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23rd European Meeting of Environmental and Engineering Geophysics
- Conference date: September 3-7, 2017
- Location: Malmö, Sweden
- Published: 03 September 2017
101 - 120 of 157 results
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High-frequency Surface Wave Measurements for the Investigation of Pavement Structure
Authors T. Inazaki, H. Kisanuki, K. Hayashi and S. ImamuraSummaryWe have developed a non-contact type surface wave measurement tool using arrayed microphones for the investigation of pavement structure. The tool comprises a pair of microphone arrays, each of which is composed of a total of 24 microphones aligned at 5 cm intervals. The microphone arrays are suspended 2 cm above the pavement surface in a sound insulation box cart. The air-coupled microphone arrays can detect imperceptible sounds of leaky surface waves masked by overwhelming shot coherent air wave noises. Field measurements demonstrated that the tool could observe the surface waves which showed clear dispersions in the frequency range from 40 Hz to 20 kHz. To confirm whether the dispersion characteristics derived from the observed sounds were valid, we conducted comparative measurements with high-frequency accelerometers adhered to the pavement surface. We had conducted field measurements using the accelerometers on pavements and successfully delineated 2D velocity structures of the pavements. The comparative measurements elucidated that the air-coupled microphone array tool provided basically the same dispersion curves as the surface-adhered accelerometers. In conclusion, the microphone array tool was capable for the continuous or 2D structural investigation of pavements.
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Sequential Phase-velocity and Waveform Inversion of Shallow-seismic Surface Waves - A Field Example for Bedrock Mapping
More LessSummarySurface waves are widely used to determine near-surface S-wave velocity structures. The individual surface-wave phase-velocity inversion and waveform inversion suffers from relatively low resolution and high ambiguity, respectively.
We propose to adopt a sequential phase-velocity and waveform inversion of surface waves to delineate near-surface materials. The phase-velocity inversion result is smoothed and used as the initial model for waveform inversion. We applied our method to a field data collected in Olathe, USA. Shape of the bedrock is clearly delineated in the inversion result. The sequential-inversion result nicely agrees with the borehole data, indicating relatively high reliability of it. This study shows that the sequential phase-velocity and waveform inversion can provide an effective way for high-resolution imaging of near-surface materials.
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Bedrock and Fracture Zone Delineation Using Different Near-surface Seismic Sources
Authors B. Brodic, A. Malehmir and C. JuhlinSummaryTo delineate the bedrock surface and a fracture zone intersected by a well at c. 50 m depth, a seismic survey was conducted using four different near-surface seismic sources. These were a 5-kg sledgehammer, a metal I-beam hit laterally, an accelerated weight drop and a prototype source tested for the first time called Udarnik. The Udarnik source has two hammers whose impacts are initiated by an electromagnetic force of the stable coil exciting its inner moving part. Two hammers separated by a distance of approximately 50 cm successively hit two contact plates mounted on the bottom of the source. The sweep length is adjustable and maximum 18 hits can be made per second. In this study, we compare the performance of every source used and present reflection seismic sections and tomography results from the high-fold (star-type acquisition was used) combined landstreamer and wireless recorder survey. Preliminary results indicate that bedrock was well delineated both on tomography results and stacked sections for all sources and some weak reflectivity is observed where the fracture zone is expected with most of the sources used showing the potential of the seismic methods for fracture zone imaging and near-surface characterization
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Joint Shear Wave Reflection Seismic and Geotechnical Drilling for Site Investigation at Tønsberg Hospital Area
Authors U. Polom, J.S. Rønning, G. Tassis and J. GelleinSummaryQuick-clays in Nordic countries seriously affect building settlements safely, especially essential infrastructure. The detection of safe building ground for top priority infrastructure buildings is therefore essential in areas prone to quick-clay liquefaction. In 2015 the administration of the central hospital of Tønsberg (Sykehuset I Vestfold, SIV), Norway, intended expansion towards an area prone to quick-clay. Former drilling investigation indicated a dubious bedrock topography below soil, with clay, silt, and anthropogenic infills estimated up to 25 m thickness. A dense drilling grid would be required to analyze the depth to bedrock for pile foundation, but could not be realized due to the building density and the installations at and below the surface. Since Ground Penetrating Radar (GPR) failed in the area due to high soil conductivity and seismic refraction did not provide the resolution required, high-resolution shear wave reflection seismic was suggested combined with a focused geotechnical drilling campaign, to expand the drilling results in the area. This unique experiment was realized in June 2016 and led to a comprehensive interpretation of the bedrock topography and the soil thickness above. It strongly improved the planning process regarding the foundation construction and reduce the financial risk compared to conventional foundation construction.
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Stiffness of a Fracture from AVO Inversion Incorporating Linear Slip Boundary Condition - Experimental Investigation
Authors S.H. Minato, R. Ghose and G. OsukukuSummaryCharacterizing fracture is important in order to understand how groundwater is transported and stored in fractured environments, to assess contaminant transport through fractures, as well to evaluate the mechanical behaviour of a fractured rock mass. In this research, we have investigated through careful laboratory experiments the amplitude versus offset (AVO) response of seismic reflections from a fracture. We use the linear slip boundary condition at the fracture and estimate the angle-dependent reflection response due to a single fracture. The observed angle-dependent reflectivity is inverted to obtain the fracture compliance and aperture. Two detailed laboratory experiments are performed - one using laterally homogeneous fracture and another using laterally heterogeneous fracture (partly air-filled and partly water-filled). Our results demonstrate that normal compliance (inverse stiffness) of a fracture can be quite accurately estimated from the AVO inversion of P-P reflected waves. It is also possible to obtain the non-zero tangential compliance. The existence of fluid in the fracture can be predicted. Distinction of the fracture infills and quantification of the fracture aperture are possible. This finding will be crucial for numerous new applications in civil and geotechnical engineering, hydrogeophysics, as well as in other areas of earth sciences and non-destructive material testing.
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Modelling Time-lapse S-wave Velocity Changes in an Unsaturated River Dyke Due to Water Infiltration
More LessSummaryUnderstanding the effect of saturation is important in assessing the failure mechanism on the land-side of a river-dyke due to rising water level in a river after heavy rainfall. S-wave velocity is controlled by soil suction and degree of saturation. Therefore, there is a possibility to estimate the unsaturated soil properties from the temporal changes in S-wave velocity. For this purpose, we model the temporal changes in S-wave velocity due to seepage of water in a dyke under rainfall. We propose a new approach for interpolating/extrapolating experimental data, in order to obtain shear modulus as a function of suction and confining stress. The seepage analysis of a river dyke under heavy rainfall shows that the temporal change in the S-wave velocity is determined more by the shear modulus than by the density. Furthermore, the S-wave velocity at shallow depths is more sensitive to seepage than the S-wave velocity at greater depths. These, together with the fact that a simple relationship exists between the shear modulus, suction and saturation, lead us to the new possibility of predicting shear modulus as a function of suction from the time-lapse S-wave velocity monitoring.
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Electrode Influence on Micro-ERT Field Dataset Subjected to Fire
Authors C. Verdet, C. Sirieix, Y. Anguy, R. Clément and C. GaborieauSummaryIn this paper, we presented an approach on how to consider the electrode effect on micro-electrical resistivity tomography data. The case study is set on an underground quarry wall under a fire. The used dispositive is made up of 24 rode-like electrodes that reach a depth of about half the electrode spacing. We demonstrate that at such a small scale, with a spacing of 4cm and for a homogeneous model, the effect is diminished with the presence of a superficial conductive layer. We suggested here a methods to do so: by modelling the half-space with complete electrode and by searching for an equivalent EEP depth. Such depth can then be implemented on the real data field for the inversion. We demonstrate here that the electrode effect is also lowered in case of a first conductive layer on field. We also showed that the fire created a first conductive layer and an intermediate layer from 3 to 15 cm that is more resistive than before the fire, where we had a two layers medium (conductive over resistive layer).
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Challenges in Identifying Karst Porosity in a Caledonian Nappe Setting Using Gravimetric Mapping
Authors T. Solbakk, C. Fichler, W. Wheeler, S.-E. Lauritzen and P. RingroseSummaryIn this work we measure gravity field variations over an area covering a known cave, and model the observed gravity signal utilizing observed cave rooms and incorporate density variations in the host rock. The aim of the modelling is to test hypotheses of hidden karst features and variations in the lithology introduced by karstification processes that significantly lowers the density.
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Estimating Time-lapse Near-surface Velocity Models Using Ground Roll from Coarsely Sampled 2-D Land Crooked Surveys
Authors C. Barajas-Olalde, A.J. Livers, S.A.S. Burnison, J.A. Hamling and C.D. GoreckiSummaryThe ability of the surface wave methods to estimate not only near-surface properties but also implicitly characterize the surface waves, which can be used to improve the ground-roll noise attenuation step in seismic data processing, makes this type of method almost a natural option for time-lapse land surveys. As part of a time-lapse project monitoring CO2 movement within a reservoir in the Powder River Basin in southeastern Montana, USA, we used a surface wave method to estimate time-lapse near-surface velocities from 2-D crooked surveys acquired using three-component seismic nodes and an accelerated-weight-drop seismic source. As was expected, the baseline and monitoring surveys were affected by high levels of coherent and incoherent noise typical of active oil fields. Using a time-offset windowing scheme on individual surface wave events, we were able to focus the ground roll energy recorded in our sparse and irregular acquisition array and estimate near-surface velocity models. We compared these models with near-surface velocity models derived from first-break traveltime tomography. Despite the different resolution of the models, it was possible to confirm the good quality of the velocity models estimated with the surface wave method and identify the geologic formations in the near-surface of the study area.
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Monitoring of the Tidal Response of a Sea Dike Using Sea Waves as Seismic Noise
Authors M. Le Feuvre, P. Côte and A.J. JoubertSummaryAmbient seismic noise has long been considered as a nuisance for the analysis of seismograms. For some years now, it has shown a great potential for monitoring various geological objects with high accuracy. Here, using ambient seismic noise generated by sea wave impacts, we reconstruct surface waves propagating within a sea dike body, and monitor the tidal response of the structure as the water level changes. Velocity and attenuation variations are obtained respectively from delay and spectral amplitude measurements. We localize lateral areas of variation which temporal evolution is correlated with water level. These variations are interpreted as the consequence of water infiltration inside the structure.
The proposed method may provide a solution for detecting internal erosion, generally only observed at late stage by visual inspection.
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On the Accuracy of Seismic Wavefield Spatial Gradients
Authors N. Allouche, E. Muyzert and P. EdmeSummaryRotation measurements have found applications in various fields of geophysics ranging from near-surface archaeological mapping to large scale upper mantle tomographic inversion. In the absence of a cost-effective sensor sensitive over the typical seismic frequency bandwidth, rotation measurement can be approximated by finite-differencing the response of the vertical particle velocity over a short distance. Accurate estimates of the horizontal gradient of the vertical wavefield can be obtained when perturbations associated with the measurement are minimised. These perturbations can be sensor related, for example geophone sensitivity and natural frequency, and/or deployment related such as tilt and coupling errors. We developed a “sensitivity” chart ranking these perturbations according to impact. We recommend the use of geophones with low variance in sensitivity, deployed as vertical as possible (less than 5 degrees tilt error) at highly accurate position (error in the order of centimetres). Moreover, the distance between the two receivers should be about 1/8 of the minimum wavelength in order to reduce the error due to finite-differencing.
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Electrical Resistivity Tomography (ERT) for Time-lapse Mapping of in-situ Self-sustaining Treatment for Active Remediati
Authors L.M. Trento, P. Tsourlos, J.I. Gerhard, M. McMaster and A. SimsSummaryThis study evaluates the use of time-lapse Electrical Resistivity Tomography (ERT), a surface-based geophysical technique, as a complementary method to monitor the first full-scale application of Self-sustaining Treatment for Active Remediation (STAR) smouldering technology in real-time. A shallow treatment region (with ignition point at 2.4 meters below ground surface) was evaluated at an industrial site with coal tar contamination, and resistivity surveys were conducted before, during and after treatment to provide insight into the spatial extent of the STAR smouldering reaction, groundwater and gas mobility, influence of air flow and the capture zone of vapor extraction system. Results presented shows, qualitatively, a correlation between gas generated by smouldering and resistivity increases in the subsurface. Overall, the resistivity surveys provide continuous mapping of the entire treatment region, and the high-quality imagery show that ERT is a promising to evaluate in-situ STAR.
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Cave-surface Electrical Resistivity Tomography in “Castello di Lepre” Karst System (Marsico Nuovo, Southern Italy)
Authors E. Rizzo, M. Guerriero, E. Gueguen, L. Capozzoli, G. De martino and F. PercianteSummarySeveral geophysical methods, electrical resistivity tomography (ERT), gravimetric prospecting (G), Ground Penetrating Radar (GPR) and seismic methods (S), are commonly applied to characterize karst cave ( Beres et al., 2001 ; Martínez-Moreno et al., 2015 ; Bermejo et al., 2016 ; and references therein). In detail, the geophysical approach provides information on cave geometry and subsurface geological structure.
Several karst areas have a complex deep system, but the geophysical methods, such as geoelectrical ones, offer partial deep information due to a low-resolution. Therefore, this work would like to introduce a new 2D and 3D geoelectrical approach, that we call “cave-surface” ERT, which define an improvement of the deep resolution comparing to surface ERT. Finally, an application of this new methodology in the karst cave Castello di Lepre (Marsico Nuovo, Basilicata region, Italy) is described.
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Streaming Current Signals Associated with Pumping Test Experiments
By K.V. TitovSummarySelf-Potential (SP) signals of the electrokinetic origin can be detected in the course of pumping test experiments. SP signals provide useful information about space heterogeneity of hydraulic properties of aquifers. SP signals are recorded with multi-voltmeters and a number of non-polarizing electrodes located around the pumping well to compliment information provided by drawdown measurements in pumping and observation wells. However, to the best of our knowledge, the streaming current (SC) has never been measured at pumping test experiments. We recorded the current in the course of a pumping test of shallow low-permeable aquifer using an ammeter connected to two electrodes grounded to the pumping interval of the well and to the ground surface. The recorded signal contains two components: (i) of electrochemical origin, produced by different physical-chemical properties at the interfaces of the two measuring electrodes; and (ii) of electrokinetic origin, produced by SC coupled with the groundwater flow. We decoupled the SC signal from the total signal. We found a correlation between SC and the flowrate, and we obtained the respective best fit. Based on the fit slope and the hydraulic conductivity value we calculated the SC coupling coefficient value, which is in reasonable agreement with published data.
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Spatio-temporal Monitoring of Heat Storage in a Shallow Aquifer Using Electrical Resistivity 4D Imagery and DTS
Authors N. Lesparre, T. Hermans, F. Nguyen, T. Kremer and T. RobertSummaryThe design of groundwater heat pumps requires a good understanding of the aquifer and heat flow conditions. Issues of short-circuit or recycling between cold and hot wells have to be carefully considered. Surface geophysical methods allow monitoring subsurface processes without additional perturbations of the medium. Within available methods, the electrical resistivity imagery (ERI) applied in time-lapse (TL) is appropriate. Here, we monitored with ERI and distributed temperature sensors (DTS) a heat plume propagation during an experiment of hot water injection in a shallow aquifer. DTS and TL ERI measurements acquired from two boreholes provide a local estimate of the heat propagation through the medium. TL ERI were also performed from a grid at surface to follow the 3D plume shape formation and evolution through time. The different complementary data validate the potential of surface TL ERI for monitoring in 3D the behavior of shallow heat plumes. ERI highlight the heterogeneity of the aquifer by distinguishing regions with higher or lower hydraulic conductivity. In the higher hydraulic conductivity zone, the heat might be evacuated through water flow, while in the lower hydraulic conductivity area heat storage is achievable. Thus, in that last region the plume temperature decreases progressively with time.
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Integrating Small-scall Electrical Resistivity with Multi-compartment Sampler Techniques to Monitor Solute Fluxes
Authors E. Bloem, Z. Koelma, P. Fernandez and H.K. FrenchSummaryUnderstanding water and contaminant transport in the unsaturated zone is highly challenging due to spatial variability. Sufficient characterization of spatial and temporal distribution of contaminant transport requires measurements of solute fluxes at multiple locations with high temporal resolution. Solute monitoring is often limited to observations of resident concentrations, while flux concentrations govern the movement of solutes in soils. We developed a new instrument, which combines multi-compartment sampling with electrical resistivity techniques, to monitor solute fluxes.
The multi-compartment electrical resistivity sampler (MCERS) is capable of measuring downward solute fluxes at a high spatio-temporal resolution under natural conditions. The instrument consists of 100 cells (each 31.5 by 31.5 mm). For each cell, the amount of drops passing through every 5 min is recorded, monitoring directly water fluxes. Electrodes are mounted at each corner of each cell, making it possible to perform small-scale ERT measurements above each cell while water percolates through, thereby monitoring solute fluxes. By using different electrode couples, the setup can also be used to image above the MCERS. The percolate is collected from each cell and can be repeatedly extracted for additional chemical analysis while leaving the instrument buried in situ, facilitating prolonged operation times.
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Using Geophysical Hard Data to Enhance the Reliability of Hydrological Models
Authors G. De Schepper, C. Paulus, J. Molron, P.-Y. Bolly, T. Hermans and T. RobertSummaryAppropriate design of geophysical experiments combined with common hydrological measurements offer opportunities to use geophysical data as hard data in hydrological models, regarding their conceptualisation or their calibration. Two study sites located in Wallonia, Belgium, were investigated. In the first case (fractured limestone aquifer), streaming potentials were linked to piezometric measurements, allowing us to better conceptualise the local groundwater flow model and calibrate it. In the second example (alluvial sandy aquifer), the use of 4D electrical resistivity tomography and temperature measurements appeared to be a reliable methodology to predict heat storage and recovery cycles in hydrological models with a better constrained calibration process.
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Geostatistical Regularization and Cross-gradients Operators for Joint Inverse Problems on Irregular Meshes
Authors C. Jordi, J. Doetsch, T. Günther, C. Schmelzbach and J.O.A. RobertssonSummarySolving joint inverse problems requires appropriate regularisation and coupling operators. In particular, when the inversion is performed on irregular meshes where cell sizes vary throughout the domain, the operators should be designed to be as independent of the discretisation as possible. We define regularisation operators for inversions on irregular meshes based on a geostatistical correlation model. The same correlation model is combined with a neighbourhood approach for gradient-calculation to produce cross-gradient operators with a geostatistical footprint. We apply geostatistical regularisation operators to a 3D synthetic cross-hole ERT example and show how they can improve the resulting tomogram compared to anisotropic smoothing. In a synthetic study, we show that the geostatistical cross-gradient operators are much less dependent on the discretisation and improve the accuracy of the calculated cross-gradient field. The geostatistical regularisation and cross-gradient operators will be the base for a joint inversion formulation on irregular meshes.
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Towards 3D Probabilistic Inversion with Graphcuts
Authors N. Linde, M. Cardiff, G. Mariethoz, J. Bradford and G. PirotSummaryFor appropriate uncertainty quantification in hydrogeological applications (e.g., contaminant plume forecasting), it is essential to infer subsurface models that feature geologically realistic geometries and property contrasts. Recently, an efficient multiple-point statistics probabilistic inversion approach, with model proposals based on graph cuts, has been shown to provide posterior model realizations that honor pre-defined geometrical shapes and property contrasts. It has been tested for both synthetic and field examples involving crosshole ground-penetrating radar. Here, we present the approach and proceed with initial findings on how to extend this method to 3D and hydraulic tomography data. Improvements and modifications in the Markov chain Monte Carlo algorithm are proposed that allow for appropriate acceptance and convergence rates. We also discuss possible ways to circumvent long computing times, for example, by including physical approximations and machine learning techniques, or to focus on global optimization rather than Bayesian posterior sampling.
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Processing and Modeling of Time Domain Induced Polarization Data
Authors A. Nivorlis, P. Tsourlos, G. Vargemezis, G. Tsokas and J.H. KimSummaryOver the past years, the leaning of the research over the Induced Polarization method has been focused on the Time-Domain Induced Polarization and the calculation of the spectral content that is embedded into the measurements. To catch up with this trend we developed a tool for modeling real 3D spectral Induced Polarization data, based on a model (e.g. the Cole-Cole model) given the subsurface distribution of the electrical properties and the model parameters. To evaluate the results the central line of each model was extracted and inverted by introducing smoothness constrain not only in space domain but also in channel domain by inverting the available channels simultaneously. The inversion results can be used to form the decay curves for each model cell. An optimization tool for fitting the model that best describes the curve of each cell was developed, to calculate the spectral information using the Particle Swarm Optimization algorithm. The results with synthetic, experimental and real data suggest that the presented approach is efficient and relatively robust given the complexity of the SIP data.
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