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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
41 - 60 of 134 results
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Mapping the Far Western Bushveld Complex Using Legacy 2D Seismic Reflection and Petrophysical Data
Authors T. Nadan, M. Manzi and S. Scheiber-EnslinSummaryThe Bushveld Complex is very well known for its significant contributions to the world’s Platinum-Group Element’s and chromium reserves. The Far Western Limb is an extension of the complex and has not been extensively explored due to the Quaternary sedimentary cover in the area which covers the contacts between the different lithological units. Outcrops of the chromium within the Far Western Limb have already been identified and mined. This study aims to identify the shallow subsurface lithologies and evaluate the potential for future mining of chromium ore in the Far Western Limb. The results show that the bedrock is very shallow (< 80 m) which implies that the Quaternary cover in the Far Western Limb is thin. This allows for further investigation through near-surface geophysics and drilling as the subsurface lithologies that host the chromium ores can be found close to the surface.
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Drone Geophysics for Forecasting and Monitoring Natural Hazards
Authors B. Dupuy, A. Tobiesen, A. Grøver, A. Einbu and A. RomdhaneSummaryWe present an innovative approach to combine different sensors on a flexible drone platform. The goal is to record repeatable data that can be used to forecast and monitor natural hazards such as snow avalanches and landslides.
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The Abra Xcite AIP Modelling Case Study
Authors A. Viezzoli and A. MenghiniSummaryThe role of AEM in mineral exploration is ever increasing, both internationally (e.g., AusAEM) and within the European context (Smart exploration, Infact). Acquisition with lower base frequency and modelling IP effects stand out among the recent different technical developments in AEM. We present a case study from the Abra sedimentary-hosted deposit from Western Australia, with IP modelling of an Xcite dataset, accompanied by extensive ancillary geophysical information.
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Data Fusion of ERT and Infiltration Tests, Using Bayesian Maximum Entropy to Mapping Saturated Hydraulic Conductivity
Authors S. Rabouli, M. Serre, V. Dubois, J. Gance, H. Henine, P. Molle, C. Truffert and R. ClementSummarySoil infiltration is used as an outlet for the discharge of domestic treated wastewater. The sizing of these infiltration areas in a heterogeneous soil context is difficult, due in particular to the spatial measurement of saturated hydraulic conductivity, which is costly and time-consuming to implement. Nowaday, there is no adequate technique for evaluating the variability of satured hydraulic conductivity for a heterogeneous soil. The development of a methodology for the spatial characterization of soil properties is essential. We demonstrate here the benefit of using Bayesian Maximum Entropy data fusion to obtain a 2D representation of the infiltration capacity of heterogeneous soils. This tool opens up prospects for improving the sizing of infiltration areas, in context of heterogeneous soil.
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Inversion of VLF Data Using a Non-Linear Smoothing Operator
Authors M.A. Uge, G. Karcioglu, A. B.Tekkeli and M.S. ArslanSummaryInversion of electromagnetic induction data, including VLF, is generally realized using smooth inversion methods. The smoothness of the recovered models and the regularization of the ill-conditioned problem is ensured with smoothing matrices. Smoothing matrices are simple linear derivative matrices penalizing the resistivity differences between adjacent cells. Since these matrices are linear operators, they are calculated once at the beginning of the inversion process. Considering its structure, smoothing matrices can be considered similar to low-pass Gaussian filters. Similarly, it’s possible to define a non-linear smoothing operator based on rank order filtering. We have defined a non-linear smoothing constraint based on these filters and penalized the differences from the cells corresponding to the desired rank value. Since the defined constraint is non-linear it is re-calculated as the model parameters change. The defined constraint is tested on synthetic data and its results are compared to the results obtained with a traditional smoothing matrix. Accordingly, the defined non-linear rank order smoothing constraint can provide relatively focused, amplified structures, and can increase blockiness.
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Seismic Monitoring of a Maritime Pine Root-System Failure During Its Overturn: A Field Experiment
Authors V. Allègre, A. Denis, A. Cointe and J. CoureauSummaryThe mechanical response of a few maritime pines was investigated by a series of experiments at the field scale. During the overturning of two trees, a seismic array was deployed at the ground surface to continuously monitor the micro-seismicity associated with the failure of their root-system. We identified seismic events during the traction process applied to each tree. A threshold force value and a significant duration of the traction process seem to be needed in order for cracks to happen, and for seismic events to be recorded. We identified tens of seismic events, especially during the most advanced stages of the experiments, prior to complete failure of the root-system. However, a few events were observed during early stage of the experiment, when the mechanical solicitation was still in the elastic regime. Using pairs of geophones and seismic intensity ratios, we localized the epicenters of most of the events within the seismic network. According to the sensitivity of the array, all epicenters are roughly localized within a 1 square-meter area. The estimation of epicenters show that the events are aligned to the direction of traction, on the opposite side of the tree.
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EASYMAG: A Versatile Solution for High-Quality UAV Magnetic Acquisitions
Authors J. Mercier de Lepinay, T. Fréville, B. Gavazzi, B. Kiemes, L.M. Sanabria and H. ReillerSummaryTerremys has sought to develop EASYMAG, an unmanned aerial system fully dedicated to magnetic surveying in a variety of contexts ranging from small and shallow objects or pipelines detection to large-scale geological mapping. Four major requirements are identified if one wishes to efficiently acquire quality magnetic data at such variable scales: 1- the sensitivity of the magnetic acquisition must be greater than the lowest expected signal according to the application (usually a few nT or lower); 2- The navigation and positioning must be precise enough to compete with ground operators with centimetric accuracy; 3- the flight flexibility must allow contrasting flight domains with topographic draping at any height; 4- the efficiency on the field must compete with existing vectors. Each of these aspects are investigated to demonstrate Terremys’ ability to perform quality magnetic surveys. A case study of drone magnetic acquisition for archaeological purposes is presented in comparison with a ground survey acquired in the same area. Ground and UAV surveys are of comparable quality and they can lead similar interpretations.
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New Insights into a Very-Large, Slowly Moving Landslide (Hell-Bourg, Reunion) from High-Resolution Seismic Surveys.
Authors C. Rault, K. Samyn, B. Aunay, A. Bitri and M. DelatreSummaryIn November 2020, a passive and active seismic investigation was applied to the large slow-moving landslide of Hell-Bourg, Réunion Island. The objective of this survey is to provide relevant information for the characterisation of the internal structure and hydrogeology of the landslide. The first joint application of geophysical methods, geomorphological surveys, field observations and measurement has already led to the general characterisation of this complex landslide up to 200 m depth. The initial results from the seismic survey obtained from the P-wave refraction tomography allow confirming the existence and continuity of the main sliding surface, which had previously pointed out by resistivity model from aerial electromagnetic data. Furthermore, thanks to this data discontinuities that affect both the landslide and the substratum up to 450 m deep have been detected, suggesting a strong relation between the dynamic of the landslide and the substratum geometry. These initial results are promising and thus suggest that the seismic data we are currently processing will reveal new insights to improve our knowledge of the structure and dynamics of the Hell-Bourg landslide.
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Is It Redundant to Use Model-Based Subtraction Together With the Reference Noise Cancellation?
Authors L. Liu, M.P. Griffiths, M.Ø. Vang, D.J. Grombacher and J.J. LarsenSummarySurface NMR is a competitive method in hydro-geophysical investigation due to its directive sensitivity to the water content, permeability and hydraulic conductivity of the aquifer. One of the main obstacles to expand its application in many area of interest is the low signal-to-noise ratio (SNR) of measurement. To improve the SNR by mitigating noise, numerous approaches has been proposed, for example remote reference noise cancellation (RNC) and model-based powerline harmonics subtraction (MB). In this abstract, the question whether MB should be applied before RNC is investigated. First, the theoretical length of Wiener filter (WF) in RNC is analysed by observing harmonics relationship between primary and reference channels. Then the benefits of applying MB before RNC is given within multiple noise sources are present. Last, the noise level of field measurements processed by RNC and MB followed by RNC are compared.
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Correlation between Distributed Rayleigh Sensing (DRS) and Moisture Sensors as Indicators of Slope Instability
Authors P. Clarkson, R. Crickmore, A. Godfrey, C. Minto, J. Chambers, B. Dashwood, D. Gunn, L. Jones, P. Meldrum, D. Morgan, A. Watlet and J. WhiteleySummaryThis paper describes the verification of Distributed Rayleigh Sensing (DRS), traditionally associated with acoustic sensing, for monitoring low frequency activity on a slope prone to landslides that is used as the British Geological Survey’s landslide observatory at Hollin Hill, North Yorkshire, U.K. The observatory is monitored using a variety of geological survey instruments and provides a unique opportunity to compare measurement systems that have very different principles of operation.
Previous studies of the slope have shown good correlation between the low frequency strain and temperature measured using the fibre with prior knowledge of the geology of the site and longer-term measurements made on more established geological survey instruments. This paper presents a more detailed comparison of measurements made on the DRS system over the winter of 2020/2021, with measurements of soil moisture content made on point sensors and estimates of ground movement measured using GPS marker posts.
The DRS system is sensitive to multiple important indicators of slope instability and can monitor ground movement effectively. Areas of unstable ground can be clearly identified by the larger changes observed in the fibre output in those regions.
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Cross-Borehole ERT: Sensitivity, Model Resolution, and Field Data Quality
Authors L.M. Madsen, A.K. Kühl, L. Levy and A.V. ChristiansenSummarySensitivity, resolution, and data quality are important parameters to consider when designing an ERT cross-borehole survey. We present an open-source algorithm for computing 2D and 3D sensitivity patterns of any borehole setup and use this to compare single-borehole and different cross-borehole electrode configurations, which both show complex patterns. To study the resolution capability of an entire survey, the model resolution matrix is computed for a field dataset. The dataset is split into different electrode configurations and the model resolution matrices of the different configuration are compared. The results show that the sensitivity and resolution decreases very quickly away from the electrodes, especially for the single-borehole configurations. In the studied field case, this means cross-borehole configurations are needed to correctly image the area between the boreholes, even though the cross-borehole data often are associated with a lower signal-to-noise ratio due to near-zero potential measurements and generally more outliers. The study concludes that in a production mode, where acquisition time and thus the number of possible data points are limited, the combination of electrode configurations must be carefully considered and a trade-off between resolution and data quality must be evaluated.
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Discrete Cosine Transform Reparameterization for Bayesian Time-Lapse ERT Inversion
Authors A. Vinciguerra, M. Aleardi, A. Hojat and E. StucchiSummaryTime-Lapse electrical resistivity tomography (TL-ERT) is used to monitor dynamic processes through mapping the resistivity variations in the subsurface. Inversion of TL-ERT data is a highly non-linear and ill-conditioned problem characterized by non-unique solutions. For this reason, an accurate uncertainty appraisal is essential to quantify the ambiguity affecting the estimated resistivity model. We present a probabilistic TL-ERT inversion in which the Differential Evolution Markov Chain (DEMC) algorithm samples the posterior probability density function, while the Discrete Cosine Transform (DCT) is used to compress the model space. The model compression aims at mitigating both the ill-conditioned nature of the inversion problem and the curse of dimensionality issue. On the other hand, the DEMC combines principles coming from metaheuristic optimisation methods and Markov Chain Monte Carlo algorithms to speed up the probabilistic sampling. To draw essential conclusions about the reliability and applicability of the implemented algorithm, we focus on synthetic inversion experiments in which we simulate two data acquisitions at different time instants (t0 and t1) and we jointly estimate the resistivity model at t0 along with the resistivity changes at t1. The results demonstrate that the implemented method provides accurate model predictions and uncertainty estimations with an affordable computational cost.
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Ensemble-Based Time-Lapse ERT Inversion with Model and Data Space Compression Through Deep Variational Autoencoders
Authors A. Vinciguerra and M. AleardiSummaryTime-lapse electrical resistivity tomography (TL-ERT) aims to image resistivity changes in the subsurface. This is an ill-posed and non-unique inverse problem and hence the estimation of the model uncertainties is of crucial importance. To reduce the computational cost of the probabilistic inversion, model and data can be re-parameterized into low-dimensional spaces where the inverse solution can be computed more efficiently. Among the many compression methods, deep learning algorithms based on deep generative models provide an efficient approach to reduce model and data spaces. Here, we propose a TL-ERT probabilistic inversion where the data and model spaces are compressed through deep variational autoencoders, while the optimization procedure is driven by the ensemble smoother with multiple data assimilation, an iterative ensemble-based algorithm that performs a Bayesian updating step at each iteration. This method provides multiple realizations for the quantification of the uncertainty by iteratively updating an initial ensemble of models that we generate according to previously defined prior model and spatial variability pattern. A finite-element code constitutes the forward operator. We test the method on synthetic data computed over a schematic subsurface model. Our tests demonstrate the applicability and the reliability of the proposed TL-ERT inversion.
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Low-Frequency Drone-Borne GPR for Soil Conductivity Mapping
Authors K. Wu, E. Jacquemin, L. Palt, L. Ory, T. Parizel, V. Vincent Dienst and S. LambotSummaryIn this study, we proposed a low-frequency drone-based GPR for soil conductivity mapping. The lightweight radar system consists of a handheld VNA, a microcomputer, a differential GPS and a power bank. A 5-meter half-wave dipole antenna is used to provide the operating frequency range of 15-45 MHz. For the data processing, antenna-drone effects are filtered out with the antenna transfer functions generated from calibration, and 3-D LUT is used for fast and robust inversion. The soil conductivity is obtained from point measurements using kriging interpolation. The result shows a good spatial correlation, and reasonable conductivity values comparing with EMI, which initially proves the concept of drone-borne GPR for soil conductivity mapping.
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Transept Foundations of a 12th Century Chapel Revealed by Geophysical and Photogrammetric Prospection
Authors C. Fauchard, L. Aillaud, A. Legrand, R. Antoine, V. Guilbert, C. Ledun and B. BeaucampSummaryThis study presents the work carried out at the Notre-Dame-du-Val chapel located in Sotteville-sur-Mer (Normandy, France). Local authorities are considering restoration work and have requested a geophysical survey of the surroundings. First, aerial photography by drone yields a Structure from Motion (SfM) model of the exterior of the chapel, completed by a 3D interior model. Geophysical prospecting consists in imaging shallow surface of surroundings, with help of Ground Penetrating Radar (GPR) and Electrical Resistivity Imaging (ERI).
GPR measurements clearly reveal former apses on both sides of the chapel, corresponding to the transepts meets in perfect agreement with the presence of filled-in openings visible in the 3D interior photogrammetric model. The ERI measurements underline the presence of a very resistive substratum, probably corresponding to the local chalky formation where the chapel was built. Besides, resistive formation underlines the potential earthworks carried out to support the transepts meets, correlated with GPR signals. Several GPR and ERI anomalies have not been interpreted and deserve deeper investigation: archaeological excavations remains the only solution to confirm their nature.
The restitution within the photogrammetric model allows a very explicit display of the results, integrating the architecture of the chapel and geophysical measurements.
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Preliminary Evaluation of Geothermal Energy Potential in Western Part of Dahomey Basin, Southwestern Nigeria
Authors E.A. Ayolabi, O.B. Balogun, M.O. Okunubi and R.P. AkinwaleSummaryGeothermal Energy, Dahomey Basin, Curie Point Depth, Temperature Gradient, Heat Flux
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Application of Satellite Imagery Landsat-8 to Identify Surface Hotspot as Preliminary Survey on Ranau
Authors T. Taufiq and M. MaharaniSummaryRanau Geothermal Prospect Area is located at borderline South Sumatera and Lampung Province on the Southwest Sumatera Island, bordered by three clusters of young volcanic, namely Mt. Pugung on Southwest, Mt. Seminung on South, and Mt. Raja on Northeast. With this condition, based on geological map, Ranau Area is dominated by volcanic breccia, tuff, and andesite-basaltic lava (Qhv) from Seminung Volcanoes, Andesite-Basaltic (Qv) with thickness 300 m from Pugung Volcanoes, contained by sulphide minerals and quartz veins (Tomh), and Rhyolitic tuff, pumiceous tuff, welded tuff with carbonaceous claystones intercalations (QTr). On surface mapping and based on reference found three geothermal manifestation such as Kota Batu hotspring on northeast of Seminung Mountain, Ujung hotspring on the west seminung, and wahid hotspring on northwest area. Based on Land Surface Temperature Map, we obtain the high temperature indicated by dark-red color with value is 35-50ºC. In this research, we would to correlate the result of landsat-8 satellite imagery (processed into land surface temperature map) with geothermal manifestation (three hotspring) based on reference to know the hotspot zone as a preliminary study of geothermal and geo-tourism prospect area.
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Integrating Electrical Resistivity and Seismic Refraction Tomography at an Active Landslide Site
Authors J. Whiteley, A. Watlet, S. Uhlemann, P. Wilkinson, J. Boyd, C. Jordan, M. Kendall and J. ChambersSummaryThis study demonstrates a procedure for preparing co-acquired ERT and SRT data from an active landslide for joint quantitative interpretation. We show the results of integrating these co-located datasets using a simple clustering approach to improve the interpretation of the inverted models. The results identify the major lithological units forming the structure of the landslide, and provide an example of the benefits of quantitative integration and interpretation.
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Geophysical Investigations to Study the Celtic Open Settlement of la Peyrouse (Dordogne, France)
Authors J. Hantrais, V. Mathé, P. Corfmat, G. Sheehan, C. Chevillot, R. Chapoulie and E. HiriartSummarySince 2019, geophysical archeological surveys conducted in the Celtic and Gallo-roman site of La Peyrouse (Dordogne, France) within the collective regional program RAPSODIE and a PhD research. The aim of this study is to explore, investigate and document an unfortified economic center dated from the 3rd century BC setting extensive near-surface techniques, particularly geophysical surveys. While excavating large-scale sites such as open Celtic settlements is a challenge for archaeologists, this protocol provides a quick understanding of the site. After five campaigns, the total area mapped in La Peyrouse with a manual geophysical system amounts to 12 ha. Numerous archaeological structures such as a craft production area, a religious building, a presumably public place, and other buried remains of the settlement were detected and mapped. Excavations on high-potential areas previously selected by geophysical surveys thus allowed an integrated archaeological interpretation. The compilation of data has provided new information on this large open agglomeration. This considerably increases our knowledge of this type of habitat still poorly known in Celtic Europe.
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Marine Gas Hydrate Deposits Study: Temperature Inversion Advantages
Authors A. Vasilev, E. Kozhuharov, N. Botoucharov, I. Genov, P. Petsinski and R. PehlivanovaSummaryThis work demonstrates the possibilities of the marine geothermy (heat flow methods) to reconstruct the formation process of heat-related geological objects with an example of 2D inversion of data from in-situ temperature measurements in seafloor sediments for gas hydrate deposit study in Danube paleodelta, Black Sea.
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