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24th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 9-12, 2018
- Location: Porto, Portugal
- Published: 09 September 2018
81 - 100 of 203 results
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Detection of Buried Mine Shafts by Ground Conductivity Mapping - Two Examples from the Walloon Coalfield (Belgium)
Authors N. Dupont, T. Martin and O. KaufmannSummaryThe presence of numerous buried mine shafts in Wallonia (South Belgium) causes significative post-mining hazards and geotechnical risks. Recently, the locations of thousands of buried mine shafts have been estimated with some uncertainties. To eliminate these uncertainties, the most suitable solution is to relocate precisely the buried mine shafts. Geophysical surveys could provide an efficient way to detect such buried mine shafts. However, the applicability depends on the local context and needs to be assessed.
In our study, we applied ground conductivity mapping to detect and locate buried mine shafts expected within two distinct sites (Sambreville and La Louvière). The application of this method gave very convincing results, with some nuance according to the site geology and to the mine shaft abandonment (backfilling or not). In addition, a significant number of unknown mine shafts was also discovered. In the future, development projects in these areas could benefit from investigations such as the ones presented here.
This is why this work could be seen as a milestone in dimensioning ground conductivity surveys to detect buried mine shafts in the regional context of the Walloon coalfield.
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Geophysical Study to the Port of Kos Island (Greece) after the Earthquake of 21st of July 2017
Authors G. Apostolopoulos, G. Amolochitis, K. Leontarakis, D. Merziotis, D. Karaiskos, A. Kamilakis and A. StergiouSummaryOn 21st of July 2017 a 6.5-magnitude earthquake between Bodrum and Greek islands ( Fig. 1a ) strikes injuring more than 200 people. Kos’s port was put out of action due to damage at the harbor. Greek Authorities commissioned ISTRIA Ltd. and the Laboratory of Applied Geophysics of NTUA to conduct sampling drilling and geophysical research to provide the appropriate information to engineers for the port repair study. The initial design of the geophysical survey was for P and S refraction seismic survey in order to detect the depth to bedrock and identify the loose formations above through their seismic velocities. The actual field conditions resulting to hidden layer problems and noisy data have led to the use of MASW method exploiting the 5 shots and all the respective seismic data. The depth to bedrock was found well as borehole data indicated but details in the loose sediments above couldn’t be found with such a seismic array setup. ERT sections have shown the top of semi-impermeable zone to saline water and lateral inhomogeneities and GPR sections near surface loose sediment zones with higher presence of saline water and zones with regular presence possibly related with hidden anthropogenic structures.
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Geophysical and Geotechnical Data Fusion for Levee Assessment - Interface Detection with Biased Geophysical Data
Authors T. Dezert, S. Palma Lopes, Y. Fargier and P. CóteSummaryTo prevent disastrous consequences imputed to levee breakage, assessment methodologies have to be improved. Geophysical and geotechnical investigation methods are usually used to make such assessments. However, the effective combination of these two specific types of data remains a challenge. We propose the fusion of geophysical and geotechnical data by means of Belief Functions. Here we demonstrate our approach on a synthetic case study including geophysical (electrical resistivity) and geotechnical (cone-bearing) data and by implementing Smets and PCR5 normalization rules. This new data combination approach allows the characterization of horizontal interfaces and of a geological structure initially hidden by the effects of a highly conductive body.
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S-Wave Imaging of Glacial Overdeepened Alpine Valleys – Multi-Components
Authors T. Burschil, H. Buness, D.C. Tanner and G. GabrielSummaryS-wave seismic imaging complements the application of P-waves with benefits, e.g., in resolution and petrophysical parameters. However S-wave imaging strongly depends on subsurface conditions. We investigate two overdeepened Alpine basins with P-wave and S-wave seismic methods using single-component and multi-components: (1) The Tannwald basin, located in the Alpine foreland, and (2) the inner-alpine Lienz Basin. These basins have a 3-D shape and a spatially-varying internal structure that constitute promising study areas.
Here, we present two profiles of S-wave seismic reflection imaging using 6-components, i.e. 3-C geophones and a horizontal vibrator sources exciting in two directions. The interpretation of coincident P-wave profiles benchmark the S-wave data. The seismic S-wave sections show most geological structures on the components that have same source-receiver orientation and, thus, corresponds to the SV- and SH-domains. We are able to detect S-wave reflections and diffractions of the basin base down to 600 m depth (2 s TWT). Reflectivity on the other (mixed) components indicate a large amount of wave conversion. The application of S-waves increases the resolution and is able to improve seismic imaging in these environments.
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Integrated Geophysical and Geotechnical Analyses at a Landslide Area - Case History in Lopare, Bosnia and Herzegovina
Authors K. Suto, J. Sugawara, M. Urosevic, S. Arsenovic and S. KomatinaSummaryA heavy rainfall in May 2014 caused an extensive damage in the Balkan area.
A combination of seismic reflection, MASW and electric resitivity surveys are carried out to supplement a surface geotechnical observation at an area in Bosnia and Herzegovina for potential of further landslides. This is a Geoscientists without Borders project.
Surface observation found soft and low strength layers in the area with multiple scarps and cracks.
The depths of slip surface are estimated at 10m at the upper part of slope and about 2m at the lower part of slope. These are indicated by low reflectivity on the seismic reflection, low S-wave velocity in the MASW and high conductivity in the resistivity tomography sections.
According to these results, several suggestions are made to local government to mitigate landslide disasters.
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New Gravimetrical Map of The Rieti Intra-Mountain Basin (Central Apennines, Italy)
Authors K. Skrame, M. Di Filippo and M. Di NezzaSummaryThis paper describes the acquisition of the gravity data and their analysis to model and map the bedrock configuration of the Rieti Basin (hereafter RB); a typical intra-mountain tectonic depression of Central Apennines in Italy, related to a still active tectonic extensional regime.
The study involved a test area of 35 km2 occupied by 110 gravity stations. The new gravity measurements were merged with the gravity data of the 358 gravity stations collected during the 1995 gravity survey ( Ciccollella et al., 1995 ). The gravity data resulted from the network adjustment were used to calculate the Bouguer anomaly map. To realize the 2-D gravimetric models of the RB, a realistic density of 2,15 g/cm3 for the unconsolidated Quaternary deposits, a density of 2.50 g/cm3 for the Travertine and a density of 2.60 g/cm3 for the Meso-Cenozoic pelagic basin deposits were used ( Skrame, 2011 ).
The models obtained matched quite well with the exiting geological and geophysical data.
The new bedrock surface topography map of RB provides a new more detailed and representative image of the buried morphology, and gives a more accurate evaluation of the thickness of the Plio-Quaternary sedimentary infilling.
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Development of Geophysical Methodologies for the Assessment of Coastal Concrete Infrastructures
Authors C. Sachet, P. Sentenac and D. LerouxSummaryDue to the geometry and sea effect, an adaptive methodology is required in the case of the geophysical assessment of coastal concrete infrastructures. Along with other observables, these distinctivenesses influence the resistivity and permittivity measurement. The procedure jointly uses the ERT and GPR geophysical methods and is directly taking into account the geometrical engineered particularities, and the presence of the sea water. With complex geometrical shapes addressing the engineering needs, measurements are directly under the effect of the general geometry and underlying structures. In order to identify the 3D structures, the acquisition and the processing steps have to take into account these complexities and the corresponding anomalies. The process considers the geotechnical and geometrical properties during the data processing and inversion problem in order to assess the coastal concrete infrastructure state.
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Selection of Field Parameters for Shallow Seismic Reflection Surveying and the Results of Two New Shallow Seismic Lines
Authors J.A. Meekes and V. VandeweijerSummarySeveral faults in the border area between Netherlands and Germany show a complex structure. Deep seismic lines do not reveal the shallow structure well enough and the density of the available lines is low.
We conducted two high resolution seismic reflection lines to clarify some of the geological questions about the fault structure. This paper presents a general discussion on the selection of field parameters for high resolution seismic surveys for shallow applications and the data acquisition and data processing of these two lines. Several aspects of the selection of field parameters for high resolution seismic surveys for shallow applications are discussed. The selection of a station interval is discussed in detail as well as the stack-array principle.
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The Contribution of Geophysical Methods to the Study of Vallcebre Landslide (Eastern Pyrenees, Spain)
Authors M. Himi, M. Sánchez, L. Rivero, R. Lovera, A. Urruela and A. CasasSummaryThe use of geophysical methods to study landslides may provide useful information on defining internal structure of a landslide, identifying geometry of sliding surface, determination of groundwater effect on landslide, physical properties of landslide material and landslide mass movement.
In this study, electrical resistivity tomography and seismic refraction tomography methods were used as complement to geotechnical and monitoring tests. The results from each geophysical study were very evident for clarifying the internal structure of the landslide, the physical properties of the rock mass involved and defining the sliding surface geometry.
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P- And S-Wave Seismic Imaging of Sinkhole Structures in the City Of Hamburg
Authors S. Tschache, U. Polom, H. Buness and C.M. KrawczykSummarySinkholes pose a natural hazard, especially in densely populated areas. In Hamburg-Groß Flottbek, two slowly subsiding sinkholes are known that lie above a salt diapir flank. The Wobbe See sinkhole is a collapse structure that has been buried and built up. The Flottbek Markt sinkhole is a suffosion sinkhole. We applied P-wave and S-wave reflection seismics on coinciding profiles to study the structure of the sinkholes and their causes. Additionally, a pseudo 3D P-wave dataset was acquired.
The P-wave and S-wave seismic sections show mostly a good accordance. S-waves enable a high-resolution imaging of the near-surface. Below the Wobbe See sinkhole, a deepening reflector interrupted by staggered faults can be interpreted, which confirms the outcome of a previous study. Down-bending reflectors and an internal bedding structure are imaged in case of the Flottbek Markt sinkhole. The P-wave data reveal reflections of the salt diapir and caprock due to their higher penetration depth. The results suggest that the combination of P- and S-wave reflection seismics is a suitable tool for the investigation of sinkhole areas, even in urban environments.
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3D-Borehole Radar - A Routine Tool for The Detailed Imaging of Salt Structures
Authors D. Orlowsky, C. Holst and B. LehmannSummaryDuring the last decade, the 3D-borehole radar technique has demonstrated to be a valuable tool for the localization of the edges and for the investigation of the internal structure of salt domes. This methodology evolved to an integral component of the standard investigation program for new cavern development projects at many salt dome sites in Europe. A 3D-borehole radar probe, which runs within a vertical drill-hole through the salt formation, emits omni-directional radar signals. To map the 3D spatial coordinates of radar wave reflectors (geological interfaces) within the salt, the directions of the incoming radar signal reflections are determined apart from the distance of the probe to the reflector. A so called cross loop antenna as receiver unit in the probe allows for the recording of three separate signal shares. Together with the knowledge of the probe’s orientation the incoming angle of each signal can be clearly set and thus the coordinates of reflectors in the 3D space can be determined
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High-Frequency GPR Investigations in Saint Vigilius Cathedral, Trento
More LessSummaryIn this paper, we present the results of high frequency GPR measurements performed on the pillars of Saint Vigilius cathedral in Trento to monitor the construction conditions as well as to detect the internal defects of the pillars. Tomographic measurements were performed using a pair of 1GHz Mala antennas at three heights along the pillars. The distribution of the propagation velocity was therefore determined in horizontal planes within the pillars and the data were then translated to images of the relative permittivity. Reflection measurements were performed using the very high frequency 3GHz IDS antenna. The results detected some minor spaces where the stones were not perfectly attached. However, significant internal defects with dimensions larger than 15cm should be excluded according to GPR results. Based on vertical profiles on all the accessible sides of the pillars, the size and layout of the stones used to construct the outer parts of the pillars were recovered. Thickness variations of two sample vaults were also monitored using the profiles measured across them.
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A Fast GPR Numerical Model Based on Machine Learning with Application to Full Waveform Inversion
Authors I. Giannakis, A. Giannopoulos and C. WarrenSummaryForward modelling of Ground Penetrating Radar (GPR) is often used to facilitate interpretation of complex GPR data and as a key ingredient of full waveform inversion (FWI) processes. As general 3D full-wave electromagnetic solvers are computationally very demanding routine application of advanced GPR modelling is not popular. A novel concept for creating a fast GPR forward model based on machine learning (ML) concepts is presented. This ML-based model is trained using a dataset obtained from a realistic 3D Finite-Difference Time-Domain (FDTD) gprMax model. The fast model is trained for a specific GPR application that can be easily parametrised and have a somewhat constrained variability. However, the training uses GPR A-Scans obtained from very realistic forward models that include all complex scattering effects and antenna coupling mechanisms. To demonstrate the efficiency of the approach an application, using real GPR data, of the fast forward solver within a FWI process, using a global optimiser requiring a great number of forward model calculations, is presented producing very promising results.
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Subsurface Utilities Mapping for Engineering Applications Using Ground Penetrating Radar (GPR)
Authors L. Adeoti, T. Oyeniran, K. Ishola, R. Adegbola and M. AyukSummaryGPR survey was conducted to investigate the presence of voids / cavities in the shallow subsurface that may compromise the construction of a landing pier/jetty/dock for small and medium sized ocean-going vessels. Two major parallel traverse lines and three minor parallel lines that cut perpendicularly across the two major lines were occupied. Panels (which are points of interest) were established on the two major traverse lines. A total of fifteen (15) GPR sections were produced and processed. Data editing, zero-offset correction, DC filtering, dewowing, automatic gain control (AGC), trace equalization, band-pass filtering in frequency domain, NMO correction and migration were applied to process the data. GPR data were constrained with a borehole log and interpreted in terms of the radar facies. Two intercalated radar facies namely; sand and clay are identified. The depth of penetration of the radar signals ranges from 4 m to 14 m. The thickness of the reinforced concrete hardstand across the study area is 0.15 m. Voids / cavities are suspected only at three sampled points with the depth range of 1.1 to 6.3 m.
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Tree Root Imaging by Ground Penetrating Radar Synchronised with Self-Tracking Total Station
Authors K. Takahashi and K. AoikeSummaryLocating tree roots in subsurface is an important task in the treatment of unhealthy trees. There are some examples of tree root imaging by ground penetrating radar (GPR) scanned in multiple straight survey lines. Tree roots tend to extend in radial direction from the trunk and the conventional straight line scanning is not optimum considering antenna polarisation. We developed a GPR system that records accurate position of antennas by tracking with a total station. The system is allowed to scan freely while data collection for 3D measurements and thus circular scanning can be employed in the tree root imaging. In this way, antennas directed normal to scanning direction can keep polarisation parallel to radially directed targets and thus receiving higher reflection amplitude from tree roots is expected compared to scanning in other directions. We used the system for trees in parks and major root systems were successfully imaged. The present paper illustrates the imaging results.
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Hydraulic Permeability Prediction from Induced Polarization Data at Field Scale
Authors G. Fiandaca, P.K. Maurya, N. Balbarini, A. Hördt, I. Møller, V. Rønde, N. Foged, P.L. Bjerg, A.V. Christiansen and E. AukenSummaryIn this study we present the prediction of hydraulic permeability (k) from time-domain spectral induced polarization (IP) data, measured in boreholes and along 2D surface profiles. The borehole data were collected with the El-log technique, which provides undisturbed “while drilling” measurements of the direct current (DC) resistivity, time-domain IP data and gamma radiation. Data were measured on unconsolidated formations at a landfill site in South Denmark, in three boreholes and along 16 2D profiles.
DC and full-decay IP data were inverted in terms of the BIC re-parameterization of the Cole-Cole model, which present smaller parameter correlations and disentangles bulk and surface conduction. Permeability values were computed from bulk conductivity and the maximum imaginary conductivity, using the empirically-derived formulae presented in a recent study without any calibration. The IP-derived k estimates were compared to those estimated using grain size analysis and slug tests, for a total of 157 comparisons.
A good correlation, on average within one decade, was found between the k estimates over four orders of magnitude, with similar depth-trends. In conclusion, IP can be reliably used for estimating hydraulic permeability on unconsolidated formations at the field scale, using the relations found in the laboratory without any further calibration.
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High-Resolution Shallow Seismic Integrated with Electrical Resistivity Method for Hydrogeological Prospecting
Authors E. Onyebueke, M. Manzi and R. DurrheimSummaryTwo perpendicular high-resolution shallow seismic and resistivity profiles were acquired to assist in imaging the near-subsurface sedimentary architecture for hydrogeological prospecting in the Nylsvley Nature Reserve. We deployed 48 channels of 14 Hz geophones with 1–2 m dense source-receiver spacing, providing fold coverage of 24. To enhance the reflected seismic signal, we employed an extensive seismic processing workflow which enhanced the seismic reflectivity on the stacked sections. The seismic reflection interpretation was constrained and integrated with refraction, resistivity tomography and the 1-D multichannel analysis of surface waves (MASW) to generate the model that best represents the real subsurface geological model. The integrated results show the bedrock-overburden contact at 8–12 m depth, which correlates well with boreholes, drilled in the area. In addition, reflection seismic and refraction tomography show the bedrock undulation and velocity changes associated with erosional surfaces or weathered/fracture systems. We further interpret these characteristics to be associated with groundwater movement and storage related to the fractured/weathered zone within the bedrock. The integrated data also delineate the interface between the unsaturated sand and saturated sand-gravel that represents the groundwater water table. This study demonstrates the potential of combining several surface geophysical techniques for near-surface investigations, especially for hydrogeological prospecting.
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Geophysical Surveys for Improving the Hydrogeological Conceptual Models in the Semi-Arid Region of Valle Alto (Bolivia)
More LessSummaryGeophysical methods provide important information for mapping the subsurface. In semiarid areas, a detailed knowledge of the aquifers geometry can assist to a better management of groundwater. In the Valle Alto (Bolivia), which is a semiarid region, there is gap in the knowledge about the aquifer system geometry and hydrogeological processes. Local reports indicated problems in some wells such as very low production and bad water quality (i.e. high concentrations of salts). In order to improve the knowledge about the aquifer system and identify the depth of the layers with saline water, the geophysical methods Electrical Resistivity Tomography (ERT) and Time Domain Electromagnetic (TEM) were applied. The main objective of this study is to refine the hydrogeological conceptual models and to delimitate the depth and thickness of saline layers. The TEM soundings are performed in a grid of 150 m separation, which provide significant information for proposing 2-D models that displays the lateral/vertical extent of the saline layers. While the ERT surveys were performed across the Valle Alto in order to obtain long cross sections where the aquifer geometry can be exposed. With the refined conceptual models, further planning and policies can be proposed for protection and sustainable groundwater exploitation.
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New Processing Schemes for the Removal of Harmonic Noise from SNMR Signals in Complex Multi-Frequency Noise Environments
Authors T. Kremer, J.J. Larsen and F. NguyenSummaryIn SNMR surveys, the electromagnetic (EM) perturbations originating from powerlines infrastructures is often the main EM noise source that contaminates the SNMR acquisition signal. The removal of these harmonic signals is therefore critical to extract the data and carry out the study. To date, some techniques exist that give satisfying results in many cases, provided that the assumptions of a harmonic noise based on a single and constant fundamental frequency are valid. However, when these conditions are not met and the characteristics of the harmonic noise become more complex, these methods will fail to remove it correctly. In this paper, we show how such complex conditions will affect the frequency spectra of the SNMR acquisition signal. Then, we address the issue of a harmonic noise composition based on two different fundamental frequencies, by solving the non-linear optimization problem simultaneously for the two frequency values, using a 2D grid-search. Then we investigate the case where the fundamental frequency of the harmonic signal is not constant but variates within the signal duration. We expose a new approach based on the Nyman and Gaiser estimator, which, associated with a signal bootstrapping strategy, efficiently handles this situation.
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Supplementary Geophysical Survey in Urban Park for New Metro, Fælledparken, Copenhagen
Authors O.F. Nielsen and K. MartinezSummaryThe new metro line in Copenhagen, Cityringen, is currently under construction and is expected finalized in July 2019. As part of the preinvestigations urban geophysical site investigations were carried out including 13.9 km of seismic lines, 29 VSP (Vertical Seismic Profiles) as well as a geo-electrical survey covering Fælledparken, the largest urban park in Copenhagen (ca. 400,000 m2).
The purpose of the geoelectrical survey was to provide detailed hydrogeological information in the urban park area where only sparse borehole information was available.
A total of 68 criss-crossing DC-soundings were collected and interpreted using a Spatially Constrained Inversion (SCI). Due to this approach and detailed planning only 4 out of the 68 soundings were affected by electrical coupling effects from buried cables.
The results showed a 5–10 m thick secondary aquifer underlaid by approximately 10 to 35 m clay till indicating a minimal recharge from the secondary to the primary aquifer. Below the lower till the top of the limestone was seen dipping from approximately 25 m depth in the eastern part of the park to approximately 35 m in the western part.
The results contributed to a detailed hydrogeological understanding of the area and therefore a more realistic hydrological model.
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