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20th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 01 Apr 2007 - 05 Apr 2007
- Location: Denver, Colorado, USA
- Published: 01 April 2007
141 - 160 of 164 results
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Revealing Of Subterranean Karst Using Modern Analysis Of Potential And Quasi-Potential Fields
More LessThe main goal of near-surface geophysical studying is development of precise and reliable physicalgeological model of the underground. Karst terranes usually differ from the host media by their physical characteristics (density, magnetization, electric and thermal conductivity, electromagnetic and seismoelectric properties, polarizability, etc.) that allowing to applying a set of corresponding geophysical methods for displaying of karst phenomenon. Potential (gravity, magnetic, thermal, selfpotential and direct current) and quasi-potential (induced polarization and VLF) geophysical fields are inexpensive, prompt and effective tool for recognizing and localization of karst terranes. The developed interpreting system (described mainly in Khesin et al. (1996)) includes special methods for advanced analysis of anomalies of the aforementioned fields observed under complicated environments: inclined relief, oblique magnetization (polarization) and unknown level of the normal field. Another component of the system is a 3-D combined modeling of gravity and magnetic fields. The developed system has been successfully applied on models of karst terranes and in real situations.
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The Effect And Mitigation Of Vine Trellising On Em38 Soil Conductivity Measurements
Authors Peter J. Clark, David W. Lamb, Ron. Bradbury and Paul FrazierModern agriculture is increasingly requiring a focused application of scarcer resources to achieve optimal or specific harvest outcomes. The widespread use of the EM38 and EM31 in mapping soil characteristics, such as texture, moisture, and salinity has outpaced the geophysical understanding of their limitations in this application. The EM38, in particular, is a robust, light, simple, and easy to operate. In addition, its data is conveniently post-processed into an easily interpretable color image. If the image is then interpreted without regard to interferences and data-artifacts, the conclusions and farm management decisions may be sub-optimal. Steel trellising in vineyards has numerous advantages in commercial agriculture. For example,
expense reduction during vineyard installation, better compliance with mechanization and increased durability. However, the presence of large electrically conductive loops of the modern steel-posted trellis systems that are typically just 3 m wide present significant data interference to an electromagnetic survey. Previous work has identified the significant effect of the steel trellising in EM38 investigations within vineyards. In particular, the conducting trellis increases the amplitude of the measured apparent electrical conductivity. This investigation quantifies of the trellis effect in EM38 data. After analysis, the investigation and testing of mitigation strategies and the development of post-processing corrections naturally follow.
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Geophysical Surveys For Assessing Levee Foundation Conditions, Feather River Levees, Marysville, Ca
Authors José L. Llopis and Janet E. SimmsLevees are a fundamental part of many flood-damage reduction projects that protect life and property and their condition and performance in emergency flooding situations is of utmost importance. The U.S. Army Engineer Research and Development Center (ERDC) is conducting research related to levee condition evaluation and assessment. A primary objective of this research is to develop the capability to obtain information about levee conditions and rapidly convey the data to decision-makers during emergency operations, particularly in cases where levee failure is possible. A field study was conducted along a 9-km stretch of the landside levee toe of the Feather River south of Marysville, CA. The purpose of the investigation was to determine the potential of rapidly assessing levee foundations using a variety of geophysical instruments: a Geophex GEM-2 and Geonics EM31 and EM34 electromagnetic (EM) induction tools, a Geometrics OhmMapper capacitively coupled and a Scintrex SARIS direct-current (dc) resistivity system. Electrically non-conductive sleds were built to tow the EM instruments behind a vehicle. The capacitively coupled resistivity system and a combination of the EM instruments provided useful and definitive information for assessing foundation conditions.
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Comparison Of Airborne Em Inversions To Cone Penetrometer Results For Identifying Clay Layers Under Rio Grande Levees
Authors Greg Hodges, Joseph Dunbar and Steve SmullenFugro Airborne Surveys flew a Helicopter Electromagnetic survey in 2001 over selected areas of levees bordering the Rio Grande River and floodways for the U.S. Army Corps of Engineers and the U.S. Section of the International Boundary and Water Commission. The goal of the survey was to identify the soils on which the levees were built by measuring the soil conductivity. In particular, it was necessary to define the location, extent, and thickness of near-surface clay layers, which can resist under-seepage. Since the original survey was flown, sections of the levees have been sampled with cone penetrometer tests (CPT). This project involved testing a wide range of inversion algorithms on the data set for effectiveness in identifying and measuring the clay layers, comparing the results of each inversion to the boring information. The inversion results shown are very consistent for the many algorithms, including the approximate resistivity sections, multi-layer depth imaging and inversion, and discrete model inversions applied. Comparisons to the CPT borings are often good, but sometimes questionable. There is,
however, some ambiguity in the double conversion from CPT resistance to soil type to electrical conductivity. Near-surface conductive layers, assumed to be clays, were identified in many locations. The study demonstrated that each type of inversion has different applications and strengths.
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Use Of Multi-Frequency Electromagnetic Profiling In A Remedial Investigation At Camp Kohler, Mcclellan Air Force Base
Authors Richard P. Allen and Ronald A. CrowsonCamp Kohler, located approximately one mile east of McClellan Air Force Base in Sacramento, CA, is a 35 acre site that was used as an internment camp for Japanese-Americans during World War II. In 1947, an Air Force hospital on the premises was converted to a base laundry. Laundry operations ceased in 1973, and the building was demolished in 1982. Clothing
contaminated with radioactive materials from radium painting operations and atmospheric testing at the Nevada Test Site was laundered at the Camp Kohler facility. Concerns regarding the release of radionuclides to the environment in and around Camp Kohler prompted the Air Force to conduct a remedial investigation (RI) to assess potential radiological releases. As part of this investigation, geophysical surveys were conducted to delineate possible disposal areas, potential underground storage tanks, and to verify the location of buried utilities. A multi-frequency electromagnetic (MFEM) survey was completed at Camp Kohler in the fall of 2005. Instrumentation consisted of a Multi-frequency electromagnetic (MFEM) unit which was mounted on a plywood sled and towed behind an all terrain vehicle. The MFEM unit was integrated with a GPS receiver. Ground penetrating radar (GPR) was also used to further delineate electromagnetic
anomalies. This paper will detail the logistics of the survey and the efficiencies that were achieved with rapid data collection along closely-spaced traverses. MFEM contour maps will demonstrate the high level of detail that can be achieved with this technology, and results and conclusions will be presented.
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A Gpr Investigation Of Buried Infrastructure At The North Unit Of Illinois Beach State Park
Authors Timothy H. Larson, Curt Blakely and Timothy C. YoungFrom the 1920s to the mid-1970s subdivisions were built along the Illinois shore of Lake Michigan for several miles south of the Wisconsin border. Natural erosional forces hastened the
destruction of many of these homes. The state responded with the purchase of the "endangered residences" thus creating the North Unit of the Illinois Beach State Park. The houses were removed but, except in the area now occupied by a marina, the sub-surface infrastructure remained. Debris spread along the beach by ongoing erosion of the old home sites was thought to pose a health threat to the public. Of particular concern was the expected existence of a relict network of water and/or sewer pipes made of asbestos-containing concrete. To search for the buried utilities in one abandoned subdivision south of the marina we acquired over 3,500 m of GPR profiles. We detected a network of buried steel pipes, a large currently used raw-water supply pipe, and small clay tiles, but we could find no evidence in the study area that asbestos-containing water supply or sewer pipe networks were ever installed in this subdivision. Differences in the signatures of steel and other types of pipes were confirmed by direct observations.
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Metallic And Plastic Landmine-Like Objects Assessment Using Gpr Technique
More LessLandmine problems are one of the most severe crises in the world. Recently, GPR is successfully applied as a noninvasive technique to detect different types of landmines. Samples of metallic and plastic landmine-like objects have been buried in dry sand similar to that exist in the western desert of Egypt. Three different GPR frequencies have been tested to locate the buried landmine and to establish the landmine objects properties. It has been found that the low frequency is not efficient in exploring small landmine targets, while very high frequency is drastically attenuating without giving an adequate resolution. The relative contrast in dielectric constant between the landmine and host soil is imperative aspect for applications of GPR technique. Most of plastic landmines showed dielectric constant close to that of the host soil. The metallic landmines were easily seen as they have much higher dielectric constant relative to the host soil. This fact is critical in choosing the season of the year which is better for conducting the GPR technique to different landmine types. The responses of GPR technique for
metallic and plastic landmine-like objects buried in wet and dry environments have been achieved using field data and synthetic simulations. The wet environment decreases the GPR resolution of metallic body, while it increases the capability of the GPR to detect plastic bodies.
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Seismic Reflection: Upstream, Downstream, And On Earthen Dams And Dikes
High-resolution seismic reflection has been used successfully to characterize material and investigate a variety of problems associated with earthen dams and abutments. Limitations and challenges of seismic reflection when interrogating these structures and lithologies are nontrivial and require very critical thinking. Seismic reflection has proved an effective tool for mapping confining units for integration into the cutoff wall at the Keechelus Dam in Cle Elum, Washington; mapping lithologies and bedrock structures for earthquake retrofitting at Bend, Oregon; delineating karst in bedrock beneath the dam core responsible for subsidence on the upstream side of a major flood control structure at Clearwater Dam, Missouri; and detecting high permeability zones within a glacial outwash embankment of a water retention dam near Enumclaw, Washington. Extreme geometries and material variability associated with any man-made structure are the most formidable challenge to seismically imaging. Inconsistent source wavelets, out-of-the-plane energy, extreme statics (topography and velocity based), and source noise
(disproportionately high percentage of surface waves) are all problems that are not unique to earthen dams, dikes, and levees, but they are certainly more prevalent with those types of structures. Success of the technique in these settings is source characteristics and spatial oversampling.
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Optimizing High Frequency Vibroseis Data
Authors Theresa R. Rademacker, Richard D. Miller and Shelby L. WaltersVibratory sources used for seismic investigation are often subjected to coupling and transmission problems between the baseplate and actuator mass system and the ground surface. This results in dramatic force amplitude fluctuations at points throughout the sweep and excessive attenuation of intermediate and higher frequencies ranging from 200 to 500 Hz. This study is focused on determining if the baseplate of a commercial high frequency hydraulic vibrator behaves like a rigid body throughout its operational frequencies. Specialty accelerometers were strategically placed to determine optimal location that best matches ground response as measured by a buried geophone. This aided in discovering aberrations within the sweep spectra related to source configuration, and provided target enhancement opportunities for higher frequencies through optimized measurements of baseplate and ground coupling. The results show a significant variation in acceleration depending on where the accelerometer is positioned on the baseplate. Moreover, the baseplate accelerometer alone is sufficient for deconvolution.
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Repeatability Observations From A 2D Time-Lapse Seismic Survey
Authors Shelby L. Walters, Richard D. Miller, Joseph B. Dunbar and Steve SmullenTime- lapse seismic surveys are particularly useful for detecting changes in material characteristics. Distinguishing changes in subsurface conditions from noise, source and receiver coupling
variability, source wavelet change, and other sources of error related to the dynamics of surface conditions in time- lapse seismic images is challenging, at best. The objective of this study is to identify factors that affect repeatability of high-resolution near surface time-lapse seismic data when using a projectile source in unconsolidated sediments and investigate minimizing the effects of these factors. Data for this study were acquired as part of a 2D time- lapse seismic reflection survey in the floodplain of the Rio Grande near Las Cruces, New Mexico. The source was a 30.06 fired into pre-drilled holes in sandy soil; the holes were used for the baseline and eight monitor surveys. Changes in wavelet characteristics clearly indicate that the source signature varied non- linearly as a function of the number of shots fired downhole. Results indicate that changes in the wavelet become increasingly non- linear at increasing distance from the source and spectrally degrade with continuous use despite attempts to maintain the same firing conditions. This change through use is caused in part by change in source signature due to degradation of the holes during initial shots. It appears necessary to fire multiple shots downhole prior to acquisition to compact unconsolidated sediments and condition holes to increase
repeatability of the data. The number of conditioning shots is likely related to soil conditions (saturation, compaction, clay to sand content, etc.).
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An Application Of Shear Wave Reflection Landstreamer Technology To Soil Response Evaluation Of Earthquake Shaking In An Urban Area, Ottawa, Ontario
Previous shear wave seismic test locations and boreholes had indicated the presence of a buried bedrock valley in the Ottawa, Ontario, suburb of Orleans. This region is in a significant high seismic hazard zone, and the surficial materials are primarily high water-content, poorly compacted Holoceneage Champlain Sea Sediments.
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Historical Overview Of The Surface Wave Method
Authors Choon B. Park and Nils RydenHistorical development of the surface wave method for engineering applications is summarized with a focus on the last two decades of routine use. Starting from the pioneering attempt with one receiver made in mid-twentieth century that soon evolved into the continuous surface wave (CSW) method, subsequent introduction of the two-receiver approach of the spectral analysis of surface waves (SASW) method, and then twelve-or-more-channel approach of the multichannel analysis of surface waves (MASW) method are described in an approximate chronological order as to their basic principles, effectiveness, and limitations. Current status of the surface wave method in general is also briefly stated with suggestions of future research directions.
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Reconnaissance Geophysical Investigations For The Assessment Of Levee Conditions At The Canal Ranch Levee, Sacramento River Delta
Authors Martin Miele, Jeremy Flemmer, Charles Nycum, Sally Lamb and Sandra A. TakataThe Sacramento River Delta is the second largest river delta in the United States. The earthen levees were built in the 1800's and were not built with today’s engineering standards. Local earthen materials ranged from a wide variety of materials and structure. Therefore, the levees are prone to failure and flooding or other catastrophic events. The State of California (Department of Water Resources) published a set of specifications in an attempt to find some of the subtle physical problems before they occur. The specifications focus on surface geophysical techniques to assess subtle levee conditions in reconnaissance mode with follow up higher resolution geophysical techniques. They concentrated on two zones. The shallow zone targets were beaver burrows and dens, disturbed material, loose sandy material, and other shallow features that may cause levee failure during winter conditions. A second and deeper zone of interest was levee materials and conditions (ergo saturated areas, voids, loose materials, etc.) throughout the thickness of the levee itself. Shaw used duo frequency GPR surveys to assess the shallow zone conditions and conducted multiple OhmMapper resistivity surveys along 15 miles of levee to assess the effectiveness of the geophysical technologies. The survey was followed by ground-truthing which correlated positively with interpretations of the geophysical data.
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Offset Selective Dispersion Imaging
Authors Choon B. Park and Nils RydenA 2-D wavefield transformation method commonly used to image dispersion patterns of surface waves recorded from a multichannel survey applies the summation of wavefields over the entire offset range covered by the input record to take a full advantage of the widest spatial bandwidth ever possible. In reality, however, this full-offset summation may not always be the best choice because the prevalent offset range for a particular mode may be confined within a certain fraction of the full range and other mode(s) that may prevail in the remainder of the offsets may hinder the effectiveness of the summation. A simple modification can be added to an existing transformation scheme in such a way that minimum and maximum offsets are determined as certain ratios (for example, 0.1 and 5.0) of the wavelength of the mode being calculated. If these ratios are properly chosen, the modification can improve the imaging resolution and this usually leads to an enhanced delineation of multi-modal dispersion characteristics, which is one of the critical factors contributing to an increased accuracy during the inversion. Both synthetic- and field-data sets are used to demonstrate the effectiveness of the offset-selective imaging.
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Comparison Of Surface Wave Dispersion Curves From Active And Passive Measurements At A Deep Soil Site
Authors Brent L. Rosenblad and Jianhua LiLow frequency surface wave measurements offer a non-intrusive and inexpensive means to characterize deep shear wave velocity profiles (VS) for earthquake site response analysis. In this study, results are presented from low-frequency (1 to 12 Hz) surface wave measurements performed at a deep soil site in the Central United States using active and passive-source methods. Active-source measurements were performed using a lowfrequency servo-hydraulic shaker and were analyzed using both the Spectral-Analysis-of-Surface-Waves (SASW) approach and the multi-channel frequency-wavenumber (f-k) method. Passive surface waves recorded using a circular array and a linear array were analyzed using the f-k approach and the Refraction-Microtremor (ReMi) approach, respectively. Dispersion curves from the two active-source methods are in good agreement over the frequency range of this study. Low-frequency passive energy in the frequency range of 1 Hz to 2.5 Hz was prevalent at this remote farm site. Dispersion curves in this frequency range developed from the circular-array passive measurements
are consistent with the active source measurements. The linear-array passive approach (ReMi) provided three estimates of the dispersion curve that are consistent with the other
methods at frequencies above 2 Hz but varied considerably at lower frequencies.
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Non-Contact Surface Wave Testing While Moving
Authors Nils Ryden, Michael J. S. Lowe, Peter Cawley and Choon B. ParkWe present experiments with a rolling multichannel microphone array where measurements can be taken continuously while moving. Leaky air-coupled surface waves are measured with ordinary nondirectional audio microphones. Results show that microphones can be successfully used to produce a realistic phase velocity spectrum even while moving along the surface. The paper presents some theoretical background along with initial experimental results using the moving microphone array.
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Shear Velocity Profiles Obtained From Microtremor Array Data With An Example From Direct Fitting Of Spac Curves
More LessAnalysis of passive seismic (microtremor) array data using either beam-forming or the Spatial AutoCorrelation (SPAC) methods successfully establishes shear-wave velocity profiles at scales from a few metres to order 1 km depth. The SPAC method is enhanced by fitting the observed coherency spectrum directly with a modelled SPAC spectrum. The method yields shear-wave velocity versus depth profiles with a minimum of bias and also minimizes the number of array spacings required, compared with that required for beamforming methods. The method allows interpretation of Vs to a precision of 5-10%, facilitates recognition of higher-mode energy, and the correct interpretation of near-surface low-velocity layers. The method is demonstrated in blind and comparative studies on Holocene-Pleistocene alluvial sediments of the Santa Clara Valley, California, resolving boundaries at depths from 2 metres to 550 metres. The method is also capable of resolving a near-surface buried low-velocity layers of thickness 3-5 m (presumably silt) with Vs 100 to 150 m/sec, beneath 7 m of sands.
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Data Resolution Matrix Of High-Frequency Rayleigh-Wave Phase Velocities
Authors Jianghai Xia, Richard D. Miller and Yixian XuHigh-frequency (≥ 2 Hz) Rayleigh-wave phase velocities have been utilized to determine shear (S)-wave velocities in near-surface geophysics since the early 1980s. For a given nearsurface
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 the
match may only be possible for data in a certain frequency range because the sensitivity of Rayleigh-wave phase velocities due to changes in S-wave velocities varies with frequency. The
data resolution matrix is not a function of the data, but only of the data kernel (the Jacobian matrix, determined by a geophysical model and a priori information applied to the problem). The
data resolution matrix of high-frequency Rayleigh-wave phase velocities, therefore, offers a quantitative tool for design of field surveys and prediction of the match between calculated and
observed data. We employ the data resolution matrix to discuss the resolving power of phase velocities at different frequencies and the advantages of higher modes. Our discussion provides
insights into the inversion of Rayleigh-wave phase velocities. Because of restrictions of the data kernel of the inversion system, each near-surface geophysical problem can be resolved only with Rayleigh-wave phase velocities in a specific frequency range and higher modes normally possess higher resolving power than the fundamental mode.
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Assessing The Benefits Of Using Multiple Modes In Surface Wave Inversion
Authors Yosep E. Supranata and Michael E. KalinskiThe use of multiple-mode dispersion data in surface wave inversion to derive shear wave velocity (vs) profiles has been recognized as a means to improve the accuracy of the inversion result.
However, the error associated with non-uniqueness in multiple-mode inversion has not been quantified, so this research was performed to assess this type of error. In this research, inversion of surface wave dispersion data was performed using either one or two modes. Two different vs profiles were assumed, and “experimental” multiple-mode dispersion data were synthesized from these profiles. The dispersion data were then inverted using the Levenberg-Marquardt method. The results demonstrate that, as expected, inclusion of a second mode does not improve the uniqueness of the inversion for regular profiles where stiffness increases with depth. However, inclusion of a second mode significantly improves the uniqueness of the inversion for irregular profiles, where stiffness decreases with depth. For irregular profiles, the match between the true and inverted vs profiles is greatly improved by including a second mode in the inversion. The results also demonstrate that regardless of the nature of the profile, the uniqueness of the inversion improves when the starting profile more closely matches the true profile.
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Probabilistic Approach To Seismic Waveform Inversion
Authors Rambod Hadidi and Nenad GucunskiElastic waves carry substantial information about the characteristics of the medium they propagate in. However, most of the current methods which use elastic waves for engineering
evaluations, only utilize a limited portion of this information, such as peak return frequency or dispersion curve. Seismic waveform inversion is a relatively new technique for engineering applications, which seeks to use full information content of the seismic waveform. The objective of the waveform inversion is to find a reasonable subsurface model which its theoretically predicted waveforms match reasonably well the observed waveforms. The inversion of seismic data in general and waveform inversion in particular can be carried out using either a deterministic or a probabilistic approach. In the deterministic approach, a single model is identified as the problem solution, which implicitly assumed there are no uncertainties in the problem. However, in the probabilistic approach uncertainties in the data and forward model are included in the analysis and their effects on the obtained results are evaluated as a part of the solution. The probabilistic seismic waveform inversion is introduced in this paper as a robust technique for subsurface evaluations in engineering applications. To provide the background, the probabilistic approach to inversion is presented in general terms and a technique for its numerical implementation using Monte Carlo Markov Chains (MCMC) with Neighborhood Algorithm
(NA) approximation is outlined. Seismic waveform inversion for shallow subsurface evaluation is then described, and the results of a waveform inversion experiment using synthetic data are presented.
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