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23rd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 11 Apr 2010 - 15 Apr 2010
- Location: Keystone, Canada
- Published: 11 April 2010
1 - 20 of 131 results
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Not Such an Easy Target: A Student Case Study using Ground Penetrating Radar to Locate Buried Mortars at West Point
More LessOne, or possibly two, of the 12-inch coastal artillery mortars used for cadet artillery instruction between 1902 and 1931 remain buried roughly two to six meters below the surface of the northern edge of the Plain at the United States Military Academy at West Point, New York. This survey used a Sensors and Software PulseEKKO 1000 Ground Penetrating Radar (GPR), 225 MHz antennae, together with geo-referenced imagery to attempt to locate these mortars. The intrinsic value of 132 tons of buried iron, the historic value of the mortars, and sheer curiosity provided the impetus for this study. Geo-referenced aerial photography of Battery Byrne provided a starting point for the search, and we developed a data collection grid to estimate the most likely location. We collected data in conjunction with independent study classes from September 2007 through April 2008, collecting a total of 7,690 GPR traces. After processing initial data with EkkoView and EkkoMapper, we found a distinct hyperbola at Trace 386 of Line 16. We then constructed a tighter grid to characterize our point in higher resolution, but the reflection we identified may be too shallow to be the first mortar. For the second mortar, we focused on a location identified from a 2002 GPR survey, which indicated a mortar near the corner of a gravel pit on the grass of the Plain. The data indicated a small hyperbola shape in the East-West transect of Line 12, so we designed a second tighter grid to further investigate. Again, the results were not as conclusive as we had hoped. Additional research with another ground-truthing method, such as a magnetic or a gravity survey, could validate our findings. This survey has opened the way for future work. More GPR data with a different frequency would be beneficial, preferably 110 MHz to achieve greater penetration. Alternate methods of near surface geophysics would be very valuable, such as magnetic, gravity, or even seismic refraction. Additionally, a partnership with the Department of Civil Engineering could determine the costs, benefits, and feasibility of an excavation project. The West Point Historian may eventually want to erect a plaque describing the historical importance of Battery Byrne for the artillery instruction of cadets. Finally, locating these mortars would provide an interesting geophysics laboratory exercise for future West Point cadets in science and engineering classes.
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Ground Penetrating Radar: Utility/Rebar/Cable Detection and Concrete/Subgrade Debonding
Authors Neil L. Anderson, Evgeniy Torgashov and Oleg KovinGround penetrating radar (GPR) profiles acquired during seven separate field investigations are presented. The intent is simply to illustrate that GPR can be used to detect rebar, pre-tensioned tendons, deteriorated and/or detached concrete/fill, buried utilities, underground storage tanks, and buried foundations. The objective of each investigation is summarized, accompanied by brief descriptions of the nature of the target of interest. The application of GPR to the investigations described in this paper is not new. However, the presentation and illustration of multiple applications of this imaging technology in a single manuscript is somewhat unique.
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Ground Penetrating Radar as a Sentinel Device
Authors Peter J. Hutchinson and Bryan J. TeschkeGeophysical methods provide a non-destructive non-intrusive method of subsurface investigation. Oftentimes, however, the target is difficult to “see” due to lack of contrast between the target and background. This lack of contrast can be rectified through the use of a sentinel device. A series of sentinel devices were used in downtown Pittsburgh, Pennsylvania to determine potential subsidence beneath a building plaza. This sentinel strategy was performed to predict future deformation of the sidewalk and to prevent foot-traffic hazards.
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Using Shear-Wave Velocity to Detect Void-Induced Chages in Stress
By Steven SloanThe absence of geologic materials associated with subsurface anomalies such as voids creates an abrupt and distinct change in material properties. These voids can be challenging to image directly using geophysical techniques, but their effect on surrounding materials may produce physical property variations that are detectable. This has been described as the “halo” effect, where changes in density, stress, fracturing, etc. may produce a geophysical anomaly larger than the actual target. The removal of geologic material causes an increase in effective stress as the load previously borne by the removed material is transferred to the sidewalls and roof of the void. Shear-wave velocity (Vs) is directly related to stress and it is not unreasonable to suggest that localized changes in stress will result in Vs variations. Examples from seismic surface wave and reflection studies showing detectable increases in Vs coincident with zones of expected stress build up are presented.
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Joint Land and Shallow-Marine Seismic Investigations of Landslide Processes in the Bay of Trondheim, Mid-Norway
More LessThe urban development of the city of Trondheim in the last century has extended onto the near-shore areas (Figure 1). In the last 15 years, land reclamation has been intensified for construction work and building activity. Historical landslides are known to have occurred along the shore (e.g., L’Heureux et al., 2007). Damages to coastal infrastructures and loss of life resulted from these events. In one case (1888) a devastating tsunami wave struck the shoreline (Figure 1). The combination of historical landslides and urbanization has increased the concerns about the stability of the near-shore slopes. Therefore, numerous geological, geophysical and geotechnical investigations were carried out in recent years to explore in detail the stratigraphy of the fjord-valley fill and to investigate the stability both on land and in the fjord. One of the purposes of this paper is to show the ability of the shear-wave seismic method to study fjord-valley sediments from paved areas. In addition, we demonstrate the exceptionial potential for correlation between land-seismic and shallow-marine seismic data sets.
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Application or Radiomagnetotellurics on Geomorphological Studies
By B. TezkanA radiomagnetotelluric (RMT) survey is carried out to solve geomorphological studies concerning the evolutionary history of the terraces of the river Rhine in the Lower Rhine Bay next to Cologne, Germany. Geomorphological studies concern e.g. the evolutionary history of the valleys and the classic geomorphologic approach is to take drilling samples to examine the spreading of the covering sediments. The geophysical task is to map continiously the two boundaries between tertiary sands, fuvial gravel and loess. The measurements were realized using the new tensor RMT-device developed recently. Horizontal components of the magnetic and electric fields were observed in the frequency range from 10 kHz to 1MHz. Military and civilian radiostations broadcasting in this frequency range were used as transmitters. Transfer functions (e.g. apparent resistivities and phases) are determined by spectral analysis using a newly developed processing software. The observed apparent resistivities and phases were reliable and they showed a smooth frequency dependence. The data was inverted for 2D conductivity models and compared with two-dimensional inversion results of DC measurements and with existing boreholes. The good comparability indicates that radiomagnetotellurics can be optimally applied to this kind of geomorphological questions.
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Advanced Inversion Strategies using a New Geophysical Inversion and Modelling Library
Authors Thomas Günther and Carsten RückerGeophysical inverse problems often suffer ambiguity and yield fuzzy subsurface images. Often satisfactory results can only be obtained if additional information is incorporated in the inversion. The latter can be structural information about known boundaries or information about the parameters or their limits. However, this is rarely done by the available inversion software packages. We present an extremely versatile inversion and modeling framework for solving inverse problems on arbitrary geometries. Irregular meshes are used to incorporate known discontinuities. The generalized minimization scheme allows for controlling every model cell and every cell boundary individually. Moreover the subsurface can be subdivided into regions that represent different geological units or different physical properties. For each region the model transformation function can be set, e.g. for incorporating petrophysical relations, logarithmic barriers or a combination of it. The constraint type, e.g. smoothness, and the strength and characteristics can easily by varied in the course of iterations. Different joint inversion schemes are easily derived from that. By hand of a synthetic ERT study we show how the approach can be used to imagine small contrasts within a contrasted environment and to monitor small changes with time-lapse inversion.
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Borehole Flows, Hydraulic Heads, and Fracture-Zone Connection in The Potsdam Sandstone at The Altona Flat Rock Research Site in Northern New York
Authors John H. Williams, Frederick L. Paillet, David A Franzi and Edwin A RomanowiczNumerous borehole-geophysical methods are available for the characterization of fractures penetrated by boreholes but relatively few of these methods help to characterize the connections between fractures in the rock surrounding boreholes. Unfortunately, most studies in fracture-rock hydrology conclude that the characteristics of connections between locally transmissive fractures have a greater influence over aquifer-scale permeability than the local aperture of individual fractures (Long and others, 1996; LeBorgne and others, 1996; Shapiro and others, 2007). The study described here focused on the application of borehole-flow methods in the characterization of possible hydraulic connections between fracture zones penetrated by open bedrock boreholes.
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Comparison of Borehole Flow Measurements Obtained by Heat Pulse Flowmeter and Dilution Logging in a Fractured Bedrock Aquifer
Authors Frederick Paillet, John Williams and Edwin RomanowiczWe compare flow measurements made under ambient and pumping conditions in a bedrock borehole using high-resolution flow (heat pulse flowmeter) and fluid column dilution (repeat conductivity profiles after brine emplacement) methods. A second round of brine emplacement was avoided by using the fluid column conditions at the end of the ambient part of the dilution experiment as the initial condition for the pumping part of the dilution experiment. The estimates of flow by both methods yielded results that were in close agreement. The dilution method yielded direct estimates of flow based on the movement of interfaces, but logistics would have required the experiment to be repeated at two different pumping rates for most accurate results. The flowmeter yielded interval-averaged estimates of flow that agreed with the dilution results, but there was scatter of as much as 100% among the individual flow measurements. The flowmeter also required measurements under more than one pumping rate, and this was logistically much easier to accomplish than for the dilution experiment. We conclude that the methods are largely equivalent, with offsetting advantages and disadvantages for each method such that best results would be obtained using both methods in the same borehole.
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Borehole Geophysical Investigation: Seminoe Dam, WY Characterization of Internal Fracturing and Dynamic Moduli Reduction of Mass Concrete Undergoing Aar
More LessAlkali aggregate reaction (AAR) is the general term for a slowly occurring chemical reaction in which highly alkali cement paste reacts with concrete aggregate. AAR is a common problem in concrete poured prior to the 1950’s. The reaction leads to the formation of an alkali silicate gel at the interface of the aggregate and cement. This gel product is less dense than the reactants, causing expansion. The gel product also increases in volume with water. During July of 2009, five six-inch diameter boreholes with varying depths were drilled into Seminoe Dam. Down hole optical and acoustic imaging tools were utilized to locate and characterize fractures within the concrete surrounding the borehole. Data collected from sonic and density logging tools were used to calculate the in-situ dynamic modulus values of the concrete surrounding the borehole wall. Imaging data was analyzed for fracture dip, orientation and frequency. Sonic data was analyzed for the concrete’s compression and shear wave velocities. Concrete density was computed from gamma ray logging data. In-situ shear, bulk and Young’s modulus values were determined throughout the depth of the boreholes. Characterization of the fractures and in-situ dynamic modulus values of the concrete are used in comparison with past investigations of Seminoe Dam to gauge the progression and effects of AAR throughout the structure.
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Characterization of a Slim-Hole Gamma-Ray Sonde for Potash Exploration Applications in a Simple Test Pit Environment
Authors Stefani D. Whittaker, Ryan Sharma, Daniel Hallau, James P. Lewis and Robert M. CluffPrior studies of gamma-ray tools have all been focused around instruments that typically evaluate sandstone, shale, and limestone formations that are encountered in oilfield logging. For this study, a set of experiments were conducted to characterize the lateral and vertical response functions for a slim-hole gammaray sonde used in the mining industry to locate potash mineral deposits. The experiments were conducted in an indoor warehouse environment utilizing a set of large plastic tanks that were filled with light evaporite minerals (granular halite and sylvite) in differing arrangements to simulate various possible formation configurations. Measurements were taken while using a centralized slim-hole gamma ray tool in an air filled 4.5 inch plastic borehole. Sequential tests were run to establish the linearity of tool response, the radial depth of investigation, the vertical response function, and the repeatability of the measurement. Radial depth of investigation was measured using concentric radioactive rings of increasing diameter with two possible intermediary substances, air and halite. To test vertical response a column structure was built using halite as the bottom “bed” and a sylvite layer was systematically added in known quantities to acquire a response function of increasing thicknesses. Repeatability of the measurements was verified by logging several points with multiple tools of the same model for all the various experimental setups. Results were corrected for background radiation to predict the response in solid subsurface conditions without incident surface radiation seen in the experiments. The findings were also corrected for the differences between low porosity subsurface conditions and the unconsolidated granular products that were used in the simulated formation. The experimental results were surprisingly close to theoretical tool response for an oil field sonde, as well as to published specifications of major oilfield logging vendor’s tools. Consequently, gamma-ray logs collected with a slim-hole tool in shallow mineral core holes should be directly comparable to oilfield gamma-ray tools run in open-hole wellbores, once corrections for borehole size and fluid content using best practices are applied.
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Practical Focusing of Surface-Wave Inversion to Image Levees in Southern New Mexico
Authors Julian Ivanov, Richard D. Miller, Shelby Peterie and Joseph B. DunbarExisting algorithms for inversion of dispersion-curves estimated from analyzing the seismic surface-wave data provide a reasonable representation of the overall shear-wave velocity structure. However, in some cases these results may not meet the resolution requirement of the survey objectives targeting the upper portion of the subsurface section. Near-surface geophysical surveys often look for greater detail in the very near surface, such as the case for levees, while still being interested in the overall velocity structure at greater depth. The proposed method for applying surface-wave inversion revolves around applying a greater degree of focusing on the very near-surface component of the inversion model through the practical use of existing algorithms. Specifically by eliminating the low frequency portion of the dispersion-curve data and using a shallower model. The proposed inversion technique is demonstrated using the multi-channel analysis of surface waves (MASW) inversion algorithm. As a result of using this near-surface focusing approach, surface-wave velocity estimates of the levees possess greater detail and resolution. Results presented here demonstrate the potential of the proposed approach to more accurately image the very shallow subsurface top portion of other near surface MASW sites.
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Geophysical Surveys, Levee Certification Geophysical Investigations, DC Resistivity
More LessThe Yuma Area Levee system, running along the northern boundary of the city of Yuma (Arizona), trends parallel to sections of the Gila and Colorado Rivers. These Levees constructed from the surrounding earth, are composed of an assortment of sands, silts, clays, and quarried rock (which armors the river side of each levee structure). Sections of the levee system, and portions of the levee system near Walker’s Ranch have been chosen for investigation to determine structural integrity and areas susceptible to slope failure or internal erosion in the event of flooding. By using direct current resistivity (DCR), an assessment of the subsurface levee structure can be made, identifying material properties within the levee itself that indicate the presence of sands, clays, or anomalous void spaces if present. This data is used to help determine locations for levee remediation. This report discusses results from the DCR surveys performed at various locations along the levees. In general, areas of high electrical resistivity can represent locations of clean sands and gravels which may be prone to internal erosion at flood stage river levels. Areas of localized very low resistivity can indicate the presence of steel or other metallic materials such as culverts. This report provides a table of all direct current resistivity soundings and their quality of processed data. A few of the soundings revealed very high root mean square (RMS) error. These soundings will be disregarded. Direct current resistivity soundings with lower RMS error more accurately reveal areas of high resistivity sands and gravels within the levee structure. Only soundings classified as Good Data and Very Good Data will be used in evaluating the subsurface conditions when considering possible levee remediation locations.
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Time-Lapse Monitoring of the Slope Failure Process of a Model Levee
Authors Tomio Inazaki, Makoto Inoue, Yukiko Saito, Satoshi Arakane and Naoto YoshidaA half-shaped model levee 3 m high and 20 m long with a 3.5-m wide crest was time-lapse monitored over a period of four days. The purpose of the monitoring was to clarify the behavior of water migration in the levee during the deformation process. The time-lapse data included the height of the water table in the levee body measured using a water gauge array, apparent resistivity along a line set on the slope shoulder, surface temperature, and slope surface topography scanned by means of a 3D laser imaging system. Data were acquired basically at 1-hour intervals. Groundwater conditions in the levee body were controlled by infiltration from a perforated tank set at the rear side and supplemental surface watering. Surface deformation was recognized 5 hours after the surface watering, and it took another 2 hours until small slope failure occurred. Resistivity data strongly indicated that infiltration of surface water through the vadose zone and its connection with the water table in the body triggered the surface slope failure.
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A Case Study of Self-Potential Detection of Seepage at the Junction of Two Embankment Dams
This paper deals with a case study of the self-potential (S.P.) method applied to the detection of seepage occurring at the junction of two embankment dams. We took advantage of the controlled progressive emptying of the upstream reservoir to monitor changes of S.P. together with changes in water level. Seepage flow rate was also monitored. DC resistivity tomography was used to control changes in electrical resistivity. Repeated measurements in time with dual non-polarizable electrodes before the outset of the experiment showed that the repeatability of the measurements is better than 3 mV. The mapping of the electric potential showed an upstream low and a downstream high as expected. We have computed the effective electrokinetic coupling coefficient from the variation of potential with water level and flow rate. Horizontal gradients of potential were computed to display directions of preferential flow through the dam.
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Airborne Electromagnetic Surveys by The U. S. Geological Survey over Concealed Glacial Aquifers, Central United States
Authors Bruce D. Smith, Jared D. Abraham and Scott C. LundstromThe U.S. Geological Survey (USGS) over the last decade has performed several airborne electrical resistivity (helicopter electromagnetic, HEM) surveys over buried glacial features that constitute major aquifers in different regions of the central United States (Figure 1). These surveys have been conducted to map geology, contaminants, and hydrogeology for groundwater studies. Glacial deposits also can contain significant aggregate resources in the United States (Langer, 2002). This paper summarizes the USGS HEM applications in three different glacial terrains. The HEM systems applied in the surveys described here used frequency domain methods that, in electrically conductive areas may be limited to mapping depths less than 60 m. The frequency range typically is from about 100,000 Hz to 400 Hz.
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Airborne Electromagnetic Systems and their Engineering and Environmental Applications
Authors Dima Amine and Greg HodgesAirborne geophysical surveys have been used in the mining industry for over 50 years as a standard part of exploration programs (Fountain, 1998). Only recently have airborne geophysics surveys been used to help solve engineering and environmental problems.
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AEM Data for Assessing Irrigation Channel Leakage – A Meritorious Approach in an Australian Setting?
Authors Tim Munday, Mike Hatch, Andrew Fitzpatrick and David AllenGeophysical technologies, and in particular electrical and electromagnetic methods, have the potential to provide a rapid and relatively inexpensive approach to determining the location and extent of seepage along irrigation canals or channels. Although showing potential, the application of airborne electromagnetic (AEM) systems for these purposes has been very limited, in part because of the fine scale information required and also the costs associated with acquisition. However, recent developments in AEM system technologies have contributed to substantial improvements in the definition of conductivity at shallow depths and we believe these trends have made these systems a more relevant technology for the systematic mapping and detection of variations associated with irrigation infrastructure. In this paper we examine that potential through the analysis of high resolution HEM data for an irrigation system located in Victoria, Australia. Inverted data from a RESOLVE FDHEM survey along an irrigation channel were compared with an inverted ground resistivity array data set. Results demonstrated that the spatial patterns and magnitude of conductivity variations are generally comparable. The ground geophysical technique benefitted from being able to map variations at finer scales. However, there may be merit in considering the deployment of airborne mapping methods if large surveys are considered and a rapid turn-around of information is required.
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Transient Electromagnetic Mapping of Clay Units in the San Luis Valley, Colorado
Authors David V. Fitterman and V.J.S. GrauchTransient electromagnetic soundings were used to obtain information needed to refine hydrologic models of the San Luis Valley, Colorado. The soundings were able to map an aquitard called the blue clay that separates an unconfined surface aquifer from a deeper confined aquifer. The blue clay forms a conductor with an average resistivity of 6.9 ohm-m. Above the conductor are found a mixture of gray clay and sand. The gray clay has an average resistivity of 21 ohm-m, while the sand has a resistivity of greater than 100 ohm-m. The large difference in resistivity of these units makes mapping them with a surface geophysical method relatively easy. The blue clay was deposited at the bottom of Lake Alamosa which filled most of the San Luis Valley during the Pleistocene. The geometry of the blue clay is influenced by a graben on the eastern side of the valley. The depth to the blue clay is greater over the graben. Along the eastern edge of valley the blue clay appears to be truncated by faults.
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Direct and Inverse TDEM Method for Near Subsurface Resistivity Imaging
By I. MerhasinWe investigate the applicability of TDEM technology for near subsurface mapping. In the direct method, we use time domain response of a multi-layered earth to a dipole excitation based on Fréchet derivatives. Expected sensitivity of detection of a low-conducting layer is analyzed for various ground conductivities and layer depth. Our inversion method is based on Marquard technique and gives 1-D conductivity depth profile for each time sequence measurement.
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