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27th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP)
- Conference date: 16 Mar 2014 - 20 Mar 2014
- Location: Boston, USA
- Published: 16 March 2014
53 results
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A Description Of An Effective Sinkhole Investigation Approach: A Case Study Of A Site In Greene County, Missouri
Authors S.C. Nwokebuihe, E.V. Torgashov and N.L. AndersonA sinkhole had developed and led to the subsidence of a portion of the parking lot of a residential building in Nixa, Missouri. The sinkhole was characterized using a combination of the geophysical data (multichannel analysis of surface waves and the electrical resistivity tomography) and supplemental data (aerial photographs, topographic and geological maps and borings). The interpretation of the data indicated that a fault trending northwest – southeast runs across the site and is underlain by shallow, fractured limestone bedrock of the Burlington Keokuk formation. The sinkhole is understood to have been triggered by the excavation of a surface drainage channel and pit adjacent to the building in order to check flooding. However, the action led to the increased concentration of flow of surface water into the fault zone, hence causing an increased dissolution of bedrock and raveling of sediments into the formed cavities and subsequently sinkhole development.
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Reflection Of Winter Road Salinity In P-THEM Data
By A. VetrovDuring the development of the P-THEM system, the author and his colleagues have made numerous test surveys on different sites in Quebec and Ontario. One of the test sites, the Alliston area, has been selected for the P-THEM system tests in June 2010. The area is a developed urban zone in the Greater Toronto Area, Ontario, Canada. Geologically, the area is represented with Quaternary sediments with underlying bedrocks of the Ordovician formation. The close distance of the area to Holland Landings airport made it preferable for testing system functionality. After processing observation data, the salty soil along roads where the salt is spread during winter time has been detected on the Tau-map. Another test flight in January 2013 showed an identical image. The achieved results of the surveys show the possibility of the time-domain electromagnetic (TDEM) airborne system for applications in the characterization of environmental properties, such as the detection of surface and near surface pollution.
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Optimization Of A Mobile Capacitatively-Coupled Geophysical Survey Used For Electrical Resistivity Tomography
Authors T.C. Goode, T.P.A. Ferré and A.C. HinnellImproving the efficiency of geophysical surveys is becoming more important with the increased application of geophysical methods in subsurface investigations. With this in mind, we propose a relatively simple optimization approach for mobile geophysical surveys. Our approach specifically evaluates the use of a capacitatively-coupled resistivity (CCR) array for electrical resistivity tomography (ERT). We demonstrate our approach through simulating a small number of CCR surveys used to identify and discriminate a target tunnel of interest from other similar tunnels in the subsurface. These examples provide insights into the practical application of the optimization process developed through this study. The mobile electrode arrays that were evaluated used a dipole-dipole configuration with a maximum survey length of 50 m. By evaluating different CCR survey designs and their respective measurement responses to buried tunnels of different size and depth, we show a means to identify and discriminate among tunnels using only voltage measurements. The differences in voltage between surveys were used to create an error response surface for a range of tunnel sizes and depths. These error response surfaces can be converted into target discrimination maps which identify tunnels that can and cannot be discriminated from a selected target tunnel. The fractional discrimination for each tunnel obtained from the target discrimination maps is used to create target identifiability maps which indicate the ability for each survey to discriminate among a range of target tunnels. Considering all possible target tunnels, the average target discrimination for a survey was selected as a measure of survey efficiency. Using this measure of survey efficiency, we provide examples of the potential impact of survey design and speed on the discrimination of target tunnels. In turn, we identify the survey with the highest efficiency and the least number of survey measurements as our optimal survey design.
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Classification Of Cued Metalmapper Data Using Data Mining Techniques
By D. HallA total of 1444 MetalMapper cued soundings were inverted for polarizability transients and then analyzed using data mining techniques to classify anomalous responses as either likely targets of interest or unlikely to be targets of interest. Polarizability transients were parameterized using six scalar moments covering size, shape, and persistence. Additional parameters were generated using a curve fitting algorithm to calculate the best fit coefficients of a polynomial function matching the polarizability transients in log/log space. Parameters were then evaluated using principal component analysis and visual observation to reduce the parameter set to the most definitive elements. Data mining algorithms including the clustering approach K means, and the classifier multi-layer perceptron were used to evaluate training data with known anomaly sources. Approaches developed and refined using training data were then applied to responses with unknown anomaly sources to determine if they represented targets of interest or were unlikely to be targets of interest. Anomalies were ranked according to the likelihood that they represented a target of interest. All of the targets of interest were recovered within the first 13 percent of the ranked list.
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Critical Depths For Higher Modes By Minimally-Invasive Seismic Profiling: Simulations And Field Test
Authors J.C. Ashlock and S. LinTo measure more complete multi-mode dispersion data and thus improve the accuracy of inversion profiles, the authors recently developed a minimally-invasive hybrid surface-and-borehole method. The new method employs a borehole geophone at selected depths to record seismic waves from a moving source on the soil surface. The goal of this paper is to develop a procedure for estimating the ranges of optimum geophone depths for capturing the higher modes. Stiffness matrix and finite element-based numerical simulations of the hybrid testing method are performed to identify the relationships between critical geophone depths and cut-off frequencies. A preliminary field test is conducted using a vertical geophone placed at five depths while impacts are applied to the soil surface over a range of offsets. Dispersion images from the five geophone depths were superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only MASW method. A comparison of the experimental and theoretical cut-off frequencies of higher modes is used to validate the prediction of critical depths by the stiffness matrix method. Matching of such experimental and theoretical cut-off frequencies is proposed as additional optimization constraints to reduce the uncertainty of final inversion profiles.
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Comparison Of MASW And MSOR For Surface Wave Testing Of Pavements
Authors S. Lin and J.C. AshlockThis paper presents a computational and experimental study on seismic stiffness profiling of pavements using the multi-channel analysis of surface waves (MASW) and multi-channel simulation with one receiver (MSOR) testing procedures. Development of a new custom-programmed data acquisition system for MASW and MSOR testing using MATLAB software and National Instruments hardware are detailed. Effects of different receiver coupling methods on the test results are examined. The cross-correlation function is employed to statistically quantify the repeatability of impacts, which is critical for MSOR tests in which multi-channel records are simulated by performing multiple impacts over a range of incremental offsets from a single fixed receiver. Experimental dispersion data from MASW and MSOR tests performed at the same site with the same testing system are compared, and MASW is found to enable measurement of dispersion data to much higher frequencies than MSOR. Inversion results from MASW and MSOR data at the same site are compared, and it is found that MASW is able to provide measurements of the stiffness of the surface layer with reduced variability.
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Method For Isolation Of Gravity Signatures Due To Major Earthquakes From Satellite Gravity Data
Authors A. Pant and R.G. SastryOften, satellite gravity (GRACE Satellite) is employed to infer near surface fluid exchanges and budgets on a global scale. However, devastating major earthquakes in offshore regions involving mass-wastage must also have a significant gravity signals in satellite gravity campaigns. But their isolation is masked by other time-varying fluid exchange signals near earth’s surface. Here, our differential data analysis (Spherical harmonics) attended by spectral filtering has successfully isolated weak gravity signals in micro-Gal range for three major earthquakes. For illustration purpose we include Sumatra earthquake (2004). Our method could infer the rupture zone in the source region. Presently, a combined seismological and gravity modeling of satellite gravity derived gravity signals are underway. These efforts if successful could be a huge step in predicting earthquakes.
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Web-Based Database Of Integrated Geophysical Method For Levee Safety Assessment
Authors K. Hayashi, T. Takahashi, T. Inazaki, K. Kitao and T. KitaIn order to evaluate the safety of levees based on an integrated geophysical method, a web-based database containing results of the integrated geophysical method, boring logs and levee safety assessment records was constructed. We collected the results of surface wave methods and resistivity methods performed at 37 Japanese rivers as well as the results of borings conducted on survey lines of geophysical methods. A land streamer and a capacitively-coupled resistivity method were generally used in the data acquisition of the surface wave methods and the resistivity methods, respectively. Analyzed results were saved as a standard XML format defined by The Society of Exploration Geophysicists of Japan. The web-based database consists of four parts: A) a database of the geophysical investigation results, B) a database of levee safety assessment and maintenance records, C) a database of blow counts (N-value), soil type, S-wave velocity (Vs) and resistivity, and D) a database of Vs, resistivity and laboratory tests. Data registered for the database is as follows: length of about 650 km of geophysical investigation results, about 400 boring logs, about 4,000 relationships of Vs, resistivity, N-value and soil type, about 1,000 relationships of Vs, resistivity and laboratory tests, 340 km of levee safety assessment and maintenance records. The database was constructed on a web server using MySQL and PHP so that users access to the database through the internet. This paper presents an outline of the database, an example of data, and an application example of the database and proposes a management framework of the database in the future.
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Monitoring Of Joint Systems Time-Lapse Behaviour Via Ert
Authors J. Jirku, J. Barta, J. Knez and J. VilhelmElectrical resistivity tomography (ERT) as a tool for monitoring the processes and time-related changes in geological environments has made great progress in recent years and has become standard for observing natural phenomena. This method is simple to use and it often provides high-quality results that are well interpreted. Our research is based on observing time-lapse changes of the physical parameters (conductivity, for instance) of joints systems (mostly in crystalline massifs). The primary aim is to develop a monitoring system mostly for the needs of deep repositories of nuclear waste. Geophysical research of such repositories has so far dealt only with one-off research (no time-monitoring) of potential host rock´s properties. Contrary to this, our developed system and methodology is unique in continuously measuring the physical properties (in this case, electrical resistivity) of the rock massif. This system will be permanently fixed in the field and by observing changes in measured data reports if any remarkable occurrence in the EDZ zone is or was happening (for example, opening or closing of the joints or micro-fractures). Today, our monitoring system is being developed and tested at the field base in Bedrichov (northern Bohemia). The Bedrichov gallery is approximately 2.5 km long and continuously leads through one type of granite massif. Point 792, where a significant geological fracture zone goes through, was chosen as an ideal place for taking repeated measurements via our ERT system. Currently, we test the measurement quality, such as whether our system provides real data and is able to reliably observe resistivity changes measured at ERT profiles.
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Optimized Interpretation Of Sageep 2011 Blind Refraction Data With Fresnel Volume Tomography And Plus-Minus Refraction
More LessWe improve the resolution of subsurface P-wave velocity tomograms with Fresnel Volume Tomography and Wavepath Eikonal Traveltime inversion, by iteratively decreasing the wavepath width. We use the SAGEEP 2011 blind refraction synthetic traveltime data to compare our tomograms with the known true model. We compare weighting of the wavepath velocity update with a Ricker wavelet vs. weighting with a Gaussian bell function. Plotting Plus-Minus refractors obtained with layer-based interpretation on the 2D velocity tomogram better visualizes both methods. Tomograms from an iterative approach of wavepath adjustment show improvement over the standard ray-based method.
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Use Of An MASW Survey To Assess Flood Damaged Road – A Case History
Authors K. Suto and R. KristinofAn MASW survey was carried out over and around a road severely damaged by flood waters, and in the creek-bed below. The MASW technique was adopted because a significant amount of the site was accessible only by foot. The survey comprised 936 metres in six lines, on which 85 points were analysed for S-wave velocity profile. From the S-wave velocity, Young’s modulus and pseudo-N values were estimated, assuming density and Vp/Vs ratio and an empirical equation. While the data quality was reasonable and free of background noise, the analysis was challenging due to quick lateral variation of the S-wave velocity. The use of relatively fewer traces for lateral resolution results in a low signal-to-noise ratio, and identifying the fundamental mode Rayleigh wave in the frequency-phase velocity domain becomes difficult. To combat this, a number of mute parameters were tried to improve the dispersion images. The results show S-wave velocity sections with rapid variation and reasonably consistent confidence levels. The S-wave velocity sections were used to infer the depth to bedrock beneath the road, providing information on foundation depths on which gabion slope retention structures could be founded. Information on depth to bedrock was also used to inform how much a particular section of the road could be lowered, thereby reducing the height of planned slope retaining structures, and thus the overall capital construction cost of the project. The line along the creek bed was difficult to analyse due to the weak signal to the distant geophones, however the results provided a reasonably accurate interpretation of the thickness of fluvial sediments, which was proven during construction. Overall the results of the survey correlated well with boreholes drilled in areas where vehicular access was possible; following close correlation in those areas, the survey data was successfully extrapolated to other areas where access was only possible by foot.
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Sub-Slab Characterization Of Gavins Point Spillway With Ground Penetrating Radar Mapping
Authors G. Byer and R. GrabowskiHistoric Missouri River flooding in June 2011, prompted the USACE1 to conduct physical inspection of exposed spillway slabs, supplemented with concrete coring and Ground Penetrating Radar (GPR) testing, as part of an overall risk evaluation of the six Missouri River Main Stem Dams in Montana, North Dakota, South Dakota, and Nebraska (USACE, 1998). In addition, the USACE mimicked operational conditions during the flood event to evaluate effects on the vertical/lateral subdrain systems beneath the spillway slab at Gavins Point Dam, near Yankton, South Dakota. The spillway gates were operated at 12,000 cfs flow with various configurations of gates discharging. Anomalous conditions observed during the half-day test suggested pressurization beneath the spillway slab. Further evaluation was performed with GPR on normally inundated portions of the spillway slab to evaluate the effects of this condition. During an 8-hour period, the Missouri River was lowered, exposing the upper spillway slab. Three teams simultaneously operated three 250-MHz GPR systems to gather bidirectional data on 2-foot centers in an area of approximately 54,590 square feet. Concurrently, concrete coring was performed near damaged vertical drains found during the half-day test. The GPR data were subjected to thorough data processing including: 1) amplitude mapping; 2) digitization and mapping of the top of bedrock; and 3) line-by-line inspection for trains of multiple reflections, shadows, and pull-up/push-down in travel time to bedrock. Based on knowledge regarding construction details, a conceptual model was developed from which prediction of the behavior of the GPR wavelet was made under various scenarios where void thickness and fill (water or air) were considered. Based on this conceptual model, the aerial extents and thicknesses of voids beneath the spillway slab were interpreted. Post-interpretation sonic drilling and borehole geophysical logging were performed to help constrain the conceptual site model and GPR anomalies. Interpretation of the void extents and a hydraulic explanation of void formation by considering multiple lines of geophysical, hydraulic, and other observational evidence provided an enhanced understanding of subslab conditions. This improved understanding was used to inform decisions on post-flood repair strategies as well as the implementation of risk reduction measures to improve the function and reliability of the slab subgrade.
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Interpretation Of Seismic Tomography Results Using Data Quality And Residual Error Maps
Authors . Fechner, L. Karl, S. Mackens, W. Albers and D. TweetonIn seismic borehole tomography, the interpretation of the results is commonly limited to the comparison of the velocity map, the ray coverage, and the global root mean square RMS residual. However, the quality of the seismic data has a significant influence on the accuracy of the arrival time picking but is generally not considered in the inversion. This paper presents an enhancement of the inversion taking into account the data quality, based on the signal-to-noise ratio, by using it to weight the travel time residuals in each iteration step. This implementation also calculates the spatial distribution of the data quality and the distribution of the residual remaining at the end of the inversion, which are used to support the evaluation of a velocity map. The effect of the data weighting is studied on a field data set. Quality and residual maps are given and their relevance for the interpretation is discussed. The results indicate that areas of exceptionally high signal attenuation can be identified by means of the quality information.
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Intelligent Meshing For Geophysical Inverse Problems Using Unstructured Meshes
Authors T.-K. Chou, M. Chouteau and J.-S. DubéMesh generation to solve geophysical forward problems is a thoroughly studied area that has seen the development of many methods and techniques. In geophysical inverse problems, a priori structured mesh is often used for inversion because the geometry of the underlying subsurface structures is unknown and mesh refinement is applied if needed by the user only after observing the inversion results. We present an intelligent meshing approach for an electrical resistivity tomography inverse problem. This new approach uses the Harris corner and edge detectors that are based on the local autocorrelation function of a signal (Harris and Stephens, 1988). The process optimizes the size of the inverse problem by refining areas where the boundaries of physical structure seems to be important and generates a more appropriate and optimum mesh for the inverse problem. The performance and robustness of the proposed algorithm are determined through a series of tests using 2D ERT modelled data and survey data. Tests on modelled data have demonstrated that the proposed meshing technique can reduce data misfit, produce a better model reconstruction, minimize the size of the inverse problem and reduce computational resource requirement. Tests on survey data from application such as ground water mapping have demonstrated that this new meshing approach produced data fit and inverse solutions that are comparable to conventional meshing and fine meshing techniques while minimizing the size of the inverse problem.
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3D Stochastic Gravity Inversion On Unstructured Meshes
Authors P. Shamsipour, M. Chouteau, D. Marcotte and E. SchetselaarThe stochastic gravity inversion on unstructured meshes is presented. The unstructured meshes are used because they provide the flexibility required to closely approximate complicated geological structures. Sharp topographic relief and geological bodies of complex shapes are usually more accurately described by unstructured meshes rather than with regular grids. The forward method has a closed form solution and gives accurate results. A geostatistical method is applied to invert the gravity data.
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Potential Field Inversion On Nodes For Stochastic Inversion Models
Authors D. Marcotte, P. Shamsipour and M. ChouteauInstead of applying gravity inversion on a large number of small prisms which is computationally very expensive, it is proposed to perform inversion on a limited number of nodes which include all the available point density data (if any) plus points sampling the inversion space fairly. It is also possible to include sets of linear constraints in the inversion. If no noise is present, the inverted field reproduces exactly the added constraints and the gravity field. These methods can be applied to the magnetic potential as well as the joint inversion of gravity-magnetic field data.
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The Application Of Monte Carlo Simulation To Borehole Gamma-Gamma Density And Spectral Gamma Calibrations
Authors J.R. Stowell and L.M. MartinezThe application of the Monte Carlo statistical processing method, originally developed during the Manhattan project in the 1940s, was originally restricted to government lab work with applications focusing on nuclear weapon design, nuclear reactor design and other radiometric processes. With the development of ever more powerful computers, numerical processing that required a room-full of main frames 25 years ago can now be accomplished on a high end lap-top. While the learning curve on the use of MCNP modeling can be steep, the availability of modern PCs and access to the code now allows others to make use of this powerful tool to assist in the development of better borehole nuclear measurements. It is now possible to find commercial providers who will model downhole tools and predict their response to natural gamma radiation in earth formations, and provide real time spectral concentrations based on MCNP and correlation with known physical models. Stability and repeatability of downhole spectral measurements has been difficult to accomplish and these methods promise much better results. In addition, the application of MCNP to other borehole radiometric measurements, such as gamma-gamma density and litho-density may provide users with much needed confirmation of field calibrations results and the normal drift associated with source half-life, borehole corrections, and overall tool performance. MCNP modeling may also be used to design better source shields and calibration fixtures to allow more accurate and safer field operations. This paper will review some of the current MCNP borehole spectral applications, as well as present examples of MCNP applied to slim-hole gamma-gamma density tools, source shields and calibration fixtures.
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Electrical-Leak Testing Of Geomembrane-Lined Basins
Authors P.J. Hutchinson, B.J. Teschke and M.H. BeirdImpermeable geomembrane-lined basins are used to contain waste water and solid waste. Postinstallation penetrations, seam failures and imperfections in the geomembrane sheet are not uncommon and provide an avenue for waste water and leachate to escape into the environment. Electrical leak testing of geomembrane integrity is based upon the mise-á-la-masse or ‘chargedbody potential’ electrical resistivity test. The leak test of a geomembrane liner consists of placing a current electrode within the basin and another current electrode in a semi-infinite distance away from the basin. Penetrations of the geomembrane liner provide a current pathway that can be located and mapped.
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High-Frequency Surface Wave Measurement For The Pavement Structural Analysis
Authors T. Inazaki, K. Aoike, T. Kita and K. HayashiDevelopment of non-destructive and high performance testing methods has been requested for the pavement assessment. While FWD (Falling Weight Deflectometer) has been widely utilized as such NDT equipment in Japan, the inversion process of FWD is still ambiguous. Dynamic response of the surface pavement is simplified to static torsional deformation. In contrast, high-frequency surface wave measurements are capable to provide detailed information on the pavement. The authors have started a research project on non-contact, continuous surface wave measurement using arrayed microphone. At the initial phase, we conducted fundamental measurements using piezoelectric accelerometer array. A total of 48 accelerometers were pasted on the pavement surface with clay pad at 10 cm intervals. Observed data showed clear dispersion in the frequency range from 40 to 5000 Hz, and analyzed as A0 mode Lamb waves or normal mode surface waves using a generic algorithm (GA) technique. Parameter study was conducted on the surface pavement structure to the responses of dispersion curves of surface waves. Combined with parameter sturdy, GA inversion successfully reconstructed surface pavement Swave velocity structure. The estimated structure was verified by comparative measurement using double frequency (300 and 800 MHz) antenna system GPR as well as check drilling.
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Cross-Plot Analysis By Using Rock Physics-Based Thresholds For An Evaluation Of Unsaturated Soil
Cross-plot of resistivity and S-wave velocity (Vs) is one of the effective methods to evaluate earth structure, e.g. river levee, because both properties are relatively easy to be obtained by geophysical methods such as capacitive resistivity survey and surface wave method. From the cross-plot of the two geophysical properties, we can interpret the soil types and looseness based on the observations that resistivity and Vs are sensitive to the grain size and looseness of the soils, respectively. For example, dividing the cross-plot by two individual thresholds for resistivity and Vs is the easiest way to interpret the data; however, setting appropriate thresholds is the most challenging issue in this methodology. In order to overcome this problem, in this study, I propose the way to determine the objective thresholds by using rock physics models. Particularly, the effect of water saturation is significant to interpret shallow subsurface; thus I modify the parallel circuit model for the resistivity prediction to account for the saturation effect. For the Vs prediction, I apply the soft sand model with considering clay content. These rock physics models enable us to connect the two properties by two common parameters: porosity and clay content. Both parameters are included in the models. Therefore, if we assume that porosity and clay content are proxies for looseness and grain size respectively, we are able to select certain values of porosity and clay content as the thresholds to discriminate the data indicating vulnerable area in the cross-plot. Preliminary analysis shows that the segmentation done by the rock physics-based thresholds gives a reasonable result that agrees with a previous study and the geophysical dataset obtained at various river levees.
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Complementary Pavement Subsurface Assessment Using Mobile Acoustic Subsurface Sensing And Ground Penetrating Radar Systems
More LessPavement subsurface condition inspection is critical for transportation infrastructure health assessment. Mechanical properties, layer picking and de-bonding/voids detection are of specific interest. Previous research work either measured mechanical properties using acoustic surface waves methods in a slow contact sensing way, or detected pavement layers with a commercial Ground Penetrating Radar (GPR) system. This presentation explores a joint analysis of pavement subsurface properties using two subsurface sensing systems, a Mobile Acoustic Subsurface Sensing (MASS) and an air-coupled GPR with innovative design. MASS measures the near-surface radiating surface waves excited on the pavement with sound enclosed directional microphones. The radiating surface waves carry the pavement profiles information of mechanical properties as elastic modulus through dispersion features. The dispersion curve could be accessed rapidly through acoustic signal processing such as windowing, filtering and spectrum analysis. The challenge is the rapid inversion for irregular pavement profiles with no prior knowledge of layering. GPR is able to collect reflected electromagnetic signal from the interfaces of different layers with dielectric contrasts. Layer boundaries can be estimated by cross correlation with transmitted signal such that we can get the traveling time of radar signal within different layers. As well, the velocity of radar signal within each layer can be calculated from reflection coefficient by deconvolution. Therefore, we can identify the thickness of each layer of pavement. This work presents the joint analysis of complementary MASS and GPR measurements. First the layer boundaries are determined from the GPR data. This information is used as starting values in the acoustic inversion determining the mechanical properties. Results show improved inversion results using the GPR layer information. The subsurface assessment can be achieved rapidly in a mobile dual system strategy. This joint NDT strategy would supply an alternative solution for faster assessment for pavement subsurface with both thickness and stiffness profile.
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Electrical And Gravity Mapping Of A Sinkhole In State College, Pennsylvania
Authors P.J. Hutchinson and H.L. KrivosMemorial Athletic Field, State College, Pennsylvania was installed within an existing sinkhole. Though its early history included such uses as waste disposal and limestone mining, the Works Project Administration in the late 1930s converted the sinkhole into Memorial Stadium. Subsequently, a surface opening (throat) on the north side of the field was connected to a series of pipes that drain 50 acres of street runoff. A major issue with exploiting the throat for runoff disposal is that the void beneath will grow due to erosion, dissolution, and subsidence. Through electrical imaging (EI) and gravity mapping, the top of the limestone within the athletic field is funnel-shaped and the interior of the “funnel” is clay-filled. The throat used for street runoff is rimmed with a 5-foot thick cap or roof of dolomite/limestone. Profiles of the gravity data using a forward modeling program show a well-developed clay-filled sinkhole beneath the bleachers. The microgravity data also shows the presence of a north-south deep-rooted fracture that probably opens to a larger series of voids at depths of greater than 60 feet.
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Efficient Underwater Site Characterization Using A Layered And Integrated Technology Approach
Authors J. Foley, J. Jacobson, D. Jennings, M. Miele and J. SteinbergsSite characterization is inherently difficult in underwater settings. Unlike for terrestrial sites, essential information impacting the effectiveness of sensor platform deployments, caused by variable vegetation and topographic conditions, cannot be defined through simple site visits or review of widely available airborne or satellite imagery. An underwater site is usually “dark.” An investigation team may have access to bathymetry from the National Oceanic and Atmospheric Administration (NOAA), or local sediment conditions from prior biological evaluations, but rarely does a site present itself for investigation with detailed bathymetry, sediment conditions and geo-registered locations of materials on the seafloor.
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Advanced Information Management To Facilitate Geophysical Anomaly Classification At Munitions Sites
Authors J. Foley, P. Hille, M. Miele, J. Jacobson and H. NgoThis paper presents an advanced information management system deployed to facilitate and improve geophysical data collection and analysis for production surveys where munitions anomaly classification is required. This work demonstrates that comprehensive and rigorous information technologies are essential to effectively integrate geophysical classification methods at Military Munitions Response (MMR) sites. Recently, advanced electromagnetic (EM) sensors utilizing multi-axis, time-domain transmitter and receiver arrays have proven effective for classifying buried targets as either munitions and explosives of concern (MEC), including unexploded ordnance (UXO), or non-threatening debris or scrap. Coupled with sophisticated analysis methods, also proven effective, this methodology has potential to significantly reduce UXO remediation costs. One major barrier to wide use of classification technologies is the challenge related to management of diverse and voluminous information developed during the surveys. Demonstrations of HDR’s new Classification Information Management System (CIMS) were conducted at recent on-going MMR sites. Test grids were selected to conduct anomaly classification utilizing CIMS as part of ongoing projects. HDR deployed its MetalMapper survey to collect data and perform in-field quality control (QC), and CIMS was utilized to streamline collection, classification, data management and reporting. Evaluated elements of the geophysical classification included; (1) project planning and site suitability for sensor platforms; (2) instrument verification to validate system performance; (3) HDR3D MetalMapper data collection and dissemination; (4) in-field QC of collected data; (5) production of daily QC reports; (6) tracking anomalies requiring re-interrogation; (7) off-site anomaly classification; (8) QC target selection; (9) intrusive groundtruth collection; and (10) data and information presentation for review and reporting. Data management during classification surveys is critical, and the CIMS architecture proved effective for comprehensive data management. The CIMS manages all survey information from original digital geophysical mapping (DGM) surveys through to intrusive actions using a comprehensive database architecture and associated geographical information system (GIS) access and reporting tools. The CIMS also establishes a data manifesting and communication framework allowing access to essential classification information during dynamic and on-going projects.
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Application Of Potential Geophysical Fields In Ore Deposits: Inverse Problem Solution Under Complex Conditions And 3-D Gravity-Magnetic Field Modeling
More LessOre deposits usually occur in mountainous conditions characterized by rugged relief, complex and variable geological medium. As a rule, application of various modifications of seismic methods in such conditions is not effective. Rugged relief usually limits using such geophysical methods as induced polarization, mise-al-la-masse, transient electromagnetic field and some other methods associated with employing cumbersome equipment. Among the geophysical methods applied in ore deposits may be distinguished mobile, inexpensive and effective potential geophysical fields: magnetic, gravity, selfpotential and temperature. The developed non-conventional interpreting system includes: (1) elimination of various kinds of noise; (2) probabilistic-informational methods for revealing useful information about the hidden targets; (3) quantitative procedures enabling to interpret observed anomalies under complex environments (oblique magnetization [polarization], rugged terrain relief and unknown level of the normal field) and (4) improved 3-D combined modelling of magnetic and gravity fields. Performed analysis permitted to reveal some common peculiarities between the magnetic and other abovementioned potential fields. These common aspects make it possible to apply the quantitative procedures developed in magnetic prospecting to the abovementioned geophysical methods. These procedures (improved modifications of characteristic point, tangent and areal methods) have been successfully tested on models and effectively applied in the real situations (gold, copper, pyrite and polymetallic ores, kimberlite pipes and other economic deposits). Application of these interpretation procedures permits not only to localize buried ore targets under the complex environments, but also to determine definite quantitative parameters of these objects.
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An Expert-Free Technique For Live Site UXO Target Classification
Authors J.B. Sigman, Y. Wang, K.A. O’Neill, B.E. Barrowes and F. ShubitidzeIn this paper we examine methods of automatic classification applied to Unexploded Ordnance (UXO) across data sets from a live site. All sensors used are time-domain Electromagnetic Induction (EMI) sensors. We solve for target extrinsic and intrinsic parameters using the Differential Evolution (DE) and Ortho-Normalized Volume Magnetic Source (ONVMS) algorithm. This inversion provides target locations and intrinsic time-series total ONVMS principal eigenvalues. We fit these to an empirical power decay model, the Pasion-Oldenburg model, providing dimensionality reduction for a Machine Learning (ML) approach. We group anomalies by the unsupervised Weighted-Pair Group Method with Averaging (WPGMA) algorithm. After requesting Ground Truths (GT) for the central element of each cluster, we train a supervised Gaussian Mixture Model (GMM), in which each class of UXO is represented by a multivariate Gaussian probability density. We request Ground Truths in rounds until we are confident there are no remaining Targets of Interest (TOI) in our survey of the site. Our system for UXO cleanup is fully automatic and expert free, and uses a priori knowledge combined with a learned algorithm.
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Observations From Borehole Dilution Logging Experiments In Fractured Crystalline Rock Under Ambient And Pump Test Conditions
Authors P.T. Harte, J.A. Anderson, J.H. Williams and A. FullerIdentifying hydraulically active fractures in low permeability, crystalline-bedrock aquifers requires a variety of geophysical and hydrogeophysical borehole tools and approaches. One such approach is Single Borehole Dilution Tests (SBDT), which in some low flow cases have been shown to provide greater resolution of borehole flow than other logging procedures, such as vertical differential Heat Pulse Flowmeter (HPFM) logging. Because the tools used in SBDT collect continuous profiles of water quality or dye changes, they can identify horizontal flow zones and vertical flow. We used SBDT with a food grade blue dye as a tracer and dual photometer-nephelometer measurements to identify low flow zones. SBDT were conducted at seven wells with open boreholes (exceeding 300 ft). At most of the wells HPFM logs were also collected. The seven wells are set in low-permeability, fractured granite and gneiss rocks underlying a former tetrachloroeythylene (PCE) source area at the Savage Municipal Well Superfund site in Milford, NH. Time series SBDT logs were collected at each of the seven wells under three distinct hydraulic conditions: (1) ambient conditions prior to a pump test at an adjacent well, (2) mid test, after 2-3 days of the start of the pump test, and (3) at the end of the test, after 8-9 days of the pump test. None of the SBDT were conducted under pumping conditions in the logged well. For each condition, wells were initially passively spiked with blue dye once and subsequent time series measurements were made. Measurement accuracy and precision of the photometer tool is important in SBDT when attempting to detect low rates of borehole flow. Tests indicate that under ambient conditions, none of the wells had detectable flow as measured with HPFM logging. With SBDT, 4 of the 7 showed the presence of some very low flow. None of 5 (2 of the 7 wells initially logged with HPFM under ambient conditions were not re-logged) wells logged with the HPFM during the pump test had detectable flow. However, 3 of the 5 wells showed the patterns of very low flow with SBDT during the pump test including pumping induced changes of inflow and outflow patterns at one well.
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TSWD – State Of The Art And Current Developments
Authors I. Kreutzer, A. Radinger, E. Brückl, W. Chwatal and D. KostialThe Tunnel Seismic While Drilling (TSWD) method has been developed to predict the geological situation ahead of the tunnel face without disturbing the construction progress in the case a Tunnel Boring Machine (TBM) is operating. The vibrations of the TBM are used as the seismic source and processing of the seismic data is targeted to the detection of reflections from fault zones ahead of the tunnel face. Modern TBM`s achieve advance rates up to 50 m per day. Near real time processing and at least daily prediction is essential. These circumstances put high demands on instrumentation, data transmission, processing, and logistics. The principles of the methodology and the state of the art will be presented. Until now we applied TSWD at four tunnel construction sites in Austria. Optimized adaptation of the TSWD technique to the particular TBM type represents a further challenge. It will be shown how instrumentation and processing have to be adapted individually for each tunnel. The final interpretation of the seismic data contains a prognosis about 100 m ahead of the TBM including location, thickness, and geometry of construction relevant geological features. A close collaboration with geologists on site supports an adequate interpretation. Current developments which aim further improvements will be addressed. The comparison of the TSWD results with the encountered geology shows that wider fault zones over a thickness of 10 m can be successfully resolved, smaller fault zones are partly detected. General, there is a prediction accuracy of about 5 m for the beginning of the structures. It was possible to detect 80% of the prominent faults or transition zones at all tunnel sites. On the strength of past experiences faults with a high risk impact on the tunnel construction can be clearly defined.
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Cloud Based Electrical Geophysical Monitoring
Authors R. Versteeg, D. Johnson, A. Henrie and T. JohnsonElectrical geophysical monitoring data contains a wealth of information on subsurface processes. The challenge being faced by end users of electrical geophysical monitoring surveys is how to efficiently and in a timely manner gain access to this information. As this extraction requires the efficient management and processing of frequently updated electrical geophysical data sets (as well as the use of auxiliary data from different sources) a manual approach will be very time and resource intensive. We have implemented a cloud based software package for electrical geophysical monitoring through which data are automatically transferred, processed, visualized and delivered to end users. Users can see system status, configure settings and access results and data through a standard browser. This approach substantially increases the efficiency (and reduces the cost) with which electrical geophysical monitoring efforts can be performed, allows for a rapid turn around between data acquisition and result delivery, and provides the different types of users and stakeholders involved in electrical geophysical monitoring with the tools allowing optimal use of electrical geophysical monitoring data.
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Resolving The Irresolvable Through Data Integration For The Transmitter Site, Bucks Harbor Me
Authors D. Clemens, M. Thompson and S. MillerThe Transmitter Site is one of three formerly used defense sites in coastal Machiasport ME, located on glacial sediments underlain by Devonian igneous rocks. Integrating all available surface and borehole geophysics, hydrogeologic and historical site data in three dimensions produced a defensible Transmitter site conceptual site model meeting many of the requirements for a Technical Impracticability (TI) and TI Zone at a fractured rock site where the task was not thought possible. A perchloroethylene (PCE)/trichloroethylene source area is within the current Cold War era septic system, leaching transformed contaminants (trichloroethylene) into the underlying glacial sediments, and fractured bedrock. The septic system overlies a north-northeast/south southwest fracture zone or pathway potentially contaminating groundwater used by homes built on Yoho Head Road since 1996. A second shallow valley or bedrock trough following the west-southwest transmissive fracture trend may also be potentially contaminating groundwater used by homes built on Yoho Head Road since 2009. The defensible, data based three dimensional conceptual site model identifies several residential well monitoring data gaps, and meets many of the TI Waiver and TI Zone requirements. The TI waiver is a more feasible site closeout option when visualizing the data in the context of the site’s current conditions and path forward. Removing the shallow PCE-TCE contaminated soil follows the TI Waiver practice at many sites with contaminated fractured bedrock aquifers. The model allows for quick TI zone determination, key requirements for a TI Waiver, especially when done after the soil removal. Adding domestic wells potentially located along the newly identified fracture zone trends improves public health protection, verifies site contaminants are not reaching these homes, and establishes the basis to take appropriate actions if TCE is found.
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A Combined Joint Diagonalization-Music Algorithm For Estimating Locations Of Subsurface Targets
Authors Y. Wang, J.B. Sigman, B.E. Barrows, K.A. O’neill and F. ShubitidzeIn this paper we present a method for extracting target location from data measured by Time- Domain Electromagnetic Multisensor Towed Array Detection System (TEMTADS). The TEMTADS is consisted of square-loop transmitters and 3-D receivers, arranged in a 2×2 array, which generates multistatic response (MSR) matrices of 4×12 in Ng time channels. The data collected first goes through a linear combination process to form a square matrix so we can apply joint diagonalization (JD), a technique that finds the eigenvectors which diagonalize the entire set of MSR matrices. A filtering process is embedded in the JD to enhance signal-to-noise ratio (SNR). The eigenvectors arouse from Targets of interest (TOI) and from noise can be identified after applying JD, and this information is passed to a multiple signal classification (MUSIC) algorithm to separate the signal and noise subspaces. MUSIC algorithm then projects the noise subspace onto a theoretically calculated Green’s Function array. Due to their orthogonality, the target locations can be estimated by looking for maximums when we invert this multiplication. With the Green’s Function pre-calculated, the method can be carried out fast enough to perform targets mapping in real or near real-time.
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Use Of Time-Domain Electromagnetics (Tem) And Passive Seismic Methods To Characterize The Subsurface In East Falmouth, Massachusetts
Authors C.D. Johnson, E.A. White, D.R. Leblanc and S.L. Morton and J.W. LaneTime-domain electromagnetic (TEM) and passive seismic methods were used to characterize the subsurface along the seacoast near Falmouth, Massachusetts. The coastline is characterized by a series of peninsulas, or necks, that extend as much as 4 km southward from mainland Cape Cod toward the ocean and are bounded laterally by shallow saltwater embayments. The necks are comprised of unconsolidated glacial deposits up to 100 m thick that overlie crystalline bedrock. The water table beneath the necks is generally about 1 to 8 m below land surface, or less than 2 m above sea level. The fresh groundwater in the glacial deposits overlies saline groundwater, although few data are available on the depth to the freshwater/saltwater interface. Most residences on the densely-populated necks are supplied by municipal water and dispose of wastewater through individual septic systems. The onsite wastewater disposal contributes to increased discharge of nitrates and other wastewater-related contaminants into the embayments, which have been degraded ecologically by excessive nutrient inputs. Actions to reduce these inputs must be based on sound scientific understanding of the hydrogeology of the necks, but the distribution of geologic materials and subsurface freshwater is largely unknown in this part of Cape Cod. This investigation provided preliminary characterization of the subsurface electrical conductivity of Davis Neck in East Falmouth.
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Detailed Characterization Of Pavement Surface Structure Using High Resolution GPR
Authors K. Aoike, H. Saito and T. InazakiThe authors have started a research and development on nondestructive measurements of pavement using GPR and high-frequency surface wave method. Our goal is to estimate S-wave velocity of each pavement layer such as asphalt concrete or road base, and to evaluate deterioration and damage of the pavement. To that end, we think that it is necessary to estimate thickness of pavement accurately by GPR. We conducted some field experiments including GPR measurement which was carried out on a pavement where cracks and wheel ruts were obvious. First, we detected the wheel ruts on a road surface by using vehicle mounted 3D Laser Scanner, and we extracted cracks by visual observation of the road surface. Next we carried out GPR survey of 0.25m grid by using an antenna which has a central frequency of 800MHz. From the analysis result of GPR, two subsidence areas of road base along the longitudinal direction were caught. The magnitude of the subsidence did not match the depth of wheel ruts. We analyzed the relations between the thickness of road base and the crack distribution. We found that the crack went on increasing at the thinner layer of road base. In order to validate the results of GPR, we conducted borehole camera inspection at several check drillings on the pavement. We confirmed that the estimated thickness of road base was consistent with borehole camera image. In the next phase of application to the high-frequency surface wave method, if we use a priori information of those layer thicknesses as strong constraints, we think that it will be possible to determine more accurate S-wave velocity of the pavement by inverting high frequency dispersion curves.
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Borehole Surveys For Determining Depth Of Sheet Piles: Non-Optimal Geometry
By M. CarnevaleThere are many situations where less than optimal survey geometry is implemented due to site constraints. The purpose of this discussion is to illustrate the effects of non-optimal survey geometry on survey results from parallel seismic, borehole magnetometer, and borehole radar surveys designed to locate the depth of metal sheet piles. Three PVC-lined wells were drilled adjacent to the sheet pile to depths of approximately 100 feet. The wells were 4 inches in diameter. The optimum distance from the sheet pile for well placement is typically 3 to 5 feet. The seismic surveys included variations in sensor types, source types, and source locations. Hydrophone and tri-axial geophone sensors were used with sledge-hammer or accelerated weight drop sources. Source locations included topside locations with sheet-pile coupling occurring through concrete piers and pile caps, and under-pier locations with source coupling on the side of metal sheetpiles. Magnetometer surveys were performed using a borehole deviation probe and/or acoustic televiewer, both containing three-component magnetometers. Radar surveys were performed with a 500-MHz borehole radar probe. Constraints to the borehole methods included: magnetic interference from local utilities and a former battleship conductivity of the sea water producing radar signal attenuation, and boring to sheet-pile separations of up to 8 feet; variation in soil seismic velocity no direct access to the top of steel sheet piles to induce seismic signals; i.e., poor coupling of seismic source with structure (thick concrete cap and asphalt base) poor coupling of borehole casing with soil (i.e., receiver coupling) excessive noise/vibration interference (trains, highways, bridge piles, nearby drilling) The effectiveness of the borehole radar method was severely reduced by the conductivity of the intervening saturated soil and sea water. Away from the former battleship, the results from seismic and magnetometer surveys were in good agreement. Sheet-pile depths derived from magnetometer surveys near the battleship were consistently 1 to 2 meters deeper than depths derived from the seismic method.
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Subsurface Characterization For Pipeline River Crossings Using Surface “Water-Coupled” Ert: Comparison With Other Geophysical Methods
By A. BullerWith an aging utility infrastructure and the need to replace utilities, proper site characterization is crucial, especially in the case of drilled pipeline river crossings. The use of surface geophysics is well-established as a means of site characterization. However, the challenge arises when attempting to characterize sub-bottom conditions at river crossings. This is especially true in small to mid-size municipal projects, where rivers are less than 1000 feet wide and where in-river conditions are variable. Although other geophysical techniques, such as seismic and GPR methods, can be used to characterize river sub-bottom conditions, the variable nature of river conditions poses unique challenges for these techniques. The goal is to find a reliable, time effective method that can overcome these obstacles and still achieve satisfactory results. The electrical resistivity method was tested at two river crossings where the electrodes on the array cable were coupled directly to the water’s surface. One site contained a wide marshy flood plain but a narrow active water channel (10-20 feet wide). A combination of land- and water-coupled electrodes was used. While lacking a flood plain, the other site consisted of a wide controlled river requiring an electrode array of 945 feet, completely within the bounds of the active river. Obstacles at these sites included, but were not limited to: access to the river, variable water depths, cable management within strong river currents, and variable bedrock depths. However, in both cases, the deployment of the electrode array was relatively quick. Direct water-coupled electrodes require less time to deploy than most land-based electrode surveys, particularly where excessive electrode tip resistance must be minimized by saturating soil around poorly coupled electrodes. The duration of data collection is similar between land- and water-based surveys. Results of the two surveys revealed complementary, if not clearer, findings to other geophysical techniques used at each of the sites. Based on these results, the use of electrical resistivity should be considered as a viable time-effective component to any program requiring the use of geophysics over river crossings.
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Global Joint Inversion Of Tomographic Data: Appraisal Of Model Reconstruction Ambiguity
Authors H. Paasche and J. TronickeGeophysical model reconstruction by data inversion is usually ill-posed and suffers ambiguity due to limited number and accuracy of the available observations. Joint inversion of different data sets allows for mutually improved reconstruction of physical parameter models underlying each of the available data sets, but considering the limited number and accuracy of available observations, some ambiguity remains. Here, we use particle swarm optimization to jointly invert synthetic GPR and Pwave crosshole tomographic data sets. Model parameterization is guided by the results of a zonal cooperative inversion based on local search optimization of an initial guess. Global optimization is first done to explore the Pareto front of the joint inverse problem in a very efficient way. Consecutively, the area behind a selected location of the Pareto front is explored to be able to assess the model reconstruction ambiguity inherent to the available data and chosen parameterization.
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The Research And Practice Of Nuclear Magnetic Resonance (NMR) Signals In Frozen Soil Layer Structure At Theqinghai-Tibet Plateau
More LessThe Tibetan Plateau is the largest permafrost regions of the world with the highest altitude in the low latitudes. It’s accounting for 7% of the permafrost area in the world. Analysis of nuclear magnetic resonance (NMR) detection technology is discussed for the permafrost regions of the Qinghai-Tibet Plateau. Response signal characteristics and a study of feasibility in permafrost structure are also discussed. This thesis focuses on some experiments about frozen soil, whose signal characteristics changes with temperature and moisture. In addition, a field experiment was conducted in the plateau permafrost region. Combined with the known drilling data, comparative study of interpretation of surface nuclear magnetic resonance (SNMR) inversion, we finally determine signal characteristics of frozen soil layer, and draw the distribution of frozen soil layer. The SNMR method is used to detect distribution of the permafrost area. This is a new attempt, which also has important scientific significance and application value.
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Geoelectric Imaging Scores Over MASW In Geotechnical Site Characterization
By R.G. SastryThe routine point-based geotechnical site testing for assessing the bearing capacity of near-surface soil masses in major civil engineering construction sites succeeds in simple 1-D geological situations. However, the Multichannel Analysis of Surface Waves (MASW) method is routinely used in geotechnical engineering, which determines the shear wave velocity variation with depth. Earlier, our regression analysis based on the geoelectric imaging methods of electrical resistivity tomography (ERT) and induced polarization imaging (IPI) along with point geotechnical tests predicted 2-D geotechnical test results. Our prediction method is based on site-specific validated regression equations describing actual correlations of geo-electrical and geotechnical data and site-independent well established empirical relations of Standard Penetration Test, SPT ‘N’ with different formation and geotechnical parameters. In this present effort, an inter-comparison of relative performance of ERT, IPI and MASW in predicting geotechnical test results such as Standard Penetration Test (SPT), Dynamic Cone Penetration Test (DCPT) and Static Cone Penetration Test (SCPT)) is undertaken in a case study. The RMS error estimates in predicting SPT, DCPT and SCPT results along a profile in the study region establish the supremacy of ERT/IPI over that of MASW. This can be explained on the basis of higher depth resolution offered by geoelectric imaging and similarity of factors affecting both geotechnical field tests and geoelectrical imaging methods.
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Self-Adaptive Method For High-Frequency Dispersion Curve Determination
By Z. LuWhen high-frequency (from 50 to 500 Hz) MASW is conducted to explore soil profile in the vadose zone, existing rules for selecting near offset and receiver spread length cannot satisfy the requirements of planar and dominant Rayleigh waves for all frequencies and will inevitably introduce near and far field effects as well as spatial aliases. To solve the problems, a self-adaptive MASW method is developed to determine high-frequency dispersion curve. An initial dispersion curve obtained by a fixed-offset MASW is used to estimate wavelengths at all frequencies. At each frequency, near offset and spread length are then set to be the ratios of the wavelength. In other words, near offset and spread length are self-adaptive to the corresponding wavelength. Receiver spacing is either fixed or linearly increased to reduce spatial aliases. A case study is presented, which demonstrates the capability of the self-adaptive method to preferentially identify the dispersion curves of either the fundamental mode or higher modes of Rayleigh waves. A nonlinear phenomenon has also been observed which deserves future investigation.
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Geophysical Investigation Of Arural Water Point Installation Program In Nampula Province, Mozambique
Authors F.J. Chirindja, B. Andersson, T. Björkström, T. Dahlin and D. JuizoThere are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the province of Nampula and Cabo Delgado in crystalline rock zone. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the suitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50%. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate of boreholes. In total, nine boreholes with sufficient yield, and five boreholes with insufficient yield were investigated. In both VES and ERT, the resistivity values indicate 3 different layers. One surface layer with resistivity between 220-5000+ Ωm, a second layer with lower resistivity value, varying from 10- 220 Ωm, less than 10 Ωm in some places, and a third layer with high resistivity values, 220-5000+ Ωm, increasing with Depth. Due to lateral variation, the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate.
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Application Of Electrical Resistivity Imaging To Evaluate The Geometry Of Unknown Bridge Foundations
Authors B. Kermani, J.T. Coe, J.E. Nyquist, L. Sybrandy, P.H. Berg and S.E. McInnesElectrical Resistivity Imaging (ERI) was utilized to image the subsurface at two bridge sites owned by the Pennsylvania Department of Transportation. Data were acquired using 28 electrodes and an Advanced Geosciences (AGI) SuperSting R8/IP resistivity meter, which allowed multiple array configurations (e.g., dipole-dipole, Schlumberger, and Wenner). Site conditions, hardware configuration, and testing procedures are presented, followed by a discussion of data analysis and interpretation. The purpose of the ERI testing was to determine the bridge foundation geometry, including configuration, depth to bottom, and dimensions, at two bridges where design information was available to determine the feasibility of using this method for bridges with unknown foundations. Typical surface methods for evaluating unknown foundations are often unable to recover information from below a pile cap. ERI presents an opportunity to address this limitation and provide geometry information without the need for drilling operations. The ERI results were promising at one of the sites and provided a reasonable estimate of pile cap dimensions (8.5 m x 7.0 m) compared to actual dimensions (7.3 m x 5.5 m). ERI also detected the presence of battered piles at an approximate 1:2.5 batter angle, which compared favorably with the 1:5 batter angle shown on the foundation plans. Interpretation of the ERI images at the second site proved problematic. This was likely due to issues with data inversion when creating the resistivity section since the resistivity values of the soils were extremely low throughout the site. Moreover, foundation bottom predictions were negatively impacted at both sites by poor signal to noise ratio likely resulting from high levels of background electrical noise from nearby utilities and fences. Despite these issues, ERI shows potential as a tool for evaluating unknown bridge foundations, particularly in less urban areas where noise levels could be more manageable.
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Imaging A Soil Fragipan Using A High-Frequency MSAW Method
Authors Z. Lu, G.V. Wilson and C.J. HickeyThe objective of this study is to noninvasively image a fragipan layer, a naturally occurring dense soil layer, using a high-frequency (HF) multi-channel analysis of surface wave (MASW) method. The HF-MASW is developed to measure a soil profile in terms of shear (S-wave) wave velocity at depths up to a few meters. While conventional MASWs use geophones as surface vibration sensors, the present MASW uses an accelerometer as a sensor to detect Rayleigh wave propagation generated by an electromechanical shaker operating in a chirp mode to achieve high frequency and high spatial resolution. With the method, the subsurface soil properties at a test site were measured, visualized, and evaluated. A 2-dimensional S-wave velocity image was obtained and from the contrast of the image, the presence, depth, and extent of a fragipan were identified. The HF-MASW result was compared with those of site characterization made by invasive methods and a 2-dimensional image obtained by a penetration test. The results from the HF-MASW and soil characterization were in good agreement. The study demonstrates the capability of the HF-MASW technique for detection and imaging subsurface layers such as a fragipan.
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Seismic Characterization Of Near-Surface Anisotropic Structure
More LessIn many geophysical applications, the neglect of anisotropy is somehow an oversimplification. The mismatch between the theory and the observation of near-surface seismics indicates the need for the inclusion of medium anisotropy. In this paper, surface wave (Love wave) dispersion properties are used to estimate the anisotropic structure of the near-surface layered earth, which is modeled as vertical transverse isotropy (VTI), a reasonable assumption for the vertically symmetric anisotropic medium. First, the dispersion curves, which are the numerical solutions of the dispersion equation, are obtained by a graphic-based method. Compared with traditional root-finding algorithms, this graphic-based method is simpler, faster, and more precise. Then, very fast simulated annealing (VFSA) algorithm is used to invert velocity structure and anisotropy structure simultaneously. The advantages of VFSA are two-fold: 1) high capability to find global minimum, and 2) independence to the initial model. The proposed algorithm is verified by the synthetic dispersion curve generated by a VTI medium model. Finally, the estimation of shear wave velocity and anisotropy structure of the field data at a site of sediments in the Connecticut River Valley shows this approach’s feasibility and efficiency.
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2.5D Resistivity Inversion In Anisotropic Media: Numerical Experiments
Authors T. Wiese, S. Greenhalgh, B. Zhou, M. Greenhalgh and L. MarescotWe present a set of 2.5D synthetic inversion experiments for a model comprising an isotropic block embedded within an anisotropic background. We examine and compare the image reconstructions obtained using the correct anisotropic code and those obtained using code based on the inappropriate but widely adopted isotropic assumption. Superior reconstruction in terms of reduced data misfit, true anomaly shape and position, and anisotropic background parameters were obtained when the correct anisotropic code was employed for media characterized by moderate to high coefficients of anisotropy. However, for low coefficient values, the isotropic inversion produced slightly better results because there are fewer parameters to determine. When an erroneous isotropic inversion is performed on medium to high level anisotropic data, the images are dominated by patterns of banded artefacts and high data misfits.
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Pavemon: A Gis-Based Data Management System For Pavement Monitoring Based On Large Amounts Of Near-Surface Geophysical Sensor Data
Authors S.S. Shamsabadi, M. Wang and R. BirkenPAVEMON is a GIS-based data management system for PAVEment condition MONitoring. It is designed to visualize and perform spatial analysis on large amounts of multi-modal sensor data that contain surface and subsurface information of pavements. PAVEMON, built on the GIS platform, writes huge amounts of data (raw, processed, fused) to an Oracle database, and makes them accessible across the web for visualization and spatial analysis. These data are collected by a multi-modal mobile sensor system, mounted on a vehicle to enable a continuous network-wide health monitoring of the roads. Multi-channel raw sensor data (microphone, accelerometer, tire pressure sensor, video) and processed results (road profile, crack density, road roughness, micro texture depth, etc.) are available. Such a system can produce huge amounts of data (TB) in a day. Considering that multiple such systems might collect data simultaneously it is crucial to automate the data flow through the system from the data acquisition to where it is automatically processed and placed into the database and on the map. PAVEMON fetches these data streams and geo-references them based on the available positioning data collected in tight time synchronization with the sensors. Each of these streams holds an aspect of knowledge required for assessing road conditions which PAVEMON deals with accordingly. From correlating sensor data with the corresponding images, collected by a camera installed on back of the vehicle, data mining algorithms can be developed and validated. Captured every 1.2 m, these images have been geo-tagged and mosaicked together providing a resolution far beyond the aerial photography. Google Street-view images have also been integrated into PAVEMON as a third party API to further verify sensors’ data. Accessing all these layers through PAVEMON has greatly enhanced the pavement management capabilities.
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Tracer Technologies: Possibilities In Reservoir Engineering
Authors L. Anisimov and I. VorontsovaThis paper outlines a methodology for analyzing the interwell space flow characteristics of productive reservoirs using tracer-determined parameters. Tracer experiments are conducted in different clastic and carbonate productive reservoirs. The conclusions about field development can be supported by water tracing experiments that were undertaken to get information about the catchment area of the producing wells and to trace the water flows from injection to producing wells. The repetition of the tracer tests from the beginning waterflooding process shows the evolution of water channels in the oil-saturated zones of reservoirs.
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Two-Dimensional Joint Inversion Of ZTEM And MT Plane-Wave EM Data For Near Surface Applications
Authors P.E. Wannamaker and J.M. LegaultThe performance of two-dimensional (2-D) joint ZTEM/MT inversion was tested using synthetic brick structures below a hill and valley model. Subsequently, separate and joint inversion of coincident ZTEM and Titan dense array MT data over the Johnston Lake district, Saskatchewan, were performed. A result of this effort is that only very few (e.g., three) MT stations may be needed to correct for background resistivity effects in a ZTEM survey provided the MT sites are appropriately spaced.
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Too Thin To Be Detected: When ERT Surveys Can Fail To Assess An Aquiclude Layer Interposed Between Two Aquifers: The Sunceri Test Site (Honduras)
Authors P. Torrese, M.L. Rainone, F. Colantonio and P. SignaniniERT surveys were undertaken at the Sunceri test site located in the city of San Pedro Sula (Honduras). Four deep pumping wells are located at the site, together with 26 shallow and 12 deep piezometers drilled through alluvial deposits. These deposits are 100-150 m thick and overlie intrusive and low grade metamorphic rocks. The site is a major public water supply (PWS) for the city with a total groundwater abstraction of 160-200 l/s. The main objective of this paper is to identify if ERT surveys can determine the occurrence and continuity of a clay layer with a variable thickness and an average resistivity of 17 Ω·m. Based on borehole logs, the clay layer is located at a depth of approximately 25 m. This layer is widespread across the entire area. It separates an upper unconfined aquifer from a deeper confined aquifer. The aquifers have different piezometric levels and hydro-chemical features. It is essential to correctly assess the thickness and paramatise the aquiclude layer so that a correct vulnerability assessment of the groundwater resource can be undertaken. Indeed, pollution of the heavily abstracted deep aquifer from contaminated shallow groundwater should be prevented. The inverted resistivity sections reveal the presence of the clay layer. However, they fail to show that the clay layer is discontinuous across the area. In particular, the clay layer doesn’t appear in the middle portion of the surveyed area where its thickness falls below 4.9 m as revealed by 2D synthetic dataset modelling. 1D modelling indicates a slightly lower value of 3 m. The fact that the thinning of the clay layer (that has a resistivity consistent with a 37 % clay content, i.e. 4 ·10-3 m/d hydraulic conductivity) occurs in an area affected by a severe drawdown of the piezometric level due to the interference between the depression cones of the pumping wells, increases the vulnerability of the deep aquifer under the current abstraction rates.
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The Importance Of A Single Transmitted Waveform In The Characterization Of Discrete Conductors
Authors A. Bagrianski, A. Prikhodko and J.M. LegaultRecently the idea of using dual transmitted waveform systems for airborne EM time-domain surveys has gained popularity with the opinion that such systems could provide the best of both worlds – strong dipole moment for deep penetration and early time gates for better definition of near surface features. Although at first glance the idea of dual waveform might seem to be attractive, more detailed analysis shows that the dual waveform has serious disadvantages, especially for mining applications where conductors are discrete and highly conductive in many cases. The single waveform system delivers better definition for the discrete conductors, the situation typical for mining applications.
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3D ERT Imaging Of The Fractured-Karst Aquifer Underlying The Experimental Site Of Poitiers (France): Comparing Wenner-Schlumberger, Pole-Dipole And Hybrid Arrays
Authors P. Torrese, M.L. Rainone, P. Signanini, P. Greco, F. Colantonio, G. Porel, B. Nauleau, D. Paquet and J.-L. MariElectrical Resistivity Tomography (ERT) surveys were undertaken to investigate the Dogger Limestone fractured-karst aquifer at the Hydrogeological Experimental Site (HES) of Poitiers, France. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected along five parallel 470 m long profiles with a 50 m line spacing. A 3D block measuring 515 x 203 m in size with a maximum depth of 100 m was surveyed. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. This paper compares the imaging obtained from different array sequences. Calibration of the 3D resistivity block with well logs indicates that: the Wenner-Schlumberger (WS) array shows the tendency to enhance layering, to locate bodies at a shallower depth and to laterally extend them; the Pole-Dipole (PD) array shows larger lateral heterogeneities, more compact and vertically extended bodies and poor data fitting; the hybrid array sequence, obtained by the combination of WS and PD array sequences, despite a poor data fitting, similar to PD, shows a better correlation with respect to well log results. In this setting, the hybrid array sequence shows better imaging, due to the combination of the large vertical resolution of WS, large lateral resolution and penetration depth of PD. It allows passing through the thick, low resistivity shallow layer. Indeed, the results are affected by the occurrence of the shallow, 30 m thick, low resistivity argillaceous limestone that reduced the investigation depth as revealed by synthetic datasets modelling and sensitivity analysis. Modelling also revealed that the occurrence of the argillaceous limestone led to a severe underestimate of the Dogger Limestone resistivity values with respect to well resistivity logs; it also allowed verifying the detectability limits when investigating shallow karst limestone intervals located at depths of up to 50 m.
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Simulation Analysis For Under Pavement Drainage Detection By Ground Penetrating Radar (GPR)
Authors H. Bai and J.V. SinfieldSubsurface drainage systems are a critical enabler of the proper performance of roadways. Effective maintenance of these drains can significantly extend a pavement’s service life. However, many reconstructed or refurbished roads are built on top of pre-existing drainage systems that are not clearly delineated. Maintenance is thus challenging in that location and subsequent inspection of these systems can be time consuming and laborious. Ground Penetrating Radar (GPR) offers a potentially effective and efficient means to perform this task. In this paper, a group of simulations performed using GPRMAX2D software are examined to explore the influence of road cross-section designs on sub-pavement drainage conduit GPR signatures, and evaluate the effectiveness of alternate GPR antennae configurations in locating these buried conduits in different ground conditions. Two different models were explored to simulate conduit detection. In addition, different pipe and soil conditions were modeled, such as pipe size, pipe material, soil moisture level, and soil type. Four different quantitative measurements are used to analyze GPR performance based on different key factors. The four measurements are 1) signal to background ratio (SBR) in dB; 2) signal to receiver noise ratio (SNR) in dB; 3) signal energy in Volts; and 4) average signal band power in Watts.
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Results Of A Laboratory Study Highlighting The Potential Of Integrated P-Wave And Electrical Methods Applciation In Near-Surface
Authors B. Hassan, S.D. Butt and C.A. HurichIntegrated near-surface geophysical surveys can improve shallow subsurface imaging. Inversion of joint P-wave and electrical resistivity data may offer a reliable tool for detailed near-subsurface mapping, especially for saturation effects. Findings/results of an integrated imaging laboratory study are reported. Monitoring/imaging of a controlled immiscible fluid-displacement through a porous medium was simulated at lab scale. The core/analogue for unconsolidated sediment was 0.5mm spherical glass bead-pack in 45cm section of a vertical transparent PVC tube with 5.09cm inner diameter circular section, i.e. a flow-cell system. Eight electrode pairs were attached along the flow-cell, spaced equally, with a 1MHz P-wave ultrasonic source/receiver sensor pair at midspan between electrode/channel 4 and 5, where channels are numbered 1-8 upward. DC resistance and ultrasonic P-wave data were acquired simultaneously for all channels as oil was displaced with brine against gravity, at constant head conditions, until after breakthrough. Three experiments with different initial/invading flow rates (fast, intermediate and slow) were performed. Ultrasonic P-wave arrival-time/velocity, instantaneous/integrated amplitudes and logarithmic values of electrical resistance were plotted against elapsed time for inferring/correlation. Apart from pure saturations, spatially and temporally localizable distinct interface/mixed-zone and associated fluid fronts were identified by juxtaposing ultrasonograms and electrical resistance variation data, consistently in all experiments. Velocity increased in the mixed-zone, compared to velocities measured in oil, but remained slower than that of the brine saturated zone. Lowest amplitudes were observed in the mixed-zone but amplitudes of brine were higher than those of oil. For higher invading fluid flow rates an early breakthrough was also deducible by observation of viscous fingering. Sensitivity of interfacial stability/degree of mixing to flow velocity/regime with interface evolving in time was evident. Efficient displacement with stable interface was observed and/or expected for slower flow rate. Inevitably, findings confirm integrated/innovative geophysical surveys more reliably map heterogeneous near-subsurface for addressing engineering/environmental problems i.e. monitoring infrastructure health, subsurface hydrogeology/EOR process, and industrial/oil spills, and in assisting/improving unconventional depth surveys/exploration.
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Hydrological Imaging Constrained By Groundwater Flow Modeling And Laboratory Measurements Of Electrical Properties Of Undisturbed Soils At Historical Grant-Kohrs Ranch, MT.
Authors H. Bertete-Aguirre and G.D. ShawElectrical underground imaging is an important tool in hydrogeological characterization, but the ill-posed nature of the resulting inverse electrical problem limits the ability to map complex hydrogeological units. The need for regularization techniques to obtain stable images in these cases is clear, but the standard regularization terms are based in smooth functionals and as a result making difficult to separate small contrast boundaries for shallow hydrogeological units. In our work, we use constrains to the conductivity distributions in the imaging process. These constraints were obtained by combining hydrogeologic field measurements with laboratory measurements of electrical properties of undisturbed soils cores measured under similar conditions at the core location. The setting for this investigation is located at Grant-Kohrs Ranch National Historic Site, which is located in the Clark Fork River flood plain (headwaters to the Columbia River). The hydrogeophysical investigation is driven by flooding that has occurred after stream restoration and a lack of understanding of groundwater-surface water interactions. Water levels, drill cuttings, and surface water flow data is not enough to accurately characterize water mounding from an irrigation ditch and the natural variation of hydraulic conductivity. Soil cores were taken at several locations in the floodplain, and our results show that they significantly improved hydrogeological data interpretation at historical Grant-Kohrs Ranch. The coupled hydrogeologic and hydrogeophysical results is used to showed an important improvement our conceptual in the understanding of hydrologic processes and develop a to construct a groundwater flow model at Grant-Kohrs Ranch.
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