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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
41 - 53 of 53 results
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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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