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NSG2021 27th European Meeting of Environmental and Engineering Geophysics
- Conference date: August 29, 2021 - September 2, 2021
- Location: Hybrid
- Published: 29 August 2021
81 - 100 of 134 results
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A physical model to study deep contaminated sites: ERT study with surface-downhole electrode configuration.
Authors M.V. Bongiovanni, V. Grünhut and E. LópezSummarySurface electrical resistivity tomography is a widely used tool to map the subsurface. One of its limitations is the decrease in resolution as depth increases. Another limitation is that the electrodes planted on the surface can be heavily influenced by temperature, weather, and water saturation changes over time. Consequently, the data can be easily contaminated by noise and therefore, unreliable for long term monitoring. Borehole DC electrical surveying allows to extend the anomaly detection capability beyond the limits of surface electric surveying. Even more, with two wells, the cross-hole DC electrical surveying provides detailed information on the variation of electrical resistivity between the boreholes, but just in a very limited zone near them. The implementation of borehole to surface electrical resistivity tomography allows to reduce this limitation. Such an arrangement is expected to provide an increase in detection capability in the area in-between the boreholes and surface. In the present work we studied the feasibility of surface-downhole measurements to detect and estimate the dimensions of a contamination plume in a deep aquifer, performing a physical model at laboratory scale. We conclude that the detection and consequently the monitoring of contaminated deep aquifers with two wells using surface-downhole ERT is possible.
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Characterization of a complex fault system by 2D acoustic Random Objective Waveform Inversion
Authors D. Köhn, M. Thorwart, D. De Nil and W. RabbelSummaryOne key ingredient of a successful seismic full waveform inversion (FWI) is the choice of the objective function. Robust FWI approaches combine objective functions with different weights of the phase- and amplitude-mismatch between modelled and field data. Typically, the phase-mismatch is corrected first and the amplitude-mismatch later. However, the weighting of one waveform information over another can result in convergence issues. In contrast, the recently proposed Random Objective Waveform Inversion (ROWI) randomly applies different objective functions at each stage of the FWI. In this study, we created a ROWI based on the combination of the robust Global Correlation Norm (GCN) and an AGC-weighted l2-norm. While the GCN is able to fit waveforms of the direct wave and dominant reflections, the AGC-norm enhances weak reflections with the drawback of destroying the amplitude information. Therefore, both objective functions can complement each other. The proposed ROWI approach is applied to land-seismic data acquired above the Glücksstadt Graben in Northern Germany. The ROWI result reveals a complex, heterogeneous system of thrust faults and disturbed layers within the Graben, while a much simpler layering is present outside of the Graben. The ROWI result is consistent with a time-migrated reflection seismic image and borehole logs.
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Comparison between classical and geostatistical regularization methods for ERT crosshole imagery
Authors K. Tsakirmpaloglou and O. KaufmannSummaryERT imaging between boreholes is a method of imaging and monitoring that has many applications. In stratified environments with little deformation, geostatistical regularization is reputed to lead to geologically more realistic results than approaches based on classical smoothing constraints with anisotropy. Using inversions carried out on a synthetic case, we examined the impact of different regulations on the recovered model.
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Monitoring a Spatial Drilling Trajectory Deviation Using a Drill-Bit Signal as a Source.
Authors Z. Wilczynski and A. KaslilarSummaryDeviation from the desired drilling trajectory may cause operational problems. Real-time monitoring of the drill bit position helps in mitigating these issues. In this study, we suggest a seismic-while-drilling (SWD) method to detect the (near) real-time drill bit position. We use a homogenous model typical for hard rock environments and conduct 3D finite-difference modeling of elastic wave propagation. We introduce a receiver array at the surface and a drill bit source at different depths deviating from its vertical trajectory. By cross-correlating the records in respect to a virtual source location, we obtain differences of arrival times at a given trace versus arrival times at the virtual source location. The correlation image presents a hyperbola where the apex indicates the horizontal location of the source, i.e. the closes drill-bit receiver path. Considering a 2D orthogonal array, and a 3D array, we show the possibility of monitoring and estimating the horizontal location of the drill bit.
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Three decades of reflection seismic surveying at Neves-Corvo, Portugal
Authors G. Donoso, A. Malehmir, J. Carvalho and V. AraujoSummaryThe Neves-Corvo is arguably the largest and one of the most important volcanogenic-hosted massive sulphide (VMS) endowments in the world. In the last three decades three seismic acquisition programs have been carried out in the area. In 1996, six profiles covering the area, with a special interest in the tier-1 Lombador deposit. Reprocessing of this dataset was done in 2019 and showed the relevance of revisiting legacy data using current tailored processing workflows. In 2011, a 24 km2 3D survey was done with the intention of producing new targets and in particular with the discovery of the Semblana deposit in 2010, the 3D dataset showed its potential beyond the earlier 2D surveys. In 2019, new technologies for seismic exploration were tested, the new seismic survey consisted of two perpendicular 2D surface profiles positioned above the known Lombador deposit, additionally source points were activated inside the exploration tunnels and simultaneously recorded on the surface, which provided an improved velocity model for migration and time-to-depth conversion. The objective of this study is to provide an overview of these surveys and present the latest results of revisiting the 2011 3D seismic data.
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Tomographic seismic imaging of a carbonate core at the laboratory scale
Authors D. Brito, C. Shen, J. Diaz, C. Bordes, J. Virieux and S. GaramboisSummaryCarbonate rocks studies are of particular interest in exploration. Such rocks are generally heterogeneous, anisotropic, porous and fractured and imaging their petrophysical properties are usually challenging with classical techniques. We present here a laboratory study where seismic tomography, a widely used method in Earth interior imaging, is tested on a decimetric carbonate core. The goal is to obtain a high-resolution Vp tomography of the interior of the carbonate rock using innovative experimental approaches and inversion workflows, the processing being accompanied by direct numerical modeling of the wave propagation within the carbonate sample.
A point-like pulsed-laser and a piezoelectric transducer are alternatively used as seismic sources and a laser interferometer records the travelling waves reaching the periphery of the carbonate core. Tomographic images of Vp are obtained and analyzed with both sources and compared to an X-ray CT-scan tomography.
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Convolutional neural networks for the characterization of magnetic anomalies
Authors J. Cárdenas Chapellín, C. Denis, H. Mousannif, C. Camerlynck and N. FlorschSummaryDuring the last decade, deep learning architectures demonstrated to be very promising in geophysics, notably in the field of seismic interpretation. Their great power and flexibility may produce valuable contributions to finding solutions to a diversity of geophysical constraints, for example, the limitations of inversion algorithms or the fine-tuning often required on the different noises that affect the data. In our paper, we use convolutional neural networks to characterize magnetic geological bodies. This characterization consists of counting the number of dipolar magnetic anomalies produced by these bodies and predicting their corresponding parameters. After several experiments with deep learning architectures, we found that the combination of YOLO and DenseNet achieves excellent performance by reaching an accuracy of over 90% in their respective metrics. We also applied visualization tools to explain our results. We managed to visualize the discriminative area of our network with Grad-Cam, and we observed the "logic" of the predictions by using t-SNE. In future studies, we will evaluate the robustness of the proposed approach using real data, for example, in the context of pyrotechnical detection for unexploded ordnance exploration.
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On the applicability of 2D SH-FWI for high-resolution imaging of 3D subsurface structures
Authors D. Köhn, M. Thorwart, D. De Nil, J. Albert, W. Rabbel and F. SirockoSummarySeismic full waveform inversion (FWI) has been established as a tool for the characterization of near-surface structures in a wide-range of applications from engineering/archaeogeophysics to hydrocarbon exploration. To reduce computational costs, most FWI applications are using 2D medium approximations to describe seismic wave propagation in the framework of the inverse problem, even if the subsurface structures would demand a 3D medium description. In this study, we investigate the applicability of a 2D SH-FWI to map complex, 3D subsurface structures of a potential thrust fault system, located east of the Great Plön lake in Northern Germany. During two extensive, geophysical field surveys in 2017/2018, four intersecting, seismic SH-profiles were acquired, covering the study area. The independent FWI of the SH-profiles with respect to the shear-wave velocity and density model reveals complex, heterogeneous subsurface structures. Despite the simplified 2D medium approximations used in the FWI, similar features are visible at the intersection points of the SH-profiles. The near-surface area down to a depth of 15 m is dominated by thin, alternating, layers disturbed by low-velocity anomalies. The deeper subsurface consists of high-velocity anomalies dipping in SW direction. The general trends of FWI density models can be correlated with borehole density logs.
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Seismic Characterization of a historical heritage building
Authors M.A. Martinez, C. García, M. Vásconez, E. Aracil, U. Maruri and P. MartínezSummaryHistorical heritage building, Passive MASW, Vs30
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In-Mine Seismic Method for Platinum Orebody Exploration in Maseve Platinum Mine, South Africa.
Authors M.K. Rapetsoa and M. ManziSummaryIn-mine seismic experiments were conducted to delineate geological structures and possibly the platinum orebody (UG2) that lies beneath the incline shaft. Three 115 m segments of seismic lines were collected, with a spread of 355 m. The seismic lines were designed to image geological structures along the incline shaft using 10 Hz receivers and a sledgehammer. Preliminary results are presented in this study and reflections identified on the raw shot gathers are interpreted to originate from the slightly dipping Merensky reef that is few meters below the tunnel. This study demonstrates and encourages the use of in-mine seismics for future mine development and planning.
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Classification of Vineyard Soil Physicochemical Zones Using Non-Invasive Frequency-Domain Electromagnetic Induction and Ndvi Methods.
Authors P. McLachlan, M. Schmutz, J. Cavailhes and S. HubbardSummaryIn this work, a novel method for obtaining maps about the physiochemical nature of soils from electromagnetic induction (EMI) data is investigated for a French vineyard. Specifically, K means clustering is used to produce maps from both the quadrature and in-phase components of EMI measurements. The commonly discarded in-phase component of EMI measurements exhibits a higher correlation with plant vigor, as measured by NDVI. In addition to the non-intrusive characterization, extensive soil samples are used to explore the correlations and causations that link soil physiochemical properties with EMI and NDVI data. It is demonstrated that the in-phase component of EMI is related to metal concentrations at the site, these properties co-vary with crucial properties related to plant vigor, e.g. cation exchange capacity and organic matter content.
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Geophysical Imaging of the Root Zone: Methods, Implications and Outlook.
Authors G. Cassiani, B. Mary, J. Boaga, I. Barone and V. IvanSummaryThe use of non-invasive geophysical methods has been demonstrated to be of high value in root zone characterization. The potential for further development of these methods is very large and will provide indispensable information for understanding and modelling of SPAC / ECZ (Earth Critical Zone) processes.
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Sparse Data Transformation for Unsupervised Clustering for the Exploration Ahead of Tunnel Face
Authors A. Sapronova, P.J. Unterlas, J. Hecht-Méndez, T. Dickmann and T. MarcherSummaryIn collaboration with Amberg Technology, 1Institute of Rock Mechanics and Tunnelling at Graz University of Technology is developing a model to predict geological conditions ahead of the tunnel face. The model employs a cascade of unsupervised and supervised machine learning algorithms and uses the seismic data and available geological documentation at the underground construction site. At first, we use unsupervised methods to cluster the entire dataset into an arbitrarily defined number of clusters so that each cluster contains a unique label (representing geological conditions). We then train supervised classifiers to predict the label(s) for each cluster.
In this work, we elaborate on developing two critical parts of this model: pre-processing and unsupervised clustering of the dataset. We tested several methods for sparse data de-noising and clustering to answer the following question: ""given a sparse structured mesh or a graph (unstructured), what method can be used for pre-processing and unsupervised clustering, to reveal major characteristics (features) of the dataset at the low dimensional space."" Four methods that showed the most robust results were selected and used further in the model’s versioning.
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Geophysics for Cultural Heritage
By R. DeianaSummaryThe main characteristic of geophysical methods is the non-invasiveness and their consequent ability to investigate and, therefore, know a system providing information on anomalies, in a specific time or time-lapsed, studying the non-direct way the variations of one or more physical parameters. This feature is undoubtedly crucial for all investigations in cultural heritage, for a particular system or artefact under protection.
The development of new technologies, sensors, and software has led to new geophysical applications in cultural heritage analyses in recent decades. Today, thanks to geophysical methods at different scales, besides the more systematic and well-known exploration, geological, hydrogeological, and engineering applications used over the times, we can provide crucial information on the state of ancient systems and objects, fundamental for restoration and conservation strategies, then opening new frontiers for the application of these non-invasive methods in cultural heritage applications.
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Georadar Investigations in the Church of San Paolo (San Giacomo Dei Militari, Palermo).
Authors P. Capizzi, M. Marrone, C. Aleo Nero, A. Bonfardeci, A. Canzoneri, A. Carollo, R. Martorana and F. RomanoSummaryAs part of a research project aimed at drawing up a restoration project for the church of San Paolo, the monument was investigated with georadar surveys. The investigations were followed in a targeted manner by archaeological excavations, under the supervision of the Superintendence of Cultural and Environmental Heritage of Palermo.
The San Paolo church is located in the oldest part of the city. The geophysical investigations will be useful to identify any pre-existing structures on which the church could be founded. The georadar investigations focused on the central nave and one of the side aisles to identify buried structures and crypts. The data showed several anomalies. One of these, located in the middle of the central nave, has been verified with an excavation. The entrance to a crypt, not yet explored, and several bone fragments were then found.
Finally, some high-frequency profiles were acquired in the walls and columns of the church, which highlighted numerous anomalies that will be verified with exploratory tests.
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Perspectives in Ground-Penetrating Radar at High Latitudes: From Occasional Imaging to Automated Continuous Monitoring
Authors A. Saintenoy, E. Léger, C. Grenier and N.M. ThiérySummaryAmong different geophysical methods employed for glacier and permafrost characterization, Ground-Penetrating Radar has been employed many times since its first developments.
However these GPR characterization were and are still mostly performed during field campaigns which can be sparse in time, expensive and human resource demanding, and can be limited by the difficulty to access areas.
The purpose of this short paper is to present two approaches for northern latitudes features imaging during field mission and discuss the benefits brought by hardware and software improvements to develop the field of long-term monitoring. The main objective of this paper is to highlight the new paradigm that the internet of things brought to the field of surface GPR characterization of high latitude environment.
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Three-Dimensional Time-Lapse Inversion of Tem Data With Application in an Icelandic Geothermal Site
Authors L. Xiao, G. Fiandaca, P. K. Maurya, A. Vest Christiansen and L. LévySummaryThe transient electromagnetic (TEM) method is widely used in resistivity mapping but rarely applied in monitoring experiments. In this study, we present an algorithm to invert time-lapse TEM data, with inversion of both synthetic data and field data. Three main novelties distinguish this new inversion algorithm: i) a multiple-mesh approach is used for the definition of model parameters and forward modelling, ii) the forward and jacobian computations are carried out in 3D and iii) two datasets, each composed of several TEM soundings, are inverted simultaneously with a generalized minimum support norm for time-lapse changes. In the synthetic example, dense and coarse acquisition layouts are modelled, to study the effect of data coverage on model retrieval. Coarse data coverage allows to retrieve the time-lapse anomaly, thanks to the 3D sensitivity of TEM data. However, dense data coverage over the anomalies gives better resolution, as expected. In the field example, we present the time-lapse inversion results of data collected at an Icelandic geothermal powerplant in 2019 and 2020. The TEM data were acquired for defining the baseline in a monitoring an experiment of Hydrogen sulphur sequestration planned for 2021. As expected, no variations were imaged by the time-lapse inversion.
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A Theoretical Approach to Near Surface Pedophysical Permittivity Models
Authors G. Mendoza Veirana, P. de Smedt, W. Cornelis, D. Hanssens and J. VerheggeSummaryDespite well-established knowledge about the relationships between the electrical conductivity or relative permittivity of rocks, ambiguity remains as to how these relationships and knowledge can be transferred to soils. In general, there are three problems that hinder the robust application of available pedophysical models. First, these models often rely on soil properties and attributes that cannot be directly quantified in the laboratory or in the field, such as pore connectivity and depolarisation factor, which reduces their applicability. Secondly, the introduction of tuning parameters (such as the cementation exponent and Roth's exponent) tends to reduce the theoretical significance of the physical process in question. Finally, oversimplifying pedophysical relationships by considering only one soil attribute (such as moisture content) effectively ignores the effect of other relevant properties (clay content or bulk density). Here we present a theoretical approach that shows how to sort out this issues while both fitting parameters and anisotropy indicators can be derived from soil properties that are easy to measure in the laboratory.
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Interpretation of Legacy 3d Seismic Data for Underground Platinum Mines: Implication for Mine Safety
Authors S. Matloga, M. Manzi and G. BybeeSummaryMethane gas pockets are intersected during normal mining activities and this causes explosions. Methane gas migrates from deeper sources to mine working environments through faults and dykes. High-resolution 3D reflection seismic method was employed in the western Bushveld Complex to a depth of 1500 m to delineate geological features in subsurface mining environment. Strong seismic reflections were tracked on the seismic cube to create seismic horizons of the platinum-bearing horizons. Preliminary results show faults at various orientations crosscutting the UG2 and Merensky Reef located between 400 – 900 m below surface. Iron-rich ultramafic pegmatoids and potholes were mapped, the former as vertical and chaotic seismic reflection features and the latter as slump structures along the seismic reflections. Spatial correlation between these seismically detected features and methane occurrences influences this research to encourage the implementation of mitigation strategies for future mine planning and development.
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Combining Soil Sampling, EM38 and 3D GPR Techniques to Map Key Water Distribution Parameters
Authors E. Bloem, J. Sala, H. Johansen Lindgaard, I. Sturite and Ø. AustadSummaryTo understand the drainage capacity of agricultural soils in Norway, we need to understand the dynamics of water distribution in the subsoil. We need a better overview of existing drainage systems in combination with the soil types on which agriculture is performed. We have investigated an agricultural field using a combination of traditional soil sampling, EM38 and 3 dimensional Ground Penetrating Radar (3D GPR).
Combining EM38 with the soil sampling made it possible to map the lateral heterogeneity of the soil types at the experimental site. The 3D GPR added a depth resolution otherwise not available with the EM38. We have cross analysed the results from the EM38 and the depth slices from the 3D GPR and we can conclude that there is a positive match between the images obtained from the two methods. The variations in apparent conductivity tie in very well with the soil texture, in terms of scattering and attenuation, that can be appreciated in the 3D GPR data. Also, we were able to map the existing drainage system. The methods provide complementary information that can be used to better understand the dynamics of water distribution in the subsoil.
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