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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 - 100 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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Crosshole Seismic Testing on Pressure and Shear Waves - An Example of Effective Usage of the Method
Authors A.I. Konkov, A.N. Oshkin, N.A. Ragozin, V.I. Ignatev and R.Y. ErmakovSummaryThe paper describes an example of the implementation of crosshole seismic testing for pressure and shear waves at the site of proposed construction of Nuclear Power Plant “Paks-2” (Hungary). Based on the comparision of summary seismograms it was concluded that method demonstrates the best results at distances between wells of 25 27 m.
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Application of ERT to Identify Buried Covered Karst Features in the Design of Linear Constructions
Authors D.V. Makarov, I.N. Modin, A.A. Pelevin, A.D. Skobelev and K.D. EfremovSummaryThe impact of sinkholes can be extremely severe so that mapping buried karst in the karst areas is of primarily concern to construction of engineering infrastructure. Initially, potentially karst hazardous sites should be determined by geological and geomorphological approaches. Then electrical resistivity tomography (ERT), a well-established method for investigating near-surface geological and geotechnical conditions, is performed on selected sites. At the last stage, drilling is performed, confirming the presence or absence of sinkholes at a particular site. We see this sequence of steps as the most economical and efficient and show some results of ERT as applied to the karst risk assessment when designing linear engineering structures like roads, pipelines, power lines.
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Potential of Ground-penetrating Radar for Imaging Active Layer and Ice Wedges in Permafrost Areas
Authors S.S. Schennen, S.S. Bricheva and J.T. TronickeSummaryIce wedges are prominent phenomena of permafrost landscapes and cause a characteristic polygonal patterned ground. However, the existence of ice bodies in the subsurface is not always delineable based on surface data. Detection and imaging of these ice bodies is of interest to identify regions of increased subsidence potential and for scientific field work such as drilling programs. We demonstrate for two synthetic datasets of increasing complexity, that ground-penetrating radar can be used to determine ice wedge structures and to image their lateral extension. However, a structural imaging of complex ice-wedge geometries (i.e., steep dipping flanks) is beyond the potential of typical acquisition-strategies (surface-based, common-offset). While for simple wedge models synthetic data can be used to delineate information about the wedge geometry, this is not suitable for our more realistic synthetic data. However, for surface-based data acquisition more sophisticated techniques such as multi-offset-strategies may be a solution.
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Conceptual Model for Exploring Geothermal Resources in La Seva Basin (NE Spain) from Integrated Geophysical Data
Authors M. Torrens, R. Lovera, M. Himi, A. Sendrós, J.C. Tapias, X. Font and L. RiveroSummaryAn integrated geophysical interpretation has been conducted around La Selva basin that show strong geochemical and geologic evidences of hydrothermal with the intent of define a conceptual model for exploration of low and intermediate enthalpy geothermal resources. The geophysical data involved in this study consist of gravity, magnetic and geoelectrical, supplemented by geological and hydrogeochemical information. The interpretation of gravity anomalies has been useful for defining the main structural discontinuities, magnetic map show the existence of several anomalies related to mafic intrusions while geoelectrical prospecting let delineate more conductive areas which could correspond to an increase in thermal fluid circulation. The results obtained by the different geophysical methods are in good agreement and allows defining a reliable geostructural model of the subsurface of the studied area.
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The Use of Slingram Method to Map the Marine Water Intrusion: A Case Study from the Trabzon, Turkey
Authors K.H. Çoban, A.E. Babacan, E. Durmaz and S. CeylanSummaryGeophysical methods are successful in identifying of possible marine water intrusion in coastal regions. The marine water intrusion may have unfavorable effect on agricultural soil and freshwater quality. A geophysical electromagnetic survey was performed to mapping possible marine water intrusion effect along the Değirmendere River in coastal regions of Trabzon, Turkey. The survey consists of Loop-Loop electromagnetic method (Slingram). The two different study area were selected along the Değirmendere River, the first one included 3 parallel profiles from north to south (the length of one profile is 52m and the interval between profiles are 6m from west and east direction) and the second study area consist of 2 profiles from north to south direction 147m and 82m respectively. Also the study areas were about 640m and 100m distance from the marine respectively. The data were collected by EM-34-3 instrument with vertical dipole mode (VD) and horizontal dipole mode (HD) for 10m and 20m interval between transmitter and receiver coils along profiles. The marine water intrusion zones were determined by the help of apparent conductivity values in studied area. In addition, the general structure of the underground has been tried to reveal with Slingram method.
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Groundbased TEM Survey in the Subsiding Mekong Delta
Authors T. Bording, A.V. Christiansen, E. Auken, J.L. Gunnink and G.H.P. Oude EssinkSummaryThe Mekong Delta in southern Vietnam is struggling with intensive use of the groundwater due to intensive irrigation of multicycle rice cultivation and aquaculture in the coastal zone. This has led to groundwater overexploitation and salt water intrusion of the fresh groundwater resources from the coast inwards. However, the picture in this enormous delta is much more complex with many of the groundwater resources still containing residual saline groundwater from previous transgressions. Furthermore, the lowered water table has led to high subsidence rates since the early 1990’s. At present, the condition of the Mekong Delta groundwater system is poorly understood, mainly described by boreholes explaining about geology and salinity, with only few geophysical surveys to connect the geological observations. In this abstract we present the first results from a groundbased TEM survey. We have mapped a large, very conductive area, with depth of investigation reaching down to -200 m. The results can feed into geological and hydrogeological models of the area and provide a better understanding of the connection between groundwater resources.
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Probabilistic Inversion of Time-lapse EMI Data for Spatiotemporal Monitoring of Soil Moisture
Authors K.Z. Jadoon, D. Moghadas and M.F. McCabeSummaryLow frequency electromagnetic induction (EMI) systems have proven to be useful in mapping soil apparent electrical conductivity and soil moisture. Nevertheless, obtaining depth profile water content has not been fully explored using EMI. In this study, we performed time-lapse EMI measurements using CMD mini-Explorer sensor along a 10 m transect in a corn farm during a 6 day period. Meanwhile, reference data were also regularly measured at the end of the profile in an excavated pit using 5TE capacitance sensors. In order to derive a time-lapse, depth-specific subsurface image of true conductivity, we applied a probabilistic optimization approach, DREAM(ZS), on the measured EMI data. The uncertainties in measured apparent electrical conductivity, as well as inaccuracies in the inverted data, introduced some discrepancies between estimated and reference values in time and space. Moreover, the difference in measurement footprints of the 5TE and CMD Mini-Explorer sensors also led to the differences between reference and estimated data. The obtained depth profile permitted to accurately monitor spatiotemporal distribution and variation of soil water content encountered because of root water uptake and evaporation. The time-lapse monitoring approach, developed using DREAM(ZS) appears to be pertinent for accurate retrieval of spatiotemporal distribution of soil water content.
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Near Surface Geophysical Method Applicable for Water Sensitive Urban Design Projects
Authors O.F. Nielsen and S.W. NielsenSummaryGround Penetrating Radar (GPR) and Sonar has been applied for a Water Sensitive Urban Design (WSUD) project. Detailed information on an urban freshwater lake in Copenhagen was obtained. A complete bathymetric survey of the lake as well as a detailed map of the thickness of limnic deposits on the lakebed was obtained. The GPR was significantly attenuated at water depth below approximately 3.3 m. However, the combination of the methods provided a full bathymetric survey. The combined results served as basis for planning of in-situ sediment samples, for calculating volume capacity of the lake as a potential rainwater storage basin as well as for quantifying the volume of limnic deposits. This will assist in the WSUD project to optimize the solutions and thereby reduce cost and time in future phases.
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Data Adaptive Denoising of Magnetic Anomaly Maps
Authors J. Tronicke and E. LückSummaryIn the past one to two decades, a number of data adaptive denoising tools have been proposed in the image processing community. The basic idea of these filter approaches is to establish the filter weights by considering the actual sampling values, their local statistics and similarities. This helps to minimize image blurring and to preserve edges and corners. As such filter characteristics are also desirable for noise attenuation in near-surface magnetic data sets, we propose to adopt these methods for processing magnetic anomaly maps collected across archaeological targets. Here, we test and evaluate two selected methods (a generalized Kuwahara-style filter and the steering kernel method) to denoise a magnetic data set collected across Neolithic ring structure in Germany. Our results show that both methods are successful in removing prominent noise features present in our data. Concurrently, they largely preserve local structures; i.e., blurred images as typically observed after applying filters using a fixed filter mask are avoided. Thus, the methods can be considered as promising and novel approaches for denoising magnetic data sets.
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Investigation of Subsurface Archaeological Remains of Kalecik Castle Using GPR Method (Arakli, Trabzon, Turkey)
Authors A. Seren, Z. Ogretmen Aydin, A.E. Babacan and K. GELISLISummaryKalecik Castle in Kalecik town, a neighborhood connected to the Araklı district of Trabzon on northeast of Turkey, is located on a rock overlooking the sea. The castle is located far away 30 km east of Trabzon. We aimed to investigate whether the underground structure includes any structural elements in terms of archaeological aspect by using the ground radar method (GPR) in this study. For this purpose, GPR data were collected with by Mala GPR system with 250 MHz and 500 MHz shielded antennas on selected south-north directional 36 profiles and east-west directional 12 profiles in the study area. After applying the required data processing steps to these data, 2D/3D views were obtained. In 2D views, high amplitude reflections were observed at different depths with different geometries. The shapes of these reflections are highlighted in 3D slice images by their depth slice maps. Finally, test excavations proposed on the part of high amplitude reflection indicated as arrows on the time slice maps in the study area.
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The Use of VLF-EM and Electromagnetic Induction Methods for Mapping the Ancient Fort of Kuta Lubok as Tsunami Heritage i
Authors M. Yanis, M. A Bakar and N. IsmailSummaryThe 2004 Indian Ocean tsunami was not only devastating for human life and their properties but also destroying many cultural monuments in Aceh Province, Indonesia. The presence of traces of the ancient buildings along the coast of Aceh from the past has been the pride of the Acehnese today. However the giant 2004 tsunami has destroyed some historical buildings along the coastal area, for example the 13th century fort of Kuta Lubok. We have applied VLF-EM and EM induction measurements in Kuta Lubok area along 22 profiles. In order to resolve a better image of the subsurface manmade structures, distances between each profile and station were performed as close as possible, i.e. 20 meters and 5 meters. Tilt and ellipse data were measured by VLF-EM, while relative conductivities and susceptibilities data were acquired in the EM induction. We have applied Fraser filtering for VLF-EM data. In the electromagnetic induction, we have performed high pass filter in order to remove regional effect from the data. The expected buried walls were imaged clearly as shown by fraser filtered in VLF-EM data. For the EM induction, the buried wall can be tracked by relative low electrical conductivity values (i.e. -25.6 to 0.75 mS/m).
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Analysis Procedures of an ERT Monitoring Network to Assess Mountain Permafrost Degradation Rate
Authors C. Mollaret, C. Hilbich and C. HauckSummaryA long-term ERT monitoring network of mountain permafrost in the Swiss Alps allows the detection of ground ice changes. It enables to compare sites of diverse landforms which are characterized by various ground properties. Electrical resistivity is highly sensitive to unfrozen water content present in the pores and to a lesser extent to temperature change. Data quality assessment of the six sites of the ERT monitoring network involves grounding resistance analysis as well as automatic filtering procedures. The combined analysis of borehole temperature and resistivity dataset reveals spatio-temporal ground property changes. This long-term monitoring network enables e.g. to characterize the extent of permafrost thaw and/or melt of ground ice at Schilthorn, to a lesser extent at Stockhorn, and no or very small changes at rock glacier Murtel.
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Monitoring Thickness and Wetness of Snow Sheets at a Catalan Pyrenees Ski Resort Using GPR, TDR and Chemical Analisis Me
Authors J. Medina, L. Rivero, R. Lovera, M. Himi, A. Sendros, A. Urruela and A. CasasSummaryThe different snow sheets have special characteristics in order to different parameters (humidity, temperature, etc.) between the different snow falls and the final fusion in summer. This particular properties will be modified during the snow transformation process along the winter/spring seasons. We try to monitoring these properties using different geophysical and geochemical methods.
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2D-SEA Methodology for GPR Reflections
More LessSummaryWe describe a 2D-Synthetic Emitter Array methodology for improving GPR reflections altered by clutter and noise. We show examples in which the proposed methodology is applied to simulated datasets. The results of the methodology are compared to the results of the 1D and Single Offset methodologies. Significant improvement with respect to them is obtained.
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Investigation of Erzincan Airport Landing Field with GPR Method
Authors I. Saricicek, Z. Ogretmen Aydin and A. SerenSummaryIn this study, it is aimed to visualize whether the deteriorations observed from surface on the landing surface of the aircraft at the Erzincan Airport floor is continuing horizontally and vertically under the floor with GPR data. It is useful to do modeling exercises to help comment on the soil structure from the radargrams obtained. Primarily, GPR data were collected using a 800 MHz central frequency antenna. GPR modeling studies have been carried out with the finite difference time method (FDTD) in the time by obtaining from the construction project for the ground structure in the inspection area. By comparing the obtained real field data with artificial radargrams theoretically generated, the deteriorations of ground structure was determined. From the results obtained; It is observed in radargrams that ground quality is generally in good condition and ground is composed of four different units although fractures and deterioration are observed in places. It was also observed that fractures observed from the surface at 3 and 15 meters along the measurement line extended to depths of about 35 cm in the ground. In addition, in theamplitude slice maps of different depths, the lateral extensions of the fractures were also determined.
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Determination of Horizontal and Vertical Changes of Esiroglu Formation using GPR and ERT Methods (Trabzon, Turkey)
Authors Z. Ogretmen Aydin and A. SerenSummaryIn this study, it is aimed to determine the horizontal and vertical changes of the Esiroğlu formation situated Akoluk distinct within the Trabzon province borders in Eastern Black Sea Region. This site considered as a solid waste site. In engineering geology applications, particularly the depth of bedrock is very important. The location of the massive formation in 2 and 3 dimensions has been determined by multi-electrodes measurements and ground penetrating radar method, while the point measurement can only be determined by drillings. Within this scope, 5 profiles ground penetrating radar and 9 profiles electrical resistivity methods were applied in the study area. After evaluating the obtained data, the subsurface structure in the study area was tried to be revealed in 2D and 3D.
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IAG/USP Test Site - II - Construction and Preliminary GPR Results on PVC/Metal Pipes and Ceramic Tubes
Authors J.L. Porsani, V.R. Elis, A.A. Ferreira and B. PoluhaSummaryThis paper shows the construction of the second Geophysical Test Site of the Institute of Astronomy, Geophysics, and Atmospheric Science at University of São Paulo, installed at USP campus in São Paulo city, Brazil. It shows too the preliminary GPR results on buried pvc/metal pipes and ceramic tubes in the subsoil in which they simulate the gas/water pipelines and pluvial water canalization gallery underground of the great cities of the world. The targets installed in subsoil are constituted by metal and plastic tanks, concrete tubes, ceramic vases, plastic conduits etc., and its top depths varying from 0.5 to 2.0 m. The targets simulate geotechnical, environmental, and archaeological studies where its location is the goal. GPR profiles of 900 MHz and 270 MHz were acquired along of line 3 constituted by pvc/metal pipes and ceramic tubes. Preliminary GPR results show typical hyperbolic reflections pattern characteristic for each target. The IAG/USP Test Site-II was constructed with teaching and research purposes in Applied Geophysics. These results can help in location of buried utilities in subsoil of the great cities minimizing the risks of dangerous accidents.
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Investigation of Concrete Internal Structures With GPR Method
Authors A. Seren and I. SaricicekSummaryIn this study, during the maintenance of part at the basement of the Department of Geophysical Engineering of the Technical University of Karadeniz, during the maintenance of, It is aimed to investigate by means of the Ground Radar (GPR) method to shows that the damaged reinforcing bar observed in the ceiling continue in the structure. For this, data were collected using 2.7 GHz antenna. After applying the necessary data processing steps to these data, various migration techniques have been applied to move the correct positions of the scattered hyperbole, which is a symptom of reinforcing bars. From the obtained results; from the view of the radargrams obtained from the data collected from the building structure ceiling; hyperbolas are observed at the 20, 50, 70 cm and 1 meter. While hyperbolas are concentrated to more scattering points, when obtained this radargram is applied stolt migration, in kirchhoff migration, hyperbols are also gathered to a scattering point less and deteriorations are more observed in the structure. Furthermore, amplitude-slice maps composed of selected profiles in directions parallel to each other was evaluated. From all these results, suggestions for repair were made by locating solid and damaged parts of reinforcing bar.
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Mapping Bedrock Lithology in Urban Environment via Large Scale Time Domain Induced Polarization Tomography
Authors M. Rossi, P.I. Olsson and T. DahlinSummaryThe present work is an example of a Direct-Current resistivity and time-domain Induced Polarization (DCIP) survey, carried out in an urban environment. The main goal of the survey is to map a weak zone in the bedrock, through which a tunnel will be constructed.
Geophysical measurements in urban environments can be very complicated due to the restrictions of available space and the high levels of noise. A way to deal with the latter is careful processing of the recorded waveforms, if they are sampled at a reasonably high frequency (sampling >= 1 KHz). A deeper understanding of raw data leads to a more accurate use of the measured quantities, improving the reliability of electrical resistivity and IP estimations.
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BeFo 382 - Detailed Comparison Study of 3D-characterized Rock Mass and Geophysical Models
Authors P.-I. Olsson, P. Jonsson, S. Johansson and L. JohanssonSummaryThis paper presents a study where resistivity and integral chargeability three-dimensional (3D) models, representing a volume in the vicinity of a rock quarry is compared to a quasi-3D characterization of rock mass volume. By repeating geological mapping, i.e. with unmanned aerial vehicles, ground based high resolution photography and sampling, as the quarry operation reaches further into the rock volume mapped with geophysics, we retrieve a semi-continuous geological characterization of the same rock mass volume characterized by the geophysical investigations. This enables a detailed comparison between geophysical results and rock volume, where factors such as fracture and weathering zones or rock type is being considered.
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Detailed 3D Geological Modelling at a Contaminated Stream Using Geophysical, Geological and Chemical Data - A Challenge
Authors I. Moeller, P.K. Maurya, G. Fiandaca, N. Balbarini, V. Rønde and A.J. KallesøeSummaryLocalizing sources of contaminated groundwater discharging to streams may be a challenging task. A detailed geological model of the site is required for simulating the groundwater flow and the contaminant transport, in addition to chemical and hydrological investigations.
In the town of Grindsted, Denmark, contaminated groundwater discharges to a stream. Several contaminated sites are present in the area, where chemical waste from a nearby chemical factory was deposited. In order to understand the hydrological flow and transport of contaminants controlled by a few mostly continuous clay and lignite layers in an otherwise sandy environment, a detailed 3D geological model is constructed. The model integrates all available data including DCIP and EMI surveys, lithological logs from new and existing wells, hydraulic head and pore water electric conductivity data. This study addresses the challenges met while constructing the 3D geological model in an area, where inorganic contaminants are highly affecting the geophysical data.
The model ambiguities related to interpretation of clay layers versus contaminated sand layers in the DCIP resistivity sections as well as resistivity/chargeability-thickness equivalences are managed by integrated interpretation of a large amount of different geophysical, geological, hydraulic and chemical data.
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Probability of Sensitive Clay from AEM Data
Authors A.O.K. Lysdahl, A.A. Pfaffhuber, H. Anschütz, Kr.K. Kåsin and S. BazinSummaryIdentification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution Airborne Electromagnetics (AEM). However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.
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Combined Geophysical Surveys to Investigate the Cause of the Rainfall Induced Shallow Landslide
Authors Y. Okada and C. KonishiSummaryWe have conducted the seismic and resistivity surveys at the slope where the shallow landslide induced by heavy rainfall occurred in September, 2015. Resistivity profiles clearly discriminates the unsaturated top soil as higher resistivity zones. We infer the groundwater pathway beneath the top soil from the low resistivity zones. S-wave velocity is good matched with the thickness of the top soil measured by the cone penetration test. At the survey line along the slope parallel to the landslide, the low S-wave velocity and high resistivity has detected within the projected interval of the adjacent landslide. Similar geophysical properties are observed at the survey line above the landslide, which is perpendicular to the slope. These characteristics geophysical properties indicate the weak point that initiates the shallow landslide in the slope.
The combination of the two geophysical surveys helps us to infer the mechanism of the rainfall induced shallow landslides and it is also useful to identify the vulnerable points for the initiation of the shallow landslide in the future.
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Towards a Time Monitoring of the Chalk Aquifer (Beauvais, North of France) Using the Magnetic Resonance Soundings
Authors P. Lutz, J.F. Girard, L.Z. Zouhri and P.E. MeurantSummaryIn the department of Oise (Picardie, France), the agricultural activities are essential and require a good management of water resources. It is therefore essential to improve the knowledge on the chalk aquifer. This is the reason for which the UniLaSalle Institute has a hydrogeological experimental site, composed of wells and piezometers allowing to characterize the chalk aquifer by adopting a multidisciplinary approach. This approach is based on hydrogeological measurements, near surface geophysics, logging, and Magnetic Resonance Soundings (MRS). The present paper, based on the MRS 2016–2017 campaign, deals with the time monitoring of the chalk aquifer, consisting in repeating each month and a half, four MRS. These are located north of the UniLaSalle hydrogeological experimental site, where the measurements conditions are suitable, unlike near the piezometers and wells. The results, consisting in curves of hydrogeological parameters (water content, relative permeability and transmissivity) versus depth up to 90m, improve the knowledge on the chalk aquifer properties and there time variations. However these results requires a calibration by pumping tests to estimate the value of the calibration coefficient Cp. The difficulty resides in the identification of a suitable site, both for MRS (low noise level, stable magnetic field) and for pumping tests.
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Surface Nuclear Magnetic Resonance with Compact Multicomponent Receivers
Authors M.A. Kass, T.P. Irons, A. Behroozmand, D. Grombacher and B.R. BlossSummarySurface nuclear magnetic resonance surveys are a valuable technique for investigating hydrologic properties in saturated aquifers, in the vadose zone, fractured reservoirs, and in permafrost settings, to name a few. Standard field measurements typically consist of a coincident large transmitter and receiver loop; while this approach has benefits with respect to field deployment and a smooth depth sensitivity function in 1D, there are numerous advantages to a compact, multicomponent receiver. Here we discuss these advantages, focusing on the imaging kernel for inductive receiver loops in all three principal directions. Additionally, we describe the imaging kernel in three dimensions and show the added spatial resolution available by a mobile three-component receiver. This analysis provides a first step toward proper survey design for subsequent 3D inversion for hydrologic properties.
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Modelling Relaxation During Pulse Effects for the Complex Inversion of Surface NMR Data; Gaining Insight into which Rela
Authors D. Grombacher and E. AukenSummaryA shortcoming of the standard surface nuclear magnetic resonance measurement, the free-induction decay, is that the meaning of the signal’s time-dependence is uncertain. Ideally, the parameter describing the time-dependence of the FID, called T2*, carries a strong link to the geometry of the pore space. However, in the presence of background magnetic field inhomogeneity a second mechanism contributes to T-2* potentially obscuring the link to pore geometry. To improve the understanding of which mechanism controls T--2*, an approach involving direct modelling of relaxation during pulse effects is proposed. Numerical studies are presented to demonstrate that the complex inversion of surface NMR data provides the sensitivity to gain insight into the magnitude of T-2 from only FID measurements. Multiple inversions are performed for plausible T-2, given the observed magnitude of T2*, and inversions providing satisfactory data fit can help constrain T2. Relaxation during pulse effects are also shown to be a contributing factor to difficulties describing the signal phase for complex inversions of surface NMR data.
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A Multichannel, Low Noise Surface NMR Receiver System with Wireless Connections to Signal and Reference Coils
Authors L. Liu, D. Grombacher, E. Auken and J.J. LarsenSummarySurface NMR holds great promise as a tool in groundwater measurements due to its unique direct sensitivity to water, but the method currently suffers from a number of drawbacks which limits its widespread applicability. Among these drawbacks are a low signal to noise ratio which limits the use of the method in many places of interest and a low production rate which makes the method costly in field campaigns. Hence there is a need for research further advancing the technology. In this paper we report on the development of a new multichannel, low noise surface NMR receiver system with wireless connections to reference coils. The receiver system works as a completely independent add-on to existing transmitter systems and consists of a number of independently operated data acquisition boxes connected with WiFi and synchronized by GPS. The internal electronic noise level of the system is 1.2 nV/sqrt(Hz). The timing jitter between data acquired in different boxes is less than 100 ns.
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Aquifer Imaging Using 2D Magnetic Resonance Tomography with Structural Constraints from GPR
Authors C.D. Jiang, J. Igel, R. Dlugosch, T. Günther and M. Müller-PetkeSummaryMagnetic resonance tomography using elongated transmitter and in-loop receiver arrays (MRTetra) is designed to efficiently image the two-dimensional aquifer structures. However, MRT suffers from limited abilities to resolve sharp boundaries. We make use of the GPR reflections to generate structural mesh boundaries for modelling of MRTetra and apply a definite sharp boundary constraint in the inversion of the surface NMR data. A comprehensive survey including GPR, MRT and other geophysical methods has been conducted at the test site Schillerslage, Germany. Inversion results of the surface NMR show that the imaging of water content and relaxation time T2* is improved using the mesh including the GPR reflections, and further improved by applying the sharp boundary constraint. In addition, multiple datasets of MRTetra can be combined together to invert the water content and T2* of the entire profile which provides a more stable and robust result comparing with separate datasets.
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Simultaneous Reorientation and Redatuming of Very High Resolution Seismic Recordings in Shallow Water Surveys
By N. JonesSummaryWe consider the challenges associated with very high resolution (VHR) seismic profiling of near-seafloor sediments, within shallow-water environments. Specifically we introduce a method for correcting for the effects of these challenges (temporal variations in water-column velocity—depth structure and source-receiver geometry changes linked to sea-state fluctuations) during data-processing. By using numerical-modelling in a V(Z) sense, seismic reflection data are slant-stack transformed to tau-p, primarily in order to retain surface-consistency for subsequent signal processing.
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Multi-frequency Seafloor Characterization Using Seismic Sources of Opportunity
Authors M.N. Banda, Ph. Blondel, M. Burnett and R. WyattSummaryThe low frequencies used in seismic surveys mean that seafloor characterization is limited horizontally and vertically. It is possible to use multiple reflections from the seabed to hydrophones on Seismic Support Vessels to measure scattering strengths and geoacoustic properties of seabed areas far from the seismic survey. This can be done by combining lower frequencies (directed toward the seabed and with repeatable beam patterns because of sensor design), providing potential sub-surface penetration and higher frequencies (emitted in all directions, with high inter-pulse variability), directly related to surface properties (slopes, roughness, seabed type). This is demonstrated with results from a shallow-water survey, in which the useful frequency range extends from 100 Hz to 20 kHz. Seismic pulses are used as sources of opportunity, and multiple scattering contributions are corrected for variations in propagation ranges and sizes of scattering patches (related to pulse durations). Twelfth-octave frequency bands give highly accurate information of seabed properties, which can be compared to models or previous measurements. The energy distribution of all measurements can be divided into three equally contributing frequency bands, and their RGB representation enables rapid assessment of seabed properties, identifying geomorphological changes and small-scale topography variations.
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Near Surface Modelling in Shallow Marine Setting Using Turning Ray Tomography Approach
Authors R.S. Alsaad, L. Wong and A. Al-HamoudSummaryfor shallow water environment standard water correction workflow was used to address the static resulted from the wave traveling through water and with the assumption of homogenies layering below the water bottom. In Arabian Gulf area, two acquisition schemes were used based on water depth. OBC acquisition was done in areas where water depth is relatively shallow range between 5 to 35 meters and standard limited azimuth 3D streamer acquisition was done in deeper water. Merging the two datasets at the prestack domain create a major challenges where water correction was used to reference the two dataset to same reference datum. Based on the input received from interpretation team, an attempt to resolve near surface zone 100 meters below the water bottom was needed to improve the imaging and resolve the difference found between the prognoses and actual depth. We used turning-ray tomography to estimate the near surface velocities for static corrections followed by prestack time migration
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Mitigating Dredging Risks Using Enhanced Geophysical Methods - The Aquares Resistivity Method
Authors P.M. Brabers, J. Errey and G. MoermansSummaryThe development of a new or existing port involving features such as quaywalls, turning basins, access channels and reclamation areas generally is significantly affected by local geological conditions. In general these represent a high risk factor to the construction project if they are not well described. When appropriate geological ground models are available the design of the construction can profit from it, reducing dredging and construction costs. The geological model forms the basis of quantities, design and risk management.
The mitigation of geological risks in ports and marine construction can be realised by including enhanced geophysical methods in the early project state. Enhanced geophysical methods are defined as high quality geophysical methods capable of producing accurate 4D digital ground models showing sediment and rock depths and thicknesses as well as rock and sediment qualities.
The Aquares resistivity method is an enhanced geophysical method. This paper explains the principles of this method and shows how it has been used in the past as an essential and highly successful tool to generate digital 4D ground models used for mitigating geological risks in ports and marine construction projects. Inappropriate practices in current site investigation procedures are described to as ways to overcome them.
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Aquifer Characterization Using Elastic Full-waveform Inversion
Authors N. Athanasopoulos and T. BohlenSummaryThis study exploits the benefits of elastic full-waveform inversion (FWI) as an effort to improve aquifer characterization for better understanding of their properties and delineating their structure. Conventional methods (e.g. boreholes, pumping tests) have usually one-dimensional nature and cannot provide information concerning the lateral heterogeneities of such complex subsurface environments. In the past, many studies were conducted using ground-penetrating radar (GPR), electrical resistivity tomography and seismics which result in two-dimensional tomographic images and provide spatially highly resolved mapping of aquifer heterogeneities. In this research, we focus in the seismic method and in particular we apply FWI to seismic data acquired at the Krauthausen test site. We compare our findings with borehole data and GPR FWI from previous studies. We conclude that combining the results of FWI with additional geophysical techniques can provide more reliable subsurface models and reduce uncertainties on reconstructing the aquifer architecture with higher spatial resolution.
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Surface Wave FWI on Complex Models - The Robustness of the Inversion to Assumptions and Forward Modeling Approximations
Authors Z. Xing and A. MazzottiSummaryFull waveform inversion (FWI) of surface waves with genetic algorithm (GA) is able to invert complex near surface models even in the case where very limited a-priori information is available, but it requires long computing time. One way to reduce the computing time is to make assumptions on the subsurface and to simplify the forward modelling. By using a few complex near surface models, with velocity inversions, lateral velocity variations and with an irregular topographic surface, we discuss how the following issues affect the inversion results in terms of either the data misfit or the model misfit: 1) fixing the compressional wave velocities and densities to the estimated shear wave velocities according to empirical equations, instead of inverting them; 2) neglecting attenuation in the forward modelling; 3) performing 2D forward modelling and applying a 3D to 2D correction to the observed data. Although these approximations degrade model prediction, yet the main features of the shear wave models can be retrieved. Instead, the data prediction is always satisfactory, showing again that theoretical approximations in the forward modelling affect more the model misfit than the data misfit.
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Full-waveform Seismic Inversion for Estimating Aquifer Dimensions and Hydrologic Parameters
Authors T. Lähivaara, A. Pasanen, A. Malehmir and J. KaipioSummaryThis study aims at developing computational tools to estimate aquifer dimensions and hydrologic parameters using seismic data. The poroelastic signature from an aquifer is simulated and using this signature we estimate water-table level and aquifer porosity, and infer the location of aquifer-basement boundary. We use discontinuous Galerkin method to solve the forward model that characterizes the propagation of seismic waves in coupled poroelastic-elastic media. The inverse problem is solved in a Bayesian framework, which enables to take into account modeling uncertainties. For the inverse problem, we use the Bayesian approximation error method, which reduces the overall computational demand. At this stage, results for a 2D synthetic model are presented to illustrate the potential of the algorithm for hydrogeological applications.
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Particle Swarm Optimization of Electromagnetic Data with Parallel Computing in the 2D Case
Authors F. Pace, A. Santilano and A. GodioSummaryThis work presents the application to electromagnetic data processing of the particle swarm optimization (PSO) algorithm, with the advantage of its high speed of convergence to the problem solution compared with other evolutionary methods. We focus on the inversion of MT synthetic data in the two-dimensional case. The PSO problem is solved complying with upper and lower bounds and a priori information, which is initially given only to a small amount of particles of the swarm. The fitness function is properly defined including smoothing parameter (Occam’s razor). Since it is a computationally demanding problem, a practical tool of parallel computing is tested and validated, thus allowing large computation time savings. Results show encouraging outcomes in terms of minimization of fitness function and data fitting. This approach represents the starting point for the 2D PSO application to MT data, Audio Magneto Telluric data and other near surface applications implying two-dimensional interpretation.
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Bayesian Inversion of Surface-Based ERT Data Using a Structure-based Prior
Authors G. de Pasquale, J. Doetsch and N. LindeSummaryInversion results obtained by surface-based electrical resistivity tomography (ERT) are strongly dependent on model regularisation (deterministic inversion) or the prior model (Bayesian inversion). Here, we present the first results of using a structure-based prior in Bayesian inversion of ERT data using a Markov chain Monte Carlo method. The method can handle unstructured meshes, which implies that topography and internal boundaries can be accounted for. The results obtained are greatly improved compared to those obtained by a prior based on uncorrelated model parameters. In the future, we will consider the problem of inferring the sediment-bedrock interface depth (and the associated uncertainty) under strong geological heterogeneity below and above the bedrock.
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Time-lapse Electrical Resistivity Surveys and Unfrozen Water Content in Cold Continuous Permafrost
Authors G.A. Oldenborger and A.-M. LeBlancSummaryEstimates of material properties such as ice content or unfrozen water content are critical for thermal modelling of the response of permafrost to climate forcing, understanding contaminant flow and transport, or for predicting the behaviour of permafrost as an engineering substrate. We utilize time-lapse electrical resistivity surveys to examine the potential for imaging relative changes in unfrozen water content for cold continuous permafrost in the Canadian Arctic. Electrical resistivity data were collected from 2012/08 to 2015/06 at semi-regular time intervals using a permanent electrode installation at Iqaluit International Airport in Iqaluit, Nunavut. Using postinversion model differencing, we observe significant changes in electrical resistivity and we infer changes in unfrozen water content that appear consistent with temperature records. The most prevalent changes in resistivity are not limited to the active layer, but extend from 1–8 m depth in a zone of significant temperature fluctuation.
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A Time-lapse Geophysical Model for Detecting Changes in Ground Ice Content Based on Electrical and Seismic Mixing Rules
Authors C. Hauck, C. Hilbich and C. MollaretSummaryThis contribution introduces a time-lapse extension of a geophysically-based 4-phase model to calculate ground ice content changes in the context of permafrost degradation due to climate change. The model uses input data from time-lapse Electrical Resistivity Tomography and Refraction Seismic Tomography monitoring and is based on simple petrophysical relationships. In time-lapse formulation, the model is not anymore dependent on time-invariant properties such as porosity, pore water resistivity, cementation exponent and P-wave velocity of rock. The time-lapse 4-phase model was then applied to long-term ERT and RST data from the mountain permafrost site Schilthorn, Swiss Alps, where a substantial ground ice loss could be detected during the past decade.
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Spectral Inversion of Capacitive Resistivity Data for the Investigation Frozen Ground
Authors J. Mudler, G. Fiandaca, A. Hördt, C. Hauck, P.K. Maurya and A. PrzyklenkSummaryThe capacitively coupled resistivity method measures frequency dependent electrical parameters in the frequency range between 1 Hz and 240 kHz. The method may potentially be useful for permafrost research, because the electrical permittivity of frozen ground and ice exhibits a characteristic frequency dependence. There is little experience with this type of measurement, as the equipment and appropriate inversion routines have only recently become available. Here, we present a case history from the Schilthorn mountain/Switzerland, where we acquired data with a dipole-dipole configuration along a profile of 28m length. A single site spectral inversion, in which we fit a Cole-Cole model to each of the configurations, yields pseudosections of five electrical parameters. We then apply a 2-D spectral inversion, which was extended to include the Cole-Cole permittivity inversion. The depth sections obtained this way are consistent with the pseudosections obtained from the single site inversions, which is taken as an indication for the robustness of the procedure. The electrical parameters basically reveal a 2-layered structure, consistent with the known snow cover over frozen ground. The excellent data quality and the consistency encourage further research into the extraction of parameters useful for permafrost research, such as ice content.
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Re-parameterization of the Cole-Cole Model for Improved Spectral Inversion of Induced Polarization Data
Authors G. Fiandaca, L.M. Madsen and P.K. MauryaSummaryThe induced polarization phenomenon, both in time-domain (TD) and frequency-domain (FD), is often parameterized using the empirical Cole-Cole model. We here suggest three re-parametrizations of the Cole-Cole model, namely the maximum phase angle (MPA) model, the maximum imaginary conductivity (MIC) model and the minimum imaginary resistivity (MIR) model. We test these new parameterizations against the classic Cole-Cole model using a Markov Chain Monte Carlo (MCMC) inversion algorithm. The MCMC method allows us to study the probability distributions of each model parameter and gives us direct information on the parameter uncertainty. We show that models that are poorly resolved from inversion with the Cole-Cole model, e.g. due to low C values or strongly correlated parameters, can be resolved well with the new parameterizations and that the parameterizations work equally well for TDIP and FDIP data.
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A Network Model of Membrane Polarization to Simulate the Induced Polarization Response of Sandstone Samples
Authors H. Stebner, M. Halisch and A. HördtSummaryThe hydraulic conductivity of sediments is an essential parameter for many applications, and thus its estimation from induced polarization (IP) measurements is an important aim of current research. For this purpose, a comprehensive understanding of the processes at the pore scale is necessary. We suggest a membrane polarization model based on a sequence of two cylinders with different size. To account for the complexity of real rock, we combine many impedances calculated for such a 2-cylinder model into a network. We define global network parameters, which we tune in such a way that macroscopic properties, such as specific internal surface area, porosity, and fractal dimension, match those measured for real sandstone samples. Using remaining degrees of freedom of the network we are able to simulate the IP parameters, namely maximum phase shift and relaxation time, of the measured samples. Matching the IP parameters gets more difficult if we also try to constrain the network by data from µ-CT measurements. We conclude that the impedance network has a great potential to connect bulk electrical properties with microscopic geometry.
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The Potential of X-ray Tomography and Image Analysis for Interpretation of Spectral Induced Polarization Measurements
Authors S. Johansson, M. Rossi and T. DahlinSummaryThe spectral induced polarization (SIP) method is a promising method for many near surface applications, including characterization of contaminated soil. In this study, SIP measurements, high-resolution X-ray tomography and image analysis are combined to investigate clean and tetrachloroethylene (PCE) contaminated sand. This is the first known combination of these methods and the aim of this paper is to demonstrate some powerful possibilities of using X-ray tomography in SIP research. X-ray tomography enables a direct visual control of the sample conditions and quantification of relevant sample properties via image analysis techniques. The image stacks from were processed with image analysis techniques, so that individual grains and PCE phases could be extracted from the sample volume. This enabled calculations of relevant sample properties such as PCE blob volumes, grain diameter, -surface area and —eccentricity as well as bulk porosity, specific surface area to pore volume and proportion of high-density grains. We conclude that the combination of SIP measurements and X-ray tomography have a great potential of increasing the understanding of SIP mechanisms in geological materials. The tomography results can be used both to calculate properties relevant for SIP interpretation as well as 3D modelling of the pore space.
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Contribution of Stern Layer and Membrane Polarization to the Spectral Induced Polarization of Porous Media
Authors M. Rossi, A. Brovelli, G. Cassiani, S. Johansson and T. DahlinSummaryRecently there is an increasing interest in the understanding of the Spectral Induced Polarization (SIP) phenomenon in porous media. The complex electrical conductivity of porous geological materials has a frequency dependent behavior than can be associated to three main mechanisms: Maxwell-Wagner (MW) polarization, Membrane polarization, Electrical double layer (EDL) polarization.
This paper aims to develop a mechanistic model for induced polarization in frequency domain, that incorporates both grain and membrane polarization and uses a non-linear mixing rule to combine the contribution of water, air and solid grains.
The model is tested on two different lab experimental datasets. Different spectra characteristics can be understood and described by distinguished porous media behaviors.
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Simulated Time-lapse DC-IP Monitoring of Dense Non-aqueous Phase Liquids (DNAPLs) - An Initial Approach
Authors C. Power, P. Tsourlos, J.I. Gerhard and T. DahlinSummaryGeoelectrical techniques have long standing potential for imaging sites contaminated by dense non-aqueous phase liquids (DNAPLs). Recent work explored the effectiveness of time-lapse DC resistivity to monitor DNAPL source zone remediation. A fundamental component of the work was the development of a unique and comprehensive linkage between well-established DNAPL and DC resistivity models, with the coupled model serving as a cost-effective and valuable investigative tool. The objective of this current study is to evaluate the application of integrated time-lapse DC resistivity and induced polarization (DC-IP) for monitoring DNAPL contamination. This paper introduces the various modelling tools and presents an initial simplistic approach to extend the established DNAPL-DC resistivity modelling framework to incorporate time-lapse IP. Preliminary results are encouraging and ongoing work is focused on developing an advanced hydraulic-electric linkage between the DNAPL and DC-IP models to produce more realistic IP responses from DNAPL spills.
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Mapping Possible Flowpaths of Contaminants through Surface and Cross-borehole Spectral Time-domain Induced Polarization
Authors T. Bording, G. Fiandaca, P.K. Maurya and E. AukenSummaryTraditional methods for mapping possible flowpaths of contaminants in sedimentary environments by boreholes may often be insufficient. Additional information may be acquired by geophysical methods. In the present study, cross-borehole and surface measurements were performed using time-domain induced polarization (TDIP). After measurements the entire test site was dug out, and the geology was described. Despite of being above the groundwater table, it was possible to retrieve exceptionally well the distribution of sand lenses/layers at the site, with clear signature in both the DC conductivity and the IP parameters, as confirmed by the excavations carried out for verifying the geophysical results. Consequently, cross-hole TDIP imaging is a valuable tool for characterizing possible flowpaths of contaminants in in clayey moraines.
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Resolution of DC Resistivity-thickness Equivalences by Inclusion of Induced Polarization Data
Authors L.M. Madsen, G. Fiandaca, E. Auken and A.V. ChristiansenSummaryThe principle of equivalence is known to cause non-uniqueness in interpretations of direct current (DC) resistivity data. Here, we show that these equivalences can be significantly reduced by combining DC data with time-domain induced polarization (IP) measurements. An understanding of model equivalences requires a comprehensive investigation for each model when using gradient-based inversion methods. Instead, we apply a 1D Monte Carlo inversion that makes it possible to investigate the space of equivalent models. We invert synthetic DC data of a model with low-resistivity equivalences. We then include a contrast in chargeability, modelled in terms of spectral Cole-Cole IP parameters, and invert the DC and IP data together. The results show that the inclusion of IP data resolves the equivalences. The degree of resolution depends on the contrast in the chargeability and/or the other IP parameters. The contrasts required are easily expected in the field, which is justified with a field example where DC and IP data are measured on a sand formation with an embedded clay layer. Inversion results show that the resistivity data alone does not resolve the clay layer due to equivalence problems, but by adding the IP data to the inversion, the layer is resolved.
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Series Expansion Based Inversion to Process Laboratory TD Induced Polarization Data
Authors A. Kiss and M. DobrókaSummaryThe Induced Polarization (IP) method is often used in ore exploration, since the electrode polarizability provides a good basis for distinguishing the ore-rich zones from the waste rock. Several successful field applications confirmed the effectiveness of the method. Previous studies proved that the series expansion based inversion is a well applicable data processing method in case of field TDIP data. In this paper laboratory apparent polarizability data measured on core samples with Time Domain Induced Polarization method are processed in a new inversion procedure, where the time constant spectrum containing the polarization effects is determined with series expansion approximation.
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Generalized Relative and Cumulative Response Functions for Conductivity Meters Operating at Low Induction Numbers
Authors F.C.M. Andrade and T. FischerSummaryRelative and cumulative analytical response functions have been widely used as a powerful tool for forward modelling and interpretation of measurements obtained by electromagnetic induction conductivity meters operating at low induction numbers for one-dimensional layered earth models. These well-known functions were derived and should be used for the instruments laid on the surface of the earth. In this paper we extended these functions and obtained new generalized analytical expressions which can be used for instruments carried at any height from the surface. The proposed new equations were compared with numerically constructed functions, obtained using the full solution of the electromagnetic equations, and proved to be in very good agreement at low induction numbers.
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Practical Inversion of Electric Resistivity in 3D from Frequency-domain Land CSEM Data
Authors F. Bretaudeau, S. Penz, N. Coppo, P. Wawrzyniak and M. DarnetSummaryEM prospection are methods of choice for many applications such as deep water or geothermal prospection because of their sensitivity to electrical resistivity and their potential to investigate at depths of 500m or even more. However, the investigated areas in Europe are usually urbanised and industrialised so high level of cultural noise prevents from the use of MT. Land CSEM is an alternative. But cost and logistical constrains limits to the use of a small number of transmiter positions. The inversion of CSEM data in the near field using a single transmiter position suffers from critical sensitivity singularities due to the unsymmetric illumination. To overcome this problem we proposed an inversion framework adapted to this ill-conditioned inversion problem. The framework relies specificaly on a robust Gauss-Newton solver, on model parameter transformations and data reformulation under the form of pseudo-MT tensors. We describe here the modeling and inversion approach implemented in our code POLYEM3D and describe the framework proposed for its practical application. We illustrate its application on synthetic cases and then show the application of the process to a real CSEM dataset acquired for thermal water prospection at a few kilometer from a nuclear power plant in France.
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A Case-study for Comparing a Mobile DC System and a Multi-configuration EMI Sensor for Depth-to-Soil Mapping
Authors E. Lück, J. Guillemoteau and J. KloseSummaryIn the past, different types of mobile sensors were used to image the lateral heterogeneity of the subsurface electrical conductivity. Later, the development of DC (direct current) and EMI (electromagnetic induction) multi-configuration instruments permitted to get also information about the soil layering. In this study, we report a case study, which shows a good agreement between the inversion of DUALEM-21 data and the inversion of DC data for imaging lateral and vertical conductivity variations.
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GPR and Geoelectical Studies in a Historical Site in Buenos Aires, Argentina, for Locating an Old Cistern and Evaluating
Authors V. Grunhut, H.P. Martinelli, N.E. Bonomo, M.V. Bongiovanni, A. Pinio and A.M. OsellaSummaryWe performed a geophysical study in a historical site. The main objective was to localize and characterize an old water cistern, no longer in use. Considering its expected dimensions, high density GPR data and geoelectrical dipole-dipole profiles were acquired. This allowed achieving very good resolution of the subsurface structures.
We could localize the cistern and determine that it would not be risk of collapse. While a very resistive electrical anomaly related to this structure was identified (with a diameter of approximately 1.5 m and extending up to a depth of about 0.9 m), considering the GPR results, this anomaly was associated to resistive filling material and not to the presence of a void. Below that depth, the structure seems to be filled with compacted material similar to the surrounding soil. Two other anomalous zones were detected that could correspond to structures with historical lineage. Exploratory excavations are being carried out.
Regarding the geoelectrical data, first, conventional 2D inversions were done. Then, a pseudo-3D model was built by transversally interpolating those results, and also a fully 3D inversion was performed. This aided to further improve the delineation of the anomalies.
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Lake Baikal - A Unique Site for Testing and Calibration of Near-surface TEM Systems
Authors M.V. Sharlov, N.O. Kozhevnikov and R.V. SharlovSummaryThe paper discusses using Lake Baikal as a site for testing a near-surface TEM system. The water column of Lake Baikal forms a natural geoelectric model with precisely known parameters. Because of this, it has been possible to identify and assess the differences between the transient voltage responses measured with different arrays, and the corresponding modeled ones. It follows, from the field experiments conducted that the smaller the transmitter loop employed (central loop mode) the greater the difference between measured and modeled TEM responses. Since the geoelectrical model is known, this difference gives an objective assessment of the effect that the TEM system has on the overall measured response. It is notable that measured and calculated responses coincide when TEM soundings are done using separate loops (loop-loop mode).
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Increasing the Signal-to-noise Ratio of Field NMR (SNMR) Measurements by Means of Pre-polarization
By T. RadicSummaryThe Surface Nuclear Magnetic Resonance (SNMR) Method allows direct quantification and characterization of free pore-water in the soil. SNMR measurements in densely populated areas, often cause S/R ratios of less than 1. This situation is made additionally worse when the SNMR method is used to investigate the Vadose Zone. Because the magnitude of the SNMR signal is also dependent on the strength of the magnetic field, we have build and tested an SNMR Instrument which uses the Pre-Polarization technique to improve the Data quality by up to one order.
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Removal of Co-frequency Powerline Harmonics from Multichannel Surface NMR Data
Authors J.J. Larsen, L. Liu, D. Grombacher and E. AukenSummaryPowerline harmonics is often the dominant noise source in surface NMR data and can completely overwhelm the NMR signal. Several methods have been successfully implemented for removal of powerline harmonics including notch filtering, multichannel filtering and model-based subtraction. However, the performance of these methods can be problematic when one of the powerline harmonic components has a frequency close to or coincident with the Larmor frequency, referred to as a co-frequency harmonic. Removal of the co-frequency harmonic can distort the NMR signal causing erroneous estimates of water content and relaxation rates. To solve this problem we propose an extended method of multichannel model-based subtraction of powerline harmonics. In this method, the co-frequency component is modeled in the primary coil and a reference coil on noise-only data recorded immediately prior to the NMR excitation. The phase and amplitude relationships between the components measured in the two coils are calculated. The relationships are used to predict the co-frequency harmonic component parameters in the primary coil during the NMR decay based on NMR signal free, synchronously recorded reference coil data. The mathematical framework is developed and we give examples of the efficiency of the method based on synthetic NMR signals embedded in noise-only data.
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Application of Magnetic Resonance Soundings and ERT Measurement to Characterize the Transition Between Sediment and Crys
Authors C. Jodry, J.-F. Girard, J. Porté, J.-R. Dujardin and P.-D. MattheySummaryCrystalline basement rocks at the surface are both surface water resources and an analogue of deep reservoir used for geothermal exploitation. We present here a geophysical survey realized in the East of France, at an outcrop in a quarry allowing to characterize the transition zone between sediments and basement. We applied electrical resistivity measurements and magnetic resonance soundings and highlighted water-bearing structures in the sandstone and in the basement, showing lateral variations.
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Analysis of Severely Aliased Surface Waves from a 2D Seismic Reflection Line
More LessSummarySurface wave analysis was performed on a 2D seismic reflection line acquired in the context of paleo-environmental research of the Llancanelo Lake region (Mendoza, Argentina). The line consisted of 96 receivers with 10 m spacing. During reflection processing the surface waves were muted due to their severe aliasing. The current analysis aimed to use them to derive complementary information about the shallow subsurface, by inverting their dispersion curves to obtain S-wave velocity profiles.
The analysis was restricted to a 190 m segment, due to hints of strong lateral variations along the rest of the line which would violate the 1D inversion hypothesis. The dispersion curves were picked in f-k domain. This required correcting the wavenumber values to account for the aliasing. Dispersion curves of the Rayleigh fundamental and first higher modes where obtained from the spectra of 4 records, allowing the computation of average curves and statistical errors. They were inverted by means of the Nearest Neighbour global inversion algorithm.
Inverted Vs profiles are reasonably well resolved up to around 30 m deep, and show a strong contrast around 6–8 m, which is consistent with results from other geophysical methods used in the region.
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Radiomagnetotelluric Studies of Landfills of Industrial Wastes on the Karelian Isthmus
Authors A.K. Saraev, A.E. Simakov, B. Tezkan, I.V. Tokarev and A.A. ShlykovSummaryResults of radiomagnetotelluric (RMT) soundings with use of various modifications of this method (foot, mobile and controlled source ones) on landfills of industrial wastes on the Karelian Isthmus are considered. A landfill in the settlement Morozova near St. Petersburg was formed as a result of a long-term warehousing of toxic wastes by the sulfuric acid manufacture. In the adjacent to the landfill territory investigations by the foot and mobile modifications of the RMT method were conducted and a polluted zone was contoured. Another landfill of debris in a former sandy
career is located in the Vyborg area. A survey by the controlled source modification of the RMT method was fulfilled. As a result the contouring of the wastes was realized and the structure of the landfill was studied.
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Geoelectric Surveys in Handling Urban Environmental and Geotechnical Problems
More LessSummaryThe termination of industrial activities may have side effects with harmful impact on civil infrastructure. The drastic increase of ground water level may endanger the quality of the fresh water resources and the basement stabilities of the surface infrastructures. Geoelecric survey is an effective method in solving environmental or geotechnical problems in urban areas.
This study introduces two case histories on geophysical characterization of sites that are endangered with the reappearance of karstic water. Validated with drill hole data the subsurface structures were reliably imaged. Extended with laboratory analysis of water and mud samples the source of water contamination was identified. The successful designation of subsurface aquifers and flow directions gave useful information for mitigation planning.
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High-resolution Geophysics in the Investigation of the Inner Structure of Walls and Columns of Batalha Abbey
Authors M.J. Senos Matias, F. Almeida, R. Moura and N. BarracaSummaryKnowledge of walls and other structural elements construction characteristics and techniques is mandatory in the maintenance and restoration of historical buildings.
Such information is obtained from documents or by comparing with monuments of the same period. Documents are difficult to obtain and comparison with other buildings can be inaccurate. The use of direct invasive techniques may be considered but they are likely to damage structures or compromise buildings stability. Thus, indirect high-resolution methods are adapted to these investigations.
This work describes a GPR and a Seismic Transmission Tomography survey to investigate the walls and columns of the 14th century Batalha Abbey (UNESCO heritage site).
High-resolution GRP data revealed the dimensions and thicknesses of different walls and the structure and nature of materials inside them.
The Seismic Transmission Tomography survey, supported by high-resolution photogrammetry scanning for accurate positioning of sources and geophones, was carried out to investigate the Abbey columns. Seismic velocities distribution inside the columns characterized their interior and clarified the nature of the materials used. Columns construction followed the same technique and used the same materials. However, each column depicts a different velocity zonation that could correspond to different stages of deterioration for each one.
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Contribution of ERI, Photogrammetry and Surface Resistivity Probe Methods for Assessing an Underground Mine Pillar
Authors L.D. Dore, Y. Fargier, S. Palma-Lopes, R.A. Antoine and C.F. FauchardSummaryNatural degradation of underground quarries is seen as a major risk management topic in France. The aim of this study is to use resistivity properties of the rock material for assessing underground quarry pillars. It focuses on a quarry pillar (located in the French city of Vendôme), on which two methods were performed. The first one consists in using ERI and photogrammetry to image the internal resistivity of the pillar ; it turns out to be both informative and difficult to carry out (because of its cost and the time it needs to be performed). The other one consists in using a small in-line Surface Resistivity Probe. It proves to be an easier but also less informative method, as it gives only surface resistivity values. Internal and surface resistivity values are well correlated. Both methods show the presence of a very conductive area on which a vertical crack is visible.
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A New Database Structure Guiding the Combination of Geophysical and Geotechnical Methods for Levee Investigation
Authors T. Dezert, S. Palma-Lopes, Y. Fargier, P. Côte and C. JodrySummaryIn this study, we introduce a new database framework for levee investigation. We are considering on an equal footing geophysical and geotechnical investigation methods in order to assist the agencies and levee managers in selecting the most relevant methods for the levees’ characterization. For this to be possible, we are taking into account the environmental constraints, the pathologies and the failure modes of the levees. The other aspect of this work is to open new ways of reflection in the research community with an important bibliographic work being carried out. We first focus on the relations between pathological physical properties and failure modes of a levee in order to identify which properties and characteristics of a levee need to be estimated. Then, we display our new database framework using the example of the Electrical Resistivity Tomography method. This work finally insists on the benefits of such a structure to propose new method investigation combinations and to provide a significant quantity of references for the research community.
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Assessment of Physical Properties of a Sea Dike Using Multichannel Analysis of Surface Waves and 3D Forward Modeling
Authors D.P. Pageot, M. Le Feuvre, D. Leparoux, Y. Capdeville and P. CôteSummarySeismic surface waves analysis methods (SWM) have been widely developed and tested in the context of subsurface characterization and have demonstrated their effectivness for sounding and monitoring purposes. Given their efficiency, SWM have been transfered to several scale of which civil engineering structures. However, at this scale, many structures may often exhibit 3D surface variations which drastically limit the efficiency of SWM application which are are mostly developed under the assumption of semi-infinite 1D layered medium without topography. Taking advantages of high-performance computing center accessibility and wave propagation modeling algorithm development, it is now possible to consider the use of a 3D elastic forward modeling algorithm in the SWM inversion process. We use a parallelized 3D elastic modeling code based on the spectral element method which allows to obtain accurate synthetic data with very low numerical dispersion and a reasonable numerical cost. In this study, we choose a sea dike as a case example. We first show that their longitudinal geometry and structure may have a significant effect on dispersion diagrams of Rayleigh waves. Then, we demonstrate the necessity of 3D elastic modeling as a forward problem for the inversion of dispersion diagrams.
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Geophysical Measurements for Seismic Microzonation of the City of Huelva, Spain
Authors A. Macau, B. Benjumea, S. Figueras, A. Gabàs, A. Roca and F.M. Alonso-ChavesSummaryOne of the targets of the ALERTES-RIM (Regional and on-site earthquake early warning system for the Ibero-Maghrebian region) project is the characterization of the seismic soil response in the city of Huelva (Spain) in order to perform site effects quantification and microzonation mapping. The aim of this work is the application of a methodology based on combination of passive and active seismic techniques in order to define soft soil layers present in the study area. In this sense, H/V spectral ratio method is used throughout the city in order to know the soil fundamental frequency. Seismic noise array technique and surface wave analysis (MASW) are performed to obtain shear-wave velocity profiles of the soft soils at the different geological setups across the city. P-wave seismic reflections present in the active seismic dataset was used to map the overburden – bedrock interface as well as to image overburden reflectors. Finally, combining all these results together with the boreholes information has allowed delineating soft soil layers topography and the bedrock geometry all over the city. All this information will be used to compute ground motion amplification due to site effects and to perform microzonation studies in the city of Huelva.
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Variation of Ultrasonic Wave Velocity with Some Engineering Properties of Limestone - A Laboratory Study in Sri Lanka
More LessSummaryMiocene limestone which covers northern and north-western coastal belt of the Island is used as a raw material for various industries but only cement and lime are being used for the construction industry. The author carried out a series of laboratory tests to find out the mechanical properties of limestone in Sri Lanka with the aim of providing literature for the construction industry. The objective of this paper is to indicate the possibility to find out the uniaxial compressive strength of porous limestone using ultrasonic wave velocity.
Borehole samples (NX) of limestone were obtained from various drilling sites in Northern Province of Sri Lanka and selected samples were prepared for different laboratory tests after visual observations. The tests were carried out according to ASTM Standards at the geotechnical and materials testing laboratories.
The range (and average result) for each property can be mentioned here as porosity 1–13.4 (6.7) %, ultrasonic P wave velocity 4739–6337 (5730) m/s and uniaxial compressive strength 11–72 (35)MPa.
Ultrasonic wave velocity indicates that limestone is highly compacted and solid. According to the compressive strength of solid limestone a few story buildings can be constructed on it after a detail investigation.
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Complex Trace Attributes of High-resolution CRS Stacked Data to Interpret the Near-Surface - The Example of Solfatara Cr
By P.P. BrunoSummaryTwo active-source, high-resolution seismic profiles were acquired in the Solfatara tuff cone in May and November 2014, with dense, wide aperture arrays which allowed recording multi-fold reflection data spanning a large range of offsets; deep penetrating refracted waves, suitable for first-arrival travel time tomography, and surface waves. The reflection profiles provide the first high-resolution seismic images of Solfatara crater, depicting a ∼400m deep asymmetrical structure filled by volcanoclastic sediments and rocks. Seismic reflection data were interpreted using trace complex attributes which clearly locate several narrow (less than 10 m thick) areas with distinctive anomalous low amplitudes in several areas within the crater. We interpreted them as fluid-filled (both gas and liquid phases) conduits created by the intersection of NE- and NW-trending sets of sub-vertical faults. The imaged degassing pathways terminate against strong-amplitude zones created by reduction of porosity that occurs at about 100 m from crater surface and generates a high impedance contrast between fluid and dry soils.
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Hydrogeological and Geophysical Characterization of Fractured Aquifer of Òdena (Barcelona, Catalunya)
Authors M. Himi, D. Rodríguez-Fernández, A. Folch, R. Lovera, C. Domènech, M. Rosell and L. RivSummaryThe results of the geophysical survey reflects the presence of heterogeneity distribution of electrical resistivity values. This allowed to delimit the geometry of the three layers, as indicated by borehole logging. At the same time, it has been possible to highlight several zones with low electrical resistivity values in the central part of some profiles. These have been considered as fractured areas. In many cases, these areas of low resistivity fractures coincide with areas where the wells have registered low values of the electrical conductivity. This fact reveals the effect as preferential flow areas that have these fractures throughout the circulation system of the contaminant.
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Development of the Electromagnetic Monitoring System under the Conditions of Urbanized Territory
By T. LaskinaSummaryThe developed electromagnetic monitoring system, adapted for the highly urbanized territory conditions, has been considered on the example of the salt deposit investigation. It can be used while exploring rock massifs, lying in the range from the surface to some hundreds of meters, with the purpose to control different geological and technogenic processes with the optimum detail level and economic efficiency.
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Identification and Characterization of Hard Rocks Weathering Profile by Electrical Resistivity Imaging
Authors A. Portal, P. Belle, F. Mathieu, P. Lachassagne and N. BrissetSummaryThis study proposes an original method for the detailed characterization of the weathering profile in hard rock (plutonic and metamorphic) contexts by electrical resistivity imaging (Electrical Resistivity Tomography —ERT). The main objective is to identify the electrical signature of the different layers constituting the weathering profile, more specifically that of the laminated layer. Location of the latter is an important issue for geological (saprolite thickness assessment) and hydrogeological (location of productive boreholes) purposes.
The dual approach presented here firstly focuses on real datasets obtained from field measurements acquired in both granitic (1 site) and metamorphic (3 sites) environments. Then, synthetic modeling, based on real data results, were performed in order to test the sensitivity of the ERT method to identify and characterize the layers of the weathering profile depending on the geometry (thickness) and the resistivity of the laminated layer and the Stratiformed Fractured Layer - SFL.
The results so obtained highlighted for the first time (i) the complex electrical signature of the weathering profile, (ii) the resistant nature of the laminated layer and (iii) a resistivity gradient at the base of the SFL.
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Research of the Increased Induced Seismicity in Complex Geoecological Monitoring of Shallow Subsurface of Ukrainian Mini
Authors O.K. Tiapkin, O.V. Kendzera and P.H. PihulevskiiSummaryThe increase of seismicity of mining territory raises danger of exogenous geological processes. The cause of increased induced seismicity of mining regions is a violation of geodynamic equilibrium and astable state of the upper layer of Earth’s crust due to intensive long-term development of mining industry with powerful explosions in mines and open pits and moving huge volumes of rock. The increase of local induced seismicity of these regions is shown on example of old-industrial region in Central Ukraine — Kryvyi Rih iron ore basin. These data significantly improved information about local tectonic earthquakes on background of seismic events of technogenically-natural origin, including the effects of powerful mining explosions. Local earthquakes with M≥4.6 may be continued with the existing intensity of mining industry with powerful explosions in this region. Therefore it’s necessary localization of modern activation areas of tectonic structures with induced seismicity for the system of complex geoecological monitoring of shallow subsurface of the region. And first of all the urgent creation of local Kryvyi Rih seismic monitoring system (with instrumental seismological observations on the base of the seismic station UK-15 and with direct link with National Centre of seismological data, Kyiv) is necessary.
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CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel
Authors S. Mehta, M. Bastani, A. Malehmir and L.B. PedersenSummaryMore than 7% of the Scandinavian landmass is covered with fresh-water bodies in the form of lakes and rivers. This poses a unique challenge to carry out electromagnetic survey on shallow-water bodies for various purposes for example geotechnical investigations. Recently boat-towed RMT (radio-magnetotelluric) technique was introduced and used for measurements over the Lake Mälaren in Stockholm, Sweden. The RMT covers a wide range of frequencies (10–250 kHz) and provides good resolution for shallow subsurface studies although it lacks resolution at greater depths. Using controlled-source frequencies in the range of 1–10 kHz sufficient penetration depths can be achieved for most of the near surface targets. In this study, we present the results from the combined use of controlled-source and RMT (CSRMT) data that were obtained over frozen Lake Mälaren. The objective of this study was to map bedrock surface and fractures in the middle of the profile where using only RMT data these were not adequate. We demonstrate a new technique where CSRMT surveys were carried out over frozen-shallow-water bodies and we expect the idea to be used in the near future for other applications where moderately-resistive water bodies are present.
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Geophysical Investigations to Assess the Extent of Disturbance in Excavated Rock Faces at Hinkley Point C
Authors A.L. Stork, A.C. Butcher, J.P. Verdon, A. Koe and J-M. KendallSummaryInitial field trials to investigate the potential for seismic surveys to estimate the extent of damage caused during slope excavation and to estimate a disturbance factor show that a velocity contrast due weathering/disturbance effects in a cliff face is detectable with an active seismic survey with geophones attached to the face. Modelling results confirm the resolution of the method and the experimental set-up is being deployed to monitor excavated faces at Hinkley Point C construction site.
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Combined Application of GPR and ERT to the Detection of Voids During the Construction Phase of a Building Complex in Koz
Authors G. Vargemezis, P. Tsourlos, D. Aggelis and I. FikosSummarySeveral cavities were revealed during the excavation phase of the new building complex planned to host the regional University in the city of Kozani, Combined 3D geoelectrical and GPR geophysical methods have been applied at the foundation level prior the construction phase in order to detect voids. Initially, the foundation areas have been surveyed with dense GPR scans.
GPR data were processed and combined to form depth slices and 3D volumetric information. Targets revealed by the interpretation of radar data have been further investigated by the resistivity method which was applied in a full 3D mode at selected areas.
Evaluation of the results obtained by both methods led to the distinction of different type of structures related to possible voids or fractured areas. Locations proposed by the geophysical survey have been drilled and borehole data in most of the cases confirmed the geophysical results. This helped to better evaluate the geophysical anomalies and to increase the reliability of the interpreted data. The described approach resulted either into accelerating the construction of the buildings in areas that no geophysical anomalies were found or to redesign and to redesign construction in specific areas in which voids were detected.
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Shallow vs Structure Estimation for Seismic Site Response Using the Active MASW Method - A Case Study at the Urban Area
By E.A. ObandoSummaryThe multichannel analysis of surface wave method (MASW) was utilized to estimate the soil stiffness at the urban area of Managua, Nicaragua. A total of 24 sites distributed throughout the study area, were sampled using the active walk-away scheme. The results indicates that in most of the sampled sites the elastic bedrock appears shallower than 30 meters. From the soil-class viewpoint, based on the Vs30 criterion, the soils of the area of Managua can be grouped into soil classes C and D, that is equivalent to soil types II and III of the Nicaraguan code. It appears that, the softer soil deposits occur at the western part of the city with a marked strip in north-south direction. The stiffer soils predominate at the central part of the study area that seems to decrease eastwards. Finally, when considering a shallow local earthquake, the calculated amplification function, independently of the depth of the elastic bedrock, shows that the peak ground amplification surpasses the amplification factors suggested in the Nicaraguan Building Code. This may indicate that for shallow local earthquakes the current Nicaraguan Building Code does not reflect the actual soil amplification of the area of Managua.
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GeoBIM for Infrastructure Planning
By M. SvenssonSummaryThe GeoBIM concept makes joint interpretation of geophysics and ALL other geotechncially related data possible in a convenient way. It also makes COMMUNICATION of data and models easy using only web freeware. This means that all stakeholders get the same information at the same time, which make decisions quality assured and that the efficiency in the workflow in the interpretation and the design process is significantly improved.
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Reprocessing of Geophysical Data, Comparison with Tunneling Results from the Bergen Ring-road and Subsequent Refraction
Authors G.A. Tassis, J.S. Rønning, S. Rohdewald, T. Kirkeby and M. WåleSummaryThe Geological Survey of Norway (NGU) has reprocessed various geophysical data from the Knappe tunnel in Bergen in collaboration with the Norwegian Public Roads Administration (NPRA) and Geophysix AS, the Geophysical company that originally collected, processed and traditionally interpreted refraction seismic data along the route of the tunnel. In this work we have compared different inversion schemes of refraction seismic data to each other but also to classic interpretations, resistivity profiles, geological mapping and actual observations done after the completion of the tunnel. In connection to the results obtained in the work described above, modeling was planned and carried out to test the efficiency of refraction seismic data inversion when applied to the detection of fracture zones in Norwegian settings (abrupt velocity changes between fracture, bedrock and possible overburden layers). Our goal is not to assess the Rayfract(r) software but instead to try and identify patterns created by known models when moderately knowledgeable inversion schemes are used and in this way attempt to correlate geophysical anomalies (seismic velocities) with weak zones on real refraction seismic data and plan future studies.
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GPR Survey for the Detection of Voids in Embankment of a Highway - Case Study in EGNATIA Highway in Northern, Greece
Authors I. Fikos, G. Vargemezis, P. Tsourlos and D. AngelisSummaryHighway road monitoring by means of visual inspection has been and probably will remain standard procedure for many years. However numerous examples of sudden cracks and collapses of the pavement have shown that more efficient methodologies are essential. GPR has been used extensively for this purpose. The survey focuses on the parts of the highway over the embankments that are approaching the bridge from both ways as well as the bridge’s deck. The appearance of a small hole in the pavement of the new part of the highway, only few tenths of meter away from the western edge of the bridge called for immediate evaluation of the extent of the void and inspection for other potentially future failures. In parallel with the inspection for cracks and cavities the investigation aimed to provide insight on the construction of the pavement of the old bridge and the approaching areas and compensate for the lack of realistic cross section. The method of GPR was the ideal method for this task, since it is non-invasive, very fast and very accurate when good approximation on the material’s electromagnetic properties can be achieved ( Saarenketo and Scullion, 2000 )
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3D Ground Model Using Geophysical and Geotechnical Data - A Case Study from the Netherlands
Authors V. Minorenti, D.H.R. Brouwer and K. van KesterenSummaryThis paper intends to review and stress the benefit of analysing integrated (geophysical and geotechnical) data within in a 3D environment. Over the past years concrete efforts, both in new survey acquisition methodologies and 3D computing capabilities, have been made to overcome the limitation embedded in conventional 1D/2D techniques. The 3D CPT cone resistance model was built using the kriging interpolating algorithm between the known cone resistance data points by statistically predicting the value of the empty voxel cells via a variogram. Statistical analyses (model robustness tests and minimum input tests), were used to analyze uncertainties inherited in the model and to predict the lateral extent of these uncertainties along the units. Theoretical optimum between number of geotechnical locations and model distortions were analyzed based on differences between the resulting models and variogram analyses.
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Shear Wave Velocity Structure Derived from the Microtremor H/V Rayleigh Wave Energy Rich Part
Authors R. Yazdanfar and N. Hafezi MoghaddasSummaryIn order to derive 1D Vs profile, inversions have been done only on specific part of microtremor H/V curve. This part which is between first peak and first trough, here called as Rayleigh wave energy rich of H/V curve. Inversions were done based on the neighborhood algorithm. Some available data of site such as downhole seismic, geotechnical information, surface geology and soil texture, were used for building a robust model parameter. 1D extracted velocity profiles show good agreement with traditional geophysics and inversion of the ellipticity curve results.
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Bedrock Depth Estimations and Their Limitations - Comparison between Refraction Seismics and Geotechnical Investigations
Authors R. Behrendt, E. Lundberg, C. Juhlin, F. Zhang and T. LehtimäkiSummaryIn this study, we compared a model of bedrock depth derived from a high resolution seismic dataset to a very extensive set of geotechnical bedrock depth estimations. The results of both methods are in very good agreement on the larger scale, but also show considerable differences in some locations. These differences are usually attributed to errors in the refraction model since it has generally a lower accuracy and is affected by unresolved near-surface velocity variations. However, we found several locations where the data consistently indicate that the geotechnical sounding yielded significantly too shallow bedrock estimations. Therefore, we argue that both methods should be evaluated carefully and — wherever possible — should be combined to a comprehensive dataset. Since geotechnical investigations are one-dimensional and seismic measurements are areal, they provide complementing information that can be used to reduce errors.
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