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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
21 - 40 of 69 results
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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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