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First European Airborne Electromagnetics Conference
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
1 - 20 of 29 results
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Novel AEM Data Acquisition Strategies for Groundwater Resource and Managed Aquifer Recharge Mapping in the Australian Landscape Context
Authors K.C. Lawrie, N. Christensen, R. S. Brodie, K.P. Tan, L. Halas and J. MageeSummaryExperience over the past 15 years has demonstrated that the use of airborne electromagnetics (AEM) for near-surface hydrogeological investigations often requires high resolution data to map key functional elements of the hydrogeological system. In the Broken Hill Managed Aquifer Recharge (BHMAR) project, the AEM acquisition strategy was governed by the need to rapidly identify and assess a number of potential managed aquifer recharge (MAR) and groundwater resource targets over a large area (>7,500 km2) of the Lower Darling Valley, with a high degree of confidence, and in very short timeframes. A flight line spacing of 200–300m successfully mapped the key elements of the hydrostratigraphy, neotectonic features, and 14 potential MAR and groundwater targets.
Subsequently, AEM data were re-inverted, enabling comparison of line spacing of 200 m, 600 m, 1 km, 2 km, 5 km and 10 km. Utilising the information gained from this exercise, a novel survey design was developed using a systems analysis approach incorporating a range of conceptual hydrogeological and geological models, morphotectonic and structural mapping, and temporal remote sensing and hydrogeological data. Trialling this approach, a few widely-spaced AEM transects identified potential new groundwater resources over a large area of the Middle- and Upper- Darling River Valley.
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Mapping Sediment Thickness Using Airborne Electromagnetics
Authors T.M. Meglich, J.D. Abraham, A. Genco, C.P. Carney and R.E.V. WoolfSummaryAn airborne electromagnetic (AEM) survey was conducted to support a feasibility study for the placement of a riverbank filtration system. The goal of the survey was to map sediment thickness along a narrow corridor centered on the Missouri River in North Dakota. This case history presents the results of that survey. A total of 1,184 line km of data were acquired along a 116 km stretch of the Missouri River using the SkyTEM 301 time domain electromagnetic system. This system, using a combination of early time (approximately 5 microseconds for the first time gate) and spatially dense data, successfully mapped the alluvial sediment - bedrock contact. Using a spatially constrained inversion in conjunction with borehole data, the AEM data reveal a paleochannel where sediment thicknesses exceed 75 meters. Six locations were identified for future investigation based on interpreted alluvial sediment thickness, sediment resistivities, and location.
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Mapping Bedrock Layering, Fault Zones and Saline Groundwater by Airborne TEM on the Island of Gotland, Sweden
Authors L. Persson, M. Bastani, M. Erlström, C.A. Triumf, P. Dahlqvist and F. JørgensenSummaryAn airborne transient electromagnetic survey (ATEM) was carried out over four areas on the island of Gotland, Sweden, for groundwater planning purposes. The primary aim of the study was to locate areas with large groundwater capacity and to identify the depth to saline groundwater. The result from the ATEM survey indicates several sites and areas, which have been assessed as favourable for groundwater extraction. The data also reveal new and previously unknown information regarding the layering and structure of the subsurface geology down to approximately 200 m depth. An example from one of the surveyed areas shows that both the thickness of the upper limestone unit as well as the interface to the underlying marlstone and depth to saline groundwater is effectively mapped by the ATEM survey. In the second example a fault zone is clearly visualized, possibly also associated with increased groundwater circulation in a zone with fractured rock.
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Towards Using AEM for Sensitive Clay Mapping - A Case Study from Norway
Authors H. Anschütz, S. Bazin and A. PfaffhuberSummaryAn AEM survey has been carried out supporting a highway construction project in Norway in order to obtain information about possible sensitive clay (leached marine clay, also called quick clay). In addition, an ERT profile was acquired along a flight line in an area where geotechnical boreholes indicate a layer of sensitive clay. AEM and ERT data agree very well, indicating only small changes in electrical resistivity throughout the sediment layer. The AEM data show more structures than ERT, demonstrating that resolution and accuracy of AEM are high enough to resolve subtle changes in resistivity charecterizing different clay units. Based on our results, AEM can be used to target zones of possible sensitive clay which can subsequently be investigated in more detail through geotechnical analyses like boreholes and sampling. Thus, using AEM to design the geotechnical program has the potential to provide significant cost reductions for similar projects.
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Geo-steered 3D Inversion of Airborne Electromagnetic Data in Rugged Terrain
Authors C. Scholl, J. Neumann and M.D. WattsSummaryMultidimensional inversions for electromagnetic (EM) data require regularization for stabilization. A common approach is to add some smoothness criterion, which provides stable results. The downside is that any feature will be uniformly blurred and tend to not appear geologically meaningful.
We present a 3D inversion algorithm for all types of airborne EM data featuring geo-steering. This additional regularization - based on a cross-gradient concept - is used to introduce assumed or known geological structures to the inversion. Using the additional information results in geologically more plausible models and increases the effective resolution of the inversion.
The approach is demonstrated with frequency domain field data set collected in rugged topography.
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Frame Effective Tilt Correction for HEM Data Acquired over Rugged Terrain
Authors P.-A. Reninger, G. Martelet and J. PerrinSummaryIn a traditional HEM inversion scheme, the voltage data are usually normalized by the effective area of the system. This effective area depends on the tilt of the frame; the pitch and the roll of the frame are monitored during the survey with tilt-meters and used to calculate a correction term. However, using the measured tilt to calculate this term, it is assumed that the terrain is flat, which may lead to inaccuracies on the inversion results. Thus, though the correction term is generally small, it can prove important to accurately compute it in rugged terrain when high resolution is needed. The measured tilt has then to be corrected by the apparent slope of the ground in order to determine an effective tilt at each measurement location. In this work we compute the effective tilt and evaluate its contribution on inversion results. This was achieved on a recent survey conducted over La Réunion, which has a very rugged terrain. The inversion results obtained using the effective tilt were compared to the ones obtained with the measured tilt. Using the effective tilt in such environment has a clear effect in the inversion results, both on resistivity patterns and on the DOI.
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Lateral Model Correlation for 1D Inverse Modeling of Large Data Sets
Authors N.B. Christensen and K.C. LawrieSummaryIn this paper, we present novel developments of the Lateral Parameter Correlation (LPC) method for invoking lateral smoothness in model sections of one-dimensional (1D) inversion models. The LPC method is a three-step procedure consisting of (1) individual inversion of all soundings, (2) a lateral correlation procedure, and (3) a final individual inversion. The lateral correlation involves solving a simple constrained inversion problem with a model covariance matrix that ensures lateral smoothness. The method separates inversion from correlation and is much faster than methods where inversion correlation are solved simultaneously. In the new developments, a strictly horizontal correlation can be performed, thereby avoiding the model artifact sometimes seen when correlating along layers. Furthermore, a solution to the intractable computation times arising with large data sets is formulated employing a tessellation of the plane and an averaging scheme within subareas that reduces the size of the numerical LPC inversion problem while maintaining correct correlation within a very large area. A field example shows the improvements obtained with the strictly horizontal correlation. The LPC method is very flexible and is capable of correlating models from inversion of different data types including information from boreholes, and it lends itself easily to ‘embarassingly parallel’ computation.
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Rapid Inversion of Large Airborne AEM Data Datasets Utilizing Massively Parallel Co-processors
Authors C. Kirkegaard, K. Andersen, A.V. Christiansen, E. Auken and T. BoesenSummaryWhile full 2D and 3D inversion schemes are emerging for airborne TEM datasets, the workhorse of large scale surveying still remains the well established 1D forward formulation. Modern airborne TEM surveys typically span several thousand line kilometers of data, which can be very time consuming to invert even within the framework of 1D forward modeling. Here, we present how we have modified our existing inversion code to offload its 1D forward computations to massively parallel Intel Xeon Phi co-processors. A prerequisite for good performance on this type of next generation technology is for a code to provide not only good parallel scaling, but also make efficient use of vector instructions. The latter is not possible without modification to the established framework of 1D forward modeling and we demonstrate how this problem can be overcome in a straight forward manner. We show how the modified algorithm provides virtually ideal parallel scaling and almost ideal use of vector instructions on both multi-core processors and massively parallel Intel Xeon Phi co-processors. The use of vector instructions alone provide a speedup of almost 8× on the co-processor, allowing for full inversion of several thousand line kilometers of data per hour.
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2.5D Inversion of Sea Ice Thickness from Helicopter EM Data
Authors M. Vöge, A. Pfaffhuber, E. Auken, C. Kirkegaard, T. Boesen, S. Hendricks and P. HunkelerSummaryIn this paper, we present a new 2D inversion approach to invert helicopter EM data for sea-ice thicknesses. The approach uses a 2.5D finite element frequency-domain solution of the Maxwell-Equations for the secondary field. Instead of inverting for resistivity values from which the location of the sea-ice/seawater boundary is derived, we directly invert for the location of the layer boundary, using fixed values for the resistivities of air, sea-ice and seawater.
Our approach was tested on a field data set from Antarctica acquired in 2005, using a frequency domain helicopter EM system. The data set covers a 500 m long profile, which contains flat sections that are approximately 1D, as well as pressure ridges with strong 2D character. The results of the 2D inversion are compared to earlier results from various 1D inversions and a sea-ice thickness data set obtained by drillings. The comparison shows that the 2D inversion gives a clear improvement for the areas with pressure ridges, while the 1D results appear to be less noisy for flat areas.
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3-D Inversion of the Grounded Electrical-source Airborne Transient Electromagnetic (GREATE) Survey Data
Authors S. Abd Allah, T. Mogi, H. Kim and E. FomenkoSummaryStudies have shown that Grounded Electrical-Source Airborne Transient ElectroMagnetics (GREATEM) is a promising method for modelling three-dimensional (3-D) resistivity structures in coastal areas, in addition to inaccessible areas such as volcanoes, mountains and deep forest cover. To expand the application of the GREATEM system, a 3-D resistivity model that considers large lateral resistivity variations is required. In this study, we present a frequency-domain 3-D electromagnetic inversion approach that can be applied to time domain data from GREATEM. To deal with a huge number of grids and wide range of frequencies in airborne datasets, a method for approximating sensitivities is introduced for efficient 3-D inversion. Approximate sensitivities are derived by replacing adjoint secondary electric fields with those computed in the previous iteration. These sensitivities can reduce the computation time without significant loss of accuracy.
We verified our inversion approach using electromagnetic response generated from two synthetic models that consist of one conductor and two conductors buried in a host resistive half-space. The inverted results were able to recover conductive zones of potential interest within the resistive region. These results are in good agreement with the synthetic model, confirming the accuracy of our inversion approach.
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3D Multiple Body Parametric Inversion of Time-domain Airborne EM Data
Authors M.S. McMillan, D.W. Oldenburg, E. Haber, C. Schwarzbach and E. HolthamSummaryWe developed a 3D parametric inversion for time-domain airborne EM data using a skewed ellipsoid representation for multiple conductive or resistive anomalies. The approach aims to simplify the task of imaging thin, potentially highly conductive, anomalies with 3D EM inversion. The algorithm finds the optimal location, shape, size and resistivity of the anomalies in a homogeneous or heterogeneous background by employing a Gauss-Newton style optimization.
Our parametric method is tested on a synthetic and field data set. The synthetic model is composed of two narrow dipping conductive anomalies in a resistive background along with a vertical narrow conductor. The survey layout and resistivity structure is based off field data from a greenstone setting. The parametric inversion accurately recovers the spatial extent and dips of the three synthetic anomalies, although the depth extent of the anomalies is exaggerated. In the greenstone field example, the inversion defines the spatial location, extent and dips of three conductive anomalies to provide a new conductivity interpretation of an area where little information is known regarding the true nature of the conductors.
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A Statistical Approach to Interpret Regional Airborne Electromagnetic Surveys for Hydrogeological Characterization
Authors M. Dumont, P.A. Reninger, A. Pryet, G. Martelet, B. Aunay and J.L. JoinSummaryAirborne electromagnetic (AEM) methods proved their accuracy in environmental studies during last decades. Volcanic islands are characterized by heterogeneous geology controlled by various complex factors. La Réunion Island, localized in the south-west part of the Indian Ocean, is composed by two shield volcanoes. Their structures are controlled by large erosive processes driven by steep topography, and weathering in inter-tropical climate and the cyclonic regime. In these conditions, in order to help interpreting a large regional AEM dataset resulting in millions of 3D resistivity values, we propose a strategy to delimit areas where the resistivity structure of the first 100 m of the underground have comparable behaviors - and accordingly comparable geological and/or hydrogeological characteristics.
Agglomerative Hierarchical Clustering (AHC) analyzes variation between the vertical resistivity distributions of all inverted soundings. AHC results are hierarchized in clusters, each one being associated to an average depth-resistivity signature. The AHC method was applied on the AEM dataset acquired on La Réunion Island. Clusters are consistent with the geological background and inform on different geological and hydrogeological phenomena such as weathering. These subdomains will be used with the aim of evaluating various conceptual hydrogeological models throughout the island.
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Induced Current System During the On-time of an AEM System
Authors A. Smiarowski, G. Hodges and T. ChenSummaryConductivity depth transforms are a useful tool for quickly creating a subsurface image from airborne electromagnetic data. Time-domain EM (TEM) transforms have utilized the response from a step-off waveform and a thin sheet or homogenous halfspace model to determine exploration depth. The induced current system from a step-off waveform has been studied in the literature. However, the current system from other waveforms, and in particular the current system during the on-time, has not been addressed. Here, we calculate the induced current for a halfsine waveform during the off-time and the on-time. We show that the on-time current density is concentrated near surface compared to the off-time. Using field data, we then compute the halfspace apparent resistivity for an on-time channel and the first off-time channel as well as five frequencies from a frequency domain survey; the on-time apparent resistivity correlates well with the high-frequency apparent resistivity while the first off-time channel correlates with a mid-frequency apparent resistivity.
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The Development of the iFTEM AEM System
More LessSummaryThe iFTEM airborne electromagnetic (AEM) system is a new fixed-wing, towed-bird AEM system which is being developed for conductivity mapping and the detection of mineral at considerable depths. It filled in the gaps in the fields of the time-domain AEM system in China. This paper presents the design philosophy and describes the design features. The system is now in the initial test-flying stages. The iFTEM system has an high power transmitter that the peak dipole moment is about 604,800 Am2 at 25 Hz. The system bandwidth is from 25 Hz to 20 kHz and the EM sensor is a 3 perpendicular dB/dt air-cored coils in towed bird. As the data acquisition unit can recording full-waveform of the 3-component response and Tx current, the multi-components data can be used to do the bird motion compensation and target geometry discrimination.
From Dec., 2014 to Feb. 2015, several flight test has been carried out. The physical embodiment of the iFTEM system has now been completed and the system is stepping into the initial trial-flying stages. This paper presents the design philosophy and describes the design features.
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A System Response Convolution Routine for Improved Near Surface Sensitivity in SkyTEM Data
Authors K. K. Andersen, N.S. Nyboe, C. Kirkegaard, E. Auken and A.V. ChristiansenSummaryWe present a new forward modelling routine, which can calculate the secondary magnetic field for all times - also during the transmitter turn-off. This is done by convolving the step response with the time derivative of the system response. The calculations can be compared to the SkyTEM primary field corrected data to obtain a better sensitivity to the near surface, which are probed by the earliest gates in the measured signal from 0 to 10 (µs. We describe the implementation of the new routine and compare it to results with the previous routine and to data measured at the Danish test site. The agreement is very good even though the system is described in two completely different ways.
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1D and 3D Sensitivity Analysis of an Airborne VCP TDEM System
Authors G-A. Sab, C. Schamper, F. Rejiba and A. TabbaghSummaryAn airborne time domain electromagnetic (TDEM) system is currently under development. Its building setup would allow it to use a configuration that has, to our knowledge, never been used in airborne TDEM prospection. Both its transmitting and receiving coils can be simultaneously vertical, resulting in a vertical coplanar (VCP) configuration emitting and recording a horizontal EM field. The aim of the presented work is to determine the advantages and disadvantages of this kind of measure. First, a db/dt signal comparison shows that using VCP implies obtaining measures with a lower depth of investigation. This is due to the signal level and to the horizontal EM field noise level. Frequency electromagnetic (FEM) commonly uses both VCP and HCP (horizontal coplanar) configurations, with VCP usually giving better results in 3D anomalies detection. It can be wondered if the same observation can be made in TDEM. 3D forward modeling has been carried out to compare HCP and VCP sensitivities to a near-surface conductive formation. Results show that VCP has both a better sensitivity to this structure and a better lateral resolution in the flight direction. VCP, though inefficient for deep targets, could then be a way to improve near-surface TDEM prospection.
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Neural Networks for Efficient and Automatic Removal of Coupled TEM Data
Authors E. Auken, K. K. Andersen, N. Foged and C. KirkegaardSummaryModern airborne transient electromagnetic surveys typically span thousands of line kilometres requiring careful data processing. When surveys are flown in populated areas, data processing becomes particularly time consuming, since the acquired data is contaminated by couplings to man made conductors (power lines, fences, pipes, etc.). Coupled soundings must be removed from the dataset prior to inversion, but since the signature of couplings can be subtle and difficult to describe in general terms it has so far remained mostly a manual task. We train an artificial neural network to recognize coupled soundings in previously processed data and use this network to identify couplings in other data. The approach provides a dramatic reduction in the time required for data processing, since one can directly apply the network to the raw data. We present the training state and performance of the network and compare inversions based on manually processed data and ANN processed data. The results show that a well trained network can produce a high quality processing of ATEM data, which are either ready for inversion or in need of minimal manual processing. The results are very promising and can significantly reduce the processing time and cost of large ATEM surveys.
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Understanding Airborne IP
Authors T. Chen, A. Smiarowski and G. HodgesSummaryWhat is a proper model to describe the IP effect in airborne time domain electromagnetic (TEM) measurement? The Cole-Cole relaxation model which characterizes a polarizable medium can be expressed either in terms of conductivity or resistivity, depending on the energizing sources (current or voltage) to be used. In either expression, the Cole-Cole model can be decomposed into a non-frequency dependent and frequency dependent part. The response is the convolution of the energizing source with the time domain expression of Cole-Cole model. We examine the response of a dc current source and a dc voltage source. We show that the Cole-Cole model expressed ass dispersive conductivity is a proper model for obtaining the airborne TEM IP response. We demonstrate this by the responses of a halfsine transmitter current waveform over a polarizable body, simulated with a wireloop.
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Case Studies of Modelling IP Effects in AEM Data
Authors V. Kaminski, A. Menghini, A. Viezzoli and G. FiandacaSummaryThe use of a specific inversion code, able to recover Cole Cole parameters, allowed us to extract chargeability info and improved resistivity estimation, from AEM dataset.
Two case studies coming from Canada, for mining exploration, are presented: VTEM and HeliTEM data were re-processed, allowing the modeling of chargeability, confirmed by borehole or ground IP prospect.
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Black Shale Mapping by AEM for Geotechnical Applications
Authors A.K. Lysdahl, S. Bazin, H. Anschütz and A.A. PfaffhuberSummaryAirborne Electromagnetics (AEM) has been used to map the extent of environmentally harmful black shale in two geotechnical projects in Norway, supported by electrical resistivity ground surveys and laboratory measurements. The work demonstrates that resistivity is a suitable parameter for both mapping and characterizing black shales, since their high sulfide content in general means a more harmful shale and in addition gives very low resistivity (0.1–1 Ωm). Various interpretation procedures to define a black shale volume have been explored, and some of the data show vertical resolution sufficient for geotechnical applications.
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