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6th International AEM Conference & Exhibition
- Conference date: 10 Oct 2013 - 11 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 10 October 2013
69 results
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Comparing/Contrasting Manual and Stochastic Interpretations of AEM Data
Authors Jared D. Abraham, James C. Cannia, Burke J. Minsley and Akbar EsfahaniWe compare and contrast the traditional manual interpretation of AEM data of digitizing features with an automatic stochastic based method utilizing a trans-dimensional Bayesian Markov chain Monte Carlo (MCMC) algorithm. The target of interest is the base of an unconfined alluvial aquifer in western Nebraska composed of electrically resistive sand and gravels sitting upon a conductive silt and clay. The objective is to provide a 3-D surface of the base of aquifer for inclusion in groundwater models. The automatic stochastic interpretations provide a robust 3-D surface that compares well with the manually digitized surface. In some areas the stochastic methods provide a more certain interpretation than the single smooth model inversion that were used for the manual interpretation. We feel that the use of the automatic stochastic approach prior to manually inspecting the AEM sections provides substantial timesaving and confidence in the final interpreted results. . The stochastic approch provides an automatic interpretation of the layers within a section and can expedite the interpretation process. We recommend that the use of the automatic stochastic approach prior to integrating complementary data and manually inspecting AEM sections provide substantial timesaving and increased confidence in the final interpreted results. With the continued use of the AEM technique in hydrogeological framework studies a fast and efficient way of providing confident interpretations needs to be implemented.
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Developing an efficient modelling and data presentation strategy for ATDEM system choice and survey design
More LessForward modelling of airborne time-domain electromagnetic (ATDEM) data is used to compare systems and design surveys for optimum detection of expected targets. The large amount of data generated for three dimensional models as well as the system dependent nature of data presents challenges when analysing the forward modelled results. Utilizing the dimensionless quantity of signal to noise ratios and presenting data as a three-dimensional nomo-volume is proposed as an efficient tool to evaluate modelling results.
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Geologically constrained inversion of airborne TEM data
Authors P.K. Fullagar, G.A. Pears and J.E. ReidTwo computer programs have been written to perform rapid TEM inversion in a 3D framework. Both programs operate on a 3D geological model, which facilitates integrated interpretation. In greenfields areas the starting model can degenerate into a discretised homogeneous half-space, to permit “unconstrained” inversion. VPem1D performs 1D inversion at each TEM data location. It is therefore a hybrid 1D/3D approach, ideal for quasi-layered environments. VPem3D performs 3D inversion on time-integrated (resistive limit) data, and is well suited for interpretation of compact conductive targets. Because the programs can invert a geological model, they permit a variety of inversion styles. For example, if one or more geological units are considered uniform in conductivity, the optimal conductivities can be determined for the entire survey area via homogeneous unit inversion. If some geological units are variable in conductivity, heterogeneous unit inversion can be applied to those units alone. Alternatively, because geological interfaces are captured in the model, geometry inversion can be used to adjust the shape of contacts or structures. For example, interpretation of cover thickness is a natural application for VPem1D. Both programs are applicable to data from variety of systems including (but not limited to) Geotem, Tempest, VTEM, Spectrem, SkyTEM, MegaTEM and Hoistem. This paper will illustrate some of the different inversion options on Spectrem data sets from Australia and Brazil.
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Development of high dipole TDEM systems
Authors K.I. Sorensen, S. Mai, K.R. Mohr and N.S. NyboeIn mineral exploration there is a demand for high dipole airborne TDEM systems as the need for delineation of deep conducting targets is increasing rapidly. The strength of the transmitted moment is the far most important parameter when considering the resolution of deep targets. Recent developments in SkyTEM transmitter technology have led to a new generation of high dipole transmitters capable of achieving a fast turnoff while providing magnetic moments up to 1.500.000 NIA. The development process leading to this new generation of transmitters necessitated that numerous compromises were made in order to optimize deep target resolution while maintaining a high level of field efficiency and stability.
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Host medium effects on the response of subsurface conductors in helicopter borne time domain electromagnetic exploration
More LessModification in the decay behaviour of vertical magnetic fields recorded by a typical HTEM system is studied considering a plate conductor in a conducting host. Good to excellent conductors sustain the induced eddy currents and thus yield slower decay rates in the recorded TEM response. For plate (or sheet like) conductors this behaviour can be seen clearly in the conductance aperture diagrams for various TEM systems. As the plate conductance increases, the amplitudes get smaller and yield slower decay rates. Thus it becomes difficult to discriminate between very good to excellent conductors. Measurements of ‘on time’ response or ‘B’ field instead of the ‘dB/dt’ field may improve the conductance discrimination to some extent. Present study reveals that current channelling due to host significantly modifies the decay of fields recorded over good to excellent conductors. For comparison, the responses of the host medium alone and the plate conductor in the absence of conducting host (air) is also computed. As the host medium conductivity increases its response dominates the response from the plate conductor to later times. Response in the presence of a very resistive (10,000 Ω.m) host is found close to that from the plate in air. However, the conductance aperture diagrams for various host medium resistivity values reveal that there is a divergence in various channel amplitudes for very good to excellent conductors. This is particularly observed for cases when the resistivity of the host medium is towards the higher values. For the lower values of the host medium resistivity, however, the response of even very good plate conductors is dominated by the currents in the host medium. The study suggests that a mildly conducting host medium may help in discriminating the conductivity of very good to excellent plate conductors.
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A Comparison of Helicopter-Borne Electromagnetic Systems for Hydrogeologic Studies
Authors Paul A Bedrosian, Cyril Schamper and Esben AukenThe increased application of airborne electromagnetic (AEM) 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 meters. We present an evaluation of four commercial AEM systems over two test blocks in Western Nebraska, USA. The selected test blocks are representative of shallow and deep alluvial aquifer systems, and have a conductive base-of-aquifer. The aquifer units show significant lithologic heterogeneity, and include both modern and ancient river systems. We compare the various data sets to one another, and inverse resistivity models to borehole lithology and to ground geophysical models. We find distinct differences among the AEM 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 calibration or compensation procedures.
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Challenges in Integrating AEM and Borehole Geophysical and Hydrogeological Information to Produce Maps of Hydrogeological Properties and Tectonics
In this study, the SkyTEM system was used to map key functional elements of the hydrogeological system critical to the identification and assessment of managed aquifer recharge (MAR) sites and potential groundwater resource targets. A suite of customized interpretation products were produced through integration of AEM data with data from 60 sonic-cored holes and 40 new rotary mud and. Data obtained from this program drilling includes: sedimentary facies, textures (including grain size), mineralogy, redox state and whole rock geochemistry; hydrogeophysical data (nuclear magnetic resonance (NMR), induction and gamma logs); hydraulic data (from slug and pump tests), hydrochemistry (from groundwater and porefluid samples), and hydrodynamic data from the monitoring of groundwater levels in 40 boreholes pre- and post-flooding in 2010-2011. Customised interpretation products developed through integration of these datasets include: maps of the sedimentary system including palaeochannels with favourable hydraulic properties; hydrostratigraphy; confining aquitards; textural classes within the sedimentary system; tectonic elements; zones of inter-aquifer leakage and groundwater flow pathways; groundwater salinity, aquifer transmissivity; MAR storage volumes; and groundwater storage estimates in various water quality classes (0-600; 600-1200; and 1200-3000 mg/L). The products were integrated with other datasets (including time series vegetation condition, surface geomorphology, flood inundation) for MAR risk assessments of various options and targets, with a priority target identified and positively assessed. Volume estimates for fresh, acceptable and brackish groundwater stored within discrete targets was particularly challenging due to the hydrogeological complexity. The methodology used relied upon a multi-scale approach. Salinity class thresholds were estimated by comparing the pore fluid data with the AEM response. Bulk volumes for each water quality class in a target were then calculated using these thresholds on an AEM depth slice basis, which had been mapped into textural classes. Gravimetric water and textural data from sonic cores were compared with borehole NMR data and laboratory effective porosities (from Lexan-encapsulated core) to estimate an effective porosity range for each textural class. These were used to convert the depth-slice bulk volume estimates to stored groundwater volumes. Sensitivity analysis revealed that there are significant uncertainties in volume estimates using this approach. There can be orders of magnitude differences in volume calculation depending on the AEM inversions used, with significant variations also found depending on the salinity thresholds used, and the uncertainties linked to the effective porosity estimation. This study has demonstrated the importance of selecting the most appropriate AEM system and optimizing the AEM inversions for generating a wide range of customized interpretation products. For estimating groundwater quality and volumes, there are large uncertainties due to the inherent issues with individual measurement methods, compounded by integration, scaling and extrapolation issues. Such mapping and estimates of groundwater salinity and volumes are still useful guides, but ultimately, the assessment of groundwater resources identified using these datasets requires numerical groundwater and solute transport modeling . Such modeling is needed to determine the duration and rates of supply possible from the identified targets, and to assess potential environmental and resource impacts from prolonged extraction during drought conditions.
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Processing and Inversion of Commercial Helicopter Time-Domain Electromagnetic Data for Environmental Assessments and Geologic and Hydrologic Mapping
Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We have developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semi-automatically identified on the basis of slope changes in the dB/dt transients and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. As examples, the average resistivity of the thick surface layer decreased from ~1800 to ~30 Ωm, the depths to the layer boundaries were reduced by 15%–23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.
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Groundwater prospecting near Zouerate, Mauritania with airborne TDEM and magnetic data sets
Authors E.H. Stettler, J.P. Smit, R.C. Whitehead, F.E. Wiegmans, G. Canahai, S. Van der Merwe and N. VeiletteThe new Askaf mine with its estimated 3 billion ton of iron ore near Zouerate in northern Mauritania needs a guaranteed water supply of 11 million litres/day. The Touadeni Basin to the east has the potential to deliver this if the geology of the basin can be properly understood. Aeromagnetic data allowed the identification of faults and thrusts on the adjacent Reguibat Shield and tracing them into the Touadeni Basin. Two eastwards dipping conductive horizons in the Basin could be identified on TEMPEST CDI’s and existing borehole information allowed them to be correlated with dolomite layers. The contact between the dolomite and underlying sandstone-shale layers has frequent water strikes and were selected as targets especially where evidence existed of faulting. Tau maps played a major role in choosing the targets.
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Maximising the benefit of historic Airborne EM through new modelling; 36 surveys over a decade for building a basin-wide conductivity model for uranium exploration
More LessSince 2001, Cameco has been acquiring airborne electromagnetic data over their tenements in the Arnhem Land region of northern Australia to assist in uranium exploration. Airborne EM has predominantly been used for mapping the depth to the sandstone-basement unconformity, and geological structures that favour uranium mineralisation. Thirty-six surveys have been acquired using the TEMPEST® system over the last 12 years in the region. These surveys have historically been limited to individual tenements that were being explored at the time, and as such have been typically treated individually and in isolation to one another. Originally, conductivity models were generated using EMFlow on Z component data only. In 2012, Cameco re-modelled these datasets using the latest inversion algorithms utilising the non-geometry corrected “raw” X and Z component data. This has increased the spatial resolution of the resultant conductivity model, providing an improved product for geological interpretation. The individual surveys were merged together with no stitching, and the resultant basin wide compilation shows the excellent stability of the TEMPEST® system throughout the 12 years of surveying. Re-modelling has markedly improved the conductivity models from these surveys and allowed a basin wide compilation to be constructed to aid in exploration, particularly by providing a regional structural framework which is not always obvious at the tenement scale.
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A simple correction for ATEM data acquired over a linear slope
Authors J. Guillemoteau, P. Sailhac and M. BehaegelIn this abstract, we present the theoretical background for the geophysical EM analysis with arbitrarily oriented magnetic dipole. We study the case of airborne EM measurements over an inclined ground. This context can be encountered if the measurements are made in mountain area. We show in particular that transient central loop helicopter borne magnetic data should be corrected by a factor proportional to
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Extracting Geology from Airborne EM
By Jan GunninkAirborne EM provides quick and relatively cheap spatial coverage of resistivity distribution in 3 dimensions. After data processing and inversion, the main challenge is to extract relevant geological and geohydrological information from the resistivity distribution for further use in 3 dimensional modeling. Two case studies are described, the first one in which airborne EM was successfully integrated with available borehole data to create a 3 dimensional model of the distribution of clay and sand. In another study, Artificial Neural Networks were used to extract geological information from AEM resistivity Resistivity derived from AEM can be linked to geological features in a number of ways. Besides manual interpretation, statistical techniques are used, either in the form of regression or by means of Neural Networks, to extract geological and geohydrological meaningful interpretations from the resistivity model
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Inversion of AEM data to assist exploration strategies in a regolith-dominated terrain: Yilgarn Craton, Western Australia
Authors Alan Yusen Ley-Cooper and Ignacio González-ÁlvarezThe conductive regolith present in most of Australia compromises the use of airborne electromagnetic (AEM) data, limiting the method's depth of penetration and its ability to unveil basement mineral anomalies under conductive cover. Here we describe an approach where by incorporating the highly conductive EM response of the 'overburden' rather than avoiding it in the interpretation; we increase the understanding of the cover's architecture and the broader exploration scenario. A better understanding of regolith distribution and stratigraphy has significant impact in the planning of drilling targeting and geochemical interpretation of surface anomalies. Detection of potential mineral targets, data driven geological sections and constrained regolith architecture such as thickness and main stratigraphical units can be achieve by: 1) operating a deep penetrating AEM system such as SPECTREM, 2) applying an inversion algorithm which can simultaneously resolve a same 1D model from X and Z component data and address unknowns around the system's geometry and 3) combining ancillary data,
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3-D HEM Modeling Studies in Northern Germany
Authors Bernhard Siemon, Angelika Ullmann, Marion P. Miensopust and Dominik SteinmetzAEM data sets are used for both geophysical and geological modeling. For generating geophysical (resistivity) models, commonly 1-D inverse modeling procedures are applied. In many cases such 1-D models are sufficient, e.g., for horizontally layered targets and groundwater applications. In case of strong 3-D targets, however, this may result in misleading interpretation due to strange 1-D models, hence multi-dimensional modeling and inversion is required. The number of 2-D and 3-D AEM modeling and especially inversion codes is limited and they are also often subject to restrictions. Limiting are mainly the requirements related to the allowed resistivity structure but also the required memory and computation time to run models of decent size. Thus, real 3-D modeling is often restricted to ‘simple’ resistivity models. Full 3-D AEM inversion codes are generally not freely available. On the other hand, geological modeling uses diverse commercially available software packages. The direct integration of AEM resistivity models in geological models, however, is still challenging due to both appropriate codes and an often non-unique relationship between resistivity and lithology. In order to get reasonable resistivity models suitable for 3-D geological modeling we try to model AEM data in 1-D to the greatest possible extent and therefore restrict 3-D modeling to areas of strong resistivity anomalies only. Those limited 3-D areas are identified either manually or automatically. The quasi 1-D data are inverted using standard 1-D inversion procedures and the anomalous data are modeled in 3-D using forward or inverse modeling procedures. Once a satisfying 3-D model is obtained it needs to be integrated into the quasi 1-D environment of the remaining data set. The challenging task is, besides 3-D inversion itself, to define those areas where 1-D inversion fails and to extract and integrate the 3-D data and models, respectively. The 3-D geophysical and geological modeling is applied to spatial data measured by geophysical surveys, particularly an HEM survey flown by BGR in Northern Germany in 2000. There, the melt-water flow carved the North-South orientated Cuxhaven buried valley into Tertiary sediments during Pleistocene glacial regression epochs of the Elsterian glaciation.
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Airborne EM systems variety: what is the difference?
Authors A. Volkovitsky and E. KarshakovThe challenge in developing airborne electromagnetic systems was followed by the appearance of a wide variety of different kinds of such systems. Even in the case of considering active inductive systems with only carried transmitters. Sometimes it seems that these different airborne electromagnetic systems are based on completely different laws of physics. But all of them, Time-Domain and Frequency-Domain, with different transmitter-receiver geometry, with use of different primary field waveform – are based on the same principles: time-variable magnetic field generated by any transmitter induces eddy currents in buried conductors and the secondary field of these currents measured by an inductive receiver can give us information about the geology structures. The objective of this paper is to review the variety of systems from different points, especially in terms of signal structure, to analyse advantages and limitations of many existing systems and to suggest an approach that can make possible the ability to use the achievements in all of them during developing a new one. An attempt to realize this lead to the appearance of the system EQUATOR. The practice of using the system EQUATOR in both isolating and conductive environment for different type of targets fully confirms effectiveness of this approach.
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Sampling the transient decay
Authors David Annetts and Juerg HauserTypically, airborne electromagnetic (AEM) prospecting systems employ a standard number of windows to sample the transient electromagnetic decay. The number and definition of these windows is a function of the system’s noise characteristics and the structure to be imaged. Using 1D models with application to groundwater and to minerals exploration, we show that model parameter resolution can depend on the nature of the windows used to sample the decay and suggest that there may be value in optimising window parameters to suit particular transient decays.
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The importance of the TDEM waveform and characterization of discrete conductors
Authors Flemming Effersø, N.S. Nyboe and P. GisseløIn airborne time-domain EM (ATEM) the signal-to-noise ratio (SNR) is paramount for the detection of small responses from discrete conductors. In this paper it is examined how shortening the linear current turn-off can enhance the target responses significantly for discrete 3D conductors. For the objective of the paper two methods were applied (1) synthetic 2D thin sheet modelling as approximations to 3D discrete conductors and (2) comparison of real datasets collected over discrete conductors using two different ATEM systems having respectively a 200 s and a 1,200 s turn-off ramp. The findings of both the synthetic modelling and the real datasets show that the target response can be amplified by a factor 2 or even more if the length of the current turn-off ramp is shortened from 1,200 s to 200 s. The enhancement of the target response, and thereby the SNR, occurs for a large group of discrete conductors for which the time constant is comparable to or smaller than the duration of the current turn-off ramp. Shortening the current turn-off ramp will improve the capability to detect such conductors.
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Geobandwidth: comparing EM waveforms with a wire-loop model
More LessWe compare time domain systems of different waveform shape, power and receiver sampling times using a wire-loop conductor model to define a comprehensive “geobandwidth” that shows the strength of the response over a range of decay constants, analogous to a range of conductance. Frequency domain EM responses can also be calculated as a function of decay constant for a wire loop model, giving a consistent comparison method for all time domain waveforms and frequency domain. Peak decay constant or equivalent frequency can be determined analytically or numerically. Arbitrary waveforms can be modelled as a sum of simple short ramps, and the geobandwidth determined numerically. The frequency content of a time-domain system can be defined by the peak decay constant or the equivalent frequency. The results compare response amplitude across the full range of geological target conductance. Systems can be compared on the basis of signal or signal/noise ratio.
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Extending the range of conductivities detected by the Spectrem AEM System
More LessThe SPECTREM AEM system [ Annan A.P. 1986 ] generates the step response of the ground to a repetitive sequence of transmitter steps switching from positive to negative and back again. Apart from the intervals where the transmitter is in transition it is always on. This means that the ground response and the transmitter signal are present in the receiver at the same time. In order to retrieve the secondary signal alone the transmitter signal must be removed from the receiver secondary signal. Unlike a ground system like UTEM [West G.F. 1984] which also measures the secondary step response and the primary field at the same time, both the receiver and transmitter positions are fixed during the recording and the amplitude of the transmitter can thus be computed from the geometry. For a towed bird system like SPECTREM, this method cannot be used to compute the amplitude of any component of the transmitter primary at the bird as the relative positions, and rotations, of the transmitter and receiver coils are constantly changing. For a particular component (say Z) at the receiver the amplitude of the primary signal can change by a few percent from one reading to another while the changes in the secondary (ground) signal are of the order of a few hundred ppm. That is, the secondary signal of interest is buried in a not dissimilar transmitter signal three orders of magnitude larger. The traditional method SPECTREM uses for the real time processing performed in flight is to assume that near the end of each half cycle, the secondary signal due to the transmitter transition (step at the start of the half cycle has more or less decayed away and this late time signal can be regarded as primary signal only, and when subtracted from all the preceding data in the half cycle the result is deemed to be the actual secondary signal. By choosing lower base frequencies for the transmitter the separation of this late time signal from the transmitter switch- over is increased and so better estimate of the primary field is obtained, as more of the secondary signal will have died away. At a base frequency of 25 Hz the transmitter signal changes sign every 0.02 seconds ( two transitions per cycle ). Assuming the secondary signal at 0.02 seconds is just at the noise level of 100 ppm then, for an inductive limit secondary signal amplitude of say 25% of the primary field the decay constant must be less than about 3700 microseconds. Any decay rate slower than this will have a secondary signal amplitude at 0.02 second delay time, greater than the noise level and by subtracting this (non zero) signal together with the true primary signal from the preceding data it will cause them to be under estimated. If one assumes a 50,000 microsecond decay constant and the same inductive limit amplitude as the previous example, the late time amplitude at 0.02 seconds delay would be is 167,580 ppm and so all the preceding secondary data will be reduced in amplitude by this amount when it is subtracted along with the true primary. This is likely to push the signal below the noise level for nearly all the data, apart from some at early times. This results in the anomaly being unlikely to be selected as a target for further work. So the really good conductor (think Nickel) is then effectively, rejected due to poor processing. Several techniques for computing a better estimate of the primary field will be demonstrated, as well as the one currently in use by SPECTREM, ( and currently being improved upon) together with illustrations on actual field data.
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Obtaining both early and late time AEM data: the pros and cons of almost perfect systems and almost perfectly known systems
By James MacnaeBoth 'almost perfect' and 'almost perfectly monitored' approaches to getting good early time AEM data work, but have several pros and cons. Common factor is the need for accurate removal of a bias response from residual transmitter and / or bucking coil currents. To achieve this bias correction, low noise systems need either much higher altitude data as references (1600 to 2400 m rather than the usual 700 to 800 m presently collected), or in theory could use 2 or more lower altitudes to predict the primary field response at extreme altitude.
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Sharp boundaries in multi-layer models via Minimum Gradient Support regularization
We present a focusing regularization technique utilizing a multi-layer model with fixed vertical discretization, while preserving the capability to reproduce sharp vertical transitions. The method relies on minimizing the number of layers of non-negligible resistivity gradient, instead of minimizing the norm of the model variation itself. AEM methods are capable of producing extremely large datasets that are conveniently inverted for smoothly varying 1D models of fixed vertical discretization. The vertical smoothness of the obtained models stems from the application of Occam type regularization constraints, meant for addressing the ill-posedness of the problem. An important side-effect of such regularization, however, is that sharp vertical layer boundaries can no longer be accurately reproduced as the model is required to be smoothly varying. This issue can be overcome by inverting for fewer model layers using variable layer thicknesses, but having to decide on a particular and constant number of layers for inversion of a large survey can be equally problematic. Here, we present a focusing regularization technique for getting the best of both methodologies. It allows for accurate reconstruction of resistivity distributions using a fixed vertical discretization, while preserving the capability to reproduce sharp vertical transitions. 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. This approach ensures model results that are consistent with the measured data while favouring, at the same time, sharp vertical transitions. The formulation is general and can also be applied in a horizontal direction, in order to promote sharp lateral transitions such as faults. We present the theoretical framework of our regularization methodology and illustrate its capabilities by means of both field and synthetic datasets. We further demonstrate how the concept has been integrated in our existing Spatially Constrained Inversion (SCI) formalism and show its application to large scale inversions of airborne time-domain electromagnetic data.
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A Bayesian approach to the interpretation of airborne electromagnetic surveys: Quantifying data errors, model assessment, and lithology classification
Authors Burke J. Minsley, Akbar Esfahani, Maryla Deszcz-Pan and Ross C. BrodieAnalysis of uncertainty is a crucial, yet often overlooked, aspect of any geophysical inverse problem. For airborne electromagnetic (AEM) data, uncertainty analysis is compounded by the large volume of data that are typically acquired in a survey. Here, we describe a Bayesian Markov chain Monte Carlo (McMC) algorithm initially developed for the analysis of frequency-domain electromagnetic data, including AEM, along with examples where this algorithm has been used to add new insight into model uncertainty. Recent algorithm developments will also be presented, including capabilities to assess random or systematic data errors as unknown parameters, simultaneously run multiple soundings in parallel allowing for the analysis of large surveys, speed up and assess convergence of the McMC algorithm, and to implement these methods for either time- or frequency-domain datasets.
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A general approach of inferring 'derived products' from AEM inversion results
Authors Niels B. Christensen and Anders V. ChristiansenThe physical parameter found from interpreting AEM 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 translated to provide information on the actual target properties of the survey: sand/clay (geological interpretation), saturated/unsaturated and fresh/salt (hydrogeological interpretation), polluted/not-polluted (biohazard/geotechnical interpretation); estimation of hydraulic conductivity (hydraulic modeling); pristine/disturbed (forensic interpretation), etc. The parameters of these categories are often called "Derived Products". The translation process can be done in a wide variety of ways: from a predominantly qualitative approach based on professional experience to an application of rigorous quantitative relations found from scientific endeavors. In most practical situations, the number of locations with independent information on the Derived Product is considerably smaller than the number of geophysics locations; it is precisely the scarcity/sparsity of information on Derived Product that encourages the use of geophysical inversion results as a sort of qualified interpolator through a formulation of a functional relation between a geophysical parameter and the parameter of the Derived Product. We present a general, quantitative, inversion-based approach to deriving the parameter of interest. Three different methods are presented and compared through numerical simulation with a few illustrative examples.
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Comparing Integral Equation and Finite-Element Airborne Electromagnetic Modelling Using Public Domain Software
Authors E. Meunier, M.A. Vallée and C. SamsonLeroiAir and ArjunAir are two computer programs used to model airborne electromagnetic data, however based on different mathematical algorithms. LeroiAir is a 3D thin plate program developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). ArjunAir, also developed by CSIRO, is a 2.5D finite-element program. When these programs are applied to the modelling of discrete conductors, a key difference is in how they represent plates. In LeroiAir, the plates are infinitesimally thin. On the other hand, plates in ArjunAir require a minimal thickness. In this short paper, the two programs are used to model responses of simple subsurface models and these responses are compared. Both programs performed identically in models without plates, as expected. Though the results of models where plates were present were similar, a few differences could be identified. These differences, however, could be explained by physical effects (such as plate thickness or horizontal eddy currents induced on the flat top of the plates) or by modelling and discretization effects (such as number of cells, horizontal/vertical adaptation, and cell size compared to skin depth). They were not necessarily caused by the different algorithms used by the programs.
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3D sheets inversion with accurate modeling of AEM systems
Authors Cyril Schamper, Esben Auken and Casper KirkegaardModeling of 3D thin conductive plates most often considers a homogeneous very resistive background media, which is sufficiently accurate for a large of part of mining exploration surveys where resistive bedrock is very likely. However, cases exist where background earth can be conductive, e.g. with a sedimentary overburden at near surface. For such scenarios, it is necessary to at least consider a tabular background earth model (that could also vary along the flight lines) to limit the errors on the thin plate models estimation. Through different scenarios we show how important the consideration of this conductive overburden is, and how bad the determination of the thin plates’ parameters can be if this near-surface layer is not accurately determined. The inversion tests of synthetic data show that a too resistive considered overburden results in a too deep thin plate model, and a too thin overburden in a both too deep and badly oriented thin plate. Thanks to our previous experiences on accurate AEM system modeling, we are able to accurately model AEM systems to avoid misinterpretation with this new 3D sheet inversion code. We particularly insist on the importance of the system calibration. Synthetic inversion tests show that if an error in amplitude level has a negligible impact, a bad timing of ~ 5 μs leads to bad orientation estimates. The 3D sheet modeling algorithm, based on surface integral equations, has been implemented within our in-house inversion program AarhusInv. Optimization for fast solving of the scattered fields and for managing a large number of source-receiver positions have been made to compensate the computation drawback of considering a tabular background (Green functions with Hankel transforms). To improve the efficiency of the code even more, efforts have also been made to parallelize it through the OpenMP library to benefit from today’s computers, which can cheaply hold several processor units.
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Locating hidden palaeo-channels in the Cariboo Mining District, BC, Canada, using high resolution SkyTEM survey data
Authors D. Eberle, R. Schwarz and N. EbelA high resolution time domain electromagnetic and magnetic survey was flown over parts of the Cariboo mining district in British Columbia, Canada, using the helicopter-borne SkyTEM system. The objective of this survey was to locate palaeo-channels which are known for their placer gold potential in this area. Assuming conductive channel fills incised in more resistive bedrock, narrow elongated conductivity features were picked from the conductivity-depth data sets. The magnetic data set was used for structural analysis. In addition, the analytical signal of the magnetic field was computed which displays narrow increased amplitudes following the course of the previously identified conductivity features. Subsequent to the integrated interpretation of the airborne geophysical data and field mapping, a reverse circulation drilling program was launched on one of the selected conductivity features which were supposed to indicate a hidden palaeo-channel. Seven boreholes were sunk, all indicating glaciofluvial sediments, mostly clay and mud with intercalated layers or lenses of pebble and gravel. As yet, the source of the analytical signal anomalies occurring over the channel fillings has not been followed up. The combined evaluation of high resolution time domain electromagnetic and magnetic data in the search for hidden palaeo-channels in the Cariboo mining district has proven outstandingly successful.
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Country-wide airborne EM in classifying mineralized black shales in Finland
Metamorphosed sedimentary rocks rich in organic C and S, known as 'black shales', are encountered in Paleoproterozoic supracrustal rock units all over Finland. They have long been known for their ore potential, both for black-shale-hosted deposits and as an exploration tool for sulfide deposits occurring next to the black shale units. Black shales are easy to recognize on airborne geophysical maps because of their stratigraphy-related, coupled magnetic and conductive patterns. Geophysical responses of black shales vary within geological units and the measured responses are controlled by bedrock geology and structure, overburden and conductivity structures. Black shales are attributed with typical banded but irregular magnetic anomalies related to ferromagnetic monoclinic pyrrhotite, often present in mineralized environments. The regional distribution of black shales in Finland was mapped by correlating airborne magnetic and electromagnetic (EM) data in the late nineties. To our knowledge, the 1 : 1 million map was the first published nation-wide black shale map in the world. High-resolution airborne geophysical data (magnetic, frequency-domain electromagnetic and radiometric data) cover the whole country. Of the multi-frequency EM surveys, the 3 kHz data cover whole Finland. The maximum depth penetration of the system varies from 70 to 100 m, so that these data indicate conductivity variation only in the shallow subsurface. During 2009-2013 we have updated the black shale database and reinvestigated all the deep drill cores from which graphite bearing rocks or black shale had been reported during exploration. Black shale units as inferred from airborne geophysical data were verified with drill-core and outcrop information. The combination of airborne magnetic, electromagnetic and radiometric survey data has been used for the identification and classification of black shale units and mapping their regional distribution in the bedrock. Geochemical and petrophysical data was analyzed to characterize different black shale units in Finland. Petrophysical and geochemical data were analyzed with a variety of methods, for example by using non-metric multidimensional scaling (NMDS). Its application in this work led to the recognition of five black shale classes with distinctive characteristics. Even though black shales are easy to identify in airborne geophysical data, interpretation of their ore potential is challenging: their geophysical properties vary from site to site and within individual lithostratigraphic units. The surveyed airborne geophysical responses are controlled by the bedrock geology and structure, the overburden and the conductivity structures and texture. The black shale database composed of airborne geophysical, geochemical and rock physical data sets facilitates data integration, statistical analysis and interpretation. These results integrated with magnetic structural analysis provide a good tool for investigation, characterization and classification of black shales. They can be used both in predicting mineralized areas in regional context and also be applied in detailed geological and geophysical studies.
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3D-spectral CDIs... a fast alternative to 3D inversion?
By James MacnaeVirtually all AEM is interpreted using stitched 1D conductivity sections, derived from constrained inversion or fast but fairly accurate approximations. A small subset of this AEM data recently has been inverted using either block 3D models or thin plates, which processes have limitations in terms of cost and accuracy, and the results are in general strongly biased by the choice of starting models. Recent developments in spectral modelling have allowed fast 3D approximations of the EM response of both vortex induction and current gathering for simple geological target geometries. Fitting these spectral responses to AEM data should be sufficient to accurately locate current systems within the ground, and the behaviour of these local current systems can in theory approximately define a conductivity structure in 3D.
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Regional AEM Survey in NE Namibia
Authors G. Peters, G.J. Street, I. Kahimise and D. Hutchinsequence to open new areas for mineral exploration. This review of the data acquired showed that; • The TEMPEST208 system did have some noise problems but significant improvements could be achieved with minor system modifications. • TEMPEST208 can map areas of thin (0-60m), to medium (~100m) and thick cover (>150m) Kalahari Sequence. In areas of thin cover, conductors can be detected in the underlying basement. In areas of medium cover an estimate of thickness of the Kalahari (with LEI) is probably possible but in areas greater than 150m the system generally did not appear to detect the base of the Kalahari. •Comparison with detailed surveys done with standard TEMPEST and VTEM shows that TEMPEST208 detected most of the late time features seen in surveys by the more sophisticated system. •The data will be useful for explorers selecting best areas for exploration particularly where the Kalahari Sequence is less than 60m.
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Study on the potential of recovering IP parameters from Airborne TEM data in layered geology
Authors Andrea Viezzoli, G. Fiandaca and S. SergioThe possibility of extracting IP information from AEM data never stopped being of interest to both the scientific community and industry. These synthetic studies on AEM data (SkyTEM and VTEM) for simple Cole Cole models over layered earth confirm that some of the Cole Cole parameters can, in favourable conditions, be recovered from AEM data. We used both few different mineral (layered) models and permafrost models. In many cases the Cole Cole parameters associated to such units give measurable IP effects on the AEM transients, and can subsequently be recovered satisfactorily through inversion of single or multi layers models. The synthetic study also provides useful insight into different ranges/combinations of resolvable parameters. The largest IP effects, and best inversion results, are obtained with moderate to high chargeabilities, higher S/N ratio of the AEM system, and longer ramp decay times. The Laterally and/or Spatially Constrained Inversion approaches improve the model parameters recovery.
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Increasing geotechnical design efficiency by virtue of HTEM sediment mapping
Authors H. Anschütz, A. Pfaffhuber, E. Auken, J.B. Pedersen, C. Schamper, A. Sagbakken and F. EffersøA new road segment is being planned northeast of Norway's capital city, Oslo. In this context, knowledge of sediment thickness is vital, as is information about occurrence and extent of highly sensitive marine clay (so-called quick clay). Airborne EM measurements were conducted to provide information of depth to bedrock/sediment thickness between drilling sites and guide the further drilling program. AEM data indicate a variable bedrock depth with a general trend towards shallower bedrock in the northeastern part of the investigation area. Quick clay is not easily identified in the AEM data, but some possible occurences agree well with the results from drillings. Based on the AEM results recommendations for further drillings were given, thus reducing the overall costs of the project.
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Estimation of overburden thickness using airborne time-domain EM data and a few drill hole data
Authors Michel Chouteau, Zakaria Boudour, Michel Parent and Denis MarcotteWe present a technique for mapping the overburden thickness based on inversion of MEGATEM airborne electromagnetic data. These data are extensively available in many parts of Canada prospected for bedrock conductive targets in mineral exploration. The use of these data coupled with existing stratigraphic information at a few sites make possible the accurate estimation of the overburden thickness at very few expenses. Two sequential steps are used. The first step consists in inverting existing MEGATEM airborne electromagnetic data. The model to be inverted consists of two layers, a moderately conductive overburden lying over very resistive bedrock. Because the data are collected at long time gates with the MEGATEM system, the solution is non-unique, only the conductivity-thickness of the first layer can be well resolved but neither the conductivity nor the thickness. Actually the overburden can be generally modeled as a thin sheet. In the second step we use the expected correlation between depths known at a few drill holes and the estimated conductivity-thickness to estimate depths at sites where only conductivity-thickness data is available using the cokriging technique. In this study the overburden thickness known from drilling is the poorly sampled primary variable and the estimated conductivity-thickness from AEM inversion is the densely sampled secondary variable. The technique was tested on modeled data first, and then on existing survey data from the Octave river area, located in North-West Abitibi (Quebec, Canada), where overburden thickness data was available. The tests carried out in this project showed that the proposed technique allowed accurate thickness estimates even when very few drill hole data were available.
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Assessing four AEM systems; throughout their capacity to deliver target geometry and character
Authors Alan Yusen Ley-Cooper and Tim MundayIn the Musgrave Province of South Australia, local scale airborne electromagnetic data sets are being employed to build pre-competitive data bases and to develop hydrogeological conceptual models in support of the minerals industry and for regional scale water resources assessment. The work has required the re-processing of historical TEMPEST and VTEM data sets and acquisition of SkyTEM508 and SPECTREM2000 airborne EM (AEM) data. For a more precise assessment we have processed and inverted the AEM data using a same model parameterization (fixed 30 layer thicknesses) and one common inversion kernel. We inverted data from all systems using Geoscience Australia's LEI algorithm, and inverted each sample independently. The capacity to resolve targets of a different nature resides mainly on system design and its nose levels, whilst the ability to accurately model the system and account for the processing that occurs post acquisition enables a more accurate inversion of the data. Applying a common procedure to the estimate of noise-levels, tries to deliver a less bias system comparison. Data for two distinct areas containing 1) an anomaly with some of the characteristics of a mineralization target and 2) another with contrasting hydrogeology and significant sedimentary character; indicate the complex evolution of the landscape. The systems employed resolve similar structures despite their different noise-levels, geometries, induction waveforms, transmitted current, and footprints. Not surprising is the ability of certain systems to detect and resolve a more complex near-surface aquifer variability features.
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Regolith Mapping and Characterisation with AEM - A Review
By Tim MundayRegolith materials, which have been described as covering “Everything from fresh rock to fresh air”, exhibit marked contrasts in conductivity that have a profound effect on electromagnetic response. Lateral variations within the regolith can produce spatially coherent responses in EM data, and although those seeking deep targets may regard such characteristics as noise, they can be used to advantage when this information is used with other exploration technologies. Horizontal and vertical changes in conductivity may reflect changes in material type, the influence of bedrock and structure on regolith development, and groundwater condition. This information has significance from an exploration perspective, as it may help constrain geochemical sampling strategies and the interpretation of multi-element geochemical and hydrogeochemical data in regolith dominated terrains. In the last decade, significant developments in EM technology have occurred, particularly with helicopter time domain EM systems. Improvements in signal and a reduction in noise have presented the opportunity to map variability much nearer surface, and to map regolith characteristics and variations in considerably more detail to depth. These improvements, although allowing detection of conductors at greater depths, have resulted in systems also becoming sensitive to superparamagnetic (SPM) effects that may be induced by the concentration of maghemite gravels through geomorphic processes accumulating in or adjacent to palaeochannels in transported regolith. Recently advances with fixed-wing TDEM systems have afforded the opportunity to map regolith variability at regional scales and increasingly AEM data from these systems is being viewed as another part of the pre-competitive data suite offered by government agencies. Being able to measure system geometry and account for it when processing the data enables us to resolve variations in regolith cover sequences more accurately at a range of scales. Accompanying hardware and system developments has been the parallel development of robust and stable processing and inversion algorithms, that permit the more rigorous interpretation of data being acquired by airborne systems. Increased computational capabilities allow for the better definition of variations in individual transients. In some instances we can re-sample transients to emphasise variations in parts of a regolith cover sequence, depending on target needs and requirements. We also have the capability to better constrain our data through the incorporation of, for example, borehole (point) information or spatial knowledge provided by lithology, structure and groundwater character. Computation power has provided impetus to better characterise EM systems so that we can better model ground conductivity variation. It also encourages the ability to more efficiently explore model space and analyse the response of alternative models, the hallmark of Bayesian inversion approaches. The output of this type of analysis are probabilities of models given data, noise and any prior information. Although there is now a greater appreciation of AEM technologies having relevance for the study of regolith variability, in part, linked to our ability to sample more quickly and at a higher resolution, making it much easier to map variations in the near surface, there remains a need to better understand the regolith constraints on observed EM response. Until more recently we have been inclined to view regolith as a lumped entity, specifically something that is commonly conductive and something that masks potential targets at greater depth. Arguably, with the noted advances in technology, we are now better placed to define the exploration significance of regolith characteristics that can be resolved by AEM systems.
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A New Broadband Airborne VLF-LF System Developed at the Geological Survey of Sweden
Authors Henrik Johansson and Mats WedmarkA new system for acquiring and processing the three orthogonal components (Hx, Hy, and Hz) of the magnetic field in the Very Low Frequency (VLF) band (10 kHz to 30 kHz) and the Low Frequency (LF) Band (30 kHz to 300 kHz) has been developed at the Geological Survey of Sweden (SGU). The hardware is based on commercially available Analog to Digital Converters (ADCs) and an embedded computer for data recording and preliminary in-flight data processing. Presently the ADCs sample all three magnetic components simultaneously and synchronously with a 24 bit resolution at a rate of 200 000 samples per second (S/s), but they can be upgraded to 1 MS/s ADCs. All data processing is carried out in software developed at SGU and is separated into two parts: In-flight processing and post-flight processing. The former is based on Fast Fourier Transforms (FFTs) of the samples signals and it is a fast and simple approach to synchronize these data with other recorded geophysical data, as well as to carry out quality control and view preliminary results within the hour after the survey aircraft has landed. The latter processing is more demanding in terms of time and computational recourses, but in return it yields higher quality data. Here, all data processing is carried out in the time domain, beginning with data filtration by Finite Impulse Response (FIR) band-pass filters designed to match known transmitter frequencies. The filtered data is interpolated using cubic B-splines to obtain higher resolution representations of the sampled waveforms from which average amplitudes and phases can be calculated. Both the filtration and the interpolation are performed on a Graphics Processing Unit (GPU), which significantly boosts the performance compared to conventional Central Processing Units (CPUs). Average signal amplitude and phases are calculated in typically 250 ms long segments of the three magnetic components, with the strongest component chosen as the phase reference. The transmitter pair with the best signal strength, situated at an appropriate angle with respect to each other and the survey aircraft, is used to calculate quantities of geophysical interest, such as tipper values and apparent resistivity. The new VLF-LF system will be put to survey use in the Swedish summer of 2013.
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The Use of AEM as part of an Integrated Approach to Rapidly Identify and Assess Managed Aquifer Recharge Targets
To meet the challenge of rapid identification and assessment of potential MAR targets and groundwater resources over a large area (7,541.5 sq km) of the River Darling Floodplain within relatively short timeframes (18 months), it was concluded that the only cost-effective method with the ability to resolve key features of the hydrogeological system in the 0-150 m depth range was airborne electromagnetics (AEM). The helicopter-borne SkyTEM transient EM system was selected after a rigorous technology assessment exercise. The SkyTEM survey involved acquisition of 31,834 line km of data (line spacing 200-300m), and was acquired by two systems over a 9-week period. Initial Fast Approximate Inversions (FAI) provided within 48 hours of acquisition were used to target a 7.5 km drilling program (100 sonic and rotary mud holes), and complementary borehole geophysics and field and laboratory measurements. Finally, a Wave Number Domain Approximate Inversion procedure with a 1D multi-layer model and constraints in 3D, was used to produce a 3D conductivity model. The SkyTEM system successfully mapped a multi-layered hydrostratigraphic sequence of aquifers and aquifers in the near-surface (top 100 m). This included: the thickness and extent of near-surface unconfined aquifers and aquitards; the thickness, extent and internal textural variability of Pliocene sand aquifers; the thickness, extent and internal variability in upper (Blanchetown Clay) and lower (upper Renmark Group) confining aquitards that ‘sandwich’ the Pliocene Sand aquifers; and the 3D distribution of groundwater salinity (to help define fresh and brackish groundwater resources). The study revealed significant heterogeneity in the sub-surface electrical conductivity structure, reflecting a complex geology. Significant Neogene-to-Present faulting, warping and tilting are observed to disrupt hydrostratigraphic units. The survey also mapped heterogeneity within the near-surface aquifers and confining aquitards, zones of inter-aquifer leakage, and five hydraulic classes (based on grain size) within the main aquifers, as well as groundwater salinities. Locally, pump and slug tests, and NMR data were integrated with the AEM data to produce maps of aquifer transmissivity. Only in more localised domains, where groundwater quality within the Pliocene aquifers is extremely saline, did it become too difficult to resolve some of the key aquifer boundaries and internal aquifer characteristics. Initial investigations determined that the Calivil Formation aquifer was most suited to potential MAR development. AEM mapping identified palaeochannels with medium-coarse grained sands in a relatively thick (30-50m) aquifer with a high storage capacity, very high transmissivities (up to 50 l/s), and significant volumes of fresh groundwater. The aquifer is sandwiched between variably thick clay aquitards, and can be characterised as varying from a confined to a ‘leaky confined’ system. The hydraulic properties make the Calivil Formation aquifer potentially suitable for groundwater extraction and/or MAR injection, with excellent recovery efficiencies predicted. Overall, this study has shown that there are significant scientific, technical, economic and social challenges to be overcome to develop MAR options in inland Australia. Key to success in the BHMAR project was the utilization of AEM mapping and integrated assessment methodologies and workflows. It is our understanding that this is the first use of AEM as part of multi-disciplinary mapping and assessment of MAR targets. The investigations in this study also completely revised our understanding of the age, stratigraphy, structure and mode of deposition of the Darling floodplain sediments, with practical implications for the hydrogeological conceptual model underpinning the assessment of groundwater resources and MAR options.
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Not extinct yet: Innovations in frequency domain HEM triggered by sea ice studies
Authors Andi A Pfaffhuber, Yme A Kvistedal, Stefan Hendricks, Priska Hunkeler and Erik LiedWe present a new generation, frequency- domain AEM system purpose designed for sea ice thickness profiling. The system includes a combination of step stones towards a potential new generation of systems using multi-frequency, multi-component, bucking-free EM in concert with various auxiliary sensors on a small and light platform.
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The MULTIPULSE system – high resolution and high power in one TDEM system
More LessAn airborne time domain system with higher resolution as well as greater depth of exploration is always in great demand for geological mapping as well as for mineral exploration. The MULTIPULSE™ system transmits a high power half-sine pulse and one or multiple low power square pulse(s). The high power half-sine pulse ensures good depth of penetration and the low power square pulse allows a fast turn off and earlier off-time measurement as well to provide higher frequency information, thus enabling higher resolution. The spectrum of the dual pulse (a half-sine and a square pulse) clearly demonstrates increased power in the higher frequency range (> ~ 2.3 kHz) than that of a single half-sine. A simple 1D synthetic model study reveals also the superiority of the resolving power of a square pulse to that of a half-sine pulse. We field tested the system with a dual pulse configuration and compared the results with previous RESOLVE and HeliGEOTEM data. The results show comparable shallow geological resolution of the dual pulse system and that of the RESOLVE system, confirming the increased bandwidth of the dual pulse system.
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Advantages and limitations of helicopter borne TEM systems employing flying loop and grounded cable transmitters
Authors Saurabh Verma, Toru Mogi, Sabry Abd Allah, Elena Fomenko and Shashi P. SharmaThe helicopoter borne TEM (HTEM) methods have now emerged as the most popular EM exploration tool. Most of the HTEM systems fly a large loop transmitter with a small receiver coil placed either at its centre or in close vicinity. Such systems employ different types of primary pulse shapes and can have dipole moment varying over a large range. There is only one HTEM system, namely the GREATEM system, which utilizes a long grounded cable as source transmitter. Response characteristics of these two generically different classes of HTEM systems employing inductive loop and grounded cable transmitters are studied for various values of the earth resistivity. Numerically modelled transient decay responses, appropriately normalized to account for the pulse shapes and the dipole moment, are analysed to study the capability of various systems in resolving the earth resistivity. The results emphatically bring out the basic difference in the EM excitation of the earth by the inductive and galvanic systems. While the inductive systems generate only TE modes, the galvanic excitation generates both TE and TM modes. This provides a much better resolution capability to the GREATEM system. This advantage, however, should be viewed in terms of the operational flexibility of the two classes of the HTEM systems. The results are nevertheless significant as they clearly indicate the additional benefits of TM mode excitation suggesting new areas of applications. We have also analysed the ‘Tau’ (time constant) images obtained by the two classes of HTEM systems. It is found that the GREATEM system also images the effect of source cable on the host medium due to continuous ground energization. This influence dominates the ‘Tau’ imaging at late times.
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Shallow alluvial diamond exploration with GENESIS airborne TEM (225Hz) system
Authors Magdel Combrinck and Reece van BurenThe Vaalbos project area is situated in the Northern Cape Province, South Africa, and contains diamondiferous gravel deposits. Eluvial Kalahari sand covers most of the area and limits the surficial exposure of these gravel deposits. Shallow gravel channels are challenging targets to directly resolve with airborne geophysics, however bedrock geology and structure, which are controlling factors in gravel concentration, can be mapped successfully. A GENESIS electromagnetic, magnetic and radiometric survey was interpreted, resulting in a number of likely alluvial gravel deposit targets in the project area to be followed up.
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VTEM and ZTEM helicopter EM case-study over the Nuqrah Cu-Pb-Zn-Au sedex massive sulphide deposit in Western Arabian Shield, KSA
Authors J.M. Legault, A. Prikhodko, C. Izarra, S. Zhao and E.M. SaadawiHelicopter VTEM active source, ZTEM AFMAG passive source EM and aeromagnetic survey results are compared over the Nuqrah Sedex massive sulphide deposit in Western Arabian Shield of KSA. Field data and 1D-2D- inversions are used to show that all surveys map major controlling structures that host the Nuqrah deposits. VTEM directly detects more massive sulphide mineralized vent portions of Sedex orebodies; whereas ZTEM likely defining larger, less conductive and weakly mineralized distal portions of Sedex system. ZTEM also maps possible conductive down-dip extension of Nuqrah South Sedex below 750m depths.
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Time Domain Helicopter EM System Equator: Resolution, Sensitivity, Universality
Authors E. Moilanen, E. Karshakov and A. VolkovitskyTime domain helicopter borne electromagnetic system EQUATOR was developed in 2010. For past years it worked successfully on survey in various conditions. It proved its effectiveness for searching low contrast respectively small targets such as disseminated ore, kimberlites. EQUATOR’s main features are wide survey speed range and wide frequency band of receiver. First is due to construction properties. Speed variations do not affect survey results due to high precision transmitter-receiver positioning. Second advantage extends system abilities. Due to full time measurements EQUATOR provides also frequency-domain data processing. To get more informative frequency response several additional frequencies are induced. Added signal doesn't affect time-domain response because it is absent in operating impulse spectrum and can be removed in post processing. Due to small footprint EQUATOR is well suited for small sized targets exploration. Its’ measurements correspond well with ground EM and drilling data.
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Modelling the superparamagnetic response of AEM data
Authors Daniel Sattel and Paul MuttonSeveral lines of airborne EM data flown at different system elevations across a known sulphide and surface cover with elevated superparamagnetic (SPM) properties were analysed with MAXWELL, layered-earth inversions and LEROIAIR. The SPM material was modelled with frequency-dependent magnetic susceptibilities at shallow depth. The SPM responses can be confused with responses of deep conductors and vice versa. Depending on the parameter weighting used, 1D inversions model all late-time responses as deep conductive material or as surficial SPM material. However, the joint 1D inversion of data acquired at different system elevations manages to recover a deep conductor from the sulphide anomaly and elevated SPM values at the location of the SPM response. For the modelled parameters, the VTEM data require a vertical system separation of at least 5-10 m to allow for the discrimination between the SPM and sulphide responses. In the absence of vertically separated AEM profiles, the x-component data, if available, might offer some model discrimination. For laterally extensive surficial SPM material, synthetic x-data computed with a modified version of LEROIAIR show a negligible SPM response. Following the determination of SPM parameters from VTEM survey data, these values were used to predict the SPM response for other AEM systems, including MEGATEM, AEROTEM, HELITEM and RESOLVE. Whereas the transient helicopter EM systems VTEM, AEROTEM and HELITEM data can be strongly affected by SPM effects, fixed-wing MEGATEM data are unaffected, due to the high elevation and large transmitter – receiver separation of the EM system. SPM effects on frequency-domain systems such as RESOLVE data are also small.
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Application of AEM to shallow geothermal potential mapping
Authors Andrea Viezzoli, A. Menghini, A Manzella, D Montanari, S Maggi and A SantilanoThe 'VIGOR' project aims at assessing geothermal resources in 4 regions of Southern Italy. The SkyTEM survey is a part of the planned geophysical activities and has been chosen since it is able to collect resistivity data on wide areas, with great resolution and in short times. To date, the application of AEM methods to geothermal targets has been limited, due to the depth at which the target is usually found. Kirsh and Siemon (2008) suggested its application in low enthalpy study. In this project AEM is tested in an area of known geothermal activities, and where a wealth of ancillary data is also present, in western Sicily. The results are composed in a 3D model of the electrical resistivity of the subsurface, and then interpreted for geothermal potential, following 2 main paths. The first is to produce a model of heat exchange capacity. The second aims at identifying directly conductive anomalies that might be associated to locally shallow hydrothermal fluids. “Termini” area is covered by extensive geological surveys. Since geological conditions of Sicily, even at shallow depth, are very complex, this area provided a good place for defining the resistivity values of the main geological units outcropping in the region. The “Western Sicily” area covers the main thermal manifestations of Western Sicily. Based on near real time preliminary results from the more regional mapping at 1 km line spacing, three infill areas were selected as being the most promising to test the concept. The obtained resistivity volume has then been the base for a detailed lithological and geothermal interpretation. Lithological and geological maps were used to constrain surface condition and to understand the resistivity ranges of the different lithological units. On the base of AEM derived resistivity values, and of laboratory measurements of thermal and electrical conductivity on samples, it was possible to establish the main links between lithology, electrical resistivity and thermal conductivity. The work in progress aims at extending the correlation also at depth, producing a 3D model of thermal exchange capacity for the areas surveyed. This detailed interpretative modeling provides also the basis for detecting resistivity anomalies within carbonate units, which may possibly represent hydrogeological or hydrothermal bodies.
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Modelling the electromagnetic response of the helicopter-borne ZTEM system for vertical thin plate conductors buried below conductive overburden
Authors G. Connors, C. Samson, J.M. Legault and S. ZhaoA desire to reduce the need to perform forward modelling on a case by case basis has resulted in an endeavour to model the ZTEM response and produce type curves. A total of 42 models of the subsurface were analysed using the OCCAM 2D MT forward modeling code. These models investigate two types of thin-plate conductors; conductive overburden and a conductive mineralized dyke. The conductance of the target, and overburden ranged from 100,000 S to 0.01 S, and 10,000 S to 0.01 S respectively. Modelling and graphical representation of the synthetic responses allowed for the production of type curves. One type curve was created for each of the six frequencies (30 Hz, 45 Hz, 90 Hz, 180 Hz, 360 Hz, and 720 Hz) that ZTEM measures. As expected, lower frequencies were generally more effective at resolving the conductive dyke below conductive overburden to depths down to 1000 m. A steady decrease in a ZTEM response as frequency increased was usually observed. The amplitude of the tipper response generally increased with the conductance of the dyke and decreased with the conductance of the overburden. The type curves produced for each frequency ultimately represent a useful exploration tool that can be used to predict the likelihood of successful performance of ZTEM airborne surveys for conductive targets in different geological environments around the world.
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3D geological modelling using geological and EAM data
Authors J. Deparis, B. Vittecoq, T. Jaouen, F. Lacquement and J. PerrinUnderstanding the hydrogeological functioning of volcanic island is essential for an adapted management of water resource and for a better supply to population, particularly in those who have seen their population increase in years. Nevertheless, geological, hydrological or hydrogeological data are often very scarce, and appropriate approach have to be found in order to improve the hydrogeological functioning of these islands. Groundwater resource is closely correlated with the permeability of rocks, and, in basaltic volcanic islands, permeability is usually considered to be negatively correlated with age formations. Younger lavas have higher permeability than older one’s often highly weathered (Custodio et al., 1988). Existing hydrogeological models of basaltic volcanic islands describe large-scale systems at the volcano or island scale, and two end models have been described. The Hawaiian model (Meinzer, 1930) considers a low-lying basal aquifer linked to inland dike-impounded and perched aquifers overlying impervious layers and/or confined by dykes (Tabasaki and Mink, 1983). The Canary Islands model (Custodio, 1975; Custodio et al., 1988) considers a continuous and isotropic basal aquifer, and a decrease of hydraulic conductivity with the age of the volcanic rocks. Nevertheless, those models are consistent with Young Island (< 5 Ma) and seem not fully appropriate for older island such as Mayotte Island. Mayotte Island is a small French overseas territory of the Comoros archipelago in the Indian Ocean. Its population has quickly increased during the last decades and has been multiplied by four in less than 30 years. The island is now very densely populated with more than 500 inhabitants per square kilometer. Water needs are thus substantial. Surface water resource is very strongly solicited, and too dry seasons can lead to critical situations. Nevertheless, its hydrogeological functioning is not well known, and understanding the hydrogeological functioning of the island is thus essential for an adapted management of groundwater resources and to make appropriate drilling campaigns. In 2010, an helborne SkyTEM survey on the whole island has been performed. The transmitter loop is composed of four 284 m² loops that transmit a low moment with a single turn and a high moment with four turns of the loop. The current generated by the low moment is 11 A, giving a moment of about 3100 Am² and a turn off time of 10 microseconds. That generated by the high moment is 108 A, giving a moment of about 123,000 Am² and a turn off time of 38 microseconds. The first sampling gate is centred about 6.5 microseconds for the low moment and the last window of the high moment is 8 ms. A linear 3000 km of SkyTEM TDEM data was acquired in a month. The average flight line spacing was 200 metres, with local spacings of 400 metres or 100 metres. The average speed of the helicopter was 18 m/s (65 km/h) with an average ground clearance of the transmitter loop being 48 metres. The TDEM decay curves were filtered using Workbench software with 1D modelling of the curves based on the Ward and Hohmann (1988) solutions. Data inversion was done with em1Dinv software (Auken et al, 2004) using a 20-layer model with fixed depths and a spatial constraint between the model resistivities of nearby soundings (Viezzoli et al, 2008). This model was preferred to a stratified-type inversion because the development of the weathering profile is not conducive to the presence of a sharp contrast between the bodies. The aim of the research presented in this paper is to show the calibration of 3D resistivity mapping realized on another and older volcanic island, with boreholes data in a test site in the South of Mayotte Island, and how the highlighted paleo-valley structure enhance understanding the hydrogeological functioning of the South of Mayotte Island. To establish the 3D model, we used correlation between borehole, geological map and 3D resistivity grids. Comparison between TDEM data, borehole and pumping tests data have provided new evidence about the hydrogeological scheme of the island, which seems different from the two previously mentioned. This study is focused in the South of the and includes two watersheds. The surface of the Kani-Kéli watershed, the west one, is about 4.9 km², and the surface of the Mronabeja watershed, the est one, is about 4.4 km². Four boreholes have been made in the past Simplified and interpreted geological logs show that two boreholes are drilled in weathered lavas, whereas the two others encountered about 30 m of weathered lavas overlying unweathered and superposed fresh fissured lava flows, with a confined aquifer in the fresh lava. 3D geological model of this area is made with GeoModeller ® software. Two kinds of data are used: polygons extracted from the provisional geological map using ArcMap and TDEM data. One model was produced with TDEM sounding (i.e. without resistivity interpolatation). 3D modeling of different geological formations can delineate structures with a hydrogeological interest. Indeed, in healthy fractured lavas, characterized by variable resistivity of 30 to 100 Ω.m, are considered as potential reservoirs of groundwater in these formations modeled structures thus correspond to aquifer sector Choungui South. The results show that helicopter borne TDEM is particularly adapted for this volcanic island, with low resistivities soils outcropping. Data computation also allows to image geological structure in 3D and boreholes comparison allows attributing geological identification for the main ranges of resistivity data. Productive boreholes, in the axis of a resistive body, and unproductive boreholes outside, allow confirming the implication of paleo-valley as major constraint for groundwater flows and the necessity to enhance existing conceptual model. The South of Mayotte Island is thus proposed as an end member of basaltic models. These results are very hopeful, and we are now looking for similar structure in order to drill new boreholes, to confirm the replicability of this aquifer structure, and to provide water to population method.
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The Power of Frequency Domain: When you should be using it
By G. HodgesThe characteristics of frequency domain EM system give it a broad frequency range and more than 5 orders of range for resistivity. The smaller size allows a rigid geometry and thereby in-phase data. Resistivity from phase angle is very sensitive across a wide range, and relatively independent of altitude, as well as measuring magnetic susceptibility and dielectric. The smaller size also allows a coaxial (X) component and close terrain following for the best possible lateral resolution. HFEM should be used where near-surface vertical and lateral resolution are essential, and where geology is resistive or targets weakly conductive.
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Recent AEM case study examples using a Full Waveform time-domain system for near-surface applications
Authors A. Prikhodko, J.M. Legault, K. Kwan, T. Eadie, K. Fisk, G.A. Oldenborger, V. Sapia, A. Viezzoli, E. Gloaguen, B.D. Smith and M.E. BestEarly time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas but this poses difficulties due to a variety of issues, such as system bandwidth, system calibration and parasitic loop capacitance. In an effort to address this issue, a continued system design strategy, aimed at improving its early-channel VTEM data, has achieved fully calibrated, quantitative measurements closer to the transmitter current turn-off, while maintaining reasonably optimal deep penetration characteristics. The new design implementation is known as “Full Waveform” VTEM. This paper presents some case-study examples of the Full Waveform VTEM helicopter time-domain EM system for near-surface applications.
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Preliminary design parameters for a HTEM system dedicated to overburden mapping
Authors Raymond Caron, Claire Samson, Michel Chouteau and Martin BatesLarge areas of the Precambrian shield in Canada, such as in the Abitibi mining region, are covered with a thick glaciolacustrine overburden. This overburden complicates mineral exploration efforts by masking the bedrock surface and increasing the cost of exploration through expensive drilling programs. It also reduces the resolution of potential-field exploration methods by obscuring changes in bedrock topography that are known to change independently from the topography at surface. With respect to the gravity method, changes in bedrock topography can also create anomalies of the same size and magnitude of, and be mistaken for, mineral deposits (Chen and Macnae 1997). In this paper, we suggest design parameters for a HTEM system specifically dedicated to overburden mapping with the goal of using the recorded data to correct airborne gravity data for lateral variations in overburden thickness. The proposed HTEM system is a single-turn in-loop system where the transmitter and receiver are in the same plane but offset by 7m. The 7m-radius circular transmitter is energized by a square waveform with a 100 A current running at a maximum duty time duration of 0.5 ms. The transmitter moment is 15,400 A/m2. The parameters were tested using geological scenarios based on the geology prevalent throughout the Abitibi mining region. Previous electromagnetic surveys in this area identified 3 classes of glaciolacustrine sediments above the bedrock. These units of clay, sand, and till have an average resistivity of 47.3 ± 6.7 Ω•m, 251 ± 70 Ω•m, and 123 ± 35 Ω•m respectively (Palacky 1992). The bedrock was assigned a resistivity of 10,000 Ω•m. Forward modelling of the EM response of the proposed system of various 2-layer, 3-layer and 4-layer geological scenarios were conducted and then inverted in order to determine the capabilities and limitations of the HTEM system. Noise was added to the response of the forward model following the process outlined in Auken et al. 2008 in order to simulate field conditions. The inversion methodology takes advantage of the highly resistive character of the bedrock and the typical location of clay above till within the sedimentary sequence. For these simple cases, the system was able to resolve the overall thickness of the overburden within 10%. EM responses of geological scenarios composed of 5 to 8 layers with intercalations of clay, sand and till were inverted, with noise, with the same methodology. For these more complex cases, the tests showed that multiple layers are better resolved when using a similar number of layers in the inversion as in the geological model. However most important is that for any case of layering, the overburden thickness is resolved within 10% of its true value and the difference between the inverted depth and the true depth is randomly distributed. This preliminary study indicates that the proposed HTEM system is likely to be able to resolve the overall thickness of a glaciolacustrine overburden overlying resistive Precambrian bedrock.
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An efficient hybrid inversion scheme combining approximate and full forward solutions for AEM
Authors C. Kirkegaard, C. Schamper, A.V. Christiansen, G. Vignoli and E. AukenAirborne time domain electromagnetic instruments provide a widely used surveying tool for a range of applications with very varying requirements towards accuracy. The method allows for collecting extremely large datasets in relatively little time, but to be able to extract quantitative information the data typically has to undergo some kind of modelling. In mineral exploration it is often possible to identify targets directly from the data, whereas high-accuracy modelling is typically crucial in the case of eg. hydro-geophysical surveys. In order to support the requirements of these very different fields a whole range of modelling schemes exists, ranging from very fast approximate techniques to more accurate and much more time consuming full system models. We discuss how an approximate- and a full system forward model can be combined into a versatile hybrid inversion scheme that makes no compromise in the numerical formulation of the physical system. We use this hybrid scheme to examine different degrees of approximation and demonstrate the impact on accuracy and performance.
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Evolution of TEMPEST – Bird positioning
Authors S. Mulè and R LockwoodTEMPEST, a fixed wing time domain electromagnetic (TDEM) system, developed in 2000, has been applied to a range of exploration and mapping applications around the world. The systems’ versatility can be credited to its broad operational bandwidth, multifaceted software approach and distinctive calibration technique which ensures that it is able to capture and process early and late time ground responses. In the last decade the system has undergone a range of hardware and software developments with the aim of satisfying the changing landscape of geophysical exploration. Specific improvements have been made to improve the system’s ability to monitor and compensate for variations in system geometry. Inverse modelling and recent survey data are used to demonstrate the impact of system improvements on enhancing the system’s ability in a multitude of geophysical applications.
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Fast denoising of AEM data using Singular Value Decomposition
More LessAirborne Time-Domain ElectroMagnetic (TDEM) surveys are increasingly carried out in anthropized areas as part of environmental studies. In such areas, noise arises from either natural sources, such as spherics, or cultural sources, such as couplings with man-made installations. TDEM data may therefore be affected by many distortions, such as spikes, oscillations and shifts, which make the EM noise spectrum complex and may lead to erroneous inversion and subsequent misinterpretations. In such noisy environments, thresholding and stacking standard techniques, commonly used to filter TDEM data, are hardly efficient. Time-consuming and subjective manual cleaning of the data is therefore required. We propose an alternative fast and efficient user-assisted filtering approach. We adapted the Singular Value Decomposition (SVD) to denoise TDEM data. The SVD method uses the principal component analysis. It allows separating noise from geological signal extracting into components the dominant shapes from a series of raw input curves. The signal components are then used to reconstruct the EM decays without the noise. The SVD procedure was implemented in the denoising of several EM datasets acquired over anthropized areas in various contexts. The comparison between each reconstructed decay and its corresponding measured decay allows efficiently detecting noisy gates and rejecting mainly spikes and oscillations. Moreover, an ad hoc analysis of the map of weights of the components explaining noise showed high correlation with man-made installations. Thus, the SVD also provides a tool to reject most likely soundings biased by coupling noises, which may result in artefacts on the inverted models. However, some distorted decays can only be localized based on the analysis of specific SVD components. It was also shown that the maps of the weights of the components explaining the geological signal could be useful as a first rapid view of the contrasts that exist in subsurface. This established SVD based procedure is fast and provides accurate denoising tools; it makes, at least, the manual cleaning less time consuming and less subjective.
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A Bayesian Approach to the Inversion of Airborne Electromagnetic Data for a Multidimensional Earth
Authors Juerg Hauser, David Annetts and James GunningResistivity distributions for the subsurface based on airborne electromagnetic data are commonly derived using deterministic inversion methods. It is well known that this inverse problem is inherently non-unique; if one model can be found, it is likely that there exist alternative models that fit the data equally well, particularly once noise on the data is taken into account. Probabilistic approaches, like the one introduced in this work, allow exploration of the posterior distribution which represents the distribution of models that are in agreement with the data and the prior information. This work uses multipoint geostatistical models to represent the prior information, and a Markov Chain Monte Carlo technique to sample the unknown posterior distribution. The airborne electromagnetic data are predicted by employing a 2.5D forward solver, so that lateral changes in structure along the flight path are taken into account. The inversion aims at determining the probability of individual lithologies to be present. We use synthetic examples to demonstrate how the method recovers well-known facts of airborne electromagnetic imaging, for example the reduced resolution if targets are located under conductive regolith cover. Application of the method to field data collected over the Harmony Ni-S deposit in Western Australia shows that our sets of samples of the posterior distribution provides a more complete picture of solution space when compared to deterministic inversion results. Such probabilistic images of the subsurface can ultimately be beneficial for the mitigation of exploration risk, because of their quantification of uncertainties.
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Computationally efficient EM modelling using Shur decomposition applied to large scale airborne applications
Authors Trevor Irons and Yaoguo LiInversion of airborne electromagnetics problems can be extremely challenging due to the large number of transmitter locations. Differential-equation based solutions offer compelling advantages over integral equation and stitched 1D inversions, but come at a high computational cost and can struggle with the inclusion of the air layer. We present a new modelling scheme that is well-posed in the air, is extremely memory efficient. The algorithm is numerically attractive. Furthermore, preconditioners can be used to which allow for very quick convergence of iterative solvers. These preconditioners can be used across transmitter locations, source frequency, and conductivity model—making the algorithm well suited for AEM forward modelling and inversion on parallel architecture.
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Scalable, parallel constrained inversion of large AEM datasets
Authors C. Kirkegaard and E. AukenOver the past decade the typical size of AEM datasets has been growing rapidly, while at the same time targeting new applications that rely on advances in terms of resolution and accuracy. Approximate inversions and data transform techniques have previously defined the norm for interpretation of huge surveys, but rarely pose attractive solutions for modern applications such as aquifer mapping, uranium exploration, integrated modeling etc. For these applications high-resolution full system modeling techniques provide the only acceptable solution, but their refinement comes at the expense of significant added computational complexity. Further applying spatial constraints to the inversion of multiple 1D soundings facilitate a resulting quasi-3D model, however, there are severe intrinsic issues in effectively solving the underlying systems of linear equations. Here, we describe how we have attacked scalability issues of the Spatially Constrained Inversion (SCI) formalism and optimized our code to handle arbitrarily large problems on parallel multi-processor computers.
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Using the in-line component for fixed-wing EM layered inversion
More LessMany authors have discussed the utility of multicomponent measurements. Generally speaking, the vertical component couples to horizontal bodies while the in-line component couples best to vertical targets. For layered-earth cases, helicopter EM systems have little or no in-line component response and as a result much of the in-line signal is due to receiver coil rotation and appears as noise. In contrast to this, the in-line component of a fixed-wing airborne electromagnetic (AEM) system can be substantial, exceeding the vertical component in conductive areas. In this paper I compare the in-line and vertical response of a fixed-wing system to a halfspace and show the sensitivity functions. I then calculate the expected inversion model parameter uncertainty, showing that the use of both components better resolves model properties. I then compare inversion using both components to vertical component alone, providing an example using field data from the oil sands region of Canada.
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AEM Discrete Conductor Inversion
Authors M. A. Vallée, J. Lemieux and T. KimuraDiscrete object modelling has been used since the beginning of airborne electromagnetic exploration. A recent effort of integrating existing modelling and inversion tools with airborne data shows that simple models are still quite useful for exploration. They can provide simple parameters of a model with minimum a priori and still provide adequate data fitting. Approximations allow using models with reasonable computer resources. Plate models have been popular and inversion programs have been developed around them. Another inversion approach is based on approximation of sphere or plate response in free-space. Finally, a recent development is an analytic solution for a conductive sphere in a layered earth. We present some applications of this methodology.
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Accurate processing and inversion as the ultimate QC of AEM data
More Lesse ground gives higher signal, especially at early times/high frequencies , even over the same piece of ground. The readings in the Rx are the convolution of ground response and system transfer response (STF). In re-flights, what needs to be repeatable –hence precise- is the ground component part of the readings. The only way to assess precision is therefore to invert the data, uncoupling, from the measured signal, system STF and ground response. Another concept, rarely directly used in QC, but sometimes hinted at, and sometimes confused with precision, is that of accuracy. The latter is the measurement of closeness of measurement of a quantity to its actual (true) value. It should be evident that precision (repeatability) is not that useful per se, if the measured value we can repeat to a satisfying degree is not accurate (i.e., it is repeatedly far from the true value). An AEM system is well calibrated, if it is accurate. It is obvious that also in order to assess the accuracy (calibration) of an AEM system in rendering the actual distribution of the electrical resistivity, we need to work in the model space. The first approach is to invert the AEM data and compare the outcome with a relevant resistivity reference model (obtained from DC/borehole data, other EM data). The second is to forward model the reference resistivity model with the STF of the particular system in the condition of acquisition, and compare it with the actual observations of the system over it. For different reasons, the first approach tends to be the most tempting. It must however be stressed that an inversion output is the end result of a series of steps. From survey design, to data acquisition, pre-processing, post-processing, data integration (another issue that only makes sense contemplating the model space), and inversion, they all contribute to the output. Inaccurate processing and inversion can jeopardize the recovered models, and in turn also the ultimate assessment on AEM data precision and accuracy. If a problem of inaccuracy/poor calibration of an AEM system has been detected, it is again through inversions that, in some cases, the accuracy/calibration of that dataset might be improved. We will present examples that illustrate all the aspects mentioned above.
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The Nuqrah massive sulphide SEDEX deposit in Saudi Arabia - A Fugro TEMPEST perspective
Authors Darren Burrows, Reece van Buren and Emad Al SaadawiA TEMPEST airborne fixed wing Time Domain Electromagnetic (TDEM) and magnetic survey was flown over the Nuqrah – Mardah project area of central Saudi Arabia for Ma’aden Gold and Base Metals. The Nuqrah massive-sulphide sedex deposit is located within graphite schist and carbonates near the top of the pyroclastic Halaban formation. The survey has identified known mineral occurrences, and located new mineral targets. The north and south zones of the Nuqrah Deposit have been imaged, and further drilling may be indicated at Nuqrah North based on a strike length interpreted from CDI profiles of 1.6 km.
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Flying the time domain electromagnetic GENESIS system over the traditional Welkom gold mining district, Free State, South Africa, to identify contaminant seepage and acid mine drainage
By D. EberleIn May 2010 a time domain airborne electromagnetic (TD-AEM) survey was carried out for the Council for Geoscience over two selected areas in the traditional West Rand and Free State gold mining areas of South Africa. The goal of the TD-AEM survey was to acquire a 3D electric conductivity model of the survey areas reflecting the geological layering and potential pathways for rising acid mine waters or leaking contaminant fluids from tailings and slime dams. The TD-AEM Genesis system which was a fairly recent development mounted on a single engine Cessna Grand Caravan 208 was provided by Fugro Airborne Services (FAS). The survey was flown at 200 m flight line spacing, with flight lines oriented N-S and tie lines oriented W-E at 2000 m spacing. The EM transmitter and stinger mounted Caesium vapour magnetometer were flown at about 90 m terrain clearance. The EM receiver bird was closer to surface (45 m) and dragged by the aircraft. The transmitter-receiver (Tx-Rx) configuration is non-symmetric. Base frequency of the transmitter (Tx) was 75 Hz, peak moment 60300 Am2 and peak current 450 A. In high conductivity areas the depth of investigation of the Genesis system has been limited to little more than 100 m, only in low conductivity environment a maximum depth of investigation of about 300 m was achieved. Despite these constraints, the 3D conductivity voxels display the geological layering, highlight tectonic features (folding, faults) and identify individual conductive spots which are possibly associated with leaking tailings. As a whole, the usefulness of fast and cost-effective AEM surveying when tackling the issues of acid mine waters and contaminant seepage has successfully been proven. However, the use of a helicopter-borne AEM system may be preferred with regard to spatial power of resolution and system geometry.
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Helicopter AFMAG (ZTEM) Survey Results over the Ad Duwayhi intrusion related gold deposit (IRGD) in the Western Arabian Shield, KSA
Authors J.M. Legault, C. Izarra, S. Zhao and E.M. SaadawiAd Duwayhi IRGD gold deposit were designed to characterize the deposit signatures. Field data and 2D EM inversion results indicate the Ad Duwayhi intrusive centre features low magnetic susceptibilities and high resistivities that are consistent with the iron-depleted, quartz-sericite-carbonate alteration. Closer inspection of the ZTEM resistivity image suggests that the mineralized breccia zones represent weak resistivity lows within the larger resistive intrusion. The Ad Duwayhi gold porphyry lacks a visible low resistivity alteration halo that distinguishes IRGD’s from porphyry copper deposits. The combined aeromagnetic low and ZTEM resistivity high signatures might allow discrimination of other buried IRGD gold deposits regionally.
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Airborne electromagnetics in manganese exploration – a case study
Authors A Noetzli, M. Lowe, J Kita, K McKenna and N. KusumaputriAustralia since 2002 when the first Hoistem survey was flown for Pilbara Manganese. A XTEM survey was commissioned by Montezuma Mining to test the effectiveness of the technique in a new manganese province near Kumarina, Western Australia. A series of test lines was flown over areas of known manganese ore deposits and revealed a successful correlation between the electromagnetic conductors and the known manganese zones. In addition the test lines revealed new zones of conductivity where surface traces of manganese had been found but not yet drilled. Based on the preliminary results of these test lines the complete Butcherbird exploration tenement was flown in December 2010. The survey successfully mapped the extent of the known manganese zones and assisted in the identification and mapping of several new targets which were subsequently drilled and found to bear manganese oxide. The shape of the EM anomaly maps the resource outlines and the strength of the EM signal correlates strongly with the grade of the resource.
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ZTEM-VTEM-AeroTEM helicopter EM comparisons over the 501 Project Cu-Zn vms at McFauld’s Lake, northern Ontario, Canada
Authors M. Orta, J.M. Legault, A. Prikhodko, G. Plastow, S. Zhao, M. Moreton and C. UlanskyHelicopter AeroTEM, VTEM and ZTEM surveys were flown over the 501 zone in the McFauld’s Lake area, northern Ontario. The 501 zone is a relatively small VMS deposit that appears to respond well to all three active and passive airborne EM systems that have surveyed the property. Comparisons between these data sets and the geology are showcased using 1D-2D-3D EM inversion modeling.
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The analysis of ZTEM data across the Humble magnetic anomaly, Alaska
Authors Daniel Sattel and Ken WitherlyZTEM data acquired across the Humble magnetic anomaly of almost 30,000 nT were analysed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. Mag3D inversion of the magnetic data indicates magnetic susceptibility values as high as 2.0 (SI). The response of moving the receiver coil through the magnetic-field gradient peaks at 0.01 Hz and drops off strongly with frequency. Lacking information about the field strength at the base station precludes the comparison of amplitudes between computed gradient responses and the survey data, but the comparison of response shapes suggests that the gradient responses are too small to have a noticeable effect on the survey data. ZTEM responses were forward modeled with a UBC-GIF 3D algorithm that takes into account electric conductivities and magnetic susceptibilities , in order to assess the impact of the elevated values derived from the Mag3D inversion. Computing the ZTEM response for these values combined with resistive half-spaces indicates that the response amplitudes and shapes strongly depend on the half-space resistivities. Ignoring the elevated values during an inversion can result in patterns that resemble crop circles. A comparison of the synthetic data with the Humble survey data suggests that the observed responses are more affected by the subsurface conductivities than by the magnetic susceptibilities. The approximate conductivity structure of the survey area was derived with a UBC-GIF 3D ZTEM inversion, which models . Forward-model results of these conductivities combined with the elevated values derived from the Mag3D inversion indicate that the conductivities are underestimated with the assumption. For an environment such as Humble, with deep-seated zones of elevated values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.
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Voxel Inversion of AEM Data for Improved Model Integration
More Lessthe model space is usually linked to the actual observation points. For airborne surveys the spatial discretization of the model space reflects the flight lines (Figure 1). Often airborne surveys are carried out in areas where other ground-based geophysical data are available. The model space of ground-based geophysical inversions is likewise usually referred to the positions of the measurements, e.g. the electrode positions in geoelectrical investigations, and hence does not coincide with the airborne model. 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. This means that incorporating the geophysical data into the geological and/or hydrological modeling grids involves a spatial relocation of the models, which in itself is a very subtle process where valuable information is easily lost. Also the integration of a priori information, e.g. from boreholes, is difficult when the observation points do not coincide with the position of the prior information. We have developed a geophysical inversion algorithm working directly in a voxel grid disconnected from the actual measuring points (Figure 1b), which then allows for straightforward integration of different data types in a joint inversion, for directly informing geological/hydrogeological models, and for easier incorporation of a priori information. The new voxel model space defines the soil properties (like resistivity) on a set of nodes, and the distribution of the soil properties is computed everywhere by means of an interpolation function (e.g. inverse distance or kriging). Given this definition of the voxel model space, the 1D forward responses are computed as follows: 1) a 1D model subdivision, in terms of model thicknesses is defined for each 1D data set, creating “virtual” layers. 2) the "virtual" 1D models at the sounding positions are finalized by interpolating the soil properties (the resistivity) in the center of the "virtual" layers (Figure 2a). For 2D/3D forward responses the algorithm operates similarly, simply filling the 2D/3D meshes of the forward responses by computing the interpolation values in the centers of the mesh cells (Figure 2b). This definition of the voxel model space decouples the geophysical model from the position of acquired data, allowing for straight-forward integration of different data types in joint inversion and for directly informing (hydro)geological models. We believe that this new approach will facilitate the integration of geophysics, geology, and hydrology for improved groundwater and environmental management. The presented algorithm is a further development of the AarhusInv program package, which manages both large scale AEM surveys (e.g. Auken et al., 2008, Siemon et al., 2009) and ground-based data (e.g. Fiandaca et al., 2013, Behroozmand et al., 2012).
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Optimizing Airborne Electromagnetic (AEM) Inversions Through Integrating Hydrogeophysical, Hydrogeological, Hydrochemical and Hydrodynamic Data
In the Broken Hill Managed Aquifer Recharge (BHMAR) project, a large (>50) multi-disciplinary group of geoscientists (geomorphologists, sedimentologists, regolith geoscientists, hydrogeologists, hydrogeophysicists, hydrogeochemists, geospatial analysts, remote sensing specialists, geochronologists, groundwater modelers and structural geologists), worked in a team to understand the hydrogeological system and identify MAR and potential groundwater resource targets. A trans-disciplinary approach was critical to successful completion of MAR pre-commissioning maximal and residual risk assessments. Initially, a number of different (Laterally and Spatially Constrained) inversions of the AEM data were carried out, with refinements made as additional information on vertical and lateral constraints became available. However, hydrostratigraphic mapping using all of these inversions was unable to resolve fundamental aspects of the hydrogeological system, particularly in the near-surface (top 20m), where hydrodynamic data indicated a connection between the major rivers and the underlying aquifers, either through incision and/or through faults (and bypass flow). To resolve these issues, a trans-disciplinary approach was used to investigate all underlying assumptions, including the regularization used in the AEM inversion. In modern laterally-correlated inversion 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. In this study we used a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. This was accomplished using a Wave Number Domain Approximate (WANDA) Inversion procedure with a 1D multi-layer model and constraints in 3D. This inversion procedure only takes days to run, enabling the rapid trialing to select the most appropriate vertical and horizontal constraints. 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 is dependent on integrating a team of scientists, where all facets of data and interpretation are considered and questioned in a trans-disciplinary analysis of the hydrogeological system. Necessary elements for this approach to succeed are the experience and professional insights of the scientists involved and a willingness and ability of scientists from diverse areas to establish a dialogue that will question and refine the inversion constraints and the quality of the final hydrogeological conceptual model. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project. The resultant improved 3D conductivity model revealed details of the hydrostratigraphy and Neogene-to-Present tectonics. Prior to the mapping of near-surface hydrostratigraphy and structural features, it had not been possible to explain apparently contradictory data, nor develop a plausible hydrogeological conceptual model. In summary, a pragmatic approach to optimizing the constraints was achieved using an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding.
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Evidence for a Paleo-Okavango Delta and overlying mega-lake from airborne TEM data
The near juxtaposition of the Makgadikgadi Basin, the world's largest saltpan complex, with the Okavango Delta, one of the planet's largest inland deltas (technically an alluvial megafan), has intrigued explorers and scientists since the middle 19th century. It was also clear from early observations that the Makgadikgadi Basin once contained a huge lake, popularly referred to as Paleo Lake Makgadikgadi. Through a contract with a commercial company, the Botswana Department of Geological Survey has acquired a helicopter time-domain electromagnetic (HTEM) data set across the entire Okavango Delta. The HTEM data were of extremely high quality, mainly due to very low noise levels (there are no large power lines and only a small number of settlements and tourist lodges in the delta) and significant contrasts between the electrically resistive units (i.e., dry and fresh-water-saturated sand and basement) and electrically conductive ones (i.e., saline-water-saturated sand and clay). Our inversions of these data returned electrical resistivity models containing three principal layers: (i) an upper heterogeneous layer that is largely resistive, (ii) an intermediate conductive layer, and (iii) a lower resistive layer. According to borehole logs and groundwater sampling, the electrically conductive layer is due to a combination of clay and saline-water-saturated sediments. We interpret these sediments to have belonged to Paleo Lake Makgadikgadi, which would have extended into the region presently occupied by the Okavango Delta. The total area of Paleo Lake Makgadikgadi would therefore have exceeded 90,000km2, larger than Earth's most extensive fresh-water body today, Lake Superior. Our HTEM data also provide evidence for a southeast-dipping paleo megadelta or megafan beneath the Okavango Delta’s upper fan and the Paleo Lake Makgadikgadi sediments. Seismic refraction/reflection experiments and boreholes suggest that this feature is composed of fresh-water sediments, probably representing a Paleo-Okavango Delta resting on top of basement rock. If this interpretation is correct, then the clay layers of the Paleo Lake Makgadikgadi sediments act as an aquitard protecting the fresh-water sediments of the paleo-delta from salt water contamination above.
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New abilities of quadrature EM systems
Authors T. Vovenko, E. Moilanen, A. Volkovitsky and E. KarshakovIt can be said that quadrature systems failed the competition with powerful time-domain and towed rigid-boom frequency-domain systems. It seems that there is no place for them but they continued to be in use for a long time and the reasons are: first, in contrast to time-domain systems quadrature systems could be applied in weak conductive environment. But one of them, namely, EM4H is still widely used for survey – about 100 000 line kilometers in a year. Experience has shown that new advantages of EM4H have improved essentially survey results and noticeably risen the potential of frequency-domain systems with large receiver-transmitter separation. In the paper basic features of the EM4H system are described and illustrated by survey results. Particular attention is paid to estimation of effectiveness for searching conductive target depending on distance.
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New Strategies for Faster 3D Inversion of Airborne EM Data
Authors Douglas Oldenburg, Dikun Yang and Eldad HaberTraditional Gauss-Newton approaches to solving inverse problems have been successful for inverting many types of geophysical data. However, the need to finely discretize a large volume of the earth and the requirement to solve Maxwell’s equations with many sources makes 3D inversion of airborne EM data very challenging. The main computational bottlenecks concern the time to carry out forward modelling of the data, computation of sensitivities, and solution of the matrix equation to provide an updated conductivity. In previous work we used a single fine scale global mesh for forward modelling and inversion but that can become untractable for very large problems. Here we separate the inversion and forward modelling mesh. The local mesh can be designed to handle a subset of the initial soundings or it can be tailored to an individual sounding. We demonstrate the large speed-up that can be achieved by having a local forward modelling mesh for each transmitter-receiver pair. This mesh is small, easy to store, and it allows a large EM problem to be decomposed into many small highly independent problems for which massive parallelization can be effectively applied. The second improvement focusses upon how to reduce the number of transmitters at each Gauss-Newton iteration. Rather than down-sampling a-priori, we adaptively select soundings chosen at random from the entire population. The number of soundings needed at each iteration increases with the complexity of the conductivity model but the final number of soundings needed to update a model is usually a small fraction of the total number of soundings. A final forward modelling of all of the soundings in the survey serves as a check on the process. The combination of local meshes and adaptive soundings greatly decreases the computation time. Our field test example involving 14362 soundings, 8 time channels for 114896 data, and a mesh with 433520 cells required about 4.3 hours.
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