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21st EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 06 Apr 2008 - 10 Apr 2008
- Location: Philadelphia, USA
- Published: 06 April 2008
61 - 80 of 136 results
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Data Resolution Matrix And Model Resolution Matrix Of Rayleigh-Wave Inversion Using A Damped Least-Square Method
Authors Jianghai Xia, Richard D. Miller and Yixian XuEstimation of shear (S)-wave velocities by inversion of high frequency (≥ 2 Hz) Rayleigh waves has been given increasingly more attention by the near-surface geophysical community with application to a variety of near-surface geological and geophysical problems. For a given near-surface geophysical problem, it is essential to understand how well the data, calculated according to a layered-earth model, might match the observed data. It is also important to recognize that a match may only be possible for data within a certain frequency range or at specific frequencies because the sensitivity of Rayleigh-wave phase velocities due to changes in S-wave velocities varies with frequency. To provide insights into the process of inverting Rayleigh-wave phase velocities to estimate S-wave velocity structure, we introduced and discussed a data resolution matrix and a model resolution matrix of a damped least-square method. The resulting discussion indicated that each near-surface geophysical target can only be resolved using Rayleigh-wave phase velocities within specific frequency ranges and higher mode data are normally more accurately predicted than fundamental mode data because of restrictions on the data kernel of the inversion system. We proposed an inversion of data selected according to the data resolution matrix. The synthetic and real-world examples demonstrated that the proposed inversion could produce models with higher data resolution as well as higher model resolution.
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Numerical Modeling Of Wave-Propagation In Elastic-Half-Space With Imperfections
By Gokhan InciSoil deforms under impact. If the strains are small enough (5×10-4 or less), the material approximates elastic behavior. In geotechnical engineering analysis, soil properties are assumed to be
isotropic and the state of stress is often modeled as an equivalent isotropic state. Thus, the compression wave velocity and the shear wave velocity can be used to characterize the material in the stress-strain range of mechanical waves. Shear modulus and Poisson's ratio are then estimated from these wave velocities. Elastic properties of soils are important in determining the small-strain response of soils under dynamic and static loading and are of major concern in site assessment. In the field seismic crosshole and seismic down-hole tests are performed to evaluate small-strain, elastic properties of soils. Ultrasonic testing has been used to determine small-strain, isotropic, elastic properties of soils in the laboratory. Seismic testing methods have the advantage of providing fast and reliable results. In this paper wave propagation in isotropic elastic half space is simulated with Finite Element and Finite Difference numerical models. P-wave and S-wave arrival times are compared to the theoretical arrival times and elastic properties are back-calculated. Imperfections on the surface are detected in terms of change in the wave arrival time. Errors for input versus back calculated values are provided. Finite element and finite difference methods can be used to simulate complex problems. Numerical tools can assist in the interpretation of field and laboratory test results.
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Efficient 2D Inversion Of Long Ert Sections
More LessIn this work a new algorithm for the efficient 2D inversion of long ERT lines is introduced. The algorithm incorporates an experimental procedure to avoid calculation and storage of the entire Jacobian matrix. This approach speeds up the Jacobian matrix calculations and also reduces the required memory resources to store it. The efficient storage of the sparse Jacobian and Smoothness matrices and the efficient inversion using the LSQR method increase significantly the inversion speed. The application of the new algorithm to synthetic and real data sets resulted in reconstructed models of comparable accuracy to the standard inversion approach.
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ERT Monitoring Of Recycled Water Injection In A Confined Aquifer
Authors Panos Tsourlos, Jung-Ho Kim, George Vargemezis and Myeong-Jong YiThis work describes the installation and measurements of an electrical resistivity tomography (ERT) system to monitor the injection of recycled water into a confined aquifer in the area of Sindos (Thessaloniki, N. Greece). The aim is to provide, through time-lapse ERT measurements and processing, geoelectrical images of rather increased volumetric sampling around and between the holes and to obtain improved understanding of the flow and transport of the injected water. The details about the general setting, the construction and installation of the ERT cables into the boreholes are explained in full. Measurements involving single and cross-hole ERT measurements in time-lapse mode were obtained and processed with a 2D inversion algorithm to produce images of the subsurface. Results depict a very good correlation between ERT images and the lithology and resistivity logs an indication of the reliability of the approach.
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Cooperative Inversion Based On Fuzzy C-Means Cluster Analysis – Application To Field Data
Authors Hendrik Paasche, Jens Tronicke and Peter DietrichIn many near-surface geophysical applications it is now common praxis to collect co-located disparate geophysical data sets. The advantage of such multi-method based exploration strategies is the potential to reduce ambiguities and uncertainties in data analysis and interpretation. To reconstruct the underlying physical parameter distributions, usually requires the application of tomographic reconstruction techniques. To improve the reliability of the tomographic parameter models, the information content of all co-located data sets should be considered during the model-generation process. In this study, we apply a novel approach based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of crosshole seismic P-wave and S-wave traveltimes. The approach results in a single zoned two-parameter model outlining the major subsurface units in terms of the P-wave and S-wave velocity fields.
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Numerical Modelling Of Commericial GPR Antennas
Authors C. Warren and Dr. A. GiannopoulosGround Penetrating Radar (GPR) antennas are often not well described and hence modeled in the majority of GPR simulators. As a consequence results from these models do not represent reality. In this paper two detailed Finite-Difference Time-Domain (FDTD) models of commercial GPR antennas are presented. The complex geometry, material properties and feeding of the antennas are all captured in the models. The Visualisation Toolkit (VTK) format has been utilised to enable powerful three-dimensional visualisations of the complex antenna geometries, in addition to the antenna field patterns. Initial results from free-space responses (crosstalk) show good agreement between the models and data taken from the H177actual GPR systems.
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Spatially Constrained Inversion For Quasi 3D Modeling Of AEM Data
Authors Andrea Viezzoli, Anders Vest Christiansen and Esben AukenThe spatially constrained inversion (SCI) is a robust methodology for quasi-3D modeling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. It can be implemented with airborne or ground-based data, both in frequency and time domain. The airborne EM data here presented show how the SCI produces laterally smooth results with sharp layer boundaries that respect the 3D geological variations of layered settings. Information migrate horizontally through spatial constraints applied between nearest neighboring soundings, and allow to resolve layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. Data sets, models and spatial constraints are inverted as one system, producing layered sections with smooth horizontal variations. The SCI suppresses the elongated artifact commonly seen in horizontal maps (i.e., average resistivity, or saltwater boundary elevation maps) resulting from profile oriented data sets. Being an over-determined parameterized inversion problem, it produces a full sensitivity analysis of the output models, an essential tool for the evaluation of the results.
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Characterizing Submarine Ground-Water Discharge Using Fiber-Optic Distributed Temperature Sensing And Marine Electrical Resistivity
More LessSubmarine ground-water discharge (SGD) contributes important solute fluxes to coastal waters. Pollutants are transported to coastal ecosystems by SGD at spatially and temporally variable rates. New approaches are needed to characterize the effects of storm-event, tidal, and seasonal forcing on SGD. Here, we evaluate the utility of two geophysical methods–fiber-optic distributed temperature sensing (FO-DTS) and marine electrical resistivity (MER)–for observing the spatial and temporal variations in SGD and the configuration of the freshwater/saltwater interface within submarine sediments. FO-DTS and MER cables were permanently installed into the estuary floor on a transect extending 50 meters offshore under Waquoit Bay, Massachusetts, at the Waquoit Bay National Estuarine Research Reserve, and nearly continuous data were collected for 4 weeks in summer 2007. Initial results indicate that the methods are extremely useful for monitoring changes in the complex estuarine environment. The FO-DTS produced time-series data at approximately 1-meter increments along the length of the fiber at approximately 29-second intervals. The temperature time-series data show that the temperature at near-shore locations appears to be dominated by a semi-diurnal (tidal) signal, whereas the temperature at off-shore locations is dominated by a diurnal signal (day/night heating and cooling). Dipole-dipole MER surveys were completed about every 50 minutes, allowing for production of high-resolution time-lapse tomograms, which provide insight into the variations of the subsurface freshwater/saltwater interface. Preliminary results from the MER data show a high-resistivity zone near the shore at low tide, indicative of SGD, and consistent with the FO-DTS results.
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Remote Operated Vehicle Geophysical Survey Using Magnetic And Vlf Methods: Proposed Schemes For Data Processing And Interpretation
More LessAircraft geophysical investigations are recognized now as a powerful tool for geologicalgeophysical mapping, searching economic minerals, environmental studying, ecological monitoring, etc. Two main reasons limit an effective application of such conventional investigations: (a) essential impediments of performing geophysical surveys at small altitudes, (b) heightened danger for the aircraft crew, especially in the regions with a rugged topography. Remote Operated Vehicles (ROV) are free from the above-mentioned limitations. The new ROV generation – small and maneuvering vehicles – can fly at levels of few (and even one) meters over the earth’s surface (flowing the relief forms) with simultaneous performing geophysical measurements. Besides this, geophysical investigations observed at the vehicles, should have an extremely low exploitation cost (we suggest that the exploitation ratio of ROV / conventional investigations may consist of 1/20). Finally, measurements of geophysical fields at different observation levels could provide new unique geological-geophysical information. It is proposed that the most prospective geophysical integration for ROV should include measurements of magnetic and VLF electromagnetic fields. GPS (with utilization of the improved wide-band Kalman filtering) will assure an exact topogeodetic relation for the proposed observations. The nonconventional interpreting system developed for complicated environments may be successfully applied for analysis of the ROV observed geophysical data. Keywords: airborne, magnetics
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An Overview Of Ice Profiling Using Ground Penetrating Radar (GPR)
Authors Patrick I. Finlay, Neil S. Parry, Samuel A. Proskin and Robert J. MickleWinter ice roads are built on water bodies in northern regions to transport goods and machinery to otherwise isolated mines and communities. Effectively monitoring ice thickness is one of the major factors that contribute to safe ice road travel. By continuously profiling the ice with Ground Penetrating Radar (GPR), thin areas can be detected by measuring the time it takes the electromagnetic pulse to travel to the ice-water contact, then converting this time to depth using the ice's velocity. Traditionally, for calibration purposes, an auger hole is drilled in the ice and its depth is used to back calculate the velocity of the ice. This method gives a fairly accurate indication of ice thickness and allows calibration at that location. However, changes in the ice's dielectric properties do exist and are primarily a function of temperature and the material’s composition – a mixture of water, air and ice. Therefore, with larger data sets, greater separations in latitude, and differences in the ice's composition, the ice’s physical properties are in constant flux. EBA Engineering Consultants Ltd. has developed a new radar system that can directly measure the ice's velocity while profiling. This paper looks at ice road data collected with traditional systems, discusses what mechanisms lay behind changing ice dielectrics and speculates that being able to measure real time variations in ice velocity may lead to a correlation in measuring ice strength.
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Engineering Geophysics In Australia: Urban Case Studies From Downunder
Authors Robert J. Whiteley and Simon B StewartWithin Australia’s major coastal cities and urban areas geotechnical engineers are increasingly applying engineering geophysics for improved site characterisation in civil engineering projects and to assist with specific problems. Recent case studies from these cities illustrate the application of a variety of methods to a range of projects. Combining conventional surface geophysics, innovative borehole and underground geophysics with geotechnical methods increased innovative engineering opportunities and assisted the solution of specific design, construction and remediation problems for these civil projects.
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A Comparative Assessment Of Several Dipole-Based Algorithms For The Extraction Of Uxo Target Parameters
Authors D.D. “Skip” Snyder, David C. George, Scott C. MacInnes and J. Torquil SmithWith funding from the Office of Naval Research (ONR), the Naval Explosive Ordnance Technology Division (NAVEODTECHDIV), the Strategic Environmental Research and Development Program (SERDP), and the Environmental Security Technology Certification Program (ESTCP), several groups have developed physics-based interpretation software for extracting target parameters from secondary EM fields induced in UXO or other small buried metallic objects. In most cases, the software uses a point dipole with a frequency- or time-dependent polarizability tensor as a model of the EM field. As part of an effort currently being funded by ESTCP to commercialize some of the emerging hardware and software technology for UXO detection and characterization, this paper presents comparative results from the application of 3 different parameter extraction algorithms for EM data based on the dipole model. Although based on a common model, there are significant differences in the respective algorithms. We highlight these differences by applying the algorithms to a common data set. The data were acquired over the Calibration Lanes at the UXO Test Site located at the Yuma Proving Grounds using a new version of the Advanced Ordnance Locator (AOL2), a state-of-the-art time-domain EMI system consisting of 3 orthogonal transmitters and an array of 9 tri-axial receivers.
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Imaging A Shallow Paleo-Channel Using 3D Ultra-Shallow Seismic Reflection Methods
Authors Steven D. Sloan, Don W. Steeples and Georgios P. TsofliasA 3D ultra-shallow seismic-reflection survey was conducted to image a shallow paleo-channel at a field site located near Lawrence, KS. An orthogonal survey design was used, with a patch consisting of 4 receiver lines with 48 receivers each and 12 source lines with 16 source locations each. The source and receiver intervals were 0.5 m, and the source line and receiver line intervals were 2 m. The survey design resulted in a total fold of 48, covering an area of ~15.5 m x 35.5 m. Large variations in velocity were present, ranging from 300–600 m/s laterally and ~300–1600 m/s vertically. As normal moveout corrections cannot account for intersecting reflection hyperbolae due to large vertical velocity gradients, the data were processed by extracting offset-dependent subsets based on the optimum window for each reflection. The subsets were NMO corrected independently and stacked together using conventional 3D processing techniques. Despite the large lateral and vertical velocity variations, we were successful in imaging the top of the saturated zone, paleo-channel features, and bedrock located at depths of ~5, 8.2, and 14.4 m, respectively. Results of the 3D survey are in agreement with previous studies conducted at the site.
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Investigation Of The Hydrogeological Setting Of Plitvice Lakes, Croatia
Authors Bozo Padovan, Mark Blohm, Tomo Novosel, Miroslav Andric and Damir GrgecGeophysical surveys were conducted at the Plitvice Lakes in the Plitvice Lakes National Park, Croatia to help define the geological and hydrogeological setting of the Lakes.
Plitvice Lakes is one of Croatia’s most popular tourist attractions. It was set aside as a National Park in 1949, and was granted United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage status in 1979. The Lakes are located in an area dominated by carbonate rocks (limestones and dolomites). The limestone units are typically highly permeable (karstic). The Lakes are formed in depressions and separated by travertine barriers with a number of impressive cascade falls. A complex system of joints and faults are postulated to form barriers that impede water flow and allow the formation of the natural reservoirs. Below the Lakes, the excess water flows out to the Korana River, which sinks underground in the permeable limestone during dry periods. Three geophysical methods (shallow seismic reflection, geoelectrical tomography and time domain electromagnetic soundings) were used to map structural features and rock types to assist in interpretation of hydrogeological relations and to site future investigation boreholes. This poster paper provides the preliminary results of the geophysical investigations.
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Assessing The Cost-Benefit Of Multi-Core Cables And Nonpolarizable Electrodes On Shallow Time-Domain Ip Surveys
Authors Daniel F.S. Maia and Guilherme P. CastilhoIt has become a common procedure to acquire electrical data for environmental surveys with multi-core cables and multi-channel readers. These systems use pre-loaded protocols that instruct the relay box to combine the electrodes in hundreds of possible arrangements. When acquiring Induced Potential (IP) data, there is always a fear of electromagnetic (EM) coupling on the cables and polarization effects on the steel electrodes. As a precaution, current cables are usually separated from the potential cables and non-polarizable electrodes are used for potential readings. This also implies that special care must be taken when writing the acquisition sequence protocols. All these precautions increase considerably the time needed to assemble and disassemble a line thus leading to greater cost and lesser production. In this work, the effects of multi-core cables and non-polarizable electrodes on shallow time-domain IP surveys are analyzed from a practical point of view. It is shown that, if the instrument is able to perform Self Potential (SP) correction before integration, the results obtained with stainless steel electrodes and multicore cables are virtually the same as those obtained with separate cables and non-polarizable electrodes.
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Seismic Reflection And Resistivity Surveys To Map Sand Resources For New Jersey Beach Nourishment Projects
The New Jersey Geological Survey (NJGS) maps the State’s offshore geology and sand deposits by acquiring, analyzing and interpreting marine geologic and geophysical data. These data support development of a regional geologic framework that characterizes offshore sediment distribution. This framework produces a statewide assessment of offshore sand resources for beach nourishment, supports the NJGS geologic mapping program and is essential for a science-based approach to long-term resource management and contingency planning. The NJGS conducts surveys using seismic reflection methods and marine DC electrical resistivity. Geophysical data is ground-truthed with sediment vibracores. Analyses include grain size distribution, calculated interval velocity, conventional radiocarbon and amino-acid racemization (AAR) dating of sediment samples, and seismic stratigraphy. Delineating sand resources using a geologic framework permits the NJGS to identify areas strategically, obtaining detailed information where the resource potential is greatest for the least cost. This methodology enables a comprehensive assessment of sand location, quality and volume. NJGS has identified more than 274 million cubic meters (358 million cubic yards) of sand in both State and Federal waters along New Jersey's 127 miles of coastline. Data and findings are shared with the U.S. Army Corps of Engineers (USACE), NJ Department of Environmental Protection, and U.S. Department of Interior Minerals Management Service.
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Geotechnical Parameters From Integrated Geophysical Methods: Laboratory Measurements And Regression Models
Authors Frederick Owusu-Nimo, Fred Kofi Boadu and Aboagye MenyehSeismic and electrical measurements are performed in the laboratory on unconsolidated geomaterials with well-characterized geotechnical properties. The measured seismic parameters are compressional wave velocity (Vp) and shear wave velocity (Vs), and the electrical parameters include the amplitude and phase of electrical response. Measurements are conducted on 32 samples with wide variability in mineralogical composition, texture and geotechnical properties. Each sample is saturated with fresh water and subjected to varying effective stress levels of 5, 10, 15, 20 and 25MPa for measurements of the seismic and electrical parameters. The geotechnical properties, porosity, fines content, particle size distribution, plastic and liquid limits are determined for each sample. The relationship between the geotechnical properties and geophysical parameters is investigated and regression models are developed. It is shown that porosity decreases slightly with increasing fines content to a threshold of about 15%, after which the linear decrease is sharp, for all the effective stress levels. The variation of resistivity with stress is also found to be dependent on the amount of clay and fines present in the sample. Resistivity correlated all the geotechnical properties with higher accuracy. The best prediction is for fines content with R2 of 0.93. Fines content is also found to be the best geotechnical parameter which has better relationship with both resistivity and velocity. Combinations of resistivity and velocity also predicted plastic and liquid limits reasonably. The relationships developed will be useful in the geophysical interpretation of field measurements as well as the rapid estimation of geotechnical properties in the preliminary investigation of a project.
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Detecting Abandoned Air-Raid Shelter Using The S-Wave Seismic Reflection Method
Authors Shunichiro Ito, Takao Aizawa, Fumio Nakada and Ryosuke KitamuraA high-resolution S-wave survey was conducted over a known air-raid shelter to evaluate its effectiveness in detecting an underground void. It is a part of a government project of prevention of accidents of caving shelters. The survey used a mechanical impactor powered by an air compressor as the source and a purpose-built S-wave land streamer. The good quality of the data acquired warranted a focused processing to the depth of the target, and migrated stack and instantaneous amplitude sections show the target clearly.
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Magnetometric Researches Used In The Archaeological Studies Of The Greek Roman Fortress Located On The Shore Of Razelm Lake
By Sorin AnghelIn Romania, geophysical methods are normally used to estimate the distribution of cultural relics, before digging. Objects of archaeological interest are usually located within a few meters of the surface. The geophysical studies were carried out within the archaeological site both in 2005 as well as in 2006.Geophysical works were conducted using Geometrics equipment (G856 proton procession magnetometer) with a 0.1nT precision, which allowed for a highly detailed local morphology of the geomagnetic field and for the mapping of the magnetic anomaly. The working technology has been chosen to enable to emphasize mainly abnormal effects produced by sources located at depths of 0-5 m. On the south side of the late Roman fortification, outside the precinct wall, an artisanal area including a furnace for manufacturing building materials dated from the late Roman period, was found as well as some Greek furnaces for manufacturing ordinary brick. The south area of the site has been studied within this research project using the magnetometrical method (Fig. 5). Geophysical studies will prove very useful for further archaeological diggings, supplying them with a more clearly defined image on the substratum situation. Keyword ancient cities, proton pocession magnetometer, geomagnetic field, gradient system
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Inversion Of Surface Waves In Complex Structures
Authors Anouar Romdhane, Gilles Grandjean, Fayçal Réjiba and Adnand BitriSeismic techniques are nowadays widely used to image near-surface structures for both geomechanical and geological applications. However, in the context of shallow complex
structures, some limitations arise, particularly for inverting surface wave dispersion. In this paper, we introduce the full waveform inversion as an alternative to conventional MASW
(Multichannels Analysis of Surface Waves) methods typically used for subsurface prospecting. Considering 2D reference velocity models, we study the elastic wave propagation,
based on finite difference modelling in the frequency-space domain, for generating synthetic seismograms. The frequency versus phase-velocity curve, known as the dispersion curve, is afterwards computed and used to invert shear-wave velocities. Full-waveform inversion technique, based on the Born approximation, is tested using both body waves and surface waves to evaluate their respective contributions in the seismic imaging processes. The efficiency of such an inversion method compared to the classical MASW one is then evaluated.
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