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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
21 - 40 of 53 results
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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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