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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
21 - 40 of 131 results
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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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