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Second European Airborne Electromagnetics Conference
- Conference date: September 3-7, 2017
- Location: Malmö, Sweden
- Published: 03 September 2017
1 - 20 of 31 results
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Efficient 2D Hybrid Inversion of Airborne Frequency Domain Data
Authors E. Auken, T. Boesen, A.V.C. Christiansen, G.F. Fiandaca, A.A. Pfaffhuber and M.V. VögeSummaryWe present a 2D inversion code for frequency-domain HEM data designed for inverting field scale surveys on normal desktop computers. The fundamental algorithm is a 2.5D algorithm with field separation into primary and secondary fields. Due to limited memory, as well as performance concerns, sectioning is introduced for splitting large surveys into smaller sections. Sectioning is only done when calculating the 2D forward and derivatives, and it is done with a sufficient overlap, such that vital 2D information is preserved. The algorithm uses a hybrid scheme which i) starts with 1D forward and inverse calculations, ii) then switches to 2D forward calculations and 1D derivatives, and iii) finally ends with full 2D calculations. The result of this is a code which produces results like a 2D code, but with a substantially shorter computational time.
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Probabilistic Inversion of AEM Data with an Explicit Choice of Prior Information
Authors T.M. Hansen and B. MinsleySummaryMultiple approaches exist that allow inverting airborne electromagnetic (AEM) data into subsurface models of resistivity. Here we present an alternative approach to inversion of AEM data, based on probabilistic approach to inverse problems where known states of information is combined into a posterior probability density (the solution to the inverse problem) using the concept of conjunction of information. Two types of information is integrated as part of the inversion: The observed electromagnetic data with associated model of uncertainty, and an explicit choice of a priori information based on existing expert knowledge. The choice of prior model is here completely detached from the quantification of the data (and uncertainty of the data), and there is therefore no manual regularization to consider. We demonstrate the methodology on an airborne electromagnetic data from, using a variety of Gaussian based a priori models with non-Gaussian 1D marginal distributions.
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Comparison of Near-surface Sensitivity Functions of Airborne and Ground-based EM Systems
Authors L.M. Madsen, K.V. Lassen, E. Auken and A.V. ChristiansenSummaryKnowledge of the sensitivity function of an electromagnetic (EM) system helps to assess the advantages and shortcomings of the system set-up. We have computed 3D sensitivity functions of three different EM systems – one airborne (SkyTEM) and two ground-based systems (tTEM and GCM) – in order to compare the sensitivity distribution of the different transmitter-receiver set-ups. The results show that the GCM has the highest sensitivity to near-surface structures, but also has a very limited footprint depth. The SkyTEM and tTEM systems have comparable sensitivities vertically in the ground with a 90% footprint depth around 5 m for early time measurements. However, the sensitivity of the SkyTEM system is distributed must wider horizontally than the tTEM.
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3D Inversion of the Semi-airborne Electromagnetic Data from Schleiz, Germany
Authors M. Cherevatova, C. Nittinger, M. Becken and T. GuentherSummaryWe present a new semi-airborne frequency domain electromagnetic system being developed within the DESMEX project. In the system, the high-moment transmitter is positioned on the ground and the receivers (induction coil and fluxgate magnetometers) are installed in a helicopter-towed bird. The major difficulty is to overcome the problem of motion noise and motion-induced voltages, due to the pendulum-like behaviour of the bird. For this purpose, we developed a processing scheme which corrects data for motion related noise. Specifically, for processing in frequency domain we utilize only free-of-motion-noise frequencies. The initial design of the system was developed and tested in several flight campaigns. In the current paper, we present a first 3D inversion of the data acquired during the experiment in Schleiz, Germany. In the model the conductive anomalies which we interpret as alum shales, are embedded within generally resistive Cambrian basement. A comparison with the 2D electrical resistivity tomography model shows that our model represents the same resistivity structures. The experiment also proved that our system allows us to cover an area of around 36 square km during one flight (3 hours) resulting in penetration depth of 1–1.5km.
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Two-dimensional Induced Polarization Effects on Airborne Time Domain Electromagnetic Data
Authors C. Lin, E. Auken, G.H. Fiandaca and A.V. ChristiansenSummaryRecently, the interest in induced polarization (IP) phenomenon in airborne time-domain electromagnetic (ATEM) data has been significantly increased. 2D induced polarization greatly affects the ATEM data and masks underlying geological structures. In order to simulate rigorous 2D airborne IP data, a 2.5D modeling algorithm has been developed using the Cole-Cole model and the finite-element method. We verify our algorithm by comparison with the 1D solution of the AarhusInv code. 2D IP effects in the ATEM data are related to the four Cole-Cole parameters, the horizontal length of the 2D target and the distance between the receiver and the target. Compared to the 1D IP responses, the 2D responses are different especially at the stations near the edge of the 2D target. The general shape of the 2D target except for the edge can be recovered by the 1D Laterally Constrained Inversion (LCI) of AarhusInv.
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Three-dimentional Pseudo-lithology Derived from Interpretation of Airborne Geophysical Data
Authors A. Viezzoli, V. Kaminski, H. Paasche and G. MancaSummaryA VTEM survey was flown over the Tli Kwi Cho kimberlite complex in 2004. The voltage data turns negative over the two kimberlitic targets (DO-18 and DO-27). The data have been transferred to Aarhus Geophysics in order to be inverted using the Cole-Cole model. The inversion of the data allowed recovery of chargeable and conductive material, which coincides with the pyroclastic and partially volcaniclastic facies of the DO-27 kimberlite. Further the TMI data collected during the same airborne survey have been inverted using 3D inversion code, then all physical properties were combined together and populated a same mesh, then the properties were clustered to yiled a 3D pseudo-lithology distribution.
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A Novel Semi-airborne EM System for Mineral Exploration - First Results from Combined Fluxgate and Induction Coil Data
Authors C. Nittinger, M. Cherevatova, M. Becken, T. Martin, H. Petersen, A. Steuer, U. Meyer, B. Siemon, U. Matzander, B. Friedrichs, W. Mörbe, P. Yogeshwar, B. Tezkan, R. Rochlitz and T. GüntherSummaryA novel semi-airborne frequency domain system is developed within the DESMEX project. The system is designed to achieve a penetration depth of about 1km with an areal coverage of about 6×6km2 per source installation on the ground. Here, we report on first results achieved with the new system. Horizontal electric dipole transmitters were used to inject time-varying electrical currents into the ground, and both a three-axis induction coil set up (Metronix MFS-11e) and a fluxgate (Bartington MAG-03) were installed on a helicopter-towed platform. We measure the induced magnetic field in the air in a frequency range from 1Hz to 10kHz. Additionally recorded attitude data can be used for correction of motion noise and for in-flight calibration to determine orthogonality and scaling errors in the fluxgate data as well as orientation errors of all sensors within the bird. In a first flight test the corrected fluxgate data shows a good signal-to-noise-ratio in the range 10ߝ400Hz with a noise level generally below 50pT/√Hz. The coils yield superior noise levels of less than 1pT/√Hz at frequencies higher than 400Hz. Frequency-domain response functions between the magnetic field and the source current are found to be consistent with subsurface conductivity structures.
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SkyTEM High Power Systems – A New Generation of Airborne TEM Transmitters
Authors P.G. Gisselø and N.S. NyboeSummarySkyTEM Surveys has released a new generation of high power airborne TEM systems in 2017. Specifically, the new systems are the SkyTEM306HP and SkyTEM312HP high power variants, which deliver twice the magnetic dipole moment of regular SkyTEM systems. The high power variants of the two systems are capable of generating magnetic dipole moments of ∼500,000 Am2 and ∼1,000,000 Am2 respectively. A further advantage is that the high power systems have significantly lower transmitter loop self-inductance than comparable traditional SkyTEM systems. This allows for faster low moment current turnoff, which results in improved early time data and associated near-surface resolution.
The SkyTEM high power systems are mounted on an improved version of the compact SkyTEM 300 series frame. While the traditional SkyTEM 300 series frame is both rigid and aerodynamic, the newer frame is lighter due to improvements in the frame production process, which incorporates even stiffer and lighter materials. The compact, light and aerodynamic frame is key for maintaining sufficient maneuverability in challenging topographies.
We demonstrate the real-life performance of the high power systems, including measured low- and high moment waveforms, high altitude noise and bias characteristics as well as calibration data from the Danish national test site.
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An Improved Helicopter Time-domain EM System for Near Surface Applications
Authors T. Eadie, J.M. Legault, G. Plastow, A. Prikhodko and P. TishinSummaryAs part of a continued system design strategy aimed at expanding its early-time data, VTEM ET (Early Time) focuses on further improving the system’s capabilities for near surface applications, such as groundwater and environmental. The VTEM ET system improvements include: a) increased receiver bandwidth, b) microsecond resolution of early time channel measurements, and c) shorter transmitter current waveform turn-off. Each of these improvements contributes to an overall increase in the system’s sensitivity to subtle changes in the near-surface geology and improves the accuracy of data models. This is initially demonstrated using 1D synthetic forward and inversion modelling of VTEM ET data compared to the standard VTEM system. To further demonstrate increased sensitivity, inversion results are presented from the Spiritwood aquifer region of North Dakota and are compared against well log lithology.
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Recent Advances in Skytem Receiver System Technologies
Authors N.S. Nyboe and S.S. MaiSummaryThe SkyTEM system was originally designed with emphasis on mapping aquifers and their exposure to potential contaminants. This type of hydrogeophysical surveying requires a wideband receiver system in order to detect the subtle variations in early-time signal amplitude typically arising from variations in the near-surface sedimentary composition. For this reason, most SkyTEM receiver systems are capable of recording data covering a bandwidth of several hundred kHz. Recent developments in the SkyTEM receiver system further expands the achievable system bandwidth by introducing 5 MHz continuous data sampling and flexible realtime data reduction algorithms. Gate interval data are represented as polynomial coefficients, which provides more opportunities in post processing to minimise noise influence, while massively reducing the required amount of stored data. The 5 MHz sampling rate and finer signal quantization allows us to detect smaller variations in the early time signal behaviour as well as obtain system responses in unprecedented detail, which are free from implicit interpolation assumptions.
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Stretching AEM Near-surface Resolution Limits Related to Low- and Very High Resistivity Contrasts
Authors G.H. Skurdal, A.A. Pfaffhuber, A. Davis, S. Bazin, H. Anschütz, N.S. Nyboe, N. Foged, T. Thomassen and T. WiigSummaryData from AEM surveys carried out in Norway, to support ground investigations for infrastructure projects, was used in this study. In large infrastructure projects knowledge of sediment thickness is vital, as is information about possible occurrence of highly sensitive clay.
In an area with conductive shales over resistive bedrock, the recently introduced system response method was tested. It’s applied in the inversion of SkyTEM data and makes it possible to utilize the very earliest gates. The models showed to give more pronounced structures in the near-surface, reflecting true structures observed in resistivity borehole measurements. The same outcome was observed when conducting synthetic modelling. In another setting AEM measurements were carried out along a planned road project to provide information about the extent of very conductive, possible alum shale. A volume estimate of excavated masses was sought, as alum shale is decomposed to sulfuric acid by weathering. Preliminary AEM models had a tendency to overestimate the thickness of the resistive overburden. Experimenting with the inversion settings resulted in models better fitting other prior information from the area. Limited LM data was available due to a noisy environment. This affected the reliability of the models, illustrated by modelling and resulting real models.
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Developing a Communications Management Solution for Airborne Surveys in Peri-urban Environments
Authors J.A. Hodgson, E. Scanlon, M.D. Ture and S. O’BrienSummaryThe Tellus airborne geophysical survey is a national survey of Ireland and therefore low-level flying occurs over numerous dwellings, farms and livestock. A Tellus Communications Viewer has been developed to manage survey operations allowing flight plans to be integrated with digitized polygons of relevant stakeholders who can then be contacted. The tool was created using open source software - Matererialize – a front-end responsive framework created by Google. While ‘leaflet’ software is used to allow interactions with maps. The tool also acts as a database management system logging all interactions with stakeholders. The system has been developed over a number of years and was fully operational for airborne surveys conducted in 2016.
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Structural Geology Interpreted from AEM Data - Folded Terrain at the Foothills of Rocky Mountains, British Columbia
Authors F. Jørgensen, A. Menghini, G. Vignoli, A. Viezzoli, C. Salas, M.E. Best and S.A.S. PedersenSummaryThis study presents an interpretation of newly collected AEM data from the Peace Region, British Columbia, Canada. Through careful data processing and interpretation we demonstrate the potential for AEM methods to map traditional structural geology such as folds and thrust structures. The study area covers about 1,960 km2, and based on a dense grid of flight lines, a conceptual structural geological model has been developed. The geology of the region generally comprises Cretaceous rocks which are gently folded and displaced by shallow westerly dipping thrust faults. Alternating clay content of the stratigraphic formations in the area makes them detectable by the AEM data. Our structural model presents a sedimentary regime affected by gentle deformation and thin-skinned thrust faulting. We are able to resolve several anticlines, synclines and thrust faults. The main thrust fault is east-dipping, thus counter to the regional Laramide orogenic trend and likely represents a backthrust from a subcutaneous structure (triangle zone). We suggest a décollement level to be situated in or just below the clay-rich Buckinghorse Formation.
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The RESOLVE Frequency Domain EM System Applied to a Geotechnical Project
More LessSummaryHelicopter-borne frequency-domain instruments are very often the geophysical method of choice for high-resolution near-surface information. In this talk we briefly review some of the advantages of FD systems over TD (such as in-phase measurements, tuned coils, usefulness of phase angle, compact footprint) for environmental applications. We then present a case study of a recent RESOLVE survey. Here, the system was used to help plan a pipeline project through a seismically active mountain valley.
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Characterization of the Spiritwood Valley Aquifer, North Dakota, Using a Helicopter Time-domain EM System
Authors J.M. Legault, T. Eadie, G. Plastow, A. Prikhodko, D. Hisz and J.C. PatchSummaryBuried valley aquifers, consisting of permeable sand and gravel deposits in eroded bedrock valleys, are important sources of groundwater supply in many regions of the United States and Canada. But they have been difficult to define because they are often partially eroded, have complex lithology and are hidden within thick glacial overburden. Recent investigations of the Spiritwood Valley aquifer in Manitoba by the GSC using helicopter time-domain EM have provided the impetus for the North Dakota State Water Commission to fly a VTEM helicopter TDEM survey in the Jamestown, ND region in October, 2016. The VTEM data collected over the Spiritwood-JT block allowed for geological mapping from near surface to depth, in spite of relatively weak resistivity contrasts (<10X). The VTEM data were inverted with the 1D GALEISBSTDEM algorithm to produce resistivity-depth models. These models were able to resolve the location and depths to the top and bottom of the Spiritwood aquifer throughout the central portion of the block providing more detailed pictures of the aquifer’s geometry. In addition to resolving the main Spiritwood aquifer as well as its deeper channels, the VTEM data and models highlighted several smaller aquifers which cross-cut/branch-off from the main Spiritwood channel.
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Application of Helicopter Time-domain EM for Mine Operations
Authors F. Effersoe and B. BrownSummaryThis paper presents examples of SkyTEM helicopter-borne time-domain (HTEM) systems recently applied to solve a variety of environmental and engineering problems. The mining industry has traditionally used airborne geophysical methods almost exclusively for mineral exploration. However, this paper provides examples of how HTEM data can inform decisions relevant for mine operations. Examples comprise delineation of aquifers that may (1) pose a risk to underground workings and (2) may be threatened by seepage from tailing dames, delineation of clay caps that can act as shallow gas traps endangering drilling operations, and mapping of faults and taliks the location of which must be considered when placing mine infrastructure.
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Mapping of 3D Hydrogeological Framework of the Deccan Basalt Groundwater Systems Using AEM
Authors S. Chandra, S. Sonkamble, E. Auken, S. Ahmed and S.K. VermaSummaryThe SkyTEM data provides a comprehensive three-dimensional picture of the subsurface. The low moment data ensured the high resolution mapping of the near surface and the high moment data to the deeper level. The dual moment AEM data provided high resolution mapping of subsurface from top to ∼ 250 m depths. The results revealed significant contrast between the B-G layers that has a resistivity range of 10–15 Ωm and generally has groundwater yields of 6–8 lps. It is also noted that the groundwater yield (> 6 lps) increases for deeper (<150 m) B-G contact.
The potential aquifer zones such as intertrappeans, fractures in vesicular basalts and Basalt-Gondwana contact are clearly mapped in the resistivity section. The regional fault (potential aquifer) which is abutting in the SW part with its orientation SE-NW is successfully delineated in the mean resistivity maps. The aquifer systems in basalts and Gondwanas and their extensions are also clearly demarcated on the 3D map using AEM data.
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HTEM Data Improve 3D Modelling of Aquifers in Paris Basin, France
Authors G. Martelet, P.A. Reninger, J. Perrin and B. TourlièreSummaryIn Paris Basin, we evaluate how HTEM data complement the usual borehole, geological and deep seismic data used for modelling aquifer geometries. With these traditional data, depths between ca. 50 to 300m are often relatively ill-constrained, as most boreholes lie within the first tens of meters of the underground and petroleum seismic is blind shallower than ca. 300m. We have fully reprocessed and re-inverted 540km of flight lines of a SkyTEM survey of 2009, acquired on a 40×12km zone with 400m line spacing.
The resistivity model is first “calibrated” with respect to ca. 50 boreholes available on the study area. Overall, the correlation between EM resistivity models and the hydrogeological horizons clearly shows that the geological units in which the aquifers are developed almost systematically correspond to relative increase of resistivity, whatever the “background” resistivity environment and the lithology of the aquifer. In 3D Geomodeller software, this allows interpreting 11 aquifer/aquitar layers along the flight lines and then jointly interpolating them in 3D along with the borehole data.
The resulting model displays 3D aquifer geometries consistent with the SIGES “reference” regional hydrogeological model and improves it in between the boreholes and on the 50–300m depth range.
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Heliborne EM and Magnetism for Hydrogeological Characterization in an Obduction Context - A New-Caledonian Example
Authors P.A. Reninger, J. Perrin, S. Lesimple, J. Jeanpert, N. Foged and E. AukenSummaryAirborne Time Domain ElectroMagnetic (TDEM) and magnetic (MAG) surveys have successfully been used for environmental studies. In igneous provinces, TDEM and MAG data may be jointly interpreted for the characterization of the geological environment. In May 2015, a heliborne survey was conducted in New-Caledonia Island (south-west in the Pacific Ocean) over a small area nearby Koné for hydrogeological characterization. The area is hosted by pre-Late Cretaceous metamorphic unit, Cretaceous formations, two volcanic facies and peridotite massifs, relic of the ophiolite nappe. TDEM and MAG data were separately and jointly interpreted in order to characterize the Koné area. Separately, results obtained from the two methods provided new information on the hydrogeological environment and allow delineating the precise limits of the different geological formations. Jointly, the multilayered results were interpreted using a hierarchical ascendant clustering algorithm. Through a statistical classification of three MAG grids and three TDEM grids, it was possible to propose a predictive geological map for the Koné area. The input will help to better manage the groundwater resources and will be used to position new exploration wells.
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Comparison Between ATEM, RMT, ERT and CPTR Data at a Quick Clay Site in Sweden
Authors M. Bastani, L. Persson, H. Löfroth and C.A. SmithSummaryIn 2015, airborne transient electromagnetic (ATEM) surveys at four areas were conducted to study feasibility of using the method for identification of areas with possible presence of quick clay in Sweden. Here we present the results from survey in area 3. Using resistivity models from 1D inversion of ATEM data ground investigations were planned. These include radio magnetotelluric (RMT), electrical resistivity tomography (ERT), and cone penetration tests with resistivity (CPTR) measurements together with core samplings. The RMT models show the best correlation with the CPTR measurements in the upper 10 meters of the boreholes however they lack sensitivity to resolve underlying resistive bedrock. The ATEM models show the same resistivity variations seen in the CPTR data and RMT models at shallower depths with less resolution. They also demonstrate reasonable sensitivity to detect bedrock. At locations where 1D assumption is not valid the ATEM models are biased and the depths to the bedrock are underestimated and do not coincide with the observations in the boreholes. Our analyses reveal that the ATEM data contain valuable information to use for identifying areas with quick clay provided that geotechnical data are available for calibration of models and interpretations in the survey areas.
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