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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
41 - 60 of 157 results
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Detailed 3D Geological Modelling at a Contaminated Stream Using Geophysical, Geological and Chemical Data - A Challenge
Authors I. Moeller, P.K. Maurya, G. Fiandaca, N. Balbarini, V. Rønde and A.J. KallesøeSummaryLocalizing sources of contaminated groundwater discharging to streams may be a challenging task. A detailed geological model of the site is required for simulating the groundwater flow and the contaminant transport, in addition to chemical and hydrological investigations.
In the town of Grindsted, Denmark, contaminated groundwater discharges to a stream. Several contaminated sites are present in the area, where chemical waste from a nearby chemical factory was deposited. In order to understand the hydrological flow and transport of contaminants controlled by a few mostly continuous clay and lignite layers in an otherwise sandy environment, a detailed 3D geological model is constructed. The model integrates all available data including DCIP and EMI surveys, lithological logs from new and existing wells, hydraulic head and pore water electric conductivity data. This study addresses the challenges met while constructing the 3D geological model in an area, where inorganic contaminants are highly affecting the geophysical data.
The model ambiguities related to interpretation of clay layers versus contaminated sand layers in the DCIP resistivity sections as well as resistivity/chargeability-thickness equivalences are managed by integrated interpretation of a large amount of different geophysical, geological, hydraulic and chemical data.
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Probability of Sensitive Clay from AEM Data
Authors A.O.K. Lysdahl, A.A. Pfaffhuber, H. Anschütz, Kr.K. Kåsin and S. BazinSummaryIdentification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution Airborne Electromagnetics (AEM). However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.
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Combined Geophysical Surveys to Investigate the Cause of the Rainfall Induced Shallow Landslide
Authors Y. Okada and C. KonishiSummaryWe have conducted the seismic and resistivity surveys at the slope where the shallow landslide induced by heavy rainfall occurred in September, 2015. Resistivity profiles clearly discriminates the unsaturated top soil as higher resistivity zones. We infer the groundwater pathway beneath the top soil from the low resistivity zones. S-wave velocity is good matched with the thickness of the top soil measured by the cone penetration test. At the survey line along the slope parallel to the landslide, the low S-wave velocity and high resistivity has detected within the projected interval of the adjacent landslide. Similar geophysical properties are observed at the survey line above the landslide, which is perpendicular to the slope. These characteristics geophysical properties indicate the weak point that initiates the shallow landslide in the slope.
The combination of the two geophysical surveys helps us to infer the mechanism of the rainfall induced shallow landslides and it is also useful to identify the vulnerable points for the initiation of the shallow landslide in the future.
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Towards a Time Monitoring of the Chalk Aquifer (Beauvais, North of France) Using the Magnetic Resonance Soundings
Authors P. Lutz, J.F. Girard, L.Z. Zouhri and P.E. MeurantSummaryIn the department of Oise (Picardie, France), the agricultural activities are essential and require a good management of water resources. It is therefore essential to improve the knowledge on the chalk aquifer. This is the reason for which the UniLaSalle Institute has a hydrogeological experimental site, composed of wells and piezometers allowing to characterize the chalk aquifer by adopting a multidisciplinary approach. This approach is based on hydrogeological measurements, near surface geophysics, logging, and Magnetic Resonance Soundings (MRS). The present paper, based on the MRS 2016–2017 campaign, deals with the time monitoring of the chalk aquifer, consisting in repeating each month and a half, four MRS. These are located north of the UniLaSalle hydrogeological experimental site, where the measurements conditions are suitable, unlike near the piezometers and wells. The results, consisting in curves of hydrogeological parameters (water content, relative permeability and transmissivity) versus depth up to 90m, improve the knowledge on the chalk aquifer properties and there time variations. However these results requires a calibration by pumping tests to estimate the value of the calibration coefficient Cp. The difficulty resides in the identification of a suitable site, both for MRS (low noise level, stable magnetic field) and for pumping tests.
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Surface Nuclear Magnetic Resonance with Compact Multicomponent Receivers
Authors M.A. Kass, T.P. Irons, A. Behroozmand, D. Grombacher and B.R. BlossSummarySurface nuclear magnetic resonance surveys are a valuable technique for investigating hydrologic properties in saturated aquifers, in the vadose zone, fractured reservoirs, and in permafrost settings, to name a few. Standard field measurements typically consist of a coincident large transmitter and receiver loop; while this approach has benefits with respect to field deployment and a smooth depth sensitivity function in 1D, there are numerous advantages to a compact, multicomponent receiver. Here we discuss these advantages, focusing on the imaging kernel for inductive receiver loops in all three principal directions. Additionally, we describe the imaging kernel in three dimensions and show the added spatial resolution available by a mobile three-component receiver. This analysis provides a first step toward proper survey design for subsequent 3D inversion for hydrologic properties.
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Modelling Relaxation During Pulse Effects for the Complex Inversion of Surface NMR Data; Gaining Insight into which Rela
Authors D. Grombacher and E. AukenSummaryA shortcoming of the standard surface nuclear magnetic resonance measurement, the free-induction decay, is that the meaning of the signal’s time-dependence is uncertain. Ideally, the parameter describing the time-dependence of the FID, called T2*, carries a strong link to the geometry of the pore space. However, in the presence of background magnetic field inhomogeneity a second mechanism contributes to T-2* potentially obscuring the link to pore geometry. To improve the understanding of which mechanism controls T--2*, an approach involving direct modelling of relaxation during pulse effects is proposed. Numerical studies are presented to demonstrate that the complex inversion of surface NMR data provides the sensitivity to gain insight into the magnitude of T-2 from only FID measurements. Multiple inversions are performed for plausible T-2, given the observed magnitude of T2*, and inversions providing satisfactory data fit can help constrain T2. Relaxation during pulse effects are also shown to be a contributing factor to difficulties describing the signal phase for complex inversions of surface NMR data.
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A Multichannel, Low Noise Surface NMR Receiver System with Wireless Connections to Signal and Reference Coils
Authors L. Liu, D. Grombacher, E. Auken and J.J. LarsenSummarySurface NMR holds great promise as a tool in groundwater measurements due to its unique direct sensitivity to water, but the method currently suffers from a number of drawbacks which limits its widespread applicability. Among these drawbacks are a low signal to noise ratio which limits the use of the method in many places of interest and a low production rate which makes the method costly in field campaigns. Hence there is a need for research further advancing the technology. In this paper we report on the development of a new multichannel, low noise surface NMR receiver system with wireless connections to reference coils. The receiver system works as a completely independent add-on to existing transmitter systems and consists of a number of independently operated data acquisition boxes connected with WiFi and synchronized by GPS. The internal electronic noise level of the system is 1.2 nV/sqrt(Hz). The timing jitter between data acquired in different boxes is less than 100 ns.
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Aquifer Imaging Using 2D Magnetic Resonance Tomography with Structural Constraints from GPR
Authors C.D. Jiang, J. Igel, R. Dlugosch, T. Günther and M. Müller-PetkeSummaryMagnetic resonance tomography using elongated transmitter and in-loop receiver arrays (MRTetra) is designed to efficiently image the two-dimensional aquifer structures. However, MRT suffers from limited abilities to resolve sharp boundaries. We make use of the GPR reflections to generate structural mesh boundaries for modelling of MRTetra and apply a definite sharp boundary constraint in the inversion of the surface NMR data. A comprehensive survey including GPR, MRT and other geophysical methods has been conducted at the test site Schillerslage, Germany. Inversion results of the surface NMR show that the imaging of water content and relaxation time T2* is improved using the mesh including the GPR reflections, and further improved by applying the sharp boundary constraint. In addition, multiple datasets of MRTetra can be combined together to invert the water content and T2* of the entire profile which provides a more stable and robust result comparing with separate datasets.
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Simultaneous Reorientation and Redatuming of Very High Resolution Seismic Recordings in Shallow Water Surveys
By N. JonesSummaryWe consider the challenges associated with very high resolution (VHR) seismic profiling of near-seafloor sediments, within shallow-water environments. Specifically we introduce a method for correcting for the effects of these challenges (temporal variations in water-column velocity—depth structure and source-receiver geometry changes linked to sea-state fluctuations) during data-processing. By using numerical-modelling in a V(Z) sense, seismic reflection data are slant-stack transformed to tau-p, primarily in order to retain surface-consistency for subsequent signal processing.
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Multi-frequency Seafloor Characterization Using Seismic Sources of Opportunity
Authors M.N. Banda, Ph. Blondel, M. Burnett and R. WyattSummaryThe low frequencies used in seismic surveys mean that seafloor characterization is limited horizontally and vertically. It is possible to use multiple reflections from the seabed to hydrophones on Seismic Support Vessels to measure scattering strengths and geoacoustic properties of seabed areas far from the seismic survey. This can be done by combining lower frequencies (directed toward the seabed and with repeatable beam patterns because of sensor design), providing potential sub-surface penetration and higher frequencies (emitted in all directions, with high inter-pulse variability), directly related to surface properties (slopes, roughness, seabed type). This is demonstrated with results from a shallow-water survey, in which the useful frequency range extends from 100 Hz to 20 kHz. Seismic pulses are used as sources of opportunity, and multiple scattering contributions are corrected for variations in propagation ranges and sizes of scattering patches (related to pulse durations). Twelfth-octave frequency bands give highly accurate information of seabed properties, which can be compared to models or previous measurements. The energy distribution of all measurements can be divided into three equally contributing frequency bands, and their RGB representation enables rapid assessment of seabed properties, identifying geomorphological changes and small-scale topography variations.
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Near Surface Modelling in Shallow Marine Setting Using Turning Ray Tomography Approach
Authors R.S. Alsaad, L. Wong and A. Al-HamoudSummaryfor shallow water environment standard water correction workflow was used to address the static resulted from the wave traveling through water and with the assumption of homogenies layering below the water bottom. In Arabian Gulf area, two acquisition schemes were used based on water depth. OBC acquisition was done in areas where water depth is relatively shallow range between 5 to 35 meters and standard limited azimuth 3D streamer acquisition was done in deeper water. Merging the two datasets at the prestack domain create a major challenges where water correction was used to reference the two dataset to same reference datum. Based on the input received from interpretation team, an attempt to resolve near surface zone 100 meters below the water bottom was needed to improve the imaging and resolve the difference found between the prognoses and actual depth. We used turning-ray tomography to estimate the near surface velocities for static corrections followed by prestack time migration
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Mitigating Dredging Risks Using Enhanced Geophysical Methods - The Aquares Resistivity Method
Authors P.M. Brabers, J. Errey and G. MoermansSummaryThe development of a new or existing port involving features such as quaywalls, turning basins, access channels and reclamation areas generally is significantly affected by local geological conditions. In general these represent a high risk factor to the construction project if they are not well described. When appropriate geological ground models are available the design of the construction can profit from it, reducing dredging and construction costs. The geological model forms the basis of quantities, design and risk management.
The mitigation of geological risks in ports and marine construction can be realised by including enhanced geophysical methods in the early project state. Enhanced geophysical methods are defined as high quality geophysical methods capable of producing accurate 4D digital ground models showing sediment and rock depths and thicknesses as well as rock and sediment qualities.
The Aquares resistivity method is an enhanced geophysical method. This paper explains the principles of this method and shows how it has been used in the past as an essential and highly successful tool to generate digital 4D ground models used for mitigating geological risks in ports and marine construction projects. Inappropriate practices in current site investigation procedures are described to as ways to overcome them.
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Aquifer Characterization Using Elastic Full-waveform Inversion
Authors N. Athanasopoulos and T. BohlenSummaryThis study exploits the benefits of elastic full-waveform inversion (FWI) as an effort to improve aquifer characterization for better understanding of their properties and delineating their structure. Conventional methods (e.g. boreholes, pumping tests) have usually one-dimensional nature and cannot provide information concerning the lateral heterogeneities of such complex subsurface environments. In the past, many studies were conducted using ground-penetrating radar (GPR), electrical resistivity tomography and seismics which result in two-dimensional tomographic images and provide spatially highly resolved mapping of aquifer heterogeneities. In this research, we focus in the seismic method and in particular we apply FWI to seismic data acquired at the Krauthausen test site. We compare our findings with borehole data and GPR FWI from previous studies. We conclude that combining the results of FWI with additional geophysical techniques can provide more reliable subsurface models and reduce uncertainties on reconstructing the aquifer architecture with higher spatial resolution.
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Surface Wave FWI on Complex Models - The Robustness of the Inversion to Assumptions and Forward Modeling Approximations
Authors Z. Xing and A. MazzottiSummaryFull waveform inversion (FWI) of surface waves with genetic algorithm (GA) is able to invert complex near surface models even in the case where very limited a-priori information is available, but it requires long computing time. One way to reduce the computing time is to make assumptions on the subsurface and to simplify the forward modelling. By using a few complex near surface models, with velocity inversions, lateral velocity variations and with an irregular topographic surface, we discuss how the following issues affect the inversion results in terms of either the data misfit or the model misfit: 1) fixing the compressional wave velocities and densities to the estimated shear wave velocities according to empirical equations, instead of inverting them; 2) neglecting attenuation in the forward modelling; 3) performing 2D forward modelling and applying a 3D to 2D correction to the observed data. Although these approximations degrade model prediction, yet the main features of the shear wave models can be retrieved. Instead, the data prediction is always satisfactory, showing again that theoretical approximations in the forward modelling affect more the model misfit than the data misfit.
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Full-waveform Seismic Inversion for Estimating Aquifer Dimensions and Hydrologic Parameters
Authors T. Lähivaara, A. Pasanen, A. Malehmir and J. KaipioSummaryThis study aims at developing computational tools to estimate aquifer dimensions and hydrologic parameters using seismic data. The poroelastic signature from an aquifer is simulated and using this signature we estimate water-table level and aquifer porosity, and infer the location of aquifer-basement boundary. We use discontinuous Galerkin method to solve the forward model that characterizes the propagation of seismic waves in coupled poroelastic-elastic media. The inverse problem is solved in a Bayesian framework, which enables to take into account modeling uncertainties. For the inverse problem, we use the Bayesian approximation error method, which reduces the overall computational demand. At this stage, results for a 2D synthetic model are presented to illustrate the potential of the algorithm for hydrogeological applications.
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Particle Swarm Optimization of Electromagnetic Data with Parallel Computing in the 2D Case
Authors F. Pace, A. Santilano and A. GodioSummaryThis work presents the application to electromagnetic data processing of the particle swarm optimization (PSO) algorithm, with the advantage of its high speed of convergence to the problem solution compared with other evolutionary methods. We focus on the inversion of MT synthetic data in the two-dimensional case. The PSO problem is solved complying with upper and lower bounds and a priori information, which is initially given only to a small amount of particles of the swarm. The fitness function is properly defined including smoothing parameter (Occam’s razor). Since it is a computationally demanding problem, a practical tool of parallel computing is tested and validated, thus allowing large computation time savings. Results show encouraging outcomes in terms of minimization of fitness function and data fitting. This approach represents the starting point for the 2D PSO application to MT data, Audio Magneto Telluric data and other near surface applications implying two-dimensional interpretation.
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Bayesian Inversion of Surface-Based ERT Data Using a Structure-based Prior
Authors G. de Pasquale, J. Doetsch and N. LindeSummaryInversion results obtained by surface-based electrical resistivity tomography (ERT) are strongly dependent on model regularisation (deterministic inversion) or the prior model (Bayesian inversion). Here, we present the first results of using a structure-based prior in Bayesian inversion of ERT data using a Markov chain Monte Carlo method. The method can handle unstructured meshes, which implies that topography and internal boundaries can be accounted for. The results obtained are greatly improved compared to those obtained by a prior based on uncorrelated model parameters. In the future, we will consider the problem of inferring the sediment-bedrock interface depth (and the associated uncertainty) under strong geological heterogeneity below and above the bedrock.
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Time-lapse Electrical Resistivity Surveys and Unfrozen Water Content in Cold Continuous Permafrost
Authors G.A. Oldenborger and A.-M. LeBlancSummaryEstimates of material properties such as ice content or unfrozen water content are critical for thermal modelling of the response of permafrost to climate forcing, understanding contaminant flow and transport, or for predicting the behaviour of permafrost as an engineering substrate. We utilize time-lapse electrical resistivity surveys to examine the potential for imaging relative changes in unfrozen water content for cold continuous permafrost in the Canadian Arctic. Electrical resistivity data were collected from 2012/08 to 2015/06 at semi-regular time intervals using a permanent electrode installation at Iqaluit International Airport in Iqaluit, Nunavut. Using postinversion model differencing, we observe significant changes in electrical resistivity and we infer changes in unfrozen water content that appear consistent with temperature records. The most prevalent changes in resistivity are not limited to the active layer, but extend from 1–8 m depth in a zone of significant temperature fluctuation.
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A Time-lapse Geophysical Model for Detecting Changes in Ground Ice Content Based on Electrical and Seismic Mixing Rules
Authors C. Hauck, C. Hilbich and C. MollaretSummaryThis contribution introduces a time-lapse extension of a geophysically-based 4-phase model to calculate ground ice content changes in the context of permafrost degradation due to climate change. The model uses input data from time-lapse Electrical Resistivity Tomography and Refraction Seismic Tomography monitoring and is based on simple petrophysical relationships. In time-lapse formulation, the model is not anymore dependent on time-invariant properties such as porosity, pore water resistivity, cementation exponent and P-wave velocity of rock. The time-lapse 4-phase model was then applied to long-term ERT and RST data from the mountain permafrost site Schilthorn, Swiss Alps, where a substantial ground ice loss could be detected during the past decade.
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Spectral Inversion of Capacitive Resistivity Data for the Investigation Frozen Ground
Authors J. Mudler, G. Fiandaca, A. Hördt, C. Hauck, P.K. Maurya and A. PrzyklenkSummaryThe capacitively coupled resistivity method measures frequency dependent electrical parameters in the frequency range between 1 Hz and 240 kHz. The method may potentially be useful for permafrost research, because the electrical permittivity of frozen ground and ice exhibits a characteristic frequency dependence. There is little experience with this type of measurement, as the equipment and appropriate inversion routines have only recently become available. Here, we present a case history from the Schilthorn mountain/Switzerland, where we acquired data with a dipole-dipole configuration along a profile of 28m length. A single site spectral inversion, in which we fit a Cole-Cole model to each of the configurations, yields pseudosections of five electrical parameters. We then apply a 2-D spectral inversion, which was extended to include the Cole-Cole permittivity inversion. The depth sections obtained this way are consistent with the pseudosections obtained from the single site inversions, which is taken as an indication for the robustness of the procedure. The electrical parameters basically reveal a 2-layered structure, consistent with the known snow cover over frozen ground. The excellent data quality and the consistency encourage further research into the extraction of parameters useful for permafrost research, such as ice content.
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