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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
21 - 40 of 134 results
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3D-Underground Seismics in crystalline, salt and clay rocks
Authors R. Giese, S. Lueth, H. Richter, B. Wawerzinek, K. Jaksch and R. EsefelderSummaryThe modular seismic acquisition system at the GFZ is basically suitable for underground exploration in crystalline, salt and clay rocks. Special features of the application result from the different properties of the media such as degree of heterogeneity, damping, anisotropy and their interaction with the excavation zone (EDZ) induced by the cavity (drifts, tunnels). This manifests itself in different frequency damping behaviour for compressional, shear and surface waves, which require a specific use of impulse and vibration sources at the joint depending on the task. Seismic borehole measurements can contribute to a significant improvement of the resolution.
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Complex Electrical Conductivity of Kimberlite
Authors K. Titov, V. Emelianov, V. Abramov and A. RevilSummaryWe detected large values of the complex electrical conductivity of four kimberlite samples based on laboratory measurements carried out when saturating the samples with brines with the electrical conductivity ranging between 200 and 30 000 S/cm. Both the real surface conductivity and the quadrature conductivity were found much larger than that of common volcanic rocks and sediments. Based on measurements of petrophysical properties of these cores as well as on optical microscopic images we argued that this anomalous conductivity is explained by highly conductive and polarizable clayey matrix, which shows an abundance of the saponite, a mineral of smectite group. These new data confirms usability of electrical (resistivity, and especially Induced Polarization) and electromagnetic methods in kimberlite exploration.
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Delay Effects in Current Switching-off and Their Manifestation in the Early Time Tem Response
Authors M. Sharlov, N. Kozhevnikov and T. PestyurinSummaryIn the near-surface TEM sounding method, it is desirable to measure the transient response starting from the earliest possible time. This requires the current in the transmitter loop to be switched off quickly, which necessitates working with a low transmitter current. In this case, the loop manifests itself as a symmetric combination of two transmission lines, and the current at different points of the loop perimeter is turned off at different times. We present transient electromagnetic responses measured with offset receiving loops, which vary depending on the place where a pulse current source is connected to the transmitter loop. The survey was carried out on the ice cover of Lake Baikal. It has been shown that at early times a delay in turning off the transmitter loop current affects transient voltage induced in offset receiving loops unlike the central loop response, which remains the same,. This fact which should be taken into account when interpreting early-time TEM data .
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Impedance Network Modelling to Simulate the Chargeability of Sand-Pyrite Mixtures
Authors A. Maineult, G. Gurin and K. TitovSummaryInduced polarization method has long been used in mining geology due to its high sensitivity to the presence of disseminated ores. However, it is important to link the measured chargeability to the ore content. This can be done experimentally: for instance, Gurin et al. (2013) studied the evolution of the chargeability with ore content for sand-pyrite mixtures. Here we propose a new numerical approach, based on the resolution of the complex electrical problem using networks of complex impedances. We show that the predicted chargeability obtained by our methodology is very close to the observed one.
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Estimation and Confirmation of Electromagnetic Attenuation and Resistivity of MáTra Mountains Rock
Authors I. Lemperger, A. Novák, P. Ván, V. Wesztergom, P. Lévai, Á. Kis, S. Szalai and J. MlynarczykSummaryElectromagnetic variation’s amplitude decreases exponentially in conducting media. Higher conductance results higher attenuation of the electromagnetic variations vs. depth.
In this presentation an investigation is introduced which aims to estimate the electromagnetic attenuation coefficient and mean resistivity of the near surface rocky layers of the Mátra mountains, Hungary. The whole investigation consists of a confirmation, based on high frequency magnetotelluric sounding, too.
The direct estimation of the attenuation coefficient is performed by means of a signal excited by the global thunderstorm activity in the ground-ionosphere cavity, the so called Schumann resonance. The method basis on the direct determination of the ELF signal’s amplitude decay between a surface and the corresponding subsurface site. In case of parallel recording at the two sites, the attenuation could basically be computed from the rate of the variation’s spectral amplitude at certain frequencies.
As the surface and the subsurface observation could not be performed at the same time due technical reasons, an ELF reference observation site had been involved in the investigation. The attenuation can then be estimated by comparison of the transfer functions related to the ‘subsurface station - reference site’ and the ‘reference site – surface station’ relation.
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Characterization of Undercover Karst Morpholoies by 3d Geostatistical Modeling of Ert Data
Authors C. Verdet, C. Sirieix, A. Marache, J. Riss and J. PortaisSummaryThe study of undercover karst systems and their morphology is difficult to assess using punctual geotechnical tools. Here we present a method using a geostatistical 3D reconstitution based on electrical resistivity tomography (ERT) data to assess the high variability of the karst. We applied this method on the Lascaux hill site (south-west France) in order to: (1) detect karst morphologies under a detrital cover (pinnacles, shelves, karrens); (2) redefine the limit between two limestone formations; (3) define the geometry of the limit between two domains: limestone and detrital materials.
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Electrostatic Profiling and Mapping of Electrical Resistivity and Dielectric Permittivity in an Urban Context
Authors C. Schamper, A. Tabbagh, S. Flageul, C. Benech, Q. Vitale, C. Benjamin, M. Dabas, C. Parfant and L. Perruchon-MongeSummaryElectrostatic profiling and mapping have been carried out in an urban context with two different geometries allowing the prospection of the first 3–4 meters. Despite the larger size of the prospection devices, compared to ground-penetrating radar (GPR), it has been successfully applied at almost all GPR locations in parallel within the same few days’ survey. At an intermediate operating frequency of 15 kHz, the method provides information on both electrical resistivity and “low frequency” effective dielectric permittivity (mainly linked to polarization in clayey materials), and is therefore complementary to the GPR method primarily used in urban prospection.
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Monitoring Underground Heat Storages by Means of Borehole Electrical Resistivity Tomography. A Model Test.
Authors S.L. Fischer, E. Erkul, M. Gräber, B. Wang, S.A. Al Hagrey, S. Bauer and W. RabbelSummaryUnderground heat storages are one out of several possibilities to store energy on a large scale. Using the subsoil as such a storage, it is necessary to monitor the expansion of the temperature field and to look out for possible leakages. In this study, geothermal experiments are carried out on an idealised meso-scale sand pit named "GeoModel". The sample is heated up and cooled down by circulating hot and cold water in a Borehole Heat Exchanger (BHE). Simultaneously, one half of the sand body is monitored by borehole electrical resistivity tomography including several cross-hole and in-hole configurations. Additionally, the temperature of the other half of the sample is measured by numerous sensors at different depths and distances to the BHE. According to theory, the results show a strong anti-correlation between electrical resistivity values and measured temperatures. Repeated heating and cooling cycles demonstrate reproducibility of the investigations. A leakage of the BHE, which occured during an active cooling phase, could be detected at an early stage. Inversion results are shown to be suitable for a qualitative observation of three-dimensional temperature fields in the sand pit.
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Electromagnetic-Interferometric Direct-Wave Suppression for Detection of Shallow Buried Targets With Gpr
Authors F. Balestrini, D. Draganov, D. Ngan-Tillard and F. HansenSummaryOne of the most common methodologies utilised for shallow geophysical and archaeological investigations is ground penetrating radar (GPR). GPR is a popular tool due to its non-invasiveness and capacity for real-time data analysis, widely used for the study of near-surface structures and the detection of buried targets. However, the direct wave propagating along the surface can completely cover the shallowest targets and, thus, needs to be suppressed. Electromagnetic interferometry (EMI) can be utilised to retrieve an estimate of the direct wave. Subsequently, this estimate can be adaptively subtracted from the field data to effectively eliminate the direct wave. We apply this methodology to a GPR dataset acquired at the Jewish Cemetery in Naaldwijk, The Netherlands, in August 2020. The main objective of the survey is to identify locations of possible old, buried tombstones. After applying the proposed methodology, our results show the direct wave is well suppressed. The earliest diffraction events that were covered by the direct arrival are visible after this step. The survey revealed anomalies where buried tombstones might be expected. This work serves to demonstrate the applicability of the GPR survey to detect shallow targets and the utilisation of EMI for direct-wave suppression in GPR data.
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Results of Performance Tests of Electrodynamic Vibratory Seismic Sources
Authors T. Burschil, H. Buness, P. Leineweber and U. PolomSummarySmall-scale electrodynamic vibratory seismic sources (micro-vibrators) are efficient devices for near-surface investigations up to several hundreds of metres depth. Size, resolution, signal repeatability, and production rate are superior to commonly used source systems in this engineering depth range class. Compared to hydraulically driven mini-vibrators, which are magnitudes beyond in price class and logistics effort required, the micro-vibrators benefit in performance of field operation and produce less near-offset source generated noise, demonstrated by the ELVIS micro-vibrators. Even without an expensive vibratory source similarity control system, which is required for hydraulic vibrators, the simultaneous operation of several electrodynamic micro-vibrators excite improved signal energy into the subsurface and increases the signal strength significantly. A test using four source devices simultaneously shows up to 50% increased signal amplitude strength and higher signal-to-noise ratio compared to a single device.
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Reprocessing of Reflection Seismic Data to Highlight Near-Surface Glacio-Tectonic Deformations
Authors H. Buness and T. BurschilSummaryA small scaled 3-D P-wave survey (0.018 km2) in an overdeepened valley of the Rhine Glacier (Tannwald Basin) revealed glacio-tectonic structures at depths of 20–50 m, interpreted as cuspate-lobate folding. Previous 2-D lines, processed using prestack depth migration (PSDM), only displayed a region with poor imaging quality. A careful reprocessing of the 2-D data using common reflection surface (CRS) and prestack time migration (PSTM) now gives an image matching the high-resolution of the 3-D data. The glacio-tectonic structures exhibit very low velocities that were not accounted for in the PSDM processing and led to a distortion of deeper layers. Based on the PSDM migrated 2-D data, we speculated about a deep-reaching tectonic fault zone, which is not probable anymore in the light of the reprocessed data.
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Characterization of a Coastal Area From Integration of Resistivity and Active Multicomponent Seismic Data
Authors F. Da Col, F. Accaino, G. Böhm, S. Picotti, M. Giorgi and F. MeneghiniSummaryWe present an in-depth characterization of the shallow subsurface in a coastal area of the northern Adriatic. In this research, we integrated the results from the processing of active seismic data (P-, SH-, SV- wavefields, and surface waves) with those obtained from electrical resistivity tomography. Seismic data analysis provided stacked sections as well as velocity profiles from first-break tomography. Furthermore, surface-wave analysis was performed to obtain a S-wave velocity model.
The results show a highly heterogeneous subsurface, due to changes in water saturation, as well as in the properties of the sediments. This is consistent with the geology of the area, characterized by sand deposits and clay sediments.
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Preliminary Results From Real and Synthetic Data Using the Masw-Dual Streamer (Ds) Technique
Authors H. Hamdan, G. Kritikakis, M. Harb and A. VafidisSummaryThe preliminary results of a new proposed technique for the data acquisition of the 2D MASW method is discussed in this extended abstract. This technique aims at increasing the fieldwork productivity of the 2D MASW data acquisition significantly. It is estimated that this technique may increase the productivity of the filed work of more than 70% than the traditional land-streamer towed by truck technique. The basic idea is to perform two 2D MASW profiles simultaneously for each run of the truck, employing two land streamers, two seismic recording instruments, but only one seismic source for both receiver arrays, located in the middle of the two profiles. This work aims to investigate the applicability of the proposed acquisition technique and the accuracy of the acquired data, using real data taken from two different sites as well as synthetic seismic data. The dispersion curves and S-wave profile comparison between the traditional and the proposed technique for the real and synthetic data was found to be quite encouraging. The proposed technique manages to reproduce the S-wave profiles resulted from the conventional MASW deployment, in the framework of an acceptable, for the engineers, error. Further basic research concerning the MASW-DS assumptions is considered necessary.
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Assessing Groundwater-Citarum River Interaction and Groundwater Contribution to Flooding
Authors A. Ramdhan, A. Arifin and R. SuwarmanSummaryFlooding is one of the main problems in the Upper Citarum Watershed. Though it is generally accepted that it is caused by the increase of runoff during high precipitation, the contribution of groundwater discharge to the river should also be considered. However, its contribution is still little known or quantified. As a case study, groundwater and Citarum River interaction from Kertasari to Baleendah Sub-districts in West Java, Indonesia, was assessed using the SWAT-MODFLOW model. The objectives of this study are (1) to quantify the interaction between them both spatially and temporally and (2) to estimate groundwater discharge contribution to flooding. The results show that the groundwater and Citarum River interacted mostly as gaining streams with discharge rates ranged from 0 to 9,872 meter cubic per day. However, in some segments, they interacted as losing streams. Their interaction changed both spatially and temporally. The estimated groundwater discharge contribution to flooding in the Citarum River watershed is small, 2.4 to 4.1%, which confirms that the flooding is mainly caused by the high runoff. However, the small groundwater discharge contribution should still be considered in further analysis, such as flood modelling.
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Active versus Passive Seismic Monitoring of Near-Surface Arctic Thawing
Authors H.M. Stemland, T.A. Johansen, B.O. Ruud and R. RomeynSummaryTemperatures are rapidly increasing in the Arctic, but the actual impact of surface warming on the degree of freezing of permafrost is uncertain. We conducted active and passive seismic experiments on permafrost in Adventdalen, Svalbard, several times in 2019. Active seismic data can be generated at any given time and have strong signals, but the drawback is the generation of noise during acquisition. Passive seismic acquisition makes little noise and makes it feasible to acquire data over a longer period, but the signal may be weak and vary in strength throughout the year.
The resulting seismic data show strong surface waves. High-quality dispersion images can be generated from the active data at any time, and from the passive data whenever frost quakes occur. The dispersion curves from active experiments can be followed to higher frequencies than those from passive experiments, whereas the latter type of data can easier distinguish between different modes. A seasonal time-lapse effect suggests that surface seismic data may be useful for monitoring permafrost degradation, both on a seasonal scale and on a longer-term scale due to climate change.
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Investigation of Magnetic Susceptibility Effect on NMR Measurement: Case of the Volcanic Rocks
Authors N. Chibati, Y. Geraud and V. NavelotSummaryNuclear magnetic resonance (NMR) is a powerful technique for reservoir characterization. It has been largely used in the petrophysical evaluation and pore structure study of the sedimentary formation. IN contrast in the case of the materials with high magnetic susceptibility like volcanic rocks, the NMR measurement can be affected, resulting on the underestimation of the porosity from the NMR data. Here we investigate the relationship between magnitude of the internal magnetic field gradient (G) of the volcanic material and its relationship with the NMR porosity errors and the magnetic susceptibility. The obtained results shows that G in the studied samples increase (up to 217 T.m-1) due to the magnetic susceptibility increasing and induce NMR porosity error increasing.
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Multiparameter Anisotropic First-Arrival Seismic Tomography of Acoustic Laboratory Data in Carbonates
Authors M. Salcedo, S. Garambois, D. Brito and F. SanjuanSummaryActive seismic transmission data have been performed in a block of carbonates with a piezoelectric source and a laser vibrometer receiver leading to a full illumination dataset. From the picked traveltimes, the apparent velocity distribution according to angle highlighted the necessity to consider the non-point feature of the source. An appropriate source discretization leads to an apparent velocity pattern that can be approximated with an elliptical tilted transverse isotropic model. Such a model is then used in a multiparameter tomography inversion, which leads to about 7% of velocity anisotropy located in a lower velocity region of the block, interpreted as a possible fractured band.
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Regularized Gauss-Newton Iterative Scheme Applied to Shallow Subsurface Imaging
Authors Q. Didier, S. Arhab and G. Lefeuve-MesgouezSummarySubsurface imaging is tackled as a nonlinear inverse problem, since no approximation is considered on the interaction of the incident wave with the medium. A regularized Gauss-Newton iterative scheme is used for the reconstruction of the relative dielectric permittivity and electrical conductivity. The impact of the wavelength on the spatial resolution in the reconstructed medium is studied. For a given wavelength, the proposed iterative scheme is tested in terms of convergence speed and robustness to noise.
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3D image of the subsoil from complementary seismic methods
Authors M. Saade, A. Dechamp and S. RobertSummaryThis study consists of a geophysical survey, aiming to characterize the soil from a mechanical point of view by determining seismic velocities and to propose a geomechanical soil model over the first 50 meters. The survey consists of complementary seismic methods:
- Passive tomography using ambient seismic noise cross-correlations (Sissterra method, Sixense)
- H / V method
- Seismic refraction and MASW
The results of the different methods are coherent with each other and with geotechnical data obtained from boreholes. The survey allowed a complete geophysical investigation down to 80m and the highlight of the main geological features. In particular, the results of the passive tomography and H/V highlight a deepening that could be related to a sedimentary paleo-valley. The results from the active and passive seismic methods show a good correlation in velocity contrasts. They mainly indicate a horizon with strong lateral variations in Vp and Vs, probably related to water level and to the base of lithological contrast, presenting unstructured passages in the form of Quaternary paleo structures. The 3D measurement made it possible to delimit the most probable locations of these structures and highlight a velocity inversion at depth, probably due to clay passages.
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Geophysical investigations of a landslide to interpret the distortion of a railway tunnel
Authors J. Lataste and J. BruneauSummaryThe Aspe Valley (Pyrénées Atlantiques, France) is an old glaciated valley of the Würm period. Steep rock escarpments alternating with slipped less marked slopes are presented along the valley. The Peilhou rock slide is an example of gravitating instability related to glacial erosion. A consequence is the deformation of a railway tunnel, unexploited and instrumented since 1979. Geological and geophysical approaches allow observations to be correlated to disorders affecting masonry, then to interpret sliding mode of the slope. Schist and sandstone basement is fractured, folded and partly covered by moraine. The slide affects primarily a rock material, and mobilized superficial debris. Various zones are distinguished according to their geomorphological, geological and geomechanical behaviours. Use of electric profiles allows the in-depth extension of the surface observations, and complete investigations. These zones testify to the structural heterogeneity of the rock mass, in conformity with deformation observed of the railway tunnel.
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