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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 - 50 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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Equivalence Analysis of DC and EM Data for Layered Models using the Resolution Matrix
More LessRidge-regression inversion (Inman, 1975) is used to formulate and carry out the inversion of DC resistivity and Electromagnetic (EM) data. A by-product of this process is the resolution matrix, which is symmetric and defines the linear combinations of the parameters which have been resolved in the inversion process. Subtracting the resolution matrix from the identity matrix yields the linear combinations of the parameters which are not resolved. The implication is that the model can be varied indefinitely in these directions without affecting the fitting error. In practice, there can be a limit to the extent to which the model can be varied in these unconstrained directions. After obtaining a best fit and the resolution matrix, the model is varied in the unconstrained directions to find the point where the fitting error exceeds the best fit error by a specified amount or until the parameter variation exceeds a specified limit. Variation is done in both increasing and decreasing directions for each original parameter. For m parameters, the resulting collection of 2m+1 models and synthetic curves shows the range of models which satisfactorily fit the data and their corresponding synthetic curves. The results concur with known equivalence principles for thin surface and buried layers and the method provides a convenient means of illustrating the equivalence status of any inversion result.
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Calibration and Filtering Strategies for Frequency Domain Electromagnetic Data
Authors Burke J. Minsley, Bruce D. Smith, Richard Hammack, James I. Sams and Garret VeloskiTechniques for processing frequency-domain electromagnetic (FDEM) data that address systematic instrument errors and random noise are presented, improving the ability to invert these data for meaningful earth models that can be quantitatively interpreted. A least-squares calibration method, originally developed for airborne electromagnetic datasets, is implemented for a ground-based survey in order to address systematic instrument errors, and new insights are provided into the importance of calibration for preserving spectral relationships within the data that lead to more reliable inversions. An alternative filtering strategy based on principal component analysis, which takes advantage of the strong correlation observed in FDEM data, is introduced to help address random noise in the data without imposing somewhat arbitrary spatial smoothing.
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Agricultural Geophysics: Past, Present, and Future
Authors Barry J. Allred, Robert S. Freeland, Hamid J. Farahani and Mary E. CollinsGeophysical methods are becoming an increasingly valuable tool for agricultural applications. Agricultural geophysics investigations are commonly (although certainly not always) focused on delineating small- and/or large-scale objects/features within the soil profile (~ 0 to 2 m depth) over very large areas. The three geophysical methods predominantly employed for agricultural applications, both past and present, are resistivity, electromagnetic induction (EMI), and ground penetrating radar (GPR). Some of the more important past developments for agricultural geophysics include: soil water content monitoring using resistivity methods beginning in the 1930s and 1940s; soil salinity assessment with resistivity and EMI methods beginning in the 1960s and 1970s; updates and improvements in U.S. national program soil survey mapping using GPR beginning in the late 1970s and on into the 1980s; and for precision farming purposes, the delineation of spatial variations in soil properties with resistivity and EMI methods beginning in the 1990s. There has been significant recent advancements in agricultural geophysics, with resistivity, EMI, GPR, and other geophysical methods presently being used or evaluated for applications ranging from soil hydrologic characterization, determination of clay-pan depth, soil nutrient monitoring at confined animal feeding operation sites, crop/tree root biomass surveying, subsurface drainage system infrastructure detection, identification of subsurface flow pathways, soil compaction evaluation, etc. However, before agricultural geophysics can reach its full potential, new developments are needed, such as: expanding possible agricultural applications for resistivity, EMI, and GPR methods; greater employment of geophysical methods that have not traditionally been applied to agriculture; construction of multi-sensor geophysical equipment platforms, perhaps integrated with agricultural machinery; development of agricultural geophysics expert system computer software; etc. Achieving these future advancements in agricultural geophysics will require close collaboration between those in both the agricultural and environmental/engineering geophysics communities.
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Third Year of Subsurface Drip Irrigation Monitoring using Gem2 Electromagnetic Surveys, Powder River Basin, Wyoming
The National Energy Technology Laboratory and the U.S. Geological Survey are collaborating with BeneTerra LLC to comprehensively monitor a sub-surface drip irrigation (SDI) system at a site in the Powder River Basin (PRB) of Wyoming. Irrigation water for the SDI system is coalbed natural gas (CBNG) co-produced water. The study is being conducted at the Headgate Draw area, located approximately 17 km south of Arvada, Wyoming at the confluence of Crazy Woman Creek and the Powder River. The study site encompasses six fields and covers an approximate area of 1.2 km2 (Figure 1). The project is an integration of geophysical, geochemical, and soil science studies. The third year of a five year geophysical monitoring study is reported here.
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Applications of Self-Potential Method in Agriculture
Authors Larisa Golovko and Anatoly I. PozdnyakovElectrical geophysical methods are classified as methods measuring natural electrical potentials of the ground without introducing additional electrical field and methods utilizing artificial electrical or electromagnetic fields to measure soil electrical parameters. Method of self-potential (SP) measures the naturally existing electrical potentials in soils and “bio-potentials” in plant, which are important in agriculture. Despite growing popularity of electrical resistivity/conductivity methods in precision agriculture, method of self-potential is rarely used. The SP method is based on measuring the natural potential differences, which generally exist between any two points in the soil or plant. Electrical potential in Soil-Plant system is a combination of the natural electrical potential differences on the interfaces inside soil (between soil horizons or peds), on the interfaces inside growing plant (between different plant tissues), as well as between soil and plant. The largest electrical potential differences were observed inside soils between soil horizons drastically different in physical and chemical properties. In most soils topsoil has higher electrical potentials than subsoil. The highest potential difference between soil horizons reported for Spodosols (40-60 mV), decreasing to 20-40 mV in Alfisoils and to ~20 mV in Mollisols, and even lower in Aridisols. Maps of electrical potentials in topsoil help to reveal the micro-environments for plant growth and correspond to plant biomes in natural ecosystems. Electrical resistivity (ER) or conductivity (EC) maps are generally similar to the maps of self-potentials, but using combination of those methods brings more information about infiltration and subsurface water fluxes and aid in search for clogged drainage pipes and reclamation planning. Recent advances in geophysical equipment, such as LandMapper ERM-02 also allow non-invasively measure natural electrical potentials between soils and plants, which are very small (μV magnitude), but nevertheless can be used to study plant water and nutrient stresses and manipulated to facilitate plant growth.
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Development of Methods for Determining the Suitability of Ephemeral Streams for Produced Water Discharges
More LessWyoming’s Agricultural Use Protection Policy allows discharge of produced water from coal bed natural gas operations to ephemeral streams with irrigated lands, on the basis of water quality implied from root zone soil salinity measurements. Critics of the policy argue that the soil and water quality sampling methods used to set quality limits for produced water discharges do not accurately represent existing soil conditions or indicate the potential for soil damage that can result from contact with the sodic, brackish produced water. To improve the understanding of interactions between soil and the discharges of sodic, brackish water from CBNG production, the National Energy Technology Laboratory conducted airborne, ground, and borehole electrical conductivity surveys and optical remote sensing surveys over the Beaver Creek Watershed in the central Powder River Basin of Wyoming. The airborne electrical conductivity data were found to quickly map possible areas of high soil salinity (ECe) for the extensive sub-irrigated areas that provide forage for livestock. Hyperspectral remote sensing data can help identify surface soil minerals and vegetation susceptible to the influence of CBNG produced water and identifies evidence of other possible impacts along waterways. The combined methods are potentially useful in land and water management decisions related to salinization.
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Case History: Siting Shallow Groundwater Wells with the Aid Of Geophysics
Authors Norman R. Carlson, Paul G. Ivancie and Phil C. SirlesAlthough the Colorado Division of Wildlife’s Bellvue Fish Hatchery site is relatively small, covering approximately 30 acres, groundwater drilling results across the site are variable and unpredictable; the site includes both productive, artesian wells as well as dry wells. The site is underlain primarily by the Lykins Formation, which includes interbedded siltstones, limestones, claystones, and evaporites. In preparation for new wells, geophysical surveys were done in an attempt to better understand the subsurface with respect to groundwater production. Both transient electromagnetic (TEM) and galvanic dipole-dipole resistivity lines were run, and substantial variations in resistivity within the Lykins are evident (as might be expected from the prior drilling results). The first test well after the geophysical survey was sited to test a locally conductive zone; the well was successful and flowed artesian. Similarly, a second test in a conductive zone also flowed artesian, while a third hole, not based on the geophysical survey (but sited primarily on the basis of permits and logistical considerations) was unproductive. The geophysical data confirm the heterogeneous nature of the Lykins at this site, agrees well with downhole logging, and has been clearly useful in successfully siting groundwater production wells.
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Evaluating Chloride Contamination In Sand And Gravel Aquifers And Bedrock Fractures Using Terrain Conductivity And Very Low Frequency Geophysics
Authors W. Bradley, M.S. Tirone, L. Christopher, C.G. Covel and James E. HillierIn April 2009, dissolved chloride concentrations at a public school water supply in south-central Maine were found to exceed 400 milligrams per liter (mg/L). The Maine Maximum Contaminant Level (MCL) and Federal Drinking Water Standard for chloride (250 mg/L) had been exceeded, rendering the School’s water supply non-potable. Contaminated well water was blamed for: health risks for some of the students and employees; corrosion of pipes, pumps, plumbing fixtures, and school kitchen equipment; and other potential negative impacts to School infrastructure and associated property. School officials believed the problem originated with road salt stored at a Town facility located approximately six hundred feet from the school’s drinking water supply well. However, some Town officials blamed remnant salt-water intrusion that followed sea-level rise during the Pleistocene Era, or road salting performed by the State Department of Transportation. Therefore, an impartial and scientific investigation was needed to both solve the water supply problem and, correctly assign liability. The situation required developing and interpreting a substantial amount of geologic data, as well as safely locating and installing a new 20,000 gallons per day potable water supply before the fall School session commenced in early September. This would involve mapping and interpreting key characteristics of surficial and multiple bedrock aquifers in order to isolate the existing chloride plume from a target bedrock aquifer. We elected to design a site-specific investigation employing both shallow electro-magnetic terrain conductivity (TC), and deep geophysical Very Low Frequency (VLF)
investigations to achieve these goals.
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An Integrated Groundwater Study: Chasnigua, Honduras
Authors Catherine Skokan and David MunozA group from the Humanitarian Engineering Program at the Colorado School of Mines has employed a suite of geophysical tools to characterize subsurface water for the village of Chasnigua, Honduras. The village is small, with approximately 50 families (200 people) that rely on trucking their water to their homes. They have asked for assistance in developing water well, storage and distribtution systems. Honduras is economically one of the poorest countries in the western hemisphere, with about half the population below the poverty line. Consequently, the people do what they can to subsist on the land. This location is one with economic hardships and complex geology and hydrology. Because of the complex nature of the site and because of a lack of previous geophysical information, a suite of measurements was used. These included Magnetic and Magnetic Gradient Measurements, DC Resistivity, and Frequency Domain Electromagnetic Studies. Soil and water sampling and testing, as well as topographic mapping, augmented the geophysical data. These geophysical surveys have aided in the determination of a location for a water well in the volcanic/limestone terrain. Along with the geophysical data, an interdisciplinary team of senior level engineering students, working through the Humanitarian Engineering Program at Colorado School of Mines, cooperated with the people of Chasnigua and developed a design for the water treatment, storage and distribution system.
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Integrated Use Of Surface Geophysical Methods For Site Characterization -- A Case Study In North Kingstown, Rhode Island
A suite of complementary, non-invasive surface geophysical methods was used to assess their utility for site characterization in a pilot investigation at a former defense site in North Kingstown, Rhode Island. The methods included frequency-domain electromagnetics (FDEM), ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and multi-channel analysis of surface-wave (MASW) seismic. The results of each method were compared to each other and to drive-point data from the site. FDEM was used as a reconnaissance method to assess buried utilities and anthropogenic structures; to identify near-surface changes in water chemistry related to conductive leachate from roadsalt storage; and to investigate a resistive signature possibly caused by groundwater discharge. Shallow anomalies observed in the GPR and ERT data were caused by near-surface infrastructure and were consistent with anomalies observed in the FDEM data. Several parabolic reflectors were observed in the upper part of the GPR profiles, and a fairly continuous reflector that was interpreted as bedrock could be traced across the lower part of the profiles. MASW seismic data showed a sharp break in shear wave velocity at depth, which was interpreted as the overburden/bedrock interface. The MASW profile indicates the presence of a trough in the bedrock surface in the same location where the ERT data indicate lateral variations in resistivity. Depths to bedrock interpreted from the ERT, MASW, and GPR profiles were similar and consistent with the depths of refusal identified in the direct-push wells. The interpretations of data collected using the individual methods yielded non-unique solutions with considerable uncertainty. Integrated interpretation of the electrical, electromagnetic, and seismic geophysical profiles produced a more consistent and unique estimation of depth to bedrock that is consistent with ground-truth data at the site. This test case shows that using complementary techniques that measure different properties can be more effective for site characterization than a single-method investigation.
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Estimating Groundwater Storage Changes In The Western Kansas Using Grace Data
Authors Bo Chen, Jianghai Xia, Qiuge Wang, Chao Chen, Richard D. Miller and Qing LiangThe Gravity Recovery and Climate Experiment (GRACE) delivers monthly gravity fields since it was launched in March 2002, which provides a new way to monitor the groundwater storage variations for large regions. In this study, we attempt to apply the GRACE data combined with estimated soil moisture based on the water balance approach to estimate monthly groundwater changes in the western Kansas of approximately 100,000 km2. The comparison of different Gaussian smoothing radiuses indicated that a smaller filter radius (150 km) is more appropriate for this size of the study area to get more effective gravity signals. The results are compared with in situ yearly measurements of groundwater levels and show a prominent seasonal cycle. The groundwater storage changes estimated from GRACE data agree well with the measured groundwater levels during 2003 and 2008. Both of them show a decline trend. Such observation results from GRACE data will provide regional fundamental information for water resource management.
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Spatio-Temporal Monitoring With Airborne EM Data – Still Wishful Thinking, Or A Realistic Proposition?
Authors Tim Munday, Andrew Fitzpatrick, Volmer Berens, Andrea Viezzoli and Kevin CahillAlthough the notion of spatio-temporal monitoring of natural landscapes and phenomena using multi-date airborne electromagnetic (AEM) surveys has been around for some time, examples are very limited in scope, particularly when defining vertical and lateral changes with time. We demonstrate an effective procedure for defining spatio-temporal variations in ground conductivity across a salinised floodplain in South Australia, using multi-date FDHEM data. Lateral and vertical changes in the conductivity of the floodplain have been resolved. We believe the advent of improved calibration procedures, geometry correction, calibrated broad band AEM systems and advanced inversion procedures that obviate the necessity of system calibration – recalibration, such as the holistic inversion, provide for the realistic proposition of using AEM data for the semi-quantitative and quantitative monitoring of landscape change in the subsurface. However, we emphasize the need for caution when considering observed spatial variations, stressing the importance of accounting for system investigation depth and the potential for artifacts that might be introduced from noise, system geometry and/or data interpretation procedures, when comparing data and derived conductivity models from different dates.
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Redefining The Groundwater Resource Of The Eyre Peninsula, South Australia Using AEM Data
Authors Andrew Fitzpatrick, Tim Munday, Kevin Cahill and Volmer BerensThe groundwater aquifer lens systems of the lower Eyre Peninsula are an important water supply source for Eyre Peninsula. Demand from the aquifers is near the current extraction limits and there is a need to undertake further modelling of the groundwater systems to ensure confidence in the limits determined. This paper describes the use of airborne electromagnetics to map important elements of the Quaternary and Tertiary aquifer systems in the area. TEMPEST time domain EM data were acquired over the known extent of groundwater lens systems. These data were inverted using both constrained and unconstrained techniques to define the conductivity structure in three dimensions. Hydrogeologically significant bounding surfaces were then defined, through the use of a line-by-line interpretation procedure to link borehole data with the observed ground conductivity structure.
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Groundwater Purveying Using Very Low Frequency Fracture Delineation Methods
Authors Peter J. Hutchinson, Maggie H. Beird and Matthew MitchellRandom drilling for commercially-productive groundwater wells is a haphazard method within the Pennsylvanian-aged rocks of the Appalachian Plateau Region of southwestern Pennsylvania. These rocks have low permeability and porosity and the average production well produces only enough yield for homeowner use. Often these wells are installed as an open hole to 300 feet to insure an adequate water supply for the homeowner since the well bore acts as a storage reservoir during recovery and drawdown. Three sites mapped with Very Low Frequency (VLF) methods delineated fractures with the potential to maximize bedrock production through increased fracture-induced permeability. A boring was advanced from a location at each of the three sites selected through VLF mapping. The borings penetrated fractures at the anticipated depths of between 10 and 20 meters below grade. Pump tests indicate that these three borings can produce between over 1,000l/min with little drawdown. Each of the three wells is a commercial success.
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Aquifer Characterization Using Coupled Inversion Of DC/IP And Mrs Data On A Hydrogeophysical Test-Site
Authors Thomas Günther, Raphael Dlugosch, Raphael Holland and Ugur YaramanciAmong hydrogeophysical methods, Magnetic Resonance Sounding (MRS) and Direct Current resistivity / Induced Polarization (DC/IP) measurements can be particularly interesting and useful, since the underlying parameters are related to hydraulic properties. We present a combined investigation of a well-known test site with two aquifers. The inversions for MRS and DC/IP data are combined using a structural coupling and yield improved models due to common but free boundaries. A further combination is achieved by cluster analysis of the obtained resistivity, phase and water content distribution. The approach of the coupled inversion is applied to data of the test site Schillerslage (Hannover, Germany). The simple model obtained agrees with ground truth from boreholes and laboratory as well as with other geophysical measurements.
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Comparison Of 2D And 3D Electrical Resistivity Imaging Methods To Target High-Yield Water-Bearing Vertical Fracture Zones
Authors Brent B. Waters, Robert Davis, Mac G. Morrow and Colin D. LangfordIn bedrock terrain, groundwater predominantly flows through discrete fracture zones within a low permeability bedrock matrix. Fracture zones are typically long, linear, near-vertical zones of increased fracture density found in most geologic settings. These zones are typically very narrow and are often expressed as natural topographic depressions such as straight stream valley segments, swales and sags in the land surface, or as linear tonal or vegetative alignments often referred to as lineaments or fracture traces. Often, however, fracture zones do not have surface expressions. In such cases, finding and intersecting fracture zones with a well is like finding the proverbial needle in the haystack.
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Using Airborne Geophysical Surveys To Improve Groundwater Resource Management Models
Increasingly, groundwater management requires more accurate hydrogeologic frameworks for groundwater models. These complex issues have created the demand for innovative approaches to data collection. In complicated terrains, groundwater modelers benefit from continuous high-resolution geologic maps and their related hydrogeologic-parameter estimates. The USGS and its partners have collaborated to use airborne geophysical surveys for near-continuous coverage of areas of the North Platte River valley in western Nebraska. The survey objectives were to map the aquifers and bedrock topography of the area to help improve the understanding of groundwater–surface-water relationships, leading to improved water management decisions. Frequency-domain heliborne electromagnetic surveys were completed, using a unique survey design to collect resistivity data that can be related to lithologic information to refine groundwater model inputs. To render the geophysical data useful to multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inverted model, in conjunction with sensitivity analysis, geological ground truth (boreholes and surface geology maps), and geological interpretation, is used to characterize hydrogeologic features. Interpreted two- and three-dimensional data coverage provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models reflect actual hydrogeology at a level of accuracy not achievable using previous data sets.
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Groundwater Evaluation Using Vtem Helicopter Electromagnetics In The South Platte Nrd, Sidney, Nebraska
Authors Jean M Legault, Paolo Berardelli, Jared Abraham, James Cannia and Kerim MartinezA VTEM time domain helicopter-borne electromagnetic survey was undertaken for groundwater evaluation in western Nebraska with the objective to map the extent of deep freshwater aquifers. The VTEM system combines near surface high resolution resistivity imaging and deep penetration making it a low cost, rapid reconnaissance mapping tool in areas with thick, conductive terrain. The survey was preceded by a synthetic computer modeling study that proved its resolution and penetration capability below 200m depth. The test survey that followed used a new X-Z component system and caesium magnetometer that covered a 165 square km region of the South Platte NRD in two short survey days. Data analysis using a variety of 1D (layered-earth) inversion and conductivity-depth imaging (CDI) tools indicate that, in spite of significant presence of man-made culture (powerlines and metallic structures), the moderately conductive Ogallala Group aquifer was successfully mapped to a known depth of 160m above the conductive White River Group Brule Formation bedrock aquitard. Simulated penetration depths in excess of 200m appear to have been easily reached, suggesting the use of VTEM to be extended into deeper, more conductive aquifers within the Natural Resource Districts of Nebraska.
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Advances In The Evaluation Of Pile And Shaft Quality
Authors Frank Rausche and Brent RobinsonFor the past half century, great efforts have been made and progress has been achieved in developing a variety of electronic testing methods for the quality control and quality assurance of deep foundations. These developments took advantage of major advances in ever more accurate and sensitive sensor manufacturing and faster and more powerful computers. The dynamic pile testing methods were the primary beneficiaries of these R&D efforts and its application has been expanded from bearing capacity assessment of driven piles to drilled shafts, micro piles and even penetrometers. In addition to soil resistance, results from construction monitoring now provide information about stresses along the pile, pile integrity and occasionally soil vibrations. Dynamic pile testing methods also include nondestructive techniques involving sonic and ultra sonic signals. Much of the recent developments involved not only ruggedizing hardware and preparation of more user friendly software, but also deriving reliable calculation procedures and presenting results in a way which is easy for the report recipient to understand. Additionally, experiences from construction sites showed that an immediate assessment of the foundation characteristics is imperative. This requirement lead to the need for easily used simulation software and workshops. Today such training events are frequently performed over the internet. This presentation summarizes several recent hardware and software developments and shows a few typical results.
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Quality Management Of Stabilized Soil Construction Using Lab And Field Seismic Testing
Authors Nathan M. Toohey, Michael A. Mooney and Nils RydenCurrent pavement design practice for stabilized soils specifies a 28-day modulus and stiffness value. There is a need to evaluate performance after 5 to 7 days to ensure efficient constructability. There is a large disconnect between quality management programs not only for design and construction, but between lab and field evaluations during construction. The difference in design and measured parameters, lab and field, for performance evaluation does not permit a direct comparison. Current quality management techniques do not permit assessment of the true performance of the pavement construction. Seismic wave based testing offers considerable potential for quality assessment of stabilized soils. Surface wave analysis can be used to assess low-strain or seismic modulus of stabilized soils in the field while travel-time analysis and free-free resonance testing can be used to estimate seismic modulus of laboratory specimens. Lab and field seismic moduli are directly related, thus permitting seismic analysis to bridge the gaps encountered by current quality management techniques. This paper presents lab and field seismic testing protocol and results which indicate its effectiveness as a combined quality management technique.
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Integrated Three Dimensional GPR And EM Study Of Three Large Water Supply Pipes
Authors Steven P. DiBenedetto and Heron R. MochnyA new 16-inch municipal water supply line is to be installed between two siphon chambers in the right-of-way (ROW) of three very large diameter (9+ft) steel water supply conduits in the northeastern United States. While excellent as-built records of the conduits were available, given the age of the infrastructure, exact surveyed locations of the large water supply pipes were unknown within the ROW, and were required during the design phase of the project in order to safely perform geotechnical borings and facilitate the routing of the installation trench for the 16-inch line. Additionally, the mapping of any underground utilities within the ROW and adjacent to an existing pump station was required for design in advance of excavation and construction activities relating to the expansion of the pump station. A geophysical survey was performed by Underground Imaging Technologies, Inc. (UIT), acting as a subcontractor to Malcolm Pirnie along the ROW in order to determine the precise location of the large water supply pipes and also the locations of any underground utilities or other subterranean features that may interfere or otherwise impact the installation of the new waterline. Multi-channel Ground Penetrating Radar (GPR), multi-sensor Time Domain Electromagnetic Induction (TDEMI) and a Ground Conductivity Electromagnetic (GCEM) survey was performed to obtain the requisite data. The geophysical survey was performed between May 19th and 22nd, 2009 and over 182,000 square feet were surveyed. The results of the geophysical survey were able to accurately determine the horizontal positions and depths of the water supply pipes throughout the investigation area, in addition to mapping the positions of utilities in the vicinity of future construction for a pump station upgrade. Several geological anomalies were detected that warrant consideration in any excavation or construction activities. Depth below ground surface of the pipes obtained via GPR was converted to elevation by subtracting from USGS 10m DEM ground surface elevation. This allowed for the construction of a fully georeferenced, elevation-based, three-dimensional digital model of the pipes within the investigation area in AutoCAD.
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Continuous Exploration Ahead Of The Tunnel Face By Tswd - Tunnel Seismic While Drilling
Authors Ewald Brückl, Werner Chwatal, Stefan Mertl and Alexander RadingerSeismic measurements using the VSP principle have been carried out for over 20 years to investigate the rock ahead of a tunnel face and made a contribution to reduce the risks during construction for conventional tunnelling, as well as for tunnelling with a Tunnel Boring Machine (TBM). However, reflecting horizons intersecting the tunnel axis obliquely cannot be imaged at their intersection with the tunnel. This circumstance imposes a major uncertainty on the prediction of the geological situation ahead of the tunnel face. A possibility to image all reflecting horizons, regardless of their orientation, at least near to their intersection with the tunnel axis is offered by continuous monitoring with the drilling head of a TBM as the seismic source (Tunnel Seismic While Drilling - TSWD). In this study we present continuous TSWD data from a gallery drilled in limestone and dolomite of the Northern Calcareous Alps, Austria. This gallery intersects a deeply incised valley, filled with sediments. The two main goals for the processing were the derivation of high signal to noise seismograms from the pilot and receiver signals and the removal of the first arrivals from the data in order to uncover reflected phases, even from reflectors very near to the TBM drilling head. Both goals were achieved satisfactorily and the main geological structure was well resolved. Geophysical issues which need further investigation are addressed. The results of our study suggest that substantial risk reduction could be achieved by continuous TSWD. However, the high production rate of modern TBMs imposes a major challenge on real time monitoring, processing and prediction.
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Geoelectrical Imaging For Detection Of Water Migration In A Bioreactor Landfill
Authors Håkan Rosqvist and Torleif DahlinIn this paper we describe field investigations of leachate recirculation at a bioreactor landfill using geoelectrical imagining technique (i.e., electrical resistivity) combined with a tracer test. The use of geoelectrical imagining techniques is an established practice for environmental investigations and monitoring of various landfill processes and in recent years also the bioreactor landfill concept has been emphasised. In the study, the electrical resistivity technique was evaluated and the possibility to detect water and gas migration in the waste mass was investigated. Results showing moisture migration through the bioreactor landfill, during leachate flushing and during a tracer test, are presented. Also results indicating the resistivity technique being useful for biogas detection are shown.
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An Evaluation Of The Potential Of The Geoelectrical Resistivity Method For Mapping Gas Migration In Landfills
Authors Håkan Rosqvist, Virginie Leroux, Torleif Dahlin, Sara Johansson and Mats SvenssonMethane is a powerful greenhouse gas and growing concern regarding global climate changes over the last years has pointed out the need to quantify and control the leaking of methane into the atmosphere. Landfill gas is regarded as one of the major sources for methane migration to the atmosphere. In this study we present research work with the objective to evaluate the use of geoelectrical resistivity to detect gas migration in landfills. Extensive field experiments were conducted at the Filborna landfill site in Helsingborg, Sweden, in August 2008. In general, the resistivity measurements showed results corresponding to results reported from previous investigations in waste. However, also large variations in resistivity were indicated. Relatively high variability and high mean resistivity in the surface-near layers clearly indicate influence on the resistivity in the upper zone of the landfill. The variability and high resistivity may partly be explained by appearance and migration of landfill gas.
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Case History: Monitoring Resin Injections With The Aid Of 4D Geophysics
Authors Justin B. Rittgers, Phil Sirles, Gianfranco Morelli and Marco OcchiGround improvement is an important practice in areas where the combined presence of expansive soils and fluctuating moisture content often result in differential heaving and settling soils and variable confining forces exerted on foundations. In fall of 2009, three-dimensional (3D) seismic pressure wave (p-wave) refraction and 3D electrical resistivity tomography (ERT) surveys were performed to determine the effectiveness of seismic refraction tomography in imaging the permeation of resin injected during ground improvement efforts within problematic soils. A test site near Piacenza, Italy was selected for this undertaking: Data were collected around the perimeter of an existing house suffering damage due to differential ground settlement. Three independent yet identical surveys were conducted using both methods, producing 3D images of the spatial distribution and variations in subsurface material properties before, during and after resin injection. The site is underlain primarily by a two to three meter thick layer of silt and clay with some sand lenses over an alluvial silty sand base layer. Prior to resin injection, substantial variations in both electrical resistivity and p-wave velocity are evident beneath the particular corner of the structure that exhibits the highest extent of damage (e.g., cracking and settling). During and after resin injections and curing, resistivities and velocities can be seen to change in the surrounding vicinity of injection points, and the magnitude of change in both data sets appears to be related to the amount of resin injected at each point. Geophysical measurements confirm the heterogeneous nature of the near-surface soil at this site, results from both methods correlate well with each other and additional field data (i.e., CPT data), and resultant 3D models are clearly useful in siting injection points as well as time-lapse monitoring the extent of resin permeation.
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Identification Of The Polaris Fault Using Lidar And Shallow Geophysical Methods
Authors Lewis E. Hunter, Michael H. Powers and Bethany L. BurtonAs part of the U.S. Army Corps of Engineers’ (USACE) Dam Safety Assurance Program, Martis Creek Dam near Truckee, CA, is under evaluation for earthquake and seepage hazards. The investigations to date have included LiDAR (Light Detection and Ranging) and a wide range of geophysical surveys. The LiDAR data led to the discovery of an important and previously unknown fault tracing very near and possibly under Martis Creek Dam. The geophysical surveys of the dam foundation area confirm evidence of the fault in the area.
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Internal Structure Of An Ilmenite Mine Waste Rock Pile Modelled From Ip Imaging And Laboratory Measurements
Authors Michel Chouteau, Rachid Intissar and Michel AubertinNickel concentrations occasionally above the allowed norm have been measured in water samples in the vicinity of the Petitpas waste rock pile at the Tio mine, Québec, Canada. In order to understand the chemical and hydrodynamic mechanisms responsible for the nickel dissolution and release in the environment a large hydrogeological/geotechnical/geophysical study was undertaken on the very large rock pile (600 m x 300 m x 150 m). Geotechnical and physical properties were measured in the laboratory on samples of waste with various moisture content and salinity. Induced polarization allowed the determination of electrical resistivities and chargeabilities that were to be used to interpret time-domain IP tomography surveys carried out on the waste rock pile. IP survey data were inverted in 2-D. The resistivity models for all survey lines show common features. From surfae to 25-30 m depth, the subsurface is resistive and show lateral changes which suggests large grain size heterogeneities and low water content; below and up to 50 m, a quasi uniform layer with resistivities in the range 50-250 ohm.m suggests fine-grained material with increased saturation. Finally below a depth of 50 m, the subsurface becomes very resistive again, indicative of coarse low-moisture content material. The chargeability model shows no structural correlation with the resistivity model. In general, chargeabilities are very large (up to 100 mV/V) and show no layering. From the laboratory and the survey results, we interpret the resistivity to be sensitive to water content and salinity while chargeability is sensitive to the metal concentration. The geoelectrical model and the interpreted hydrogeotechnical model will be useful for modeling water flow and reactive transport through the waste rock pile.
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Mapping Bedrock Topography Beneath Glacial Overburden Near Cape Horn, South America
More LessPlacer gold mining operations were carried out on several of the small islands between Tierra del Fuego and Cape Horn in the late 1800’s, just before the Klondike gold rush. As soon as the Klondike rush was announced to the world many of the miners and prospectors working in this isolated region went north. The area was quiet for nearly a hundred years before there was any renewed interest in it. This was due to the dispute between Chile and Argentina over ownership of these islands. Although the Chilean government was eventually granted ownership of these islands it is only recently that any exploration activity has occurred. A placer gold exploration program that included surface sampling, geological mapping and bulk sampling was carried out in the early 1990’s. The project looked promising but there was difficulty in unravelling the glacial history and obtaining estimates of gravel thickness for tonnage calculations. Bemex Consulting International was asked to set up a program to map the bedrock topography and estimate gravel thickness. The project was carried out in two phases. The first phase was carried out during the summer season (December) to determine if time-domain EM soundings would provide the necessary accuracy to meet the above objectives. At the end of this phase the conclusion was the Geonics EM-47 system was capable of mapping the gravel thickness and bedrock topography. The second phase was conducted the following summer (January and February) and consisted of carrying out more than two hundred 40 m x 40 m loop soundings at the site. The results of this survey were verified with a series of manholes dug to depths varying from 15 m to more than 30 m. The depth to bedrock in these holes was consisted with the 1D interpretation provided from the Interpex software.
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Earthquake Hazard Maps Of The City Of Ottawa, Canada, Using Near-Surface Geophysical And Geological Methods
The city of Ottawa, Ontario, Canada, is in an area of elevated earthquake shaking hazard and is considered to be the Canadian city third-most at risk to damage (after Vancouver and Montreal). Maps showing regional variation of Vs30, fundamental site period, and an example map of the spectral accelerations at Sa=0.2s have been completed for the city at a reconnaissance scale. These have been developed from combined geological information from ~21,000 boreholes (water wells and geotechnical borings) within the city limits as well as ~700 surface and borehole shear wave seismic measurement sites. Borehole and surface shear wave measurement techniques developed for the operational area included: surface reversed refraction and reflection sites, MASW, downhole shear wave and horizontalto- vertical spectral analyses of ambient noise. The surficial geology was subdivided into three basic units based on their geotechnical properties: (i) soft, fine-grained, post-glacial sediments, (ii) glacial till and coarse-grained, till-derived sediments, and (iii) firm Paleozoic and PreCambrian bedrock. A series of shear wave velocity-depth functions were assigned to each borehole based on interpolation from proximal geophysical sites. The resulting three-dimensional shear wave velocity database was then used to determine parameters such as Vs30 and fundamental site period. The Vs30 map of the city is subdivided in terms of the U.S. National Earthquake Hazard Reduction Program (NEHRP) zones which have been adopted by the 2005 National Building Code of Canada. Since the City of Ottawa has a large areal extent and is close to the West Quebec Seismic zone, the base accelerations for the 2% in 50 year
earthquake event vary extensively within the city boundaries. Hence an example map of the spectral accelerations at Sa=0.2s has been developed to demonstrate regional variations and the effects of NEHRP zones.
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Finding Faults Using High-Resolution Aeromagnetic Data In Great Sand Dunes National Park And Vicinity, San Luis Valley, Colorado
Authors V.J.S. Grauch, David V. Fitterman and Benjamin J. DrenthHigh-resolution aeromagnetic data reveal faults near the Great Sand Dunes National Park and Preserve (GRSA) and vicinity that offset both bedrock and sediments, even under the dunes themselves. Complicated fault patterns evident in the aeromagnetic data over the exposed range front may represent multiple-age events. The range-front and parallel faults to the west together suggest that basement steps down along high-angle faults. A prominent linear feature parallels the range front from the dunes southward (“the parallel fault”). Preliminary analysis suggests that the top of the parallel fault is about 50-70 m deep, with near-vertical dip, and offsets sediments with differing magnetic properties as well as basement rocks at greater depths. In places, it is paralleled by less prominent aeromagnetic features, which may also be faults. TEM soundings across the parallel fault indicate that sediments juxtaposed there contain abundant clay to the west and mostly sand to the east. The clay-rich sediments are underlain by eastward-shallowing sand at depth. This result suggests that the eastern limit of the confining clay in this area is fault-bounded at shallow depths and may be interfingered with sand at greater depths. These relations have implications for groundwater modeling, geologic studies, and seismic hazards.
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