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23rd European Meeting of Environmental and Engineering Geophysics
- Conference date: September 3-7, 2017
- Location: Malmö, Sweden
- Published: 03 September 2017
1 - 20 of 157 results
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2.75-D ERT - Zigzag Electrode Acquisition Strategy To Improve 2-D Profiles
Authors A. Robbins and A. PlattnerSummaryAlthough 3-D electrical resistivity tomography (ERT) has been available for more than two decades, its widespread use has been limited by higher data acquisition and processing costs compared to standard 2-D ERT. Alternatively, the viable method of processing 2-D profiles with a 2.5-D approach has overwhelming popularity due to ease of data acquisition and processing. However, 2-D profiles do not account for resistivity variations perpendicular to the profile. This limits the retrieval of valuable information and may lead to biased resistivity profiles for subsurface objects that intersect obliquely with the survey line. In principle we could use 3-D processing to calculate a resistivity solution from the 2-D array. Unfortunately, this leads to inversion results that are symmetric with respect to the profile because the sensitivity pattern for each measurement shares this type of symmetry. We propose an acquisition strategy that has the simplicity of a 2-D profile in terms of work in the field and equipment requirements, but overcomes the symmetry issues of classical 2-D profiles. Rather than along a line, we arrange our electrodes in a zigzag pattern of alternating +/− offset along the y-axis. This approach, which we dub “2.75-D ERT” can be implemented by simply shifting the electrodes away from the center profile in an alternating pattern and does therefore not require any additional equipment or setup in the field. The resulting data needs to be processed with 3-D electrical resistivity code. With modern computers and software this does not pose an obstacle anymore even when only moderate computing power is available thanks to free high-performance programs such as BERT or E4D. In a field experiment, we compare the results of a 2-D array to a zigzag array both transecting a known target at an angle. Unlike the solutions for the 2-D array, our zigzag array captured the known target’s asymmetry with respect to the profile.
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Preliminary Results From The First Airborne EM Survey Conducted In Yellowstone National Park
Authors K. Dickey, W.S. Holbrook, C. Finn, E. Auken, B. Carr, K. Sims, P. Bedrosian, J. Lowenstern, S. Hurwitz and J. PeSummaryYellowstone National Park hosts over 10,000 thermal features (e.g. geysers, fumaroles, mud pots, and hot springs), yet little is known about the circulation depth of meteoric water feeding these features, the pathways that guide deep, hot fluids to the surface, or the separation depth of the steam that sources vapor-dominated systems. Previous near-surface geophysical studies have been effective in imaging shallow hydrothermal pathways in some areas of the park, but these methods are difficult to conduct over the large areas needed to characterize entire hydrothermal systems. Transient electromagnetic (TEM) soundings and 2D direct current (DC) resistivity profiles show that hydrothermal fluids at active sites have a higher electrical conductivity than the surrounding hydrothermally inactive areas. For that reason, airborne TEM should be an effective method to characterize large areas and identify hydrothermally active and inactive zones using electrical conductivity. Here we present preliminary results from an airborne transient electromagnetic (TEM) and magnetic survey acquired jointly by the U.S. Geological Survey (USGS) and the University of Wyoming (UW) in November 2016. The survey covers 2600 line-km of data at two scales: regional surveys with lines spaced 450 apart and three smaller, high-resolution surveys with line spacing of 150 m. The regional survey will cover northern Yellowstone Lake, the Norris-Mammoth corridor, and the Upper Geyser basin. The high-resolution surveys focus on the Upper Geyser Basin (including Old Faithful), the Norris Geyser Basin, and the Mud Volcano Area. Data was acquired with the SkyTEM 312, with a magnetic moment of 0.5 M A-m2. We will present preliminary inversions using the Aarhus Workbench software, with particular focus on the depths of vapor phase separation and the connectivity of pathways of meteoric water recharge.
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The Forgotten Holocaust in Lithuania - Geophysical Investigations at the Ponary Extermination Site
Authors A. McClymont, P. Bauman, R. Freund, J. Seligman, H. Jol, K. Bensimon and P. ReederSummaryPrior to the Nazi invasion of Lithuania in 1941 and the temporary retreat of Soviet forces, the country and its capital city, Vilnius, were home to a thriving Jewish community of just over 200,000 people. During the Holocaust more than 95% of Lithuania’s Jewish population was massacred by German and Lithuanian killing squads. Of those killed some 70,000 Jews, and a further 30,000 Poles, Russian POWs, and political dissidents were exterminated at a single site within the Ponary (or Paneriai) forest, near Vilnius. Here, a series of circular pits, originally intended for fuel storage by the Soviets, were used as mass graves. Because investigation of the site was actively discouraged during the post-war Soviet era, our understanding of the Ponary massacre is largely limited to survivor testimonies and a few eyewitness reports. Furthermore, since the surviving local Jewish community is sensitive to the further desecration of human remains at the site, traditional archaeological excavation methods have been largely prohibited at the site. Non-invasive geophysical methods can therefore provide useful information to document the past history of the site in lieu of and prior to targeted archaeological excavations. In 2016 we undertook a geophysical survey at Ponary using electrical resistivity tomography (ERT), ground-penetrating radar (GPR), and aerial drone imagery techniques. We describe the results of the 2016 geophysical investigation, including the delineation of an unmarked mass grave and the discovery of an escape tunnel, through which 11 Jewish slave labourers survived the Holocaust to tell the story of events at Ponary. By combining the results of our geophysical survey with interpretation of historical air photos and a LiDAR digital elevation model, we show how non-invasive geophysical and archaeological techniques can be used to reveal key details of the Ponary massacre and preserve the history of the Holocaust in Lithuania.
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Geophysical Characteristics of Permafrost Degradation across Boreal Landscapes after Disturbance by Fire or Water
Authors B. Minsley, N. Pastick, B. Wylie, D. Brown and A. KassSummaryFire and hydrology can be significant drivers of permafrost change in boreal landscapes, altering the availability and transport of soil carbon and nutrients that have important implications for future climate and ecological succession. However, not all landscapes are equally susceptible to disturbance by fire or hydrological processes. As fire frequency is expected to increase in the high latitudes, methods to understand the vulnerability and resilience of different landscapes to permafrost degradation are needed. We present a combination of multi-scale remote sensing, geophysical, and field observations that reveal details of both near-surface (<1 m) and deeper impacts of fire and hydrology on permafrost. Along 42 transects that collectively span more than 6,000 m at 31 sites located in different landscape settings within interior Alaska, subsurface geophysical imaging indicates locations where permafrost appears to be resilient to disturbance from fire or small streams, areas where warm permafrost conditions exist that may be most vulnerable to future change, and also where permafrost has thawed after fire or because of nearby surface water. Data collected along each transect include observations of active layer thickness (ALT), organic layer thickness (OLT), plant species cover, electrical resistivity tomography (ERT), and downhole Nuclear Magnetic Resonance (NMR) measurements. In addition, we discuss 300 km of newly acquired airborne electromagnetic (AEM) data in the western part of the lake-rich Yukon Flats that extends the coverage of an earlier 2010 AEM survey into a more ice-rich region. AEM data are used to evaluate the relationship between surface water features and deep (up to 100 m or more) permafrost extent in order to evaluate the potential for subsurface hydrologic connectivity and the potential for lateral fluxes of water.
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Shear-wave Reflection-seismic and ERT Investigation for Slope Stability
Authors G. Sauvin, A.O.K. Lysdahl and S. BazinSummaryAs part of a civil engineering project for line of communication extension, slope stability analysis was required. The ground model, based on geotechnical soundings alone failed to resolve the required parameters. Therefore, a more extensive study combining Electrical Resistivity Tomography and S-wave reflection seismic was conducted. The resulting geological model properly address the need for a comprehensive slope stability analysis. The integration of structural information from the SH-wave reflection seismic, the resistivity model and the geotechnical soundings provides a sounded geological model down to the bedrock and a clear image of the overlaying moraine, marine clay and fluvial deposits.
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An All In Resistivity and Seismic Portable Field System
Authors F.E.R. Almeida, R. Baieta and A. FerreiraSummaryA data acquisition system based on Lab View virtual instruments was developed to allow both electrical resistivity and seismic wave velocity measurements with no need for switching equipments and standard multicables. This is achieved by simply using eight electrodes as connectors for four geophones when changing from electrical to seismic methods.
The Pole Pole resistivity array was adopted using the eight electrodes for potential measurements while the current electrode was manually displaced thus using less cable. The tomographic refraction method was based on four common receiver gathers. Each receiver gather consists of 16 shots one metre apart fired manually This apparatus enables the use of a simple, lightweight and also cheap field system with a good spatial seismic and electrical parameter resolution for multipurpose site characterization. A controlled experiment was done to validate the system.
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Investigation of a Waste Fill
Authors A.C. Duncan and G.J. StreetSummaryGeophysical investigations were carried out across a sports field created in the sand dunes close to the coast in Perth, Western Australia. It is understood that the site was previously used as a dump for rubbish. Undulations have developed on the surface of the oval, which are believed to be due to differential compaction of the rubbish underneath a cover of sand. The purpose of the investigations was to determine the sites contamination, subsurface materials, stability and costs associated with rehabilitating the site to be used as playing fields.
A series of geophysical surveys were carried out using frequency domain EM, radiometrics, GPR, magnetics and resistivity. The site was then used as a test area for a new profiling TEM system called “Loupe”.
The new results are compared against previous results and the superior resolution in depth and better positional accuracy discussed.
This is work still in progress at the time of submission but the Loupe System is a new generation instrument incorporating RTK GPS, fast sampling, high bandwidth and easy portability. The ability to resolve conductivity in 3D in the near surface using a rapid acquisition system will change how many geotechnical surveys are conducted.
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Geophysical Site Investigation at Dalby-Önneslov Using Joint Inversion
Authors M. Ronczka, P.-I. Olsson, M. Rossi, A. Malehmir and T. DahlinSummaryA geophysical site investigation using ERT and refraction seismic was done to detect the bedrock interface and possible weakness zones. For the purpose of a more realistic subsurface model a joint inversion was done.
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Cluster Analysis on Geophisical Parameters for Subsoil Investigation
Authors G. Bohm, L. Baradello, A. Affatato and M.E. PoliSummaryIn this work we present an application of cluster analysis on seismic and electrical data with the aim to identify the important structures present in the investigated area. To define the clusters we use the cross-plot representation of sesmic velocity vs. resistivity, obtained from the travel time tomography and the Electrical Resistivity Tomography (ERT) respectively. The seismic and electrical data were acquired in four 2D lines. The studied area belongs to the Quaternary front of the eastern Southern Alps in Northeastern Italy, characterized by the presence of a tectonic actvity which caused deformation of the Upper Pleistocene — Holocene deposits. The applied method allowed us to separate the zones with similar seismic and electrical properties to better distinguish the different structures buried in the investigated area. The results point out a discontinuity in the shallow structures, which testifies the presence of an important fault, assumed to exist in this area.
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Risk Based Site Investigations for Horizontal Directional Drilling
Authors L Klinkby, J. Colberg-Larsen, K. Pedersen, M. Laier, S. Petersen and K. BuusSummaryIn the projects presented risks for horisontal directional drilling (HDD) have been in focus during integrated site investgations. Risks were identified based on geotechnical, geological and geophysical data and geological site inspections have been used to address the likelihood of the risk. The final list of identified risk and likelihood was evaluated in an iterative process between the geotechnical engineers and the geoscientists from both Energinet and COWI. The possible consequences have been evaluated by HDD Engineering Specialists, which also added technical risk and suggested possible mitigation measures for contractor work to the final risk assessment.
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Characterization of Holocene Sediments Using Geophysical Methods and Borehole Information - Ebro Delta (Spain)
Authors B. Benjumea, A. Gabàs, A. Macau, F. Bellmunt, S. Figueras and M. VilàSummaryIdentification of zones vulnerable to subsidence is a critical task for planning protection actions in deltas. This identification mainly relies on Holocene sediments characterization. Geophysical studies have been carried out in the Ebro Delta (Spain) with this objective. In this paper we focus in two borehole sites where several surface geophysical techniques have been applied to find geophysical patterns that help to characterize the Holocene sequence. The applied methods are: active and passive seismic (H/V and array), electrical resistivity tomography (ERT) and audiomagnetotellurics methods (AMT). Shear wave velocity from active seismic can help to discriminate between surficial sediment facies. Combining this method with array measurements allows obtaining a complete Vs profile for the Holocene sediments. Another targets included in delta sediments characterization are depth and thickness of the Prodelta marine clays due to their particular geotechnical characteristics. These sediments are the origin of dominant higher modes of surface waves which help to their identification. Finally, ERT and AMT results show a resistivity change that can be related to the transition between Holocene sediments to Pleistocene gravels. This contact also produces a second peak in H/V spectral ratio that can be used to estimate the depth of Holocene delta base.
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Developments in Resistivity Inversion with Electrode Displacements for Landslide Monitoring
Authors M.H. Loke, P.B. Wilkinson, J.E. Chambers, S. Uhlemann and P.I. MeldrumSummaryTime-lapse resistivity surveys are being used to monitor changes in the subsurface that relate to landslide hydraulic processes and movements. In geologically unstable areas, not only does the subsurface resistivity change with time, but the positions of the electrodes also change due to ground movements. If direct measurements of the electrode positions are not available immediately after the ground movements, both the changes in electrode positions and the subsurface resistivity have to be estimated from the apparent resistivity measurements. We describe a modification to the smoothness-constrained least-squares optimization method that includes the electrode positions, together with the subsurface resistivity, as unknown variables to be determined. A fast adjoint-equation method is used to calculate the Jacobian matrices associated with the electrode positions from the finite-element capacitance matrix. Ground movements in a landslide zone normally occur in the downslope direction. This information is incorporated into the inversion algorithm using the method of transformations to reduce the ambiguity in the recovered electrode positions. Tests with a field data set from a landslide-prone area showed the error of the estimated electrode positions from the inversion algorithm is about 10%.
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Comparison of Frequency-domain and Time-domain Spectral Induced Polarization Methods at Field Scale
Authors P.K. Maurya, G. Fiandaca, M. Weigand, A. Kemna, A.V. Christiansen and E. AukenSummaryIn this paper we present a comparison study of time-domain (TD) and frequency-domain (FD) spectral induced polarization (IP) methods in terms of field procedures, data quality, as well as spectral information retrieved from inversion. For this purpose we collected TDIP and FDIP surface measurements on three profiles with identical measurement setup at two field sites with different lithology. Both TD and FD data were processed and inverted using similar approaches. This comparison study shows that, given the right circumstances, both methods (TD and FD) can be readily applied on the field scale, thereby providing results that compare not only qualitatively, but also quantitatively. Furthermore, the surface inversions are in agreement with the boreholes’ inversions/lithology, proving the effectiveness of the surface imaging. However, due to the underlying technical constraints and with the instruments used in this comparison, TD measurements had a 10 fold speed advantage over the FD method, a wider spectral range and more flexibility in the array choice.
Therefore we conclude that TD measurements are a suitable tool to recover spectral IP information in the field, provided that the measurement procedures are planned accordingly.
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The Relationship between Pore Radii and Induced Polarization Studied with a Membrane Polarization Model
Authors A. Hördt, M. Bücker, K. Bairlein and H. StebnerSummaryThe induced polarization (IP) method has some potential to estimate hydraulic conductivity of the subsurface. One approach is based on theories predicting relationships between pore radii and IP relaxation time. Recently, laboratory data were discussed where several samples exhibit large relaxation times that do not correspond with large pore radii, and thus seem difficult to explain. Here, we study the relationship between pore geometry and relaxation times using a model of membrane polarization that is based on two cylinders with different pore radii and lengths. By choosing appropriate parameter combinations, we are able to match relaxation times of measured data for given dominant pore radii. The key to explaining large relaxation times without large pore radii lies in the choice of large pore aspect ratios. We also show that the set of parameters that is necessary to match the data lies in the so-called wide pore regime, where the relaxation time is controlled by the length of the wider of the two pores.
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A Newly Developed 2D DC and Time-domain IP Inversion Algorithm -Application on Data Acquired on an Ore Deposit in Turkey
More LessSummaryWe present the application of a newly developed 2D direct current (DC) resistivity and time-domain induced polarization (TDIP) inversion algorithm on synthetic and on field data. The algorithm applies local smoothness constraints and error weighting. The inversion is conducted in two steps. First, an effective resistivity model is obtained from inversion of the measured apparent resistivity data. Subsequently, a non-linear IP inversion is applied resulting in a chargeability model of the subsurface. The implemented forward calculation uses the Finite Element (FE) method. Unstructured triangular meshes are used for all forward and inverse calculations allowing the incorporation of complex surface topography in the inversion mesh.
A modeling study with synthetic data shows that the algorithm is capable of resolving chargeability anomalies independent from the corresponding resistivity anomaly. This is important for disseminated deposits, since the resistivity signature of finely distributed material might be insignificant whereas the IP effect of ores, especially sulfides, is strong.
The inversion of DC/TDIP field data acquired on a (mainly disseminated) copper ore deposit in Turkey reveals a strong IP effect, as expected, which is interpreted to be caused by sulfidic copper ore content.
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3D ERT Monitoring of Levee Flooding Experiment Using Multi-current Transmission Technique
Authors Y.Y. Yamashita, T. Kobayashi, H.S. Saito, T. Sugii, T. Kodaka, K.M. Maeda and Y.C. CuiSummaryWe conducted a field experiment to simulate levee overflow using a model bank. We built a small scale model bank and water pool for simulating water level rising at river levee. We also set equipment for artificial rainfall above the model bank. As one of monitoring measurements of model bank while rising water level and raining artificially, we conducted time-lapse ERT survey on this model bank for the purpose of capturing water infiltration inside the model bank. We set plural 2D survey lines to cover whole area of model bank. To realize fine time resolution ERT measurement of plural survey lines, we applied multi-current transmission instrument, which has 8 transmitters and 8 receivers. With this instrument, we can improve measurement efficiency by injecting current at plural current electrodes simultaneously. We conducted a set of measurement which has 3072 data at the interval of 20 minutes. We performed 2D and 3D resistivity inversion. We can capture the water infiltration into the model bank by fine time resolution ERT measurement. The brackish saline water worked as a good tracer of ERT measurement in this case. 3D inversion using 2D survey lines data contributed to understand the spatial distribution of resistivity time variation.
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Which EM Properties with Multi-frequency Low Induction Number Slingram Apparatus?
Authors F.X. Simon, J. Thiesson and A. TabbaghSummaryIn the 3 kHz – 300 kHz frequency range, electromagnetic measurements achieved with limited size Slingram instruments are not only sensitive to the electrical resistivity but also to the complex magnetic susceptibility and to the electrical polarization. A process allowing the determination of all the properties is proposed and applied to case studies with the use of commercial instruments.
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Processing of Densely Sampled Electromagnetic Induction Data Collected across Peat Deposits
Authors J. Guillemoteau, P. Koyan and J. TronickeSummarySubsurface conductivity imaging with mobile electromagnetic sensor is of growing interest for characterizing soils across large areas of several hectares. In this study, we evaluate the feasibility of characterizing peat deposits with densely sampled electromagnetic induction (EMI) data acquired by a four-configuration SLINGRAM sensor on a well-controlled test-site near Paulinenaue (Germany). We specially discuss the signatures of 2D/3D subsurface structures and the removal of their effects on the 1D inversion results. At this test-site, we performed 62 boreholes in order to collect ground-truth information regarding the spatial distribution of the peat layers. We also compare our inversion results to ground penetrating radar (GPR) data recorded along one selected profile across the peat body.
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Solving the Problems of Obtaining Geological Information with Using Geophysical Methods of Research During Engineering-geological Surveys
Authors V.B. Pisetski, I.V. Abaturova, L.A. Storozhenko, I.A. Savintsev and I.G. PetrovaSummaryDespite the complexity of the relationship, and often care geology of problem solving methods of geophysics, it is necessary to find common ground, because geophysics is the oldest partner of engineering geology, it appeared in the temporary aspect of much earlier engineering geology. The report examines examples of solving geotechnical problems, which have not been solved without using of geophysical methods. At the same set of geophysical methods to the greatest extent focused on the use of the latest software and hardware tools professionally oriented geophysical companies - developers and producers (GEOSIGNAL, SIBGEOFIZPRIBOR, Geotech, etc.) And our own original designs, with international patents and scientific recognition (3D–3C seismic location underground and surface-based).
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Arctic Shelf Integrated Engineering Surveys Specifics
Authors P.S. Solovyev, D.I. Chernikov, D.A. Naumenko and M.I. ChudakovSummaryThe main objectives of the surveys included high-resolution mapping of the seabed; determination of rock mass lithology; identification of tectonic disturbances and highly fractured or iced zones; measuring the depth of the top and bottom of permafrost soil bodies; and analysis of the composition, condition and properties of frozen soils in the rock mass.
The following methods were used to accomplish the above objectives:
- High resolution seismic survey (HRS)
- Continuous seismoacoustic profiling (CSAP) at low frequencies (LF) and high frequencies (HF)
- Side-scan sonar survey
- Multi-beam echosounder survey
- Transient electromagnetic resistivity survey (TEM)
- Drilling of exploratory wells
- Temperature logging.
The drilling and temperature logging data fully confirmed the geological model of the survey area. Thus, taken together, the methods made it possible to accomplish the goal.
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