. The approach integrates the mapping of tectonic geomorphology, with mapping of sub-surface hydrostratigraphy and ‘blind’ intra-plate fault systems using AEM. Validation of fault zone geometry and displacement at local scales is provided by ground geophysics (e.g. seismic reflection, resistivity and surface nuclear magnetic resonance (SNMR)) and drilling. Fault zone hydrogeology, including permeability variability, has been assessed through the integration of geophysics with hydrochemistry, hydrodynamics, and studies of vegetation response to water availability (using Landsat time-series analysis). A combination of deterministic and stochastic approaches is then used to unravel complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and recharge. This inter-disciplinary methodology has been used to parameterise numerical groundwater flow models and target potential groundwater resources.

The selection of an appropriate instrument for the data collection is one of the most important issues to address in survey design. Both theoretical analyses of the resolution capabilities of the candidate systems and field tests should be part of a selection process. It is also quite clear that any comparison must be performed based on the specific criteria of the survey. In this presentation we will concentrate on the field test aspects of the system selection process and compare the results of inverting data from three different helicopterborne systems: the VTEM system, the SkyTEM312 system, and the SkyTEM312FAST, that were flown over the same test lines. The bench marks of the comparison were mainly the near-surface resolution capabilities, both vertically and horizontally, and the resolution at depth. The results of our study show that in certain parts of the test lines both SkyTEM systems have a better near-surface vertical and horizontal resolution than the VTEM system, but that in other parts of the test lines differences are small. The depth penetration seems to be approximately the same for all three systems. The differences between the SkyTem312 and the SkyTem312FAST are almost imperceptible.

This paper presents new developments to the lateral parameter correlation method, a method that can be used to invoke lateral smoothness in model sections of one-dimensional inversion models. The lateral parameter correlation method has three steps. First, all datasets are inverted individually. Next, a laterally smooth version of each model parameter is found by solving a simple constrained inversion problem by postulating identity between the uncorrelated and correlated parameters and solving the equations including a model covariance matrix that ensures lateral smoothness. As a final step, all sounding data are reinverted with the correlated model parameter values as a priori values to produce models that better fit the data. Because the method separates inversion and correlation, it is much faster than methods where the inversion and correlation are solved simultaneously. The new development to the lateral parameter correlation method presented in this paper is an option to perform a strictly horizontal correlation, thereby avoiding the model artefacts sometimes seen when correlating along layers, namely that formations tend to follow the topography. Furthermore, a solution to the intractable computation times arising with large datasets is formulated, employing a tessellation of the plane and an averaging scheme within the subareas that reduces the size of the numerical lateral parameter correlation inversion problem while maintaining correct correlation within a very large area. In a field example, correlation along layers and the new strictly horizontal correlation are compared, and it is demonstrated that the horizontal correlation removes the above mentioned artefacts that can appear when correlating along layers. The lateral parameter correlation method is very flexible and is capable of correlating models from inversion of different data types, including information from boreholes, and it lends itself easily to “embarrassingly parallel” computation.

A novel, fast, and approximate forward modelling routine for time-domain electromagnetic responses is presented. It is based on the separation of the forward problem into a configuration-independent part, mapping conductivity as a function of depth onto apparent conductivity as a function of time, and a configuration-dependent part, i.e., the half-space step response. The response of a layered model is then found as the half-space response for a half-space conductivity equal to the apparent conductivity. The mapping is ten times faster than traditional accurate forward modelling routines, and through stochastic modelling, it is found that the standard deviation of the modelling error is 0.7 %. The forward mapping lends itself to integration in a modern state-of-the-art inversion formulation in exactly the same way as traditionally computed responses, and a field example is included where inversion results using the approximate forward response are compared with those of an accurate forward response for helicopterborne transient electromagnetic data. In addition to being used in its own right in inversion of transient data, the speed and accuracy of the approximate inversion mean that it is well suited for quality control and fast turnaround data delivery of survey results to a client. It can also be used in hybrid inversion formulations by supplying initial iterations and high-quality derivatives in an inversion based on accurate forward modelling.

Borehole conductivity logs, besides being useful for identifying, interpreting and correlating geological formations, also find widespread use as auxiliary information in the inversion of airborne electromagnetic (AEM) data. One of the quality checks often applied to AEM inversion results is a comparison between the conductivity structures revealed by borehole conductivity logs in the survey area and the AEM inversion model closest to the borehole, often called an ‘FID point comparison’. Another use of borehole conductivity logs is found in modern AEM inversion procedures, where the borehole conductivity information is included as prior information in a laterally constrained inversion. In most former and present practices, AEM layer conductivities are compared with the measured conductivity in the borehole. However, the borehole conductivity is essentially an apparent conductivity – it is a measured data value – while the AEM layer conductivities are model parameters resulting from inverting AEM data. To avoid comparing data and model parameters we suggest a conceptually clear approach based on an inversion of the borehole conductivity data to obtain a borehole conductivity model, which in turn can be compared with the AEM model. Furthermore, the AEM forward response of the borehole model can, in a consistent way, be compared with the AEM data. In both approaches, we keep track of uncertainty and define quantitative, uncertainty-normalised measures of the difference between borehole and AEM values, and we find simple functional relationships between the two. The methodology is demonstrated on the AEM data and conductivity logs of the Broken Hill Managed Aquifer Recharge (BHMAR) project.

Planning, contracting, data acquisition and processing plus the inverter's quality assessment and inversion of a regional airborne electromagnetic survey may take some months, while the interpretation of the results is a considerably more complex and comprehensive process. Most often an interpretation necessitates additional data that are time consuming to collect and complicated to integrate into an overall model, for example borehole logs, borehole core samples, water chemistry, surface vegetation, satellite imagery plus all existing geological background knowledge. Interpretation basically has to do with identifying categories and finding boundaries between them so that depths, thicknesses and a whole range of other model attributes can be estimated, qualitatively and/or quantitatively. I present two methods using the continuous wavelet transform of finding attributes intended to assist the interpreter: one finds layer boundaries in the smooth multi‐layer models that are most often used in the inversion of large airborne electromagnetic data sets, and the other finds the natural categories of the model parameter. Naturally, being based on the subsurface conductivity distribution, the boundaries and categories suggested are useful only to the extent that they coincide with geological/hydrogeological boundaries and categories – which is for the interpreter to decide.

High-resolution hydrogeophysical data are increasingly acquired as part of investigations to underpin groundwater mapping. However, optimization of AEM data requires careful consideration of AEM system suitability, calibration, validation and inversion methods. In modern laterally-correlated inversions of AEM data, the usefulness of the resulting inversion models depends critically on an optimal choice of the vertical and horizontal regularization of the inversion. Set the constraints too tight, and the resulting models will become overly smooth and potential resolution is lost. Set the constraints too loose, and spurious model details will appear that have no bearing on the hydrogeology. There are several approaches to an automatic choice of the regularization level in AEM inversion based predominantly on obtaining a certain pre-defined data misfit with the smoothest possible model. However, we advocate a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. In this approach, in a process of both confirming and negating established interpretations and underlying assumptions, the inversion results are judged by their ability to support a coherent conceptual model based on all available information. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project.

First of all, we would like to express our appreciation for the thorough Comment by Adam Smiarowski and Shane Mule, CGG, challenging some of our methods of analysis and conclusion in our paper: Chr...

Coastal hydrology is becoming the focus of increasing interest for several reasons. Hydrogeological models need good boundary conditions at the coastline, and with the expected sea level rise due to climate changes, it becomes increasingly important to grasp the dynamics of coastal hydrology in order to predict the consequences of sea level rise for nature and society. We present a helicopterborne transient electromagnetic survey from a region at the North Sea coast in western Jutland, Denmark, carried out at a seriously polluted site with the dual purpose of assessing the extent of the pollution and mapping the coastal hydrogeology to provide data for remediation activities. Data are subjected to constrained inversion with one- dimensional multi-layer (smooth) models. The extent of the pollution plume estimated from a conductive anomaly in the survey results is mainly in accordance with results from other investigations, but also points to hitherto unknown directions of seepage. The interleaving of freshwater extending under the offshore shallow sea and saltwater infiltrating under the onshore freshwater aquifer can be clearly discerned and preferential flow channels are revealed.

Any geophysical measurement is a filter through which the distribution of a certain physical parameter in the subsurface is seen, and the sensitivity function is a characteristic of the method that reveals the nature of this filter. Insight into the structural pattern of the sensitivity function pertaining to a certain transmitter-receiver configuration provides the geophysicist with an image that allows an immediate qualitative understanding of the characteristics of the method. The assets and shortcomings of different measuring configurations can be discussed and understood, and the sensitivity function permits qualified predictions about resolution characteristics of new configurations and measuring strategies. I evaluated a rapid and accurate method for calculating 3D sensitivity functions of a homogeneous half-space model for a wide variety of transient electromagnetic configurations using the central loop and an airborne offset loop configuration as examples. Computations of 3D sensitivity functions were performed as convolutions in the time domain between the electric fields from the transmitter and the receiver, had it been used as a transmitter. The 2D and 1D sensitivity functions are found through numerical integration of the 3D functions. Beside offering insight into the resolution capability of the measuring configuration, the sensitivity functions lend themselves to rapid calculations of approximate responses and derivatives in various modeling and inversion strategies.

SUMMARY We present a derivative analysis method that automatically detects and selects layers in any geophysical borehole trace. Using a wavelet analysis, we delineate relevant boundaries from inflection points. This allows for the automatic, objective detection of layers. Our software classifies layers based on importance in the geophysical data, and allows a user to select blocked layers based on total number of layers detected, a portion of the total layers, minimum layer thickness or the number of layers detected using a minimum operator width. We demonstrate the effectiveness of the layer blocking technique with some field examples in Western Australia and New South Wales for aquifer detection and soil classification.

AEM inversions often involve linearised approximations to the calculation of the Jacobian and Hessian matrices in the forward solution. This results in a second order Taylor series expansion of the estimation of an error surface when calculating the misfit between forward model data and measured data. In the vicinity of a minimum in the error surface, the first-order terms drop out and only second-order terms are present. This guarantees that model parameters resulting from the inversion will be Gaussian distributed with mean model parameter values and model parameter variance terms. The first set is the output of the inversion that is most often used, and we produce conductivity-depth sections from the mean model parameters. However, we are then neglecting the fact that those parameters contain variances which are also of value. In reality, because of the way we have constructed the inversion scheme, the model variance terms contain information about how each model parameter interacts with each other model parameter for a given inversion output result. The collection of variances is most easily assembled in the posterior covariance matrix, where terms on the main diagonal are the autocorrelation values and terms off the diagonal are the cross-correlation or covariance terms. We examine the posterior covariance matrix terms, and exploit the well known characteristics of Gaussian statistics to ask meaningful conditional and marginal probability questions from the inversion data. This is done so that we can ask questions such as: 'Given that the aquifer I am interested in has conductivity ranges between x_low and x_high, where, how deep and how thick is the aquifer?' The result is a probability map that shows the most likely location of the structure of interest given the conditional statements made when posing the question.

Over the past decades, airborne electromagnetic (AEM) surveys have mostly been used in connection with mineral exploration and a variety of issues in hydrogeophysical mapping. However, increasingly, AEM is used for a wide range of geotechnical purposes: pollution mapping, geotechnical assessment on road and freeway alignments, bathymetry, and depth to bedrock. We present an investigation using a helicopter-borne transient electromagnetic system along the planned trace of a gas pipeline. Oil and gas pipelines are often buried at a depth of a few meters and the cost of construction depends critically on whether the subsurface is composed of soft sediments that can be easily excavated or hard rock formations that require much heavier equipment and possibly have to be blasted. The aim of the AEM survey was to distinguish between the soft, relatively conductive sediments and the hard, relatively resistive bedrock in the upper few meters of the subsurface. Data were collected with a rather small transmitter moment, but a high repetition frequency that simultaneously allowed high acquisition speed, and reliable data quality. Measurements were inverted with 1D models with both vertical and lateral constraints to produce model sections along flight lines. A novel method of statistical analysis of the set of equivalent models for each inverted model, calibrated against boreholes, improved the estimates of the presence of hard rock along the flight lines.

The choice of an appropriate airborne electromagnetic system for a given task should be based on a comparative analysis of candidate systems, consisting of both theoretical considerations and field studies including test lines. It has become common practice to quantify the system resolution for a series of models relevant to the survey area by comparing the sum over the data of squares of noise-normalised derivatives. We compare this analysis method with a resolution analysis based on the posterior covariance matrix of an inversion formulation. Both of the above analyses depend critically on the noise models of the systems being compared. A reasonable estimate of data noise and other sources of error is therefore of primary importance. However, data processing and noise reduction procedures, as well as other system parameters important for the modelling, are commonly proprietary, and generally it is not possible to verify whether noise figures have been arrived at by reasonable means. Consequently, it is difficult – sometimes impossible – to know if a comparative analysis has a sound basis. Nevertheless, in the real world choices have to be made, a comparative system analysis is necessary and has to be approached in a pragmatic way involving a range of different aspects. In this paper, we concentrate on the resolution analysis perspective and demonstrate that the inversion analysis must be preferred over the derivative analysis because it takes parameter coupling into account, and, furthermore, that the derivative analysis generally overestimates the resolution capability. Finally we show that impulse response data are to be preferred over step response data for near-surface resolution.

SUMMARY The SkyTEM helicopter transient electromagnetic system has been used to map the environs of a number of tailings dams in Australia. Residues from mining and industrial processes typically have high conductivity and can be effectively mapped using airborne electromagnetic methods. Airborne surveys allow complete coverage of tailings sites otherwise inaccessible to surface geophysics. The SkyTEM system is capable of operating in a dual moment mode, in which measurements with low transmitter current, fast switch off and early-time sampling are interleaved with those performed using a high transmitter moment and measurements to late delay times. This provides high near-surface resolution and the capability to penetrate through thick accumulations of highly-conductive tailings. Surveys around tailings sites are characterised by significant surface infrastructure, which presents hazards to airborne operations and results in numerous cultural electromagnetic anomalies unrelated to the subsurface. These obstacles can be overcome with appropriate survey planning and careful editing of the airborne data prior to interpretation. A brief field example from a coal fly-ash dam in Australia illustrates the ability of the SkyTEM system to provide detailed hydrogeophysical information. Useful data were obtained even in close proximity to metallic infrastructure and high-voltage power lines. The SkyTEM survey was able to identify several zones of anomalous conductivity at points downstream of the embankment, which are a priority for future ground follow-up.

SUMMARY We present a new, efficient and accurate forward modelling and inversion scheme for magnetic resonance sounding (MRS) data. MRS, also called surface-nuclear magnetic resonance (surface-NMR), is the only non-invasive geophysical technique that directly detects free water in the subsurface. Based on the physical principle of NMR, protons of the water molecules in the subsurface are excited at a specific frequency, and the superposition of signals from all protons within the excited earth volume is measured to estimate the subsurface water content and other hydrological parameters. In this paper, a new inversion scheme is presented in which the entire data set is used, and multi-exponential behaviour of the NMR signal is approximated by the simple stretched-exponential approach. Compared to the mono-exponential interpretation of the decaying NMR signal, we introduce a single extra parameter, the stretching exponent, which helps describe the porosity in terms of a single relaxation time parameter, and helps to determine correct initial amplitude and relaxation time of the signal. Moreover, compared to a multi-exponential interpretation of the MRS data, the decay behaviour is approximated with considerably fewer parameters. The forward response is calculated in an efficient numerical manner in terms of magnetic field calculation, discretization and integration schemes, which allows fast computation while maintaining accuracy. A piecewise linear transmitter loop is considered for electromagnetic modelling of conductivities in the layered half-space providing electromagneticmodellingofarbitraryloopshapes.Thedecayingsignalisintegratedovertime windows, called gates, which increases the signal-to-noise ratio, particularly at late times, and the data vector is described with a minimum number of samples, that is, gates. The accuracy of the forward response is investigated by comparing a MRS forward response with responses from three other approaches outlining significant differences between the three approaches. All together, a full MRS forward response is calculated in about 20 s and scales so that on 10 processors the calculation time is reduced to about 3–4 s. The proposed approach is examined through synthetic data and through a field example, which demonstrate the capability of the scheme. The results of the field example agree well the information from an in-site borehole.

The East Kimberley Region of north-western Australia has been identified as a priority for potential agricultural development. Within this region, the Ord Bonaparte Plain is remote, with limited access in an area of great cultural and environmental sensitivity. Initially, spatio-temporal mapping using remote sensing (and potential field) data, combined with data on the deeper basin geology was used to plan an airborne electromagnetics (AEM) survey. The relatively resistive nature of the basin sediments has enabled the AEM to map the hydrostratigraphy to depths of 300-500m, except in the coastal zone affected by seawater intrusion. Two overlying aquifers, separated by a faulted, ‘leaky’ aquitard, have been identified. The AEM and remote sensing data were subsequently used to plan a ground magnetic resonance (GMR) survey. The latter has enabled a water table map to be constructed in an area with almost no drilling, while also enabling key aquifer properties to be determined. The target aquifer has a high free water content and high transmissivity. The GMR results have been validated by drilling, borehole Nuclear Magnetic Resonance (NMR), and induction logging. Integration of AEM, GMR and temporal (Landsat) remote sensing data has enabled rapid mapping and characterisation of the groundwater system in a data-poor, culturally and environmentally sensitive area. These data have also revealed complex faulting within and bounding the aquifer system, delineated the sea-water intrusion interface, and mapped groundwater dependent ecosystems. These data have been used to target drilling and pump testing that will inform groundwater modelling, water allocations and development decisions.

In the Howards East Aquifer (HEA) in Darwin’s Rural District, groundwater resources in a dolomitic and karstic aquifer system provide important water security for Darwin and a large horticultural i...

The presence of Neogene fault systems can have a significant impact on hydraulic connectivity of aquifers, juxtaposing otherwise disconnected aquifers, enhancing recharge and/or discharge or acting as barriers to flow and consequently compartmentalising groundwater resources. Previously, regional airborne electromagnetics (AEM) transects allied with groundwater investigations have pointed to the potential for localised compartmentalisation of the Daly River Basin groundwater systems. However, existing data is sparse, and equivocal. In this context, the main aim of the Daly River Basin Project is to determine if compartmentalisation of the aquifers is a significant factor and thus should be explicitly considered in groundwater modelling and water allocation planning. The objectives of the project main goals of the project are to: (1) map Neogene faults through the use of airborne electromagnetic (AEM) and morphotectonic mapping, and (2) assess the permeability and transmissivity of mapped fault zones and their role in potential groundwater system compartmentalisation. Data acquisition includes 3325 line-kilometres of new AEM and airborne magnetics, ground (ground magnetic resonance (GMR)), and borehole geophysics, drilling, groundwater sampling and hydrochemical analysis, geomorphic and morphotectonics mapping. Hydrogeophysical, geomorphic and hydrogeological data will also be used to better understand groundwater-surface water connectivity and the potential for managed aquifer recharge schemes to replenish extracted groundwater resources. The outcomes of this project will inform decisions on water allocations and underpin effective and efficient groundwater use. This paper specifically reports on the ability of AEM and morphotectonics mapping to identify Neogene fault systems in the Daly River Basin.

The Australian Government’s White Paper on Developing Northern Australia recognised that expanding the size of the Ord irrigation area in the Kimberley Region, northwest Australia offers the potential to significantly enhance both the profitability and economic resilience of the region. This paper reports on the preliminary results of hydrogeological investigations in the Ord Stage 3 development area, a 14,500 ha area of black soil plains in the Keep River floodplain, Northern Territory. Previous investigations in the Keep River floodplain identified potential for groundwater salinity, soil salinity and seawater intrusion (SWI) hazards. These earlier studies recognised that more comprehensive investigations were required to fully assess the risks of large-scale development of irrigated agriculture on groundwater quality and quantity. The Keep River Salinity Mapping Project has been established to provide baseline data on the groundwater system in the Keep River floodplain including aquifer and aquitard distribution and properties, and potential salinity hazards. Specifically, the main aims of the project are to: (1) map the 3D architecture and hydraulic properties of the soil, sub-soils and underlying paleovalley system; (2) map the SWI interface and variations in groundwater salinity; (3) identify potential surface water inundation risks; (4) identify groundwater-dependent ecosystems; and (5) carry out a hydrogeological assessment. Investigations include a programme of airborne electromagnetics (AEM), ground geophysics (ground magnetic resonance (GMR), passive seismic and microgravity), drilling and borehole geophysics, hydrogeological and hydrochemical investigations, and regional soils, geological and morphotectonic mapping. Products generated in this project will be used to parameterise a numerical groundwater model.

Neogene fault systems are increasingly recognised as an important control on hydraulic connectivity in some of Australia’s energy rich basins. However, accurate delineation of these faults systems is challenging and expensive. In this context, the main objective of the Exploring for the Future (EFTF) Surat-Galilee Basin (Phase 1) Project is to test novel methods for more cost-effective mapping of Neogene fault systems in the Coal Seam Gas (CSG) basins of eastern Australia. Methods assessed in this project include morphotectonic mapping using temporal remote sensing data and high-resolution terrain mapping techniques, airborne electromagnetics (AEM), and the use of earthquake databases to inform active tectonic and geomechanical analysis. The project is funded by Geoscience Australia (GA) as part of its EFTF Programme, and is focussed on exemplar areas in the Surat and Galilee Basins where Neogene fault activity has been interpreted on high-resolution 2D and 3D seismic reflection surveys. This paper reports on the use of airborne electromagnetics (AEM) for detecting near-surface (

This study reports the results of a comparative evaluation of 1D, 2.5D and 3D AEM inversions for resolving hydrostratigraphy and structural elements in two contrasting settings: unconsolidated Quaternary floodplain sediments affected by Neogene deformation; and a tectonically inverted Palaeozoic sedimentary basin. Previous studies have demonstrated the importance of airborne electromagnetic (AEM) data optimization to ensure that key elements of the hydrogeological system, including geological faults, are appropriately represented in inversion models. In the inverted sedimentary basin study, 1D inversions of AEM data indicated greater structural complexity than previously known. Initially, a suite of equivalent 1D inversion models produced very similar inversion model results. However, 2.5D inversions produced a disparity in solutions in key locations. To resolve these differences, 3D AEM inversion methods have been trialled. In the second study (floodplain setting), 3D inversions have helped resolve the geometry of hydrostratigraphic units and tectonic elements (folds and faults). In both study areas, independent validation of inversion results has involved an inter-disciplinary approach incorporating a range of borehole and ground geophysics techniques (e.g. passive seismic and Ground Magnetic Resonance (GMR)), tectonic mapping and analysis, hydrochemistry and drilling. In summary, comparative evaluation of 1D, 2.5D, and 3D AEM inversions in two contrasting settings demonstrates the importance of optimizing inversion procedures, taking into consideration all available geological, hydrogeological and tectonic data. The benefits of using 2.5D and/or 3D inversion procedures are particularly evident in areas of structural complexity. Confidence in 3D inversions is maximised when all elements of the system response are modelled appropriately.

In Australia’s semi-arid and arid interior, groundwater resources provide water supply security for agriculture and community consumptive use and are critical for underpinning economic development. The Southern Stuart Corridor Project in central Australia, is an inter-disciplinary study which aims to better characterise regional groundwater systems and identify the location, quantity and quality of new groundwater resources. The main aims of the project are(1) to de-risk investment in development of a potential agricultural precinct in the Western Davenport Basin, and expansion of horticulture in Ti-Tree Basin, (2) to identify future water supplies for Alice Springs and Tennant Creek, and (3) for regional water supplies for mineral resource development. The project is funded by Geoscience Australia (GA) as part of the Exploring for the Future (EFTF) Programme. The project integrates airborne electromagnetic (AEM), ground geophysics (ground magnetic resonance (GMR) and borehole geophysics (Induction, gamma and nuclear Magnetic Resonance (NMR)) with drilling and pump testing; hydrochemistry and geochronology; and geomorphic, geological, hydrogeological and structural mapping and modelling. Advancements in temporal remote sensing technologies for surface hydrology, vegetation and landscape mapping are also used to facilitate the identification of recharge and discharge zones and groundwater-dependent vegetation. This paper reports on initial AEM inversion results for the Alice Springs, Ti-Tree Basin, Western Davenport and Tennant Creek areas and the use of a machine learning approach for rapid geological and hydrogeological interpretation of the AEM data. These machine learning approaches have the potential to significantly reduce interpretation time and facilitate the rapid delivery of project results.

Wepresentanew,fastandversatilemethod,thelateralparametercorrelationmethod, of invoking lateral smoothness in model sections of one-dimensional (1D) models. Modern, continuous electrical and electromagnetic methods are capable of recording very large data sets and except for a few cases, standard inversion methodology still relies on 1D models. In environments where the lateral rate of change of resistivity is small, 1D inversion can be justified but model sections of concatenated 1D models do not necessarily display the expected lateral smoothness. The lateral parameter correlation method has three steps. First, all sounding data are inverted individually. Next, a laterally smooth version of each model parameter, one at a time, is found by solving a simple constrained inversion problem. Identity is postulated between the uncorrelated and correlated parameters and the equations are solved including a model covariance matrix. As a last step, all sounding data are inverted again to produce models that better fit the data, now subject to constraints by including the correlated parameter values as ap riorivalues. Because the method separates the inversion from the correlation it is much faster than methods where the inversion and correlation are solved simultaneously, typically with a factor of 200‐500. Theoretical examples show that the method produces laterally smooth model sections where the main influence comes from the well-determined parameters in such a way that problems with equivalence and poor resolution are alleviated. A field example is presented, demonstrating the improved resolution obtained with the lateral parameter correlation method. The method is very flexible and is capable of coupling models from inversion of different data types and information from boreholes.

We present a fast approximate method for 1D inversion of frequency domain data and apply it to frequency domain helicopter-borne data from the Bookpurnong area of the Murray River, South Australia. The method is based on fast approximate forward computation of transient electromagnetic step responses and their derivatives with respect to the model parameters of a 1D model, with the frequency domain responses and derivatives then found through Fourier transformation of the time-domain counterparts. The inversion is carried out with multi-layer models in an iterative, constrained least-squares inversion scheme including explicit formulation of the model regularization through a model covariance matrix. The method is 30 times faster than conventional full inversion for a layered earth model and produces model sections of concatenated 1D models and contoured maps of mean conductivity in elevation intervals almost indistinguishable from those of a conventional full inversion. In a theoretical forward and inverse modelling study, the fast approximate and conventional computation methods are compared demonstrating the applicability of the approximate method and its limitations. Applied to the Bookpurnong RESOLVE ® FDHEM data set from South Australia, the inversion produces model sections and conductivity maps that reveal the distribution of conductivity in the area and thereby the distribution of salinity. This information is crucial for any remediation effort aimed at alleviating the salinization of the river and the degradation of floodplain vegetation and associated ecosystems.

Since 1979 the Laboratory of Geophysics, University of Aarhus, has been developing a new prospecting tool for obtaining information on the topmost 100 m of the earth. The method is an extension of the conventional geoelectric sounding method, but instead of direct current (DC) the AC-geoelectrical sounding method uses alternating current (AC) with frequencies in the range 100 Hz to 20,000 Hz. The use of alternating current adds an inductive contribution to the ordinary galvanic electric field, thus producing two different sorts of information about the underlying earth structure. These two sets of information are, in many cases, of complementary nature, which enables determination of the ground parameters much more accurately than would otherwise be possible from ordinary DC-geoelectrical soundings. Among these cases is the high resistivity equivalence which appears so frequently in Danish Quaternary deposits.

The resolution capabilities of airborne electromagnetic (AEM) frequency-domain systems are traditionally analyzed in terms of the footprint, which provides a simple measure of the lateral extent of the earth volume involved in a given measurement. However, considerably more detailed insight into the system resolution capabilities can be obtained by studying the 3D sensitivity distribution as defined by the Fréchet derivatives. A qualitative analysis of the 3D sensitivity distributions for six typical magnetic dipole-dipole configurations demonstrates that the spatial resolution characteristics differ widely and that the optimal coil configuration for practical investigations depends on the expected target characteristics. For all six coil configurations, the 3D sensitivity distributions reveal significant sign changesdownwards and outwards from the center, stressing the necessity of reliable starting models for successful inversion of frequency-domain AEM data. Likewise, the central zone of sensitivity for the in-phase component is always larger than for the quadrature, indicating an inferior lateral resolution of the former. A new sensitivity footprint is defined, based on the at-surface behavior of the sensitivity distribution, simply as the area where the sensitivity is at least 10% of its maximum value. For the vertical coaxial (VCA) coil configuration, the size of the sensitivity footprint in the [Formula: see text]-direction (perpendicular to the flight path) is approximately a factor of two larger than in the [Formula: see text]-direction (along the flight path), while there is virtually no difference for the horizontal coplanar (HCP) coil configuration. The ratio of the HCP to VCA sensitivity footprint exceeds one in both [Formula: see text]- and [Formula: see text]-directions, suggesting that the VCA coil configuration has the best lateral resolution.

We propose an approximate two-dimensional inversion procedure for transient electromagnetic data. The method is a two-stage procedure, where data are first inverted with 1D multiple-layer models. The 1D model section is then considered as data for the next inversion stage that produces the 2D model section. For moving platform data there is translational invariance and the second part of the inversion becomes a deconvolution. The convolution kernels are computed by perturbing one model element in an otherwise homogeneous 2D section and calculating full nonlinear responses. These responses are then inverted with 1D models to produce a 1D model section. This section is the convolution kernel for the deconvolution. Within its limitations, the approximate 2D inversion performs well. Theoretical modeling shows that it delivers model sections that are a definite improvement over the 1D model sections. A field example shows ....

Electrical and electromagnetic (E&EM) methods for near-surface investigations have undergone rapid improvements over the past few decades. Besides the traditional applications in groundwater investigations, natural-resource exploration, and geological mapping, a number of new applications have appeared. These include hazardous-waste characterization studies, precision-agriculture applications, archeological surveys, and geotechnical investigations. The inclu-sion of microprocessors in survey instruments, development of new interpretation algorithms, and easy access to powerful computers have supported innovation throughout the geophysical community and the E&EM community is no exception. Most notable are development of continuous-measurement systems that generate large, dense data sets efficiently. These have contributed significantly to the usefulness of E&EM methods by allowing measurements over wide areas without sacrificing lateral resolution. The availability of these luxuriant data sets in turn spurred development of interpretation algorithms, including: Laterally constrained 1D inversion as well as innovative 2D- and 3D-inversion methods. Taken together, these developments can be expected to improve the resolution and usefulness of E&EM methods and permit them to be applied economically. The trend is clearly toward dense surveying over larger areas, followed by highly automated, post-acquisition processing and interpretation to provide improved resolution of the shallow subsurface in a cost-effective manner.

Ground-based electrical and electromagnetic methods are used systematically for quantitative hydrogeologic investigations in Denmark. In recent years, a desire for faster and more cost-efficient methods has led to growing interest in the possibility of using airborne systems, and in 2001 a number of test flights were performed using a frequency-domain, helicopter-borne electromagnetic (HEM) system. We perform a theoretical examination of the resolution capabilities of the applied system. Quantitative model parameter analyses show that the system only weakly resolves conductive, near-surface layers but can resolve layer boundary to a depth of more than 100 m. Modeling experiments also show that the effect of altimeter errors on the inversion results is serious. We suggest a new interpretation scheme for HEM data founded solely on full nonlinear 1D inversion and providing layered-earth models supported by data misfit parameters and a quantitative model-parameter analysis. The backbone of the scheme is the removal of cultural coupling effects followed by a multilayer inversion that in turn provides reliable starting models for a subsequent few-layer inversion. A new procedure for correlation in the model space ensures model sections with slow lateral variations in resistivity, normally assumed in sedimentary environments. A field example from a Danish survey demonstrates that the interpretation scheme can produce satisfactory results within the limitations of the system.