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25th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 8-12, 2019
- Location: The Hague, Netherlands
- Published: 08 September 2019
21 - 40 of 141 results
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Mine Bench-Tunnel Seismic Data Acquisition for Characterizing Shear Zones in the Siilinjärvi Phosphate Mine, Finland
Authors A. Donczew, A. Malehmir, E. Koivisto, M. Savolainen and B. BrodicSummarySiilinjärvi in central Finland, is currently the only phosphate mine with significant phosphorus production in western Europe. Phosphate rock and phosphorus are critical for the EU because of their supply risks and economic importance. The phosphate bearing rocks in Siilinjärvi are within a major Archean alkaline and carbonatite complex deformed and intruded by several shear and dyke systems. By understanding their spatial and temporal relationships an improved exploration and extraction of this critical ore would be possible. The objective of the study was to employ a novel seismic survey using the existing mine infrastructure for better planning and geological understanding. An innovative in-tunnel seismic survey was conducted in the Siilinjärvi open-pit mine in October 2018. A water-drainage tunnel nearly in the bottom of the pit intersecting two major shear zones was used to enable bench-tunnel seismic data acquisition. High-quality data were acquired using 144 receivers inside the tunnel, with the sources located both inside the tunnel (Bobcat-mounted vertical drophammer) and on the surface (combined explosives and Bobcat). Two reflections interpreted to originate from subvertical shear zones intersecting the tunnel were observed illustrating the importance of such surveys for shear-zone imaging and site characterization.
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A Geophysical Approach to the Phenomenon of Ground Fissures at the East Thessaly Basin (Greece)
Authors J. Alexopoulos, S. Dilalos, G.S. Mitsika, E. Vassilakis and S.E. PoulosSummaryThe disastrous phenomenon of aseismic ground fissures along the eastern Thessaly basin has deteriorated since 1989. The main reason for these fissures is the over-pumping, which leads to differential vertical compaction of the aquifer system and subsidence on the land surface. In this paper, we present the results of a combined geophysical survey applied in the area (VES and TEM soundings), in order to investigate the subsurface geological conditions. The geological regime of the area is comprised of the alpine basement, transgression formations and finally a package of post-alpine deposits with respectful thickness. A peudo-3D representation of resistivity maps for several depths of investigation was produced. Additionally, a dataset of deep boreholes was used for the calibration of the geophysical data. All the borehole and sounding interpreted data were grouped into three categories in order to produce the Lithology Model of the area. The alpine bedrock was adumbrated only at the southeast and central part of the basin, where we do not observe ground fissures. The absence of alpine bedrock for depths up to 300 meters, the thick and coarse-grained deposits and finally the over-pumping seem to contribute to the creation of the ground fissures.
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Sensitivity and Uncertainty of Petrophysical Models to Predict Storage Properties in Weathered/Fractured Hard-Rock Aquifers
Authors J.A. Mézquita González and J. ComteSummaryWeathered/fractured hardrock aquifers are complex, heterogeneous groundwater systems with overall low storage. This work develops a methodology using computer coding to estimate hardrock aquifers’ storage properties applying Archie (AR) and Waxman & Smits (WS) petrophysical models, and further assess their sensitivity and uncertainty. The data consists in both surface and borehole hydrogeophysical measurements in a micaschist aquifer in Ireland. In this case study, WS appears as most suitable because it allows to account for the significant clay content present in the subsurface as a result bedrock weathering. This property is not accounted for in AR, which therefore largely overestimates the pore space. An important finding is the high sensitivity of both models to the cementation factor. WS models are in addition highly sensitive to the clay properties, namely the cation exchange capacity. Uncertainty analyses determined the higher uncertainty in the deep, high resistivity, massive bedrock, with WS being the most affected. Availability and accuracy of spatial data on the cementation factor and clay properties are key to achieve realistic storage models with high confidence.
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A Multipurpose Platform Towed TEM-System Imagining the Top 100m of the Subsurface; Surface, Water and Snow
More LessSummaryWe present new developments of the versatile towed transient electromagnetic system (tTEM). The system now meets the demand of near surface mapping down to 100m and is suitable for application not only ground-based but also in aquatic (rivers, lakes etc.) and snow environments. The receiver technology of the system has been upgraded and it has a 4 times lower noise level compared to previous system, increasing the depth of investigation down to 100 m. The latter two system are called FloaTEM and SnowTEM, respectively. FloaTEM surveys were conducted in several parts of United States including the Mississippi delta region for mapping and characterizing the alluvial aquifer system below the river beds. The results show that the FloaTEM system produces detailed subsurface resistivity images in quick time compared to other geophysical methods. We conducted over 300 km of survey along several rivers within a week in the Mississippi delta region. The SnowTEM system uses a 4 turn, 4 m x 4 m transmitter coil and a specially designed low noise receiver coil. It has been put on skies and is towed behind a snow scooter or a snow robot.
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Early-Time and Late-Time Limitations on the Performance of Near-Surface TEM Measuring Systems
Authors N. Kozhevnikov and M. SharlovSummaryReducing the exploration depth of the TEM sounding method by measurement at earlier times necessitates decreasing size of the transmitter loop. This causes problems in measuring TEM response at both early and late times. The early time problem is associated with fundamental constraint caused predominantly by the transmitter loop inertia. As the transmitter loop decreases, its intrinsic response decreases much slower compared to that of the earth. At late times, instead of measuring the earth’s transient response one measures the receiving unit’s intrinsic response. Its initial amplitude is small, but it decreases so slowly that, eventually, its contribution to the total transient response becomes predominant. This problem is technical. It can be solved by eliminating or reducing the impact of the voltage pulse induced in the receiver loop during the current turn-off in the transmitter loop.
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Application of Time-Domain Electromagnetic Survey for Seafloor Polymetallic Sulphides in the Okinawa Trough
Authors K. Nakayama, M. Moroori and A. SaitoSummaryJOGMEC (Japan Oil, Gas and Metals National Corporation) vigorously promotes bathymetric survey, geochemical exploration, geophysical exploration and drilling to evaluate the amount of the resources of the seafloor polymetallic sulphides in Japanese EEZ. Until today several promising ore deposits have been discovered in the Okinawa Trough and Izu-Ogasawara area, and electromagnetic surveys were conducted in several areas. In this presentation, we will introduce some of the WISTEM (Waseda Integrated Seafloor TEM) survey results in the Okinawa Trough in 2018.
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A 3D Inversion Algorithm Using a Triple Mesh Approach and Domain-Decomposition for Fast Computations
Authors K.W. Engebretsen, B. Zhang, G. Fiandaca and E. AukenSummaryWith the computer power available today full 3D inversions of transient electromagnetic data (TEM) is no longer a dream of the past. Although it is possible to perform these inversions, the problems still scale in three dimensions making large datasets slow to invert. We here propose a new triple mesh method for inverting TEM datasets with multiple transmitters and multiple receivers per transmitter. The code is relative fast and with a manageable memory consumption. In this new approach we show that by using a decoupled regular structured model mesh and two finite element forward meshes, one with a coarse discretization and one with a fine discretization, we get a substantial speed up in calculations times without sacrificing much in terms of how well we fit the data. We show that we can invert large datasets by decomposing our domain and applying this triple mesh method on each domain separately.
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Geophysical Monitoring of Redox Influenced Degradation Near the Water Table
Authors P.M. Fernandez, E. Bloem, H.K. French and A. BinleySummarySoil contamination of organic origin is a major environmental challenge. One of the most common reactions in nature is the electron exchange between chemical species, also known as a reduction-oxidation (redox) reaction. Redox reactions drive organic matter degradation in the ground. Monitoring these reactions is therefore needed to protect soil and groundwater. This work aims to provide non-invasive monitoring methods to study redox state changes that occur in the near-surface interface, at the boundary between aerobic and anaerobic conditions, as well as a potential remediation solution to attenuate the negative impact of anaerobic condition.
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Geophysical Monitoring of Initiated In-Situ Bioremediation of Chlorinated Solvent Contamination
Authors A. Nivorlis, T. Dahlin and M. RossiSummaryContaminated ground is a severe problem which is present in many countries, and it is of great importance to find efficient solutions to deal with it. In Sweden, there is a trend in the industry to move towards alternate remediation methods and on that scope in-situ bioremediation has received a lot of attention. The main challenge with in-situ bioremediation is to verify that the treatment has the intended effect, and it is hence important to understand the changes that happen in the subsurface and monitor them over time. The scope of our work is to use geophysics to extrapolate the punctual information from groundwater and soil samples and acquire a better understanding of the changes in the subsurface during in-situ bioremediation of the contaminated soil. In this work we have made a permanent installation in one of our field sites, in Alingsås, where a pilot bioremediation test is taking place to treat the contaminated soil. The autonomous system can measure the resistivity and chargeability distribution in the subsurface. The results indicate promising correlations with the geophysical signature and the contaminant. Currently we are investigating the correlations between the groundwater samples and the time-lapse imaging acquired while working towards real-time monitoring.
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Mapping Sediment Contamination with a Gamma-Ray Spectrometer, Underwater and on Land
Authors R. Koomans, Van der Veeke, P. Jacobs and H. LimburgSummaryEnvironmental contamination due to human activity is recognized as one of the modern dangers of our industrial advancements. The current approach to the management and monitoring of contaminated aquatic and non-aquatic sediments involves invasive, often labor-intensive methods. The method is time-consuming and costly and the resulting contours of the contaminant distribution are subject to a number of potential errors, with the potential for ‘hot spots’ to remain unmapped. At present, there are no means for the passive, in-situ mapping of contaminants in aquatic and non-aquatic sediments.
Our method addresses the main problems associated with the current approach. We propose a method that provide rapid, spatially detailed data on contaminant distributions and concentrations in sediments by using a gamma-ray spectrometer. A gamma-ray spectrometer can be used for mapping aquatic sediments (by using a vessel as platform) and non-aquatic sediments as floodplains by using a quad-bike or drone as platform for mapping.
The study shows how these gamma-ray spectrometers have been used to map the contamination of a river basin of the Elbe tributary in Germany. The combination of all platforms result in information on contaminant levels of a river system in aquatic and non-aquatic sediments.
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An Integrated Methodology for Hydrogeological Assessment around Industrial Installations
Authors M. Claprood, C. Couegnas, E. Gloaguen, M. Krimissa and D. ParadisSummaryWe present a nested hydrogeological characterization methodology to optimize the use of existing data and better plan the acquisition of new data around man-made installations. The workflow is presented at an industrial site where the construction of deep infrastructures has disturbed the local hydrogeology settings. The first step is to lever historical data coming from hydrogeological tests and civil engineering operations before and during the construction of the industrial installations to build the frame of hydrogeological model. Based on the review of this information, new geophysical data acquisition can be scheduled to refine the interfaces between geological units. This initial model serves has a training image to simulate multiple equiprobable scenarios of the site geology while preserving the well information and the location of the buildings as, obviously, deterministic. These geological scenarios are populated with anisotropic hydraulic conductivity fields using sequential Gaussian simulation. These heterogeneous hydraulic conductivity models are ran with a flow and transport simulation algorithm to constitute an ensemble of realizations that is used in an ensemble Kalman time series assimilation scheme.
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Processing 2D ERT Data in 3D Environment - A Case Study Inside a Karstic Cave in Greece
Authors I. Fikos, G. Vargemezis, C. Pennos, B. Lønøy, K. Jensen and J. TverangerSummaryThe present work deals with the application of ERT method inside a karstic cave in Macedonia, North Greece. The cave geometry is incorporated as a priory information to 3D inversion software and the results are evaluated with forward modelling of simple realistic 2 layer models. The effect of the resistive walls and ceiling of the cave is investigated and the artefacts are discussed. Finally, the results of the 3D inversion are compared with the results of a typical 2D inversion of the same data, ignoring the 3D environment of the cave.
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ERT and SP Measurements for the Characterization of Fault-Controlled Soil CO2 Degassing
Authors C. De Paola, R. Di Maio and E. PiegariSummaryDue to the significant impact that high atmospheric CO2 concentrations can have on human and ecosystem health, a growing interest is focused on the characterization of non-volcanic CO2 degassing. The natural release of CO2 is often controlled by faults and fracture systems that favor the formation of permeable channels allowing gas migration toward the surface. In this framework, geophysical methods represent very useful investigation tools as they not only are able to identify focused gas release, but also to provide an estimate of the extent of influence areas of CO2 degassing and their preferential ascent pathways. To detect fault-controlled soil CO2 degassing, electrical resistivity and self-potential measurements were performed in a survey area located to the south of Matese Ridge (southern Apennines, Italy), where very high gas emissions are observed. Due to the nature of the investigated soils, preferential pathways associated with CO2 flux are found as resistive channels and negative self-potential anomalies.
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A New Approach to Incorporate Prior Information in MGS Inversion of ERT/IP Data
Authors R. Thibaut, T. Kremer, A. Royen, F. Nguyen and T. HermansSummaryThe current paradigm in geophysical inversions is to compute the simplest solution according to Occam's principle. The implicit assumption is that the parameters of interest have a smooth spatial distribution, which is rarely geologically plausible. An alternative is the Minimum Gradient Support (MGS), a functional that allows to compute a regularized inversion favoring sharp contrasts. Its use is currently mostly restricted to research studies and the MGS solution is highly sensitive to the selection of a variable called the focusing parameter ß. There is still no consensus on its optimal choice. To propose a methodology for applying this functional to real applications, the MGS is first used on ERT/IP synthetic data mimicking a real case study. For complex geometries, a smooth solution is first computed and used as a starting model for sharp MGS inversions. Including prior information on resistivity from drillings further improves chargeability features. The developed methodology is then applied to ERT/IP data collected on a gold deposit. The method allows different interpretations about the mineralization key properties, and seems more indicated to compute plausible electrical resistivity spatial distributions regarding the extensive prior geological knowledge. The choice of ß is challenging and should be automated in future developments.
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A Homotopy Optimization Method for Non-Liner Inversion of Geoelectrical Sounding Data
Authors Z. Esmaeili, R. Ghanati and M.K. HafiziSummaryIn nonlinear inversion of geophysical data, bad initial approximation of the model parameters usually leads to local convergence of the normal Newton iteration methods, despite enforcing constraints on the physical properties. To mitigate this problem, we present a globally convergent Homotopy continuation algorithm to solve the nonlinear least squares problem through a path-tracking strategy in model space. The global convergence of the Homotopy algorithm is compared with a conventional iterative method through the synthetic and real 1-D resistivity data. Furthermore, a bootstrap-based uncertainty analysis is provided to quantify the error in the inverted models derived from the case study. The results of blocky inversion demonstrate that the proposed optimization method outperforms the Marquardt-type algorithm in the sense of the stability and the recovered models.
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Time-Lapse Target-Oriented Crosswell Full Waveform Inversion without Downhole Sources
More LessSummaryTime-lapse seismics has a wide range of application in different scales, from near-surface to resource exploration. Crosshole sesmics is used to characterize fluid reservoirs and to obtain highly resolved rock/soil-dynamic parameters e.g., elastic moduli and Poisson ratio. Developments in distributed acoustic sensing shows the potential of deploying permanent downhole receivers at low costs. In order to achieve an efficient and accurate time-lapse seismic measurement in such scenarios, we have developed a nonlinear waveform inversion to reconstruct velocity structure between boreholes using VSP data with source located only at the surface, and no downhole sources. The new approach formulates the forward modelling using wavefield representation theorem, which enables directly estimating the velocity structure by minimizing data residuals and calculating the gradient from the adjoint state problem. We test the approach using numerical modelling of time-lapse VSP data to detect layer-specific temporal changes. A heterogeneous shallow vadose zone represents a low-velocity layer. The results show that the new approach provides more stable and more accurate temporal velocity profiles than conventional full waveform inversion, when the initial velocity model does not include the shallow low-velocity layer. The new approach is robust and highly advantageous as it does not require downhole seismic sources.
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High-Resolution Imaging of the Shallow Subsurface with Elastic Full Waveform Inversion of Surface and Body Waves
Authors A. Adwani, M. Danilouchkine, A. Soni, R. Plessix, F. Ten Kroode, S. Abri, F. Ernst and F. AlshukailiSummaryIn this abstract, we discuss an elastic full waveform inversion approach to estimate high resolution shallow subsurface elastic parameters from both surface and body waves in onshore seismic data. We have used a spacetime domain weighting function to balance the amplitudes of the dominant surface waves with respect to other body waves. This allows us to simultaneous extract information from the surface waves and from the refracted and reflected/converted body waves. The different wave types are complimentary in terms of depth and wavenumber information. The concept and results are illustrated using a complex 2D synthetic example representing a typical Middle-east geology.
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Compact Full Waveform Inversion by Model Space Re-Parameterization
Authors L. Xu, E. Manukyan and H. MaurerSummaryFull waveform inversion is an ill-posed optimization problem. To address this issue, we propose a novel compact full waveform inversion (CFWI) scheme. For that purpose, we re-parameterize the problem in an alternative model space, which is the result of two transformations, namely a 2D Fourier transformation (Hartley transform), followed by a wavelet transformation (Haar transform). In this model space, the waveform inversion problem can be appropriately parameterized using only a small number of model parameters. As a result, we can improve the robustness of FWI. We demonstrate this with a simple crosshole example, where we obtain excellent results while reducing the number of model parameters by 98%. Besides this considerable reduction of model parameters, CFWI offers new opportunities to analyze the spatial resolution, which can be extremely valuable for optimized experimental design purposes.
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Shallow-Seismic Wavefield Scattering and Implications for Viscoelastic FWI
Authors N. Athanasopoulos and T. BohlenSummaryThe aim of this study is to investigate the seismic response from shallow structures using a 2D finite-difference method. Full-waveform inversion (FWI) suffers from so-called cross-talk between viscoelastic parameters, i.e. certain combinations of viscoelastic parameters have the same scattering signature. We evaluate the scattering response from individual perturbations in the viscoelastic material parameters. By subtracting a perturbed from a background medium and solving the viscoelastic wave equation we obtain the scattering wavefield. Additionally, we evaluate the instantaneous energy density of P- and S-waves to better understand the influence of scattering. This allows us to provide some fundamental insights into the composition of seismic recordings and ideas of how to mitigate cross-talk in shallow-seismic applications of FWI. Specifically, we find that the P-wave velocity structure is constrained similarly by Rayleigh, P- and S-waves. The S-wave velocity is the most reliable parameter to invert for from FWI of surface waves due to their large amplitudes. The scattering pattern of density differs fundamentally from those of velocities and attenuation. Finally, cross-talk between attenuation and velocity for both P- and S-waves is caused by the similarity of their corresponding scattering responses.
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A Workflow Allowing Multiple-Point Statistics Realizations to Reveal Where We Lack Information
Authors M.L. Gulbrandsen, N. Jensen and T.M. HansenSummaryIn recent years, the importance of representing the uncertainty of any earth model has met more understanding. One way of doing this is by generating a series of realizations of a statistical representation of the earth model instead of just one, in some way, 'optimal' model. Modelling the subsurface in a probabilistic way like this, allow for any kind of statistical questions to be answered. These questions are often very problem-specific, however, in this study we present a methodology combining multiple-point statistical modeling and information theory allowing to quantify the information content everywhere in an Earth model, which allow us to ask the question: Where do we lack information?
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