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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
41 - 60 of 136 results
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A Modern Geophysical Approach To Mapping Clay In The Subgrade – Case Study
Authors Jim Pfeiffer, Kanaan Hanna, Khamis Haramy, Roger Surdahl and Khalid MohamedThe presence of swelling clay beneath roadways poses a significant problem to road rehabilitation design and construction. Conducting precise subgrade soil investigations to determine the
location of potential clay-rich zones rapidly and nonintrusively is of great interest to the geotechnical engineer. A multi-phase program was designed to demonstrate the effectiveness of the Electromagnetic Induction (EMI) method as a state-of-practice geophysical imaging tool for mapping the presence of clay seams in the subgrade beneath roadways. The objective of this study was to compare the results from traditional subsurface soil investigation to a nonintrusive investigation using the EMI method. The results indicated that the EMI is a promising geophysical tool to compliment the current geotechnical engineering investigations approach as a value added. It is a fast, efficient, and cost effective tool for continuous mapping of spatial variations in soil conductivity of subgrade. A strong correlation between soil conductivity and the Atterburg Limits of Soils was not established; however, a qualitative evaluation of areas with increased potential for high plasticity clay content can be estimated from the EMI data. The EMI tool can be used to investigate in-situ soil behavior and to focus the soil boring programs during project site investigations, road rehabilitation, and construction.
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A Comparison Of Shear Wave Velocities Obtained From The Crosshole Seismic, Spectral Analysis Of Surface Waves And Multiple Impacts Of Surface Waves Methods
Authors Patrick K. Miller, Nils Ryden, Yajai Tinkey and Larry D. OlsonPresented herein is the comparison of the shear wave velocity results, with respect to depth, obtained from three separate test methods at a soil test site. The Crosshole Seismic (CS) test was performed utilizing three cased boreholes nominally 10 feet apart. The direct shear wave velocity was obtained from the results of the CS test. The Spectral Analysis of Surface Waves (SASW) test method and Multiple Impact of Surface Waves (MISW) test method were also performed at the same test site for comparison purposes. Neither the SASW nor MISW test methods require the installation of boreholes, thus reducing the cost of the test. SASW and MISW differ slightly from one another in the equipment used, method of data collection, and differ significantly in data processing. This paper includes a comparison and discussion of the test results, as well as backgrounds of each test method.
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Imaging Lava Tubes For Engineering Design
Authors Jim Pfeiffer, Steve Hodges, Kanaan Hanna and Khamis HaramyIn Hawaii, lava tubes are a significant concern to highway construction and maintenance projects not only for the long-term durability of the road surface, but also for the immediate safety of the construction crews and public. Prior knowledge of the location and depth of lava tubes can greatly aid in the design of engineering structures to improve safety and mitigate road degradation. This paper covers the geophysical investigation using ground penetrating radar (GPR) technique to map the lateral extent of lava tubes beneath the roadway and to determine the depth to the top of the lava tubes at two sites in Hawaii. At the first site, a detailed survey was performed over a segment of Crater Road with known lava tubes. The Federal Highway Administration (FHWA), Central Federal Lands Highway Division (CFLHD) is currently designing a remediation structure to bridge the lava tube to protect the tube from damage while increasing the load carrying capacity of the roadway. At the second site, a segment of Saddle Road was surveyed to locate lava tubes beneath the existing roadway prior to CFLHD’s planned reconstruction project. The information obtained from the GPR surveys is of great interest to the geotechnical and design engineers during site investigations, road rehabilitation and construction.
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Modeling The Emi Decay Response Of Medium And Large Uxo With Conventional And B-Field Sensors
Authors Michael W. Asten and Andrew C. DuncanWe compare the response of a conventional dB/dt EMI system (Geonics EM63) with a new system which utilizes a 3-component fluxgate B-field sensor. The B-field system has the advantage of providing decay curve information to times a factor of 5 to 10 greater than is achieved by the conventional dB/dt system. The transition from intermediate to late-time decay behavior is predicted theoretically to occur about a factor of two earlier in time for the B-field system, and this is confirmed by plotted observed data. Longer pulse times, lower transmitter frequencies, and longer decay curves are possible with the B-field system due to its wider bandwidth and lower noise at the low-frequency end of the spectrum. The decay constant for the medium and large munitions items considered here ranges from 12 to 25 msec and all decays when excited and measured with the EM63 EMI metal detector are incorrectly estimated as too small, due to the limited pulse-length and off-time intervals in the EM63 system. We propose that the improved ability of the B-field system to estimate the fundamental decay constant of a conductive permeable object will assist in UXO target discrimination, especially for situations where decay curves at earlier sample times are perturbed by ancillary scrap, or the response of
magnetic earth.
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Turning Crosshole Sonic Logs Into Tomography Images Of Defects In Drilled Shaft Foundations
Authors Yajai Tinkey, Ph.D. Olsen and Larry D. OlsonThis paper presents velocity tomograms from the horizontal Crosshole Sonic Logging (CSL), Crosshole Tomograpahy (CT) and Angled Crosshole Sonic Logging (ACSL) tests on a mock up shaft for comparisons of defect resolution. The CSL method is used to measure the speed of sound horizontally between water-filled cast-in-place access tubes. The velocity of the sound wave which travels from source to receiver in a horizontal plane determines the presence of anomalous regions (due to water or air-filled voids or soil intrusions) and therefore the quality of the concrete. For added higher resolution of shaft integrity and defects, the CT velocity imaging method is used. The CT velocity method precisely defines the size, shape, severity, depth, and location of potential defects by determining the spatial velocity distribution of shaft concrete through analysis of numerous sound wave ray paths. However, performing the CT tests can be time consuming. The ACSL test is a compromise technique between the CSL and CT test methods in terms of speed of testing and resolution of the tomogram image. This technique uses arrays of sources and receivers (in this study, 4 sources and 4 receivers) to obtain different angled ray paths. The ACSL method turns every log into a tomogram image with a fraction of time used in the CT tests.
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Determining The Length Of H And Sheet Piles Using The Parallel Seismic Method
Authors Eugenia K. Roman, Larry D. Olson and Dennis SackMany of the older bridges in the United States have no original contract documents available and about 26,000 bridges that are rated as scour critical have unknown foundation conditons. Thus, no information is available regarding the type, depth, geometry or material of these scour critical bridge foundations. The unknown bridge foundations pose a significant problem to bridge owners because of safety concerns. The Parallel Seismic (PS) method allows for easy and accurate determination of the foundation pile tip depth. The theory of the PS test method is presented herein. Several case studies for the Parallel Seismic method for determining the unknown depths of different types of foundations are summarized in this paper.
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Geophysical Surveys At Machu Picchu, Peru: Results For Landslide Hazard Investigations
Authors Melvyn Best, Peter Bobrowsky, Marten Douma, Victor Carlotto and Walter PariGeophysical methods are being used more frequently to assess slopes for landslide hazard potential, especially in areas where traditional methods such as trenching and drilling are either difficult to employ or not allowed. This paper presents the results of joint DC resistivity and EM surveys to map fractures and zones of weakness in crystalline bedrock at Machu Picchu, Peru. DC resistivity surveys were carried out along the upper 8 switchbacks leading to the sanctuary as well as across the sanctuary. EM-34 surveys were carried out along the upper 3 switchbacks and across the sanctuary. Inversion of the resistivity data located several lower resistivity zones along the switchbacks. These zones were associated with water seeping out of the rock in ditches. The water is confined to the upper switchbacks which is consistent with the disappearance of lower resistivity zones in the lower switchbacks. EM-34 results along the switchbacks, although more subtle to recognize, located several coincident zones of lower resistivity. The DC resistivity data across the sanctuary located a lower resistivity zone on the east side of the main plaza. There is presently no information on whether any of these fractures have been active in the recent past. Consequently the results from this study are still under investigation.
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Estimation Of Bedrock Depth Using The Horizontal-To-Vertical (H/V) Ambient-Noise Seismic Method
Authors John W. Lane, Eric A. White, Gregory V. Steele and James C. CanniaEstimating sediment thickness and the geometry of the bedrock surface is a key component of many hydrogeologic studies. The horizontal-to-vertical (H/V) ambient-noise seismic method is a novel, non-invasive technique that can be used to rapidly estimate the depth to bedrock. The H/V method uses a single, broad-band three-component seismometer to record ambient seismic noise. The ratio of the averaged horizontal-to-vertical frequency spectrum is used to determine the fundamental site resonance frequency, which can be interpreted using regression equations to estimate sediment thickness and depth to bedrock. The U.S. Geological Survey used the H/V seismic method during fall 2007 at 11 sites in Cape Cod, Massachusetts, and 13 sites in eastern Nebraska. In Cape Cod, H/V measurements were acquired along a 60-kilometer (km) transect between Chatham and Provincetown, where glacial sediments overlie
metamorphic rock. In Nebraska, H/V measurements were acquired along approximately 11- and 14-km transects near Firth and Oakland, respectively, where glacial sediments overlie weathered sedimentary rock. The ambient-noise seismic data from Cape Cod produced clear, easily identified resonance frequency peaks. The interpreted depth and geometry of the bedrock surface correlate well with boring data and previously published seismic refraction surveys. Conversely, the ambient-noise seismic data from eastern Nebraska produced subtle resonance frequency peaks, and correlation of the interpreted bedrock surface with bedrock depths from borings is poor, which may indicate a low acoustic impedance contrast between the weathered sedimentary rock and overlying sediments and/or the effect of wind noise on the seismic records. Our results indicate the H/V ambient-noise seismic method can be used effectively to estimate the depth to rock where there is a significant acoustic impedance contrast between the sediments and underlying rock. However, effective use of the method is challenging in the presence of gradational contacts such as gradational weathering or cementation. Further work is needed to optimize interpretation of resonance frequencies in the presence of extreme wind noise. In addition, local estimates of bedrock depth likely could be improved through development of regional or study-areaspecific regression equations relating resonance frequency to bedrock depth.
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Effects Of Spatial Data Density, Sensor Noise And Position Errors On Uxo And Clutter Target Parameters From Inversions Of Alltem Data
More LessALLTEM is a multi-axis electromagnetic induction system designed for unexploded ordnance (UXO) applications. It uses a continuous triangle-wave excitation and provides good late-time signal-to-noise ratio (SNR) especially for ferrous targets. Inversions of field data acquired in survey (moving platform) mode over the Army’s UXO Calibration Grid and Blind Test Grid at the Yuma Proving Ground (YPG), Arizona in 2006 produced polarizability moment values for many buried UXO items that were reasonable and generally repeatable for targets of the same type buried at different orientations and depths. In 2007 we finished construction of a test stand that allows us to collect data with varying spatial data density and accurate automated position control. We have studied the behavior of physics-based nonlinear inversions of ALLTEM test stand data as a function of spatial data density, sensor SNR, and position error. These studies have been performed as part of our effort to develop quantitative confidence levels for our inversions. A high confidence level in inversion-derived target parameters will be required when a target is declared to be harmless scrap metal that may safely be left in the ground. Unless high confidence can be demonstrated, regulators will likely require that targets be dug regardless of any “no-dig” classifications produced from inversions, in which case remediation costs would not be decreased. Multi-axis transmitter (Tx) and receiver (Rx) systems such as ALLTEM provide a richer data set from which to invert for the target parameters required to distinguish between clutter and UXO. Our inversions are more tolerant of sensor noise and position error than has been reported for single-axis, single-element data inversions and thus higher confidence in calculated target parameters can be achieved. This is especially important in the case of moving platform survey mode field data.
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3D-Gpr In Transportation Infrastructure Evaluation
Authors Nenad Gucunski, Carl Rascoe and Ali MaherCommonly conducted GPR surveys on transportation infrastructure have several limitations. The first limitation stems from the use of a single antenna. This limits surveying during a single pass to a single survey line. The second limitation comes from the use of a fixed frequency antenna. Therefore, if of interest is to get a high-resolution image of a medium tested, from the surface to depths of a few meters, it is necessary to use multiple antennas of different frequencies. The third major limitation represents the antenna orientation. A single oriented antenna can detect only objects in the test line direction. A new generation of GPRs, three-dimensional (3-D) GPRs, provide significant improvements in the solution of the above problems. The 3D-GPR system presented operates in the step frequency mode, in a frequency range from 140MHz to 2GHz, and utilizes an array of 31 transmitting and receiving antenna pairs with a spatial resolution of 75 mm. The surveyed medium can be presented in terms of horizontal planes at different elevations, vertical cross sections or three-dimensional images. Applications of the system in roadway and concrete bridge deck surveys are presented.
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Fiber-Optic Distributed Temperature Sensing: A New Tool For Assessment And Monitoring Of Hydrologic Processes
Fiber-optic distributed temperature sensing (FO DTS) is an emerging technology for characterizing and monitoring a wide range of important earth processes. FO DTS utilizes laser light to
measure temperature along the entire length of standard telecommunications optical fibers. The technology can measure temperature every meter over FO cables up to 30 kilometers (km) long. Commercially available systems can measure fiber temperature as often as 4 times per minute, with thermal precision ranging from 0.1 to 0.01 °C depending on measurement integration time. In 2006, the U.S. Geological Survey initiated a project to demonstrate and evaluate DTS as a technology to support hydrologic studies. This paper demonstrates the potential of the technology to assess and monitor hydrologic processes through case-study examples of FO DTS monitoring of streamaquifer interaction on the Shenandoah River near Locke’s Mill, Virginia, and on Fish Creek, near Jackson Hole, Wyoming, and estuary-aquifer interaction on Waquoit Bay, Falmouth, Massachusetts. The ability to continuously observe temperature over large spatial scales with high spatial and temporal resolution provides a new opportunity to observe and monitor a wide range of hydrologic processes with application to other disciplines including hazards, climate-change, and ecosystem monitoring.
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Strategies For Accurate Automated ERT Data Acquisition
Authors Douglas J. LaBrecque, Paula Adkins and William A. DailyThe use of microprocessor-controlled data-acquisition systems allows the rapid collection of electrical-resistivity and induced-polarization data. Although this provides a method for cost-effective collection of multidimensional data sets, there can be significant sources of error introduced by these acquisition systems. These errors are systematic in nature and thus may be difficult to identify. Of particular concern are errors resulting from re-use of the same electrodes for transmitting and receiving and aging of those electrodes over time. In this paper, we consider results from laboratory sand-tank experiments and field studies at a number of electrical resistivity project sites. The field studies include measurements over an eight year period during the Drift-Scale Heater Test at Yucca Mountain, Nevada. The object of these studies was to understand the ability of the site to perform over a long period of time and to test a number of
methods of mitigating both systematic and non-systematic errors for real-world applications. A number of different data strategies are being tested including the effects of varying the delay time of measurements using the same electrodes and the impact of using constant-current versus constantvoltage transmitter sources. The initial results show that the relationships between errors and measurement strategies are not always intuitive. For example, a longer delay between transmitting and receiving on a given electrode pair may not necessarily reduce the errors. In addition, electrodes show long-term changes in error levels that do not show a consistent pattern or correlation to other factors.
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Multi-Faceted Characterization Of Variable Limestone Terrains
More LessVariations in composition and karstic features within limestone terrains have the potential to be significant geotechnical risks to development in certain areas of Western Australia (WA). Over the past 4 years, Geoforce conducted a series of geophysical investigations in WA to assist in the geotechnical assessment and risk mitigation of such areas. The key findings from the surveys discussed in this paper are: 1. Electrical Resistivity Imaging (ERI) is only suitable for imaging geotechnical units where an electrical contrast is expected between geotechnical units. In limestone terrains, this will typically be for cavity investigations or where the geotechnical units have marked changes in clay content (e.g. Site 3) or porosity and are saturated with conductive liquid, such as saline groundwater or seawater. ERI has not been effective in imaging geotechnical units in dry, sand/limestone environments. 2. Multi-channel Analysis of Surface Waves (MASW) is usually an effective tool for imaging geotechnical units primarily due to the strong correlation between material hardness and shear velocity. However, the method is lower
resolution than other geophysical approaches used and is not always able to pick subtle variations within the sedimentary layers overlying limestone. 3. Ground Penetrating Radar (GPR) works exceptionally well in imaging karstic features where the ground is sufficiently resistive and other forms of information are available to assist in the interpretation of the data (e.g. CPT’s,
drillholes or other geophysical datasets. GPR is unable to provide any useful information in the more clay-rich materials of the Pilbara so a solid understanding of the site conditions are required before recommending its use. An EM31 is a useful tool for quickly establishing whether GPR is likely to work in the upper 5m. 4. The use of a combination of geophysical methods was found to enhance the nderstanding of the subsurface in all three sites, from either the improved resolution of one method with the geotechnical information from another (e.g. GPR and MASW at Site 2) or the measuring of different physical properties (e.g. ERI and MASW at Site 3) to understand the relationship between geological units and geotechnical properties.
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Characterization Of Complex Hydrogeology And Geology At A Large Environmental Site In New Mexico Using Advanced Borehole Geophysical Data
By Ned ClaytonAdvanced borehole and cased well geophysical logging and analysis, using technologies developed in the petroleum industry, have been performed in over 40 groundwater monitoring wells at a large environmental site in New Mexico since 1998 as part of an environmental restoration project. Applied geophysical logging technologies include magnetic resonance for bound/moveable volumetric water content and pore size distribution (uncased wells only); fully oriented, high resolution electrical resistivity imaging for geologic structure and texture (uncased wells only); epithermal neutron porosity for total volumetric water content (uncased and steel cased wells); bulk density and photoelectric factor for total porosity, water saturation and lithology evaluation (uncased and steel cased wells); array induction resistivity for water saturation and lithology evaluation (uncased wells only); neutron induced gamma ray spectroscopy for rock/sediment matrix geochemical evaluation; and natural gamma ray spectroscopy for geologic and lithologic evaluation. In addition to individual geophysical log measurements, integrated analysis (multi-log inversion) of t+H166he acquired geophysics data has been performed to generate continuous depth profiles of key formation properties, including total and effective porosity, vadose zone moisture content, water saturation, permeability, discrete fracture identification and aperture calculation, bedding and fracture orientation, formation electrical imaging showing geologic texture and grain size, rock/sediment matrix geochemistry, and lithology/mineralogy. The geophysical logs are used to guide well design (including optimal screen placement), evaluate borehole conditions, and provide a detailed in-situ characterization of the complex geology and unsaturated/saturated zone hydrogeology.
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Geotechnical Site Characterization Of A Flood Plain By Refraction Microtremor And Seismic Refraction Methods
More LessThe objective of this study was the evaluation of quality VS models generated by refraction microtremor method (ReMi) testing 3 differents seismic sources (1 passive and 2 active ones), and its comparison with VP models generated by seismic refraction tomography, in the geotechnical site characterization of a flood plain where 2 borehole and 12 test-pits had been done in a previous geotechnical study. Nine seismic profiles were deployed with different reciever spacing (60 geophones of 10 Hz spaced 2, 1 and 0.5 meters). In each of the lines we acquired data from ambiental noise and sledgehammer hits at the beginning and the end of the seismic line to mod+H167el 3 VS profiles and one seismic refraction profile to model VP data. The ReMi and seismic refraction methods gave us good results. On the whole, both VP and VS models allowed a complete geotechnical characterization of the flood plain. All the geophysical models were compared with the geological data profiles from boreholes and test-pits and permitted a 3-D overview of the study site.
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Detection Of Buried Timber Trestles Using Surface Waves
Authors Fernando Tallavó, Giovanni Cascante and Mahesh PandeyThis paper presents results from multi-channel analysis surface waves tests (MASW) conducted on an earth embankment to detect the location of rotten buried trestles in two different sections (A and B). In Section A, the locations of the trestles are known as well as the soil properties; thus, this section is used for calibration purposes. In Section B, the trestle locations are unknown. A seismic array of 24 geophones with a geophone spacing of 0.5 m is used. Different signal processing techniques were used for the analysis of surface waves to compute dispersion curves, power spectral density functions, distance-frequency contour plot, and wavelet transforms. Numercial and experimental results show that MASW tests were able to detect the location of buried trestles. MASW tests with a low-energy source (sledgehammer test) clearly show the location of buried trestles. Timber trestles can be detected by plotting the mean square value of the vibration energy. The effects of the trestles are also observed in the dispersion curves, the distance-frequency contour plot, and the Morlet wavelet transform. Not all source locations showed the location of timber trestles, likely because of the stronger effect of the ballast layer in this test.
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GPS-Denied Geolocation For Geophysical Data Acquisition And Analysis
Authors David W.A. Taylor and Brian T. ClarkPrecise sensor positioning is critical to geophysical data collection and analysis. Positioning presents a special challenge in the woods, urban canyons and other areas where GPS does not perform well or at all. Efficient data acquisition, assurance of spatial coverage, and sensor data interpretation and analysis all require high-quality position data tightly correlated with sensor data. Improvements in underground feature/target characterization are critically reliant on the quality of the geolocation data. These approaches demonstrate the range of solutions which ENSCO has developed and demonstrated that may be considered for a particular application regardless of GPS coverage: Ranger – A local area radio-frequency positioning system, primarily applicable in wooded terrain. Integrated with an EM61 and a Geometrics G-858 for UXO surveying. Bloodhound – A GPS/inertial integrated system. Integrated with a Geometrics G-858 for meandering path data collection. SAINT – A high-precision (< 2 cm error) for local-area target characterization. Integrated with a Geonics EM61-HH for UXO characterization. RTOP – A super-precise (< 2 mm error) positioning system for ground-penetrating radar imaging. Integrated with a GSSI SIR-3000 for concrete imaging. There is no single GPS-denied technology for geophysics. The requirements for each application need to be understood and then matched to the available technology.
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GPR Stratigraphy Of Fort La Presentation: Successful Target Delineation In A Highly Impacted Cultural Site
Authors Andrew L. Kozlowski, Aaron Gore, Barry Dale, Christina Rieth and Daniel MazeauFort La Presentation, a French fort built in 1749 at the confluence of the Oswegatchie and St. Lawrence Rivers in Ogdensburg, NY. The fort was destroyed by the British and rebuilt on numerous occasions prior to American occupation. Subsequent changes to the site included alteration of the original river bank, construction of a landfill, ship yard, train yard and later a business district. Overlapping the site was a petroleum storage facility that had contaminated the soil and the shallow water table. In February of 2007 initial ground penetrating radar (GPR) surveys were conducted over a 4 acre sight suspected to contain fort remains buried at depth. Surveys were completed using a GSSI 400 MHz antenna on 0.6 meter grid spacing. Initial amplitude slice maps revealed many targets including several rectangular high amplitude features at depths ranging from 1.5-2.4 meters in depth. In September, exploration trenches guided by amplitude–slice maps revealed a cobblestone surface overlain by ash, burnt timbers, faunal remains and hand wrought nails. Within the interstices of the cobblestone surface 5 French gun flints, musket balls and pottery confirm the surface to be that of one of the original structures built as part of Fort La Presentation.
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Comparison Of Array Microtremor Survey Methods For Estimation Of Dispersion Curves In Launceston, Australia
Authors Maxime Claprood and Michael W. AstenThe geology of Launceston (Tasmania, Australia) is characterized by the presence of soft Tertiary and Quaternary sediments filling the Tamar rift valley, overlying a hard dolerite bedrock of
Jurassic age. We compare data interpreted from two microtremor (passive seismic) array processing methods at Launceston; the spatially averaged coherency (SPAC) and the frequency-wavenumber (FK) methods. We present array observations at two selected sites; GUNN Company (site GUN) located at a site with expected shallow bedrock interface, and the Detention Basin of Launceston (site DBL) inside the Tamar valley with expected deep bedrock interface. Through interpretation of SPAC data, we get a precise estimation of the shear wave velocity profile (or slowness profile) with depth, from which we derive the dispersion curve. Using FK, we estimate the shear-wave velocity and direction of propagation of the wave of most energy on the wavenumber spectra at different frequencies, also leading to the computation of the dispersion curve. At both sites GUN and DBL, we observe that SPAC analysis enables interpretation over a wider range of frequencies, while the FK method seems to overestimate the velocity at low frequency. SPAC is thus preferred for the evaluation of shear wave slowness profiles.
However, FK processing is useful for evaluating the spatial distribution of the waves direction of arrival.
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Using Geophysics To Locate Unmarked Graves To Aid In Remediation Design And Construction At An Npl Site
More LessTetra Tech EM Inc.(Tetra Tech) was engaged by the Region 6 U.S. Environmental Protection Agency (EPA) under the Response Action Contract (RAC) program to perform a geophysical survey at the Rockwool Industries Superfund site. The purpose of the survey was to delineate the lateral extent of an old cemetery adjacent to a gulley where, for remediation purposes, a culvert was to be constructed. The cemetery was known to have unmarked graves, and some of the wooden coffins had been exposed due to erosion on the steep bank of a river immediately to the north. The geophysical survey results, if successful, would be used to guide the crew in avoiding any grave sites while placing the culvert. The survey was performed on March 1 through March 3, 2005. The Tetra Tech field crew employed a GSSI SIR 3000 ground penetrating radar system with a 400 MHz antenna and a GEM 2 terrain conductivity meter. Tetra Tech ran transects spaced 2.5 feet apart in an area measuring 25 feet by 40 feet over known grave sites to test the effectiveness of the equipment. Both the GPR depth sections and time slice plots successfully identified six known grave sites and four unmarked graves, but the GEM 2 had only limited success. Upon evaluation of the test plot, Tetra Tech continued the investigation in the area of concern near the planned placement location of the culvert. The results of the GPR survey identified four marked graves, but no additional unmarked graves were located. The GEM 2 survey, while not specifically identify individual graves, did differentiate disturbed soil in the cemetery area from undisturbed soil, so was used to conservatively delineate the boundaries of the
cemetery. The construction crew used the geophysical results to guide in the placement of the culvert, and there was no incident during construction.
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