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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
121 - 136 of 136 results
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Using Resistivity Imaging Technique To Characterize Interbedded Weathered Sedimentary Rock Mass
Authors Haryati Awang, Zainab Mohamed and Mohd NawawiThis paper illustrates findings of rock mass investigation and electrical resistivity survey on a selected site at Puncak Alam, Selangor. The purpose of this study is to characterize the rock mass of interbedded sedimentary with regards to the rock type and weathering classification. To achieve the objectives direct rock mass investigation including engineering geological mapping and physical index test on the rock surface followed by field electrical resistivity measurement was carried out at a cut slope of a rock mass. Results are presented in qualitative and quantitative form. Analysis on the correlated resistivity image and actual profile of the subsurface shows a relationship between resistivity and rock types and also between resistivity and weathering classification. Preliminary resistivity index of this rock mass with respect to weathering grade are also proposed.
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Repeatability Of Towed Magnetic Data For Archaeological Prospection Within A Sand And Gravel Mineral Deposit
Authors Jennifer S. Upwood, Christopher Leech, Ian A. Hill and Neil LinfordThe Geophysical Exploration Equipment Platform (GEEP) was used to survey an area of known archaeological interest in Shelford, Nottinghamshire. The site consisted of varying depths of sand and gravel deposits reaching a maximum thickness just under 10 meters, although the archaeological remains are likely to be much closer to the surface. An array of six caesium magnetometer sensors were set up at 0.6m separations on the GEEP and towed across the site. A section of the site was surveyed again the following day to determine the repeatability of the towing method used. Sand and gravel deposits in the region produced a level of background magnetic variation across the traverses and archaeological features were clearly observed outside this trend. A differential GPS system was mounted on the GEEP to provide positional data ensuring a reliable comparison between datasets was achieved. Data obtained on the repeat survey shows extreme likeness to that observed on the original dataset indicating the repeatability of the GEEP towing method for small separation magnetic surveys in sand and gravel environments. It took the GEEP operator three and a half hours to collect the original 6 hectare dataset, and an additional forty minutes to complete the repeated section.
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Delaware Bay Benthic Mapping Project: Using Highresolution Geophysical Methods To Map Bottom Habitat And Sub-Bottom Sediments
Authors John A. Madsen, Bartholomew D. Wilson and David B. CarterThe Coastal Program of the Delaware Department of Natural Resources and Environmental Control is carrying out a bottom and sub-bottom imaging project to map the bottom habitat and subbottom sediments of Delaware Bay. A RoxAnn acoustic system measuring bay bottom hardness and roughness is correlated with ground-truth grab samples and video to classify bottom sediments. The RoxAnn data is being used to map the distribution of these sediments and has identified the spatial extent and relative density of oyster shell on the bay bottom. Sub-bottom profiles, collected using an Edgetech X-STAR chirp sonar system, are integrated with the RoxAnn data to constrain the thickness and continuity of bottom sediments and to map potential/past offshore sand borrow sites that can be/have been used for beach replenishment. The chirp data has been integrated with additional information from sediment cores to develop maps of suitable sand deposits for replenishment based upon location, thickness, overburden, and grain size. The morphology of select areas of the bay bottom is being mapped using a Reson 8124 multibeam bathymetric system. The system has been used to map high-interest areas including natural and artificial reefs, oyster beds, and critical fish habitat.
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Near-Surface Seismic Methods To Enhance Reflection Events In High Desert Region
Authors Lindsay M. Mayer, Richard D. Miller, Julian Ivanov, Tom Weis and Bob AndersonStatics are a concern on most seismic surveys and the key to addressing the statics problem is accurate estimations of near-surface velocity variability. Near-surface variabilities in weathered or sub-weathered layers can cause trace-to-trace time irregularities that adversely affect the ultimate quality and therefore useability of a CMP stacked section. In some dry, unconsolidated geologies velocity analysis can present a significant problem due to considerable overburden thickness with lateral variability and relatively shallow target depths (500-700 m). In this study we explore different methods to enhance NMO corrections by focusing on defining a detailed velocity model for the “weathered” interval. These methods attempt to increase reflection coherency by selecting a velocity function that possess realistic NMO and interval values using unfiltered data. Data for this study was acquired in the high desert of northern Nevada. Vibroseis data were recorded across a 15-150 Hz sweep range using a 240 channel recording system. Attention to variability in the velocity model both laterally and in depth, results in a superior image of the subsurface.
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Limitations Of Small Em Sensors In Resistive Terrain
Authors Haoping Huang, Maria Deszcz-Pan and Bruce SmithHandheld electromagnetic (EM) sensors, for example the GEM-2, have been increasingly used in a variety of geotechnical applications. The sensors provide users with great flexibility in
selecting operating frequencies, sensor height, and coil configuration to optimize the measurement parameters for specific targets and mapping applications. However, a good understanding of sensor limitations is needed in planning surveys for specific environments, especially when the ground is resistive. Having resistive ground and small separation between transmitter and receiver, combined with a limited frequency span, can place the instrument in the low induction number range. Within this low induction number range the sensor response is mostly dominated by the frequency used (i.e., is linearly dependent on frequency) and not by the ground resistivity. This allows for detecting conductors, but low induction numbers are not suitable for depth imaging. Theoretical diagrams may be used to establish system limits and to choose an optimal set of frequencies for specific ground resistivity. To illustrate various conditions encountered while using small EM systems in resistive terrain we will use data from two GEM-2 surveys: one conducted in an area with high environmental noise on Sacramento, California, levees, and another in a karst environment in Texas where the environmental noise was low.
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Azimuthal Self Potential Signatures Associated With Pneumatic Fracturing
Authors DeBonne Wishart, Lee Slater, Deborah Schnell and Gregory HermanPneumatic fracturing is used to enhance the permeability and porosity of tight unconsolidated soils (e.g. clays) and/or bedrock thereby improving the effectiveness of remediation treatments applied to contaminated soils. A laboratory simulation was performed whereby compressed kaolinite sediments were pneumatically-fractured and subsequently injected with an electrolyte/dye simulating a treatment. Fracture geometry was quantified via fracture strike analysis of visible fractures in the tank walls combined with optical borehole televiewer imaging. Azimuthal self potential (ASP) measurements revealed clear electrokinetic self potentials during injection that correlate with dominant fracture strikes in the clay. Polar plots show that ASP lobes coincide with azimuths of high fracture strike density and that cross plots of SP versus number of fractures display a statistically significant positive correlation. Furthermore, the magnitude of electrokinetic SP scales with flow rate for any particular fracture set, and the positive lobes of the ASP anomaly are diagnostic of the flow direction of the treatment.
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Application Of P-Wave Seismic Reflection Methods Using The Landstreamer/Minivib System To Near-Surface Investigations
Authors Susan E. Pullan, André J.-M. Pugin, James A. Hunter, Tim Cartwright and Marten DoumaLandstreamer receiver arrays greatly increase the efficiency of shallow seismic reflection surveys, and their use is becoming more widespread, particularly for the acquisition of SH-wave
reflection or MASW data. This paper presents recent results using a P-wave landstreamer coupled to an IVI minivib source to acquire shallow seismic reflection profiles for near-surface investigations. The landstreamer/minivib system has proved to be an extremely efficient method of producing high-quality P-wave reflection data in a variety of geological settings. Comparison with traditional planted geophone data using an in-hole shotgun source show that, under optimum conditions for the in-hole shotgun source, the landstreamer data shows a decrease in the frequency of the reflection signal, but there may be an increase in the reflection frequencies where surface materials are dry and sandy. In either case, the advantages of greatly increased data production and the improved multi-channel capability (number of receivers and short offsets) are very significant. The technique has been successfully used to map buried valleys, and to define glacial stratigraphy.
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Exploring The Linkages Between Geotechnical Properties And Geophysical Responses Of Unconsolidated Materials - Laboratory Measurements
Authors Fred Kofi Boadu, Frederick Owusu-Nimo and Aboagye MenyehUpon application of stress, the strength and stability of unconsolidated geo-materials are influenced by modifications of their micro-structure, texture and effective stress conditions. These modifications in earth’s subsurface often result in geo-hazards (e.g., landslides, liquefaction, debris flow) which often claim so many lives, destroy the environment and cause considerable amounts of property damage. The fundamental relations between the geo-electrical parameters and the geotechnical properties characterizing sand-clay mixtures are investigated by performing controlled laboratory experiments. Spectral electrical response (SER) measurements are performed on these mixtures over a range of frequencies (0.001Hz-10kHz). Useful electrical parameters, that is, the phase, loss tangent and capacitance are extracted from the SER measurements. Relevant petrophysical/geotechnical parameters are obtained from the mixtures which include the void ratio, dry density and the modulus of elasticity. Cross-plots of geotechnical parameters and geo-electrical parameters indicate significant correlations. These relations can be useful in predicting the physical, mechanical and geotechnical properties of unconsolidated geo-materials in transition from frame-supported to suspensions or (from high to low effective stress).
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Borehole Radiometrics- Present Trends & Future Direction
Authors John R. Stowell and James J. LoCocoThe use of borehole radiation detectors for subsurface mineral identification dates back more than 60 years. The natural gamma log is one of the most common borehole measurements and continues to be an important part of most logging programs. The recent increase in the demand for nuclear reactor fuel has created a boom in exploration and appraisal drilling of uranium prospects. The basic physics of borehole gamma detection has not changed drastically over the years, but the instrumentation, and data acquisition and processing methods have. Renewed interest in accurate spectral analysis using new scintillation detectors and high-resolution neutron activation logging for in-situ mineral assaying will provide new applications for this ubiquitous measurement in the future.
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Integrated Approach Using Body Waves, Surface Waves And Gravimetric Prospections For Solving An Urban Geology Problem: The Abbadia San Salvatore Case (Siena, Italy)
Authors Patrizio Torrese, Mario Luigi Rainone and Patrizio SignaniniThe site geometry, heterogeneity and the velocity changes with depth are key factors to select the appropriate geophysical method for use in shallow underground investigations. The limits of each technique can be stretched with the application of different methods at the same site. A reliable subsoil model can be obtained by using this approach. MASW methods are known to be unsuccessful in presence of strong lateral variation of subsurface materials, while seismic reflection shows a remarkable effectiveness in reconstructing the underground geometry in very heterogeneous media. On the other hand, the MASW technique seems to prove its effectiveness in revealing velocity inversion that is the main limit of refraction surveys. Other methods like gravimetric surveys are effective in particular geological contexts. This paper shows the application of different geophysical methods carried out at the same site within a study related to urban geology. The objective of the study was to reveal the areal extension of a shallow mud level in the urban area of the Abbadia San Salvatore (Siena, Italy) municipality. Many buildings,
including a school, are founded on a relatively thin layer of deposits that overlies this mud level. This geological configuration constitutes a potential risk for the safety of the local inhabitants. The results obtained from the combination of seismic methods, which make use of both body and surface waves combined with gravimetric surveys and calibrated by borehole drilling logs are critically discussed.
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Assessing Nondestructive Geophysical Methods For Investigating Roadway Structural Health
Authors Jonathan Miller, Gregory Schultz, Darryl Calkins and Christopher BendaRoad subsidence indicates a failing or insufficient pavement substructure. The pervasive nature of subsidence throughout the country’s aging highway infrastructure makes full-scale reconstruction of every site costly and often infeasible. A variety of conditions can lead to subsidence. Some processes, such as large void formation, may lead to sudden and catastrophic pavement failure; while other processes, such as slow migration of fine particles from the subbase, may cause gradual or seasonal subsidence. Assessing the potential for catastrophic failure of subsiding roads is critical to determining the extent and timeliness of the remediation required. Noninvasive geophysical and non-destructive testing (NDT) methods provide cost efficient alternatives to interpolating borehole data used to characterize roadway subsidence and map subsurface voids. In this study we present the results from data collected at two Vermont highway sites exhibiting pavement subsidence. We employed complementary nondestructive methods to locate subsurface voids or conditions that cause subsidence. High-resolution ground penetrating radar (GPR) grid sampling combined with capacitively-coupled resistivity (CCR) surveying over the locations of subsidence elucidated the causes of the failing pavement substructure. Spatially correlated GPR and CCR cross-sections were combined with falling weight deflectometer (FWD) test results to refine our interpretations and guide direct investigations with a cone penetrometer. Our results indicate that the conditions of the site, specifically those resulting from interim repairs, can have a large impact on the effectiveness of these methods.
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Analysis Of Multi-Sensor Emi And Gpr Array Data For Ordnance Detection
Authors Gregory M. Schultz, Joe Keranen and Jonathan S. MillerTo safely and efficiently return foreign landmine or domestic UXO range areas to public use, ordnance clearance technology must reliably detect a wide variety of ordnance while minimizing the number of false alarms resulting from non-ordnance items. Large variability in mines and other munitions of concern, target placement and orientation, as well as environmental and subsurface conditions challenge the development of robust target recognition software. While our multi-sensor approach has shown promising results, we find that the ultimate effectiveness of the multi-sensor system is determined, primarily, by the treatment of the individual sensor data. We present the analysis of stepped-frequency ground penetrating radar and electromagnetic induction array data simultaneously acquired at government ordnance test facilities with the NEMESIS unmanned vehicle platform. Although our primary focus is on improved detection and discrimination of landmines, we also report H111the potential for NEMESIS to detect unexploded ordnance and other explosive remnants of war. We describe the application of a variety of and signal processing techniques to data acquired during field demonstrations in 2007. Emphasis is placed on the new environmental calibration and “on-the-fly” discrimination methods.
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Borehole Breakout And Fracture Analyses: Jwaneng Kimberlite Mine, Southern Botswana, Africa
More LessBorehole breakouts, so named by Babcock (1978), are enlargements and elongations of a borehole in a preferential direction. These are formed by spalling of borehole fragments in a direction parallel to the minimum (least) horizontal stress. The breakouts are typically elongated in the direction of the borehole axis and can be expressed by three parameters; orientation in the borehole, the opening angle and the radial depth. Knowledge of the orientation of horizontal earth stresses derived from analysis of borehole breakouts is important to geotechnical and rock mechanical studies. The country rock model for the Jwaneng Mine is extremely complex with a number of major structural faults and splays. The main central fault that intersects the kimberlite bodies has the same orientation as the Zoetfontein fault system 55km to the north (azimuth/dip of 339°/85°). The related fault splays or fractures, are tensile with the main structures and therefore may be fluid conduits. Utilizing downhole imagery in conjunction with the more typical suite of in-situ physical rock property measurements provides constraints on these structures and associated stress fields. Inferences on water mobility through the fracture systems can be made at depth and form an integral part of mine planning, development and optimization.
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Bayesian Inversion Of Traveltimes For Cavity Location
Authors Giancarlo Bernasconi and Diego RovettaWe present a case history for the location of a cavity in the subsurface by processing the signal generated inside the cavity by a seismic source. The signal emitted by the source is recorded by surface sensors and it is processed in order to obtain direct arrival traveltimes. The traveltimes are the input of a Bayesian inversion procedure for the evaluation of the source location and of the velocity model: the results are the estimated values and their associated uncertainty. We propose a parameterization of the problem when absolute times are not available. The probabilistic approach helps the analysis of the conditioning problem, and it can aid the definition of the acquisition geometry, as a function of the source signal, the transmission medium, and the precision of the solution. The location results are accurate in the limit of seismic resolution, even in a rough environment. Other applications of the procedure are the rescue of survivors below ruins, the monitoring of underground tunnels, the positioning of the bit in trenchless excavations, and the location of microseismic events.
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An Integrated Geophysical And Hydrogeological Investigation Of The Cibolo Canyon Development Area, Northeastern Bexar County, Texas
Authors Sachin D. Shah, Jason D. Payne, Bruce D. Smith and Allan K. ClarkThe U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a geophysical and geological study of the Cibolo Canyon development area, northeast Bexar
County, Texas, to characterize hydrostratigraphic features and to map surface and subsurface hydrogeologic features of the Edwards and Trinity Aquifers. Time-domain electromagnetic (TDEM) soundings, capacitively-coupled (CC) resistivity, frequency-domain electromagnetic (FDEM) profiling, and direct current resistivity depth imaging (2D–DC) profiles were used in conjunction with new geologic mapping, and borehole logging. This study shows that near-surface geophysical methods were effective in defining electrical signatures of the various stratigraphic members of the Edwards and Trinity Groups. In general, the upper hydrostratigraphic units of the Edwards Aquifer (Kainer Formation) are less resistive than the lower units (Person Formation). The upper hydrostratigraphic unit of the Trinity Aquifer also had low resistivity. The TDEM soundings were able to delineate changes in elevation of the base of the resistive units and define the amount of vertical displacement across the faults. Both CC resistivity and FDEM methods were used to map hydrostratigraphic contacts, fault zones, and possible karst features. The 2D–DC resistivity imaging techniques showed lateral changes in the subsurface lithology that correlate well with the CC and FDEM data. The TDEM and 2D-DC methods had limited application due to accessibility and time limitations. Results from the study demonstrated that combining multiple geophysical methods and geological mapping significantly improved the hydrogeologic mapping of the Cibolo Canyon development area.
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Noise To Signal: A Microtremor Study In The New Madrid Seismic Zone
Authors Kelli Hardesty, Lorraine W. Wolf and Paul BodinWe present results of a study in the New Madrid seismic zone (NMSZ) utilizing the microtremor method. The study analyzes and interprets horizontal to vertical (H/V) spectral ratios to determine resonant periods at sites across the Mississippi embayment. Data were collected from areas that experienced earthquake-induced soil liquefaction due to strong ground motion during historic and prehistoric earthquake sequences. Results from 15 sites show resonant fundamental periods of 0.5 s to 4.3 s for embayment thicknesses of ~ 100 m to 900 m and average shear-wave velocity of 800 m/s. These fundamental periods are associated with a strong impedance contrast between embayment sediments and underlying basement rock. Other spectral peaks of 1.8 s to 3.8 s are correlated with major stratigraphic boundaries within basin sediments. The basin configuration and a strong impedance contrast between sediments and basement rocks could contribute significantly to wave amplification at these resonant periods. Results also indicate higher amplification factors and liquefaction vulnerability at sites located in meandering stream deposits near the basin axis, where embayment thickness is greatest.
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