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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
101 - 120 of 164 results
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Seismic Acoustic Research On Identification Of Archeological Sites In Submersible Zones
Authors Anghel Sorin and Ion GabrielIn Romania, geophysical methods are normally used to estimate the distribution of cultural relics, before digging. Objects of archeological interest are usually located within a few meters of the surface. The used equipment belongs to the seismic –acoustic reflexion systems category and it is usually used to detailed investigation of the submersible sediment structure. A great
contrast of acoustic impedance and therefore a correct identification of reflecting are also when in the sediment mass there are some bodies with acoustic impedance very different from that of the sediments, like archeological buildings. Keywords : acoustic impedance, vertical resolution, frequency modulation, ancient cities
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Surface-Wave Sensitivity To A High-Velocity Inclusion
Authors Xiaohui Jin, Barbara Luke and Kristina Lipinska-KalitaThe shear wave velocity (Vs) is an important mechanical property for the evaluation of dynamic soil behavior. Surface waves are often used to determine Vs. It is widely recognized that the existence of a large velocity reversal in a soil profile will give rise to significant partitioning of surface wave energy to higher modes in surface-wave testing. The authors have conducted a sensitivity study to examine the effects of thickness of a high-velocity layer (HVL), depth of the layer, and degree of velocity contrast on the surface-wave dispersion curve. Both the so-called "effective" dispersion curve, representing the superposition of energy into a single curve, and the multi-mode dispersion relation are generated. Results indicate that (a) the dispersion curve is more sensitive to the depth of the HVL than to its velocity or thickness; (b) the frequency/wavelength band within which the effective dispersion curve and fundamental-mode curve diverge is diagnostic of the position of the HVL; (c) model sensitivities are frequency-dependent; and (d) for the multi-mode case, peak sensitivities are higher for the fundamental mode than the first-higher mode.
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Modeling Seismic-Signature Wave Fields For Virtual Trials Of Sensing Algorithms
Authors Stephen A. Ketcham, James Lacombe and Roy J. GreenfieldDynamic activity from sources such as moving vehicles, walking and running humans, and stationary machines can be monitored as ground vibrations at offset distances. These signals can be processed in sensing algorithms for detection, location, and discrimination of the activity. The objective of this work is to demonstrate that seismic simulations can support sensing- algorithm development by modeling the effect of realistic environments on seismic energy as it propagates from these sources. Using massively parallel high-performance computers, the work applies a finite-difference solution to seismic propagation. Comparison with field signals shows the realism of the technique, and simulations in varied terrain reveal the nature of seismic waves as they propagate from the various sources. Measures of relative energy and signal cross correlation provide maps that reveal locations of optimal sensing. We demonstrate applications of beam forming to estimate locations of activity, and conclude that the simulation method produces realistic wave-field data for virtual trials of sensing algorithms.
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Using Surface Wave Methods For Earthquake Site Response Analyses: The Importance Of Active Dispersion Data
Authors Duane D. Bents and William M. CampSince adoption of the International Building Code, geotechnical engineers have become increasingly reliant on near-surface geophysics for determining shear wave velocities, which are needed
to evaluate site response under seismic loading. Non-invasive, surface wave methods, such as SASW, MASW, Refraction Microtremor, and MAM, are gaining in both popularity and acceptance in the engineering community for providing shear-wave velocity (Vs) profiles. Unlike traditional downhole and crosshole surveys in which Vs measurements are directly made at discrete depth intervals, surface methods utilize the dispersive property of surface waves to determine Vs at various sampled depths. In order to obtain Vs over the particular depth range of interest to geotechnical engineers (generally from ground surface to base rock), dispersion must be apparent over a corresponding range of surface wave frequencies. Surface wave methods that rely on passive source energy only (e.g, Refraction Microtremor and MAM) may not include a sufficient range of frequencies to resolve velocities within the upper portion of the soil column, which is often the most critical for site response. To provide the broadest frequency range, a composite dispersion curve from both active and passive source results should be used. The importance of coupling active with passive source surface wave data is demonstrated through comparison of response spectra based on Vs profiles obtained using only passive source dispersion curves to those using composite dispersion curves.
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Seismic Risk Assessment Of Highway Bridges In Clark County, Nevada
Authors Arya Ebrahimpour, David B. Porter, Ronald L. Sack and Barbara LukeThe Federal Emergency Management Agency (FEMA) lists Clark County, Nevada as a region of high seismicity. Slemmons, et al. (2001) show that there are eight faults in Clark County capable of producing earthquakes of moment magnitude Mw between 6.5 and 7.0. The Las Vegas metropolitan area is rapidly growing and the possibility of a large earthquake is real. Therefore, mitigation efforts are necessary to ensure the public’s well-being. Risk assessment is the first step in earthquake preparedness. To date, we have performed analyses of essential structures and highway bridges. This paper focuses on the highway bridges in which 786 bridges were considered in the study region. The risk analysis comprised of two components: (1) vulnerability analysis, and (2) importance analysis. The vulnerability analysis was completed by utilizing the FEMA software HAZUS-MH with a Level II assessment. The analysis used information on the structural systems, combined with geotechnical site characterization, which was accomplished using compiled in situ shear wave velocity measurements and well log databases. The importance analysis takes into account average daily traffic, detour length, bridge length, pedestrian traffic, railroad traffic and defense route. The final product of this study is a
bridge priority list that can help the Nevada Department of Transportation in identifying bridges with high seismic risk. Bridges receiving significantly large risk scores will be analyzed in detail to determine whether they are candidates for retrofit or replacement.
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Evaluation Of Soil Improvement Via Blasting Using Array-Based Surface Wave Tests
Authors Sungsoo Yoon and Glenn J. RixThe goal of soil improvement via blasting is to densify loose sand layers through the rearrangement of soil particles. To effectively achieve this goal, the sand must be clean, loose, and
saturated. Because the effectiveness of soil improvement via blasting is dependent on these conditions, it must be evaluated by in-situ or laboratory tests. Array-based surface wave tests are well suited to evaluate the effectiveness of blasting-based soil improvement due to their capability of mode separation in dispersion curves and their non-destructive characteristics that allow tests to be performed relatively economically and quickly without additional soil disturbance. In this study, a series of array-based surface wave tests at different sequential times following a blasting event have been performed to evaluate the effectiveness of the soil improvement with time at a landfill site in South Carolina. Surface wave dispersion curves and shear wave velocity profiles from the tests successfully captured the abrupt stiffness decrease due to blasting-induced liquefaction and the subsequent gradual increase in stiffness due to densification, the development of cementation, and the dissipation of blasting-induced excess pore water and gas pressure with time. The test results demonstrated that array-based surface wave tests
performed at various times following a blasting are an effective means to evaluate the effectiveness of blasting soil improvement.
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A Geophysical Investigation Of The Structural Controls Along The Southern Margin Of Lake Ngami, Northwestern Botswana Using Seismic Refraction And Dc Resistivity
Authors Elisha M. Shemang, Loago N. Molwalefhe and Elvis MosweuA shallow seismic refraction survey and DC Resistivity Soundings were undertaken at southeastern margin of the Palaeo Ngami Lake to determine the structure and shallow stratigraphy of the lake; determine the amount of fault displacement at the border fault of the lake. The results of the seismic refraction survey indicate that the there is a low velocity zone (~500m/s) in the area which is thinner outside the lake (less than 10m) and much thicker toward the centre of the lake (more than 30m).This low velocity zone is underlain by a higher velocity (3125 m/s) layer. Augering results within the lake indicate that the low velocity zone consists of dry clays, diatomaceous earth/silts. The high velocity layer is believed to be made up of sandstone/siltstones. Fault displacement at the margin of the lake is about 21m of downthrow. Another downthrow (about 19m) occurs at a distance of about 1300m from the margin into the lake, thus resulting in a total throw of about 50m. The results of the DC resistivity sounding indicates that the low velocity zone within the lake is characterized by 3 distinct layers; the top layer which is less than 1 m thick, has a resistivity of about 1300 Ωm (dry hard clays), the second layer about 5 m thick and has a resistivity of 34 Ωm (diatomaceous earth) and the third layer has a resistivity of about 11 Ωm (whitish soft clays interlayered with silty units).
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Time Cross-Sections Generated From Shallow Seismic Refraction Data: Preliminary Results
Authors Patrizio Torrese and Patrizio SignaniniShallow refraction seismic survey is a very common and useful subsurface investigation method. However, traditional shallow seismic refraction processing methods show long-standing
limitations due to the almost complete reliance on the first arrival signal. On the other hand, full wave form processing of data can by-pass inherent limitations and improve the imaging of the subsurface. In this sense, better results can be achieved through digital processing that has been successfully developed in reflection seismology. In particular, a time cross-section, similar to the well known reflection cross-section, can be generated through tools like CMP stacking that improves the signal-tonoise ratio. Other tools imply deconvolution and migration for improvement of, respectively, vertical and lateral resolution. Muting and dip filtering for deletion of coherent noise. This note presents a discussion on the feasibility of this kind of approach. Time cross-sections obtained with these methods are shown and compared with a HR reflection section.
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Hr Reflection Surveys For Seismic Imaging Of Unstable Slopes
Authors Mario L. Rainone and Patrizio TorreseShallow seismic reflection survey is a powerful tool in revealing subsurface geometry of unstable slopes. A more or less accurate frame on discontinuities and a dynamic characterization of the investigated lithotypes is possible with this technique. This is of great importance for the reconstruction of the geological subsoil model, especially in cases of complex landslide stability analysis under seismic conditions. It proves it's overall value when compared to more classical geologic investigations (i.e.borehole drilling) which are not univocally interpretable in complex scenarios. Both P waves or SH waves can be exploited. P waves are normally applied in the presence of a strong difference in seismic impedance (lithoid materials). On the other hand, SH waves give better results where the seismic rigidity is not high (porous media). This note presents different applications of the technique in cases of modelling of unstable slopes throughout Italy. These can be considered representative of many landslide typologies. Illustration of adopted acquisition and processing parameters is given and data relevant to the geometry of seismic profiles is presented. Results are discussed and compared with geognostic borehole data and other common geotechnical methods, achieving a satisfactory reconstruction of the subsoil model.
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Geophysics For Quaternary- Fault Mapping In Cabo Rojo, Puerto Rico
Authors Coral Roig and Eugenio AsencioTwo major seismotectonic structures have been mapped as active faults within and or near the Municipality of Cabo Rojo, Puerto Rico: (A) the South Lajas Fault, and (B) the Muertos through. The Department of Geology at the University of Puerto Rico suggests another active seismotectonic structure, the North Boquerón Bay Fault Zone (NBBFZ). This study provides new evidence that support this hypothesis. We argue that a depth converted high resolution shallow reflection seismic profile overlying a (2-D) Multi-channel Analysis of Surface Waves (MASW) shear- velocity depth-model images the fault zone. This work also provides geophysical data that delineates the fault zone.
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New Instrumentation Of Microseismic Monitoring System In The Asse Research Mine In Germany
By Hui FrickeThe former salt mine Asse II, in the ownership of the GSF-National Research Centre for Environment and Health, is a test disposal facility for low and intermediate level radioactive waste. After the research and development activities were completed, the mine was filled with residual salt material. Microseismic measurements have been performed in the Asse Research Mine since 1980. Until the final closure of the mine, which is expected in 2017, it is planned to fill the mine additionally with MgCl2-rich brine. This is necessary to avoid a possible re-liquidating process in larger sizes in the mine which would cause instability of its structural system. To enable that the microseismic monitoring system runs reliably in a salt fluid environment, new three components geophone-probes have been constructed. This is important for the ongoing observation of the geomechanical behavior response during the operational phase of the mine and for the safe work in the mine until the final closure. The renewal process has to be performed in stages without any loss of data. Twelve of these new stations, which are about half of the total number of the existing ones, have already been installed during the last year. Some of the new probes operated in parallel to the old stations. The new probes measure the microseismic activities in the mine and its close environment within a few of hundred meters accurately. The evaluation of the data includes source location, analysis and interpretation of the microseismic events. The results show the microcracking activity in the excavation damaged zone and the stabilizing effect of the filling with saline material clearly.
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Installation Of A Digital, Wireless, Strong-Motion Network For Monitoring Seismic Activity In A Western Colorado Coal Mining Region
Authors Peter Swanson, Collin Stewart and Wendell KoontzA seismic monitoring network has recently been installed in the North Fork Valley coal mining region of western Colorado as part of a NIOSH mine safety technology transfer project with two longwall coal mine operators. Data recorded with this network will be used to characterize mining related and natural seismic activity in the vicinity of the mines and examine potential hazards due to ground shaking near critical structures such as impoundment dams, reservoirs, and steep slopes. Ten triaxial strong-motion accelerometers have been installed on the surface to form the core of a network that covers approximately 250 square kilometers (100 sq. miles) of rugged canyon-mesa terrain. Spread-spectrum radio networks are used to telemeter continuous streams of seismic waveform data to a central location where they are converted to IP data streams and ported to the Internet for processing, archiving, and analysis.
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Seismic Reflection Survey Using Shear-Wave Vibrator For An Active Fault
Authors Toshiyuki Kurahashi and Tomio InazakiThis paper describes the results of a shear wave seismic reflection survey of the southern part of the epicentral area of the Northern Miyagi earthquake (Mj 6.4), which struck on July 26, 2003. The epicenters were on northern Miyagi Prefecture, about 350 kilometers north of Tokyo in Japan, and located about ten kilometers underground. After the earthquake shocks, we conducted a shear-wave seismic reflection survey using a shear-wave vibrator to locate and delineate the geological structure in the subsurface adjacent to the epicenters. We set a 4.2-km long survey line along the coast of Ishinomaki Bay. The seismic section after migration based on CMP reflection processing delineated the fold of the Neogene strata in the subsurface between Asahiyama Hill and Ishinomaki Bay. The geological interpretation shows that the Asahiyama Flexure is identified with the fold and extends into Ishinomaki Bay. The Asahiyama Flexure is more active than that estimated from the paleo-seismology.
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Surface Variations In The Earth'S Magnetic Field: Implications For Near-Surface Mri
Authors Christina L. Bray and Joseph P. HornakMillions of dollars are lost annually in the United States alone due to utility strikes during construction related excavation. Ground penetrating radar (GPR) is the most widely used geophysical
imaging tool to locate buried utilities, however, its performance in high water content, high conductivity soils is limited. Magnetic resonance imaging (MRI) may hold promise as a near-surface imaging modality in those conditions where GPR fails. An MRI based technique would produce images of voids from buried anthropogenic objects in a signal rich background from water in the soil. As a prelude to developing such a device, we measured the variations in the direction and magnitude of the Earths magnetic field in the first six meters beneath the surface of the Earth using both an Overhauser and magnetoresistive magnetometer. These static magnetic field data were then used to predict the quality of a near-surface magnetic resonance image obtained from a hypothetical system. The image simulations presented are based on relationships derived from the Bloch equations for the excitation, magnetic field gradient imposed phase and frequency encoding, and RF coil characteristics. Under certain design conditions and site requirements, these simulated images show that near-surface MRI may be feasible.
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Agricultural Soils Maps As A Framework For Conducting Shallow Subsurface Investigations In The Rio Grande Valley Near El Paso, Texas
Authors Diane I. Doser, Mark R. Baker, Richard P. Langford and Ekal M.C. ImanaFor over 10 years, we have found agricultural soil maps an increasingly useful guide in planning shallow subsurface geophysical (magnetics, DC resistivity, precision gravity, ground conductivity, ground penetrating radar) and geological (shallow coring, grain size and salinity analysis) investigations near El Paso, Texas. Our geophysical and geological studies have allowed us to tie agricultural soil classes to specific sedimentological features (e.g. flood plain, crevasse splay, abandoned channels) and enhanced our ability to predict where soil salinity, soil drainage and soil collapse problems may occur. Shallow soil structure directly controls drainage and pavement performance in new subdivision construction. The soil features also appear strongly controlled by the tectonic framework of the valley, including the location of faults that are important for earthquake hazard assessment and groundwater flow modeling. We have shared this information with city council members, city engineers and water utilities personnel to aid in planning growth within this region. We feel our methods could be applied in
other regions where agricultural soil maps are available and urban expansion is occurring.
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Geophysical Methods For Pile Length Determination: A Case Study At St. Joseph Hospital, Savannah, Ga
Authors Xiong Yu, Jian Fang, Bin Zhang, Josh Adams and Guoming LinThis paper presents a case study on the use of geophysical methods for determining the length of auger cast piles at St. Joseph Hospital, Savannah, GA. An expansion plan of the hospital involves adding one additional floor to the existing building. It is known the existing building is supported on auger cast piles, but no reliable information is available about the length and design capacity of the piles. Several geophysical methods were successfully utilized to determine the pile length and evaluate pile capacities. These methods included impact echo method, parallel seismic testing method and magnetometer method. Following a well planed testing program, all these testing were successfully conducted at the testing site. Results by these methods were found to give comparable results in estimating the length of these auger cast piles. This was then incorporated with information from site investigation to provide reasonable estimation on the bearing capacity of the existing foundation.
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New Results On Comparison Of Different Gpr Systems And Antenna Configurations At The Roman Site Carnuntum
More LessTo evaluate the suitability of ground penetrating radar (GPR) systems for archaeological prospection three different GPR systems with several antenna configurations were tested on one test area at the roman site Carnuntum under identical environmental conditions in 2004 (Seren et al. 2005). Due to developments in hardware and software within the last year these measurements where repeated and compared with the previous results. The data processing considers the needs of the different systems and antennas and aims to produce depth slices for best interpretation from an archaeological point of view. The results show advantages and disadvantages of the different systems and antennas, the need to process the data differently, and the necessity to increase the power of the antennas.
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Gpr And Dc Resistivity Application To Discontinuous Permafrost Delineation, Eielson Air Force Base, Alaska
Authors Beth N. Astley and Allan J. DelaneyDiscontinuous permafrost is present in interior Alaska to depths as great as 45 m. The surface active layer seasonally thaws to depths > 2m and in many locations groundwater can exist above, within, and below permafrost. The study area soils consist of organic material and loess draped over well sorted alluvial gravels. Vegetation is predominately black spruce forest with sedges while grasses predominate in inactive stream channels. In this setting we conducted a geophysical site investigation using ground penetrating radar (GPR) and two-dimensional DC resistivity. Measurements were made in late winter and late summer to delineate permafrost distribution beneath four parallel 500 m transects. 400-MHz GPR detected active layer depth and thaw zones while 100-MHz GPR detected thaw zones and the permafrost bottom. We compared results of Wenner and dipole-dipole resistivity with GPR and found that, in most cases, the permafrost interpretations from the dipole-dipole data closely match the GPR data permafrost interpretations; however, no boreholes are yet available for ground truth. Permafrost was detected on average between 0.7 and 20 m below the surface except along a north-south trending power line clearing where permafrost was mostly absent. Within some inactive fluvial channels
permafrost was sporadic or absent. The GPR and resistivity methods worked well in concert to delineate permafrost and stratigraphy, providing high quality data to depths of 30 m.
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Integrated Geophysical Investigation Of Preferential Flowpaths At The Former Tyson Valley Powder Farm Near Eureka, Missouri
Authors Bethany L. Burton, Lyndsay B. Ball and Gregory P. StantonThe U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, conducted surface and borehole geophysical surveys at the former Tyson Valley Powder Farm (TVPF) near Eureka, MO, to map preferential pathways on the bedrock surface for potential contaminant transport. The TVPF was formerly used as a munitions storage and disposal facility in the 1940’s and 1950’s, and the site at which the surveys were performed was a disposal area for munitions and waste solvents such as trichloroethylene (TCE) and dichloroethylene (DCE). Direct current (dc) resistivity and seismic refraction lines were acquired on the surface, and gamma, electromagnetic induction, and full waveform sonic logs were acquired in accessible boreholes. Through the combined interpretation of the seismic refraction tomographic and dc resistivity inversion results, inconsistencies in the bedrock surface were identified that may provide horizontal preferential flowpaths for dense nonaqueous phase liquid contaminants. These results, interpreted and displayed in georeferenced three-dimensional space, should help to establish more effective monitoring and remediation strategies.
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The Use Of Geophysics In Levee Assessment
Authors Joseph B. Dunbar, Steve Smullen and James E. StefanovLevee inventories are often based on visual inspection and contain only basic data about the levee, usually location and height based on traditional elevation surveys. Current levee
inventories generally do not include information about the underlying soils, or the engineering properties of the levee and its foundation. The U.S. Army Engineer Research and Development
Center, (ERDC), and the U.S. Section of the International Boundary and Water Commission (USIBWC) have conducted research on airborne and ground-based geophysical methods to
assess the condition of levees and their foundations. More than 800 km of levees in New Mexico and Texas were evaluated using airborne and surface geophysics, geologic studies, ground-truth sampling, and an Enterprise Geographic Information System (eGIS). Airborne geophysical survey methods include multi-frequency electromagnetic induction (EM), magnetics, digital
video, and light detection and ranging (LIDAR) surveys acquired at speeds up to 130 km/hr. Airborne geophysical methods permit investigation of the subsurface to depths of about 30 m.
Signatures from multi-frequency EM surveys were used to characterize soils in the levees and their foundations. Geologic studies included detailed mapping of floodplain features and the
identification of paleochannels to determine locations where underseepage potential is greatest. Ground-truth information was obtained with an instrumented cone penetrometer and direct
sampling. Geophysical, geographic, geologic, and other levee related data are managed efficiently through an eGIS, and is the basis for decisions about pre-emptive levee rehabilitation
and risk analysis. A condition assessment model was applied to levee reaches based on the results of airborne geophysics, geologic data, and historic performance and construction. This
model was used to determine levee reaches most at risk, to prioritize limited maintenance resources, and target areas for focused geotechnical studies. Subsequent geotechnical
investigations have included electrical and seismic instrumentation of levee reaches, and simulation of a maximum flood event against a levee test section to observe and monitor the
behavior of the levee soils under simulated flood conditions. This paper will present a summary overview of levee research using airborne and ground based geophysical methods and geologic
studies by ERDC and the USIBWC.
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