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25th Symposium on the Application of Geophpysics to Engineering & Environmental Problems
- Conference date: 25 Mar 2012 - 29 Mar 2012
- Location: Tucson, USA
- Published: 25 March 2012
41 - 60 of 195 results
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A NOVEL SURFACE NMR PULSE SEQUENCE FOR IMPROVED ESTIMATION OF LONGITUDINAL T1 RELAXATION TIMES
Authors Elliot Grunewald and David WalshObtaining estimates of permeability from NMR methods requires the ability to measure relaxation time parameters that are most sensitive to pore geometry. While surface-NMR (SNMR) measurements can reliably quantify the effective transverse relaxation time T2*, transverse relaxation times are sometimes dominated by the influence of magnetic geology. The longitudinal relaxation time T1 is known to provide a more robust link to pore geometry, but previous attempts to measure T1 by SNMR, have shown mixed success due to fundamental limitations the commonly applied “pseudo-saturation recovery” (PSR) experiment.
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GEOPHYSICAL RESULTS OF MAIN BARTON SPRINGS (PARTHENIA) AND THEIR IMPLICATIONS ON THE GROUNDWATER FLOW PATTERN INTO BARTON SPRINGS POOL, AUSTIN, TEXAS
More LessBarton Springs is a major discharge site for the Barton Springs Segment of the Edwards Aquifer and is located in Zilker Park in Austin, Texas. Barton Springs actually consists of four springs: 1) The Main Barton Springs discharges into the Barton Springs pool from the Barton Springs Fault and several outlets along a fault and from a cave, several fissures, and gravel-filled solution cavities on the floor of the pool west of the fault. The thin-bedded unit on the southwest side of the fault is the regional dense member, and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is between 40 and 70 feet (12 to 21 m). 2) Old Mill Springs is located in the sunken gardens southeast of the Barton Springs Pool and is primarily fed by relatively mineralized groundwater from the Saline-Line Flow Route. 3) Eliza Springs is also located along the Barton Springs Fault north of Barton Springs pool. 4) The Upper Barton Springs is located upstream of the Barton Springs pool on the south bank.
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GROUNDWATER QUALITY AND GEOPHYSICAL STUDIES IN THE VICINITY OF TAILING PONDS OF THERMAL POWER PLANTS NEAR NAGPUR IN INDIA
Groundwater quality and Geophysical studies have been carried out in a microwatershed surrounding the tailing ponds near Nagpur in India. The study assumes importance in view of the nearby habitations using the groundwater for their drinking requirements. The study area covers approximately 60 sq km. A network of observation wells (23 nos) has been set up for groundwater quality monitoring for the major cations/anions. The results indicate that SO4 concentration has been found to be very high in few wells in the downstream of the tailing ponds. Elevated fluoride concentration (>1.5 mg/L) has been observed in only one observation well.
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Shallow Electrical and Seismic Imaging of The Pineto Mud Volcano (Central Italy)
Authors P. Torrese, M. Rainone, S. Rusi and P. SignaniniShallow underground electrical and seismic imaging was undertaken at the Pineto (Central Italy) Mud Volcano site using 2D-3D electrical resistivity tomography (ERT) and 2D reflection seismic surveys. This mud volcano or mud lump (known by the local population as “Cenerone Mud Volcano”) appears as a dome of about 15x10 m in size. The height is approximately 2 m with a crater 2.5 m of diameter where emission of fluids and solids (cold brine, mud, gas) occurs. Upper Pliocene-Lower Pleistocene foredeep pelitic deposits (clays with silty-sandy levels) overlain by clayey-silty deposits outcrop in the area. Below the Middle Pliocene deposits, a NNW-SSE anticline which runs approximately 2 kilometers to the WSW of the site represents the main element of the compressional tecnonic setting. Ditchs and streams in the area run along fault systems that are oriented NW-SE and NE-SW. These could be related to the upward migration of deep mud fluids. 2D-ERT results were used to determine the geometry of the high conductivity body related to the uprising of mud fluids and detecting changes in deposits. Detailed geometry reconstruction of the shallow upward migration of mud fluids was obtained by using 3D-ERT. A shear wave reflection seismic survey was undertaken to determine stratigraphic limits and to assess the occurrence of fractured zones along which mud fluids could migrate towards the surface. The survey results revealed that the uprising of deep fluids doesn’t occur exactly below the mud volcano at present. Instead, a high conductivity body occurs at approximately 60 m to the ENE, within a fractured zone in the undercompacted clays. The probable occurrence of a high permeability layer approximately between 20 m and 30 m below ground level, confined by clay layers, gives rise to an overpressured mud reservoir. This is the source of mud fluids which flow to the surface through a mud conduit that appears nearly horizontal at depth and then inclined towards the surface up to the mud volcano crater.
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INVESTIGATION OF A COASTAL KARST AREA TO DELINEATE PREFERENTIAL GROUNDWATER FLOWPATHS USING MARINE AND TERRESTRIAL ELECTRICAL RESISTIVITY TOMOGRAPHY
Authors Yvonne O‘Connell, Eve Daly, Garret Duffy and Tiernan HenryA major karst region is located in the west of Ireland in an area comprising the Burren and Gort Lowlands. The geology of this area is dominated by Carboniferous limestones which have experienced extensive dissolution resulting in an underground network of karstified conduits and fissures that define the groundwater flow across the region. In addition, the groundwater discharge to the sea in this area is exclusively intertidal and submarine. Typical winter rainfall conditions result in the karst system becoming saturated. The gradient of groundwater flow is low resulting in temporary lakes (turloughs) in the area acting as large reservoirs which provide storage to enable the transmission of the large volumes of water in the system to the sea.
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EFFICIENT AND SCALABLE ALGORITHMS FOR THE INVERSION OF VERY LARGE AEM DATASETS
Authors Casper Kirkegaard and Esben AukenRecently the standard size of airborne electromagnetic (AEM) datasets has been growing rapidly, while many surveys at the same time focus on modern applications that require very high resolution. Often, the particular target structures of interest cannot be identified directly from the data and approximate inversions or data transform techniques also prove insufficient. This type of application includes environmental studies of aquifers, salinity and geology and also cases of relevance to the mining industry e.g. uranium exploration. In these cases highly accurate full non-linear inversion provides the only reliable solution, which is a computational challenge. In designing an inversion scheme for these types of problems it is further advantageous to utilize as many assumptions as possible. The target structures of interest can often be regarded locally one dimensional, quasi layered and spatially coherent which can be integrated in the inversion. If inverting multiple soundings at a time for a set of spatially constrained 1D models, information is allowed to propagate spatially and a vastly improved model result can be obtained. Ideally, entire surveys should be inverted at a time in huge constrained problems, but poor scaling properties of the underlying algorithms typically makes this impossible.
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COMPARISON OF TIME-LAPSE RESISTIVITY AND WELL DATA IN THE HYPORHEIC ZONE BENEATH STREAMS
Authors Jonathan Nyquist, Laura Toran, David O‘Donnell and Robert RyanCharacterization of the hyporheic zone beneath streams, where surface water and groundwater mix, is critical for understanding stream ecology and contaminant fate and transport. Tracer tests can identify storage zones, but cannot distinguish between tracer lingering in pools and subsurface storage. Wells monitor the subsurface, but prohibitively many may be required to avoid spatially undersampling the plume, and with mixed porosity sediment wells preferentially sample the more hydraulically conductive sediments. Additional information can be obtained by monitoring saline tracer using time-lapse resistivity – continuous spatial coverage and sensitivity to the tracer distribution irrespective of the hydraulic conductivity – at the expense of a more ambiguous interpretation.
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AIRBORNE ELECTROMAGNETIC SURVEYS FOR GROUNDWATER CHARACTERIZATION
Authors Jared Abraham, Burke Minsley, Paul Bedrosian, Bruce Smith and James CanniaAirborne electromagnetic (AEM) surveys provide densely sampled data over large areas (typically several hundred sq. km) that cannot be covered effectively using ground-based methods. AEM data are inverted to estimate the three-dimensional distribution of electrical resistivity structures from shallow depths to several hundred meters. These models convey unparalleled details that are used to make inferences about hydrogeologic properties and processes at the watershed and local scale. This information is being used in groundwater models that are critical to water management decisions, to better understand geologic frameworks, and to improve climate change models. The U.S. Geological Survey (USGS) has been engaged in the application of AEM to many watershed and local scale groundwater projects within United States. We present the results of several frequency- and time-domain AEM surveys acquired by the USGS that have been used for mapping alluvial valleys, buried glacial aquifers, fault-bounded basins, salinity problems, and understanding permafrost distributions.
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EXAMPLES OF THE CONTRIBUTION OF A-PRIORI INFORMATION TO IMPROVING INVERSION OF AEM DATA
Authors Andrea Viezzoli, Tim Munday, Antonio Menghini, Anne-Sophie Hřyer and Esben AukenWe present a simple yet rigorous way of adding a-priori information to inversion of Airborne EM data, using the Spatially Constrained Inversion. Resistivity models are constrained spatially to reflect the lateral homeogeneity expected from the geology. These constraints, that can be considered “soft a-priori”, are fitted together with the AEM data, during the inversion. Moreover “hard a-priori” can be added: they can be, e.g., downhole resistivity logs, geological contacts and layers, hydrogeological units. The a-priori information is treated as an extra data set, by taking into account location, values, uncertainty, and expected lateral variability. The information it contains is spread to the location of the neighbouring AEM soundings. These fields enter the SCI formulations, with the Obs matrix containing the data (AEM and a priori), the Roughening matrix the constraints, the error matrix the uncertainties for all datasets. Constraints and uncertainties are usually different depending on data types and geology. Different datasets, collected by various AEM systems, have been used.
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GROUND PENETRATING RADAR INVESTIGATIONS ON THE RELATIONSHIP BETWEEN SALINITY IN FLUIDFILLED HORIZONTAL SUB-WAVELENGTH ‘THIN-LAYER’ BEDROCK FRACTURES AND REFLECTION AMPLITUDES
Authors Carolyn Tewksbury-Christle and Gregory S. BakerSubsurface fracture characterization is crucial for hydrogeologic modeling as applied to both fresh water resources and contaminant studies. Current methods of fracture geometry estimation for incorporation into groundwater models use cores, boreholes, and surface expression that are extrapolated over the entire study area. Due to the ‘cubic law’ relationship between fracture aperture and discharge—e.g., doubling fracture aperture will result in an eight-fold increase in discharge—there is a critical need to accurately estimate the aperture. Previous theoretical formulations suggested a correlation between ground penetrating radar (GPR) reflection amplitude and fracture aperture as well as groundwater salinity.
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INFORMED INVERSION APPROACH TO ELECTRICAL RESISTIVITY IMAGING IN A FRACTURED-ROCK SETTING
Authors Judith Robinson, Timothy Johnson, Lee Slater, Dimitris Ntarlagiannis and Pierre LacombeCharacterization of fractured rock presents unique inversion challenges in electrical resistivity imaging (ERI). The standard, smoothness-constrained inversion assumptions cannot adequately account for discrete fractures. Similarly, a borehole boundary is a sharp physical contrast and is often a conductive conduit that is typically not adequately incorporated into the inversion. We conducted synthetic studies to examine alternative ERI model parameterization approaches to better represent a fractured rock setting. An unstructured tetrahedral mesh was used for the forward modeling in order to optimize flexibility in the incorporation of boreholes and known fracture locations. Modeling such small-scale features at the plot scale resulted in a finite element mesh with a large number of tetrahedral elements.
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OBTAINING VELOCITY MODELS FROM SYNTHETIC FIRST-ARRIVAL TRAVEL TIMES USING REFRACTION TOMOGRAPHY METHODS
Authors Julian Ivanov, Richard D. Miller, J. Tyler Schwenk and Shelby PeterieThe possibility to apply different approaches to first-arrival time analysis of seismic data was examined using a sample synthetic first-arrival data set calculated initially from an unknown velocity model. This model was intentionally created for the purposes of a blind test and it was designed to represent a realistic near-surface target. Using the provided synthetic first-arrival data set various velocity models were estimated based on the understanding of the wide range of possible solution that can exist when dealing with the inverse refraction-traveltime problem (IRTP). Solutions to the IRTP were obtained using the refraction-tomography inversion technique. Comparison with the synthetic velocity model showed that none of the obtained models was accurate enough. Next, efforts were applied to find out what inversion parameters, such as initial model, types and degrees of regularization, etc., are required that would obtain solutions that match closely the synthetic model. Other possible models demonstrating possible nonuniqueness will be provided for comparison, as well. Such a comparison is expected to provide more evidence about the possible degrees of nonuniqueness of the IRTP for this particular model and the key inversion parameters for finding this true synthetic model.
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MAPPING THE ARCHEOLOGICAL POTENTIAL OF THE ROTTERDAM HARBOR AREA (THE NETHERLANDS), USING MARINE GEOPHYSICS IN A MULTI-DISCIPLINARY AND MULTI-STAKEHOLDER STUDY.
Authors Marco de Kleine, Peter C. Vos, Bjorn Smit, Henk Weerts, Dimitri Schiltmans, Wil Borst and Wouter WaldusThe Port of Rotterdam (PoR) is expanding the ‘Maasvlakte’ harbor on reclaimed land. In order to connect this new land with the old harbor a deep channel (‘Yangtze Harbor’) will be dredged to a depth of 20 meters below sealevel. Previous work revealed a deltaic landscape with Late Paleolithic and Early Mesolithic archeological artifacts at depths of 17-20 meters. Prior to the dredging the archeological potential of the site had to be investigated. In order to obtain a scientific and economical justifiable approach, fitting within the limited time frame, all concerned parties worked in close collaboration: - Harbor authorities (PoR) - Cultural Heritage Agency/Authority (RCE) - The engineering company (PUMA) - Geological and archaeological research institutions Deltares, BOOR and ADC
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SIMULATING P-WAVE VELOCITY VARIATIONS DURING FLUID INFILTRATION IN THE VADOSE ZONE USING FINITE-DIFFERENCE SEISMIC MODELING IN A VISCOELASTIC MEDIA
Authors Rachel E. Storniolo and Gregory S. BakerHydrogeologic parameter estimation in the vadose zone is critical in a variety of environmental and engineering applications. However, moisture content in the vadose zone is difficult to properly constrain using traditional methods, because water saturation can be highly variable in space and time, and geologic materials can be anisotropic, heterogeneous, or both. We have previously proposed an in situ method for estimating field-saturated hydraulic conductivity using time-lapse seismic first-arrival tomography (TLSFT).
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MEC Target Classification using EM61 Interrogation during Reacquisition
Authors A. Z. Kostera, R. L. Satkin and K. W. BolerThe value of digital geophysical mapping (DGM) for the remediation of sites containing Munitions and Explosives of Concern (MEC) is well documented. In recent years, the state of the science has shifted to the application of classification techniques which limit intrusive operations to those DGM anomalies most likely to represent subsurface MEC. While new target classification techniques show significant promise, they are still impractical at many of the sites with more challenging terrain and vegetation conditions. This paper will describe the use of target interrogation during reacquisition using existing, proven technology which provides a real-time field alternative to anomaly classification applicable to most sites and with the potential to significantly reduce the number of DGM targets that require intrusive investigation. NAEVA utilized a differential reacquisition threshold and innovative methods of evaluating terrain response to eliminate subsurface targets that are unlikely to be subsurface MEC. During processing of DGM data, target selections must be made conservatively knowing that data are recorded at a single orientation (parallel to line direction) and that the instrument often will not pass over a subsurface object in a way that creates the maximum response amplitude. During target reacquisition, the original mapping instrument is maneuvered over an anomaly at multiple orientations until that maximum response is found. The first implication of this technique is that a less conservative criterion could be applied to the selection of targets than would be reasonable during data processing, thus reducing the number of targets that require intrusive investigation. In addition, the ability to evaluate anomaly characteristics from multiple orientations in real-time allows the skilled operator to make sound judgments about the nature of a subsurface anomaly in the presence of non-metallic terrain response. The size of the McClellan project offered NAEVA the opportunity to evaluate Geonics EM61- MK2 targeting and reacquisition peak response data with respect to the dig results for over 195,000 targeted DGM anomalies in terrain which is generally unfavorable for anomaly classification using existing (large) advanced sensors. An analysis of this data showed that the anomaly peak response measured during reacquisition could be utilized to eliminate a significant (10-20%) number of digs without significantly impeding the ability to detect and remove MEC. The classification of targets in areas exhibiting elevated terrain response were evaluated under two different scenarios and offered reductions in intrusive effort of more than 50%.
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A SENSITIVITY COMPARISON OF TWO COMMERCIAL TIME-DOMAIN EMI SENSORS FOR UXO DETECTION
Authors Matthew Casari and Lance BesawIn live-site unexploded ordnance (UXO) remediation, digital geophysical mapping (DGM) must be accomplished to assess potential targets of interest (TOI) in the affected area. The DGM process is extremely time-consuming and subsequently an expensive endeavor. To minimize the costs associated with DGM, towed electromagnetic induction (EMI) sensors or sensor arrays are employed to increase the coverage rates and provide large sets of data for “post-processing” evaluation. The performance of these DGM studies in effectively detecting TOI is largely dependent on the sensor characteristics, in particular the Signal-to-Noise Ratio (SNR) achieved across targets of varying size and depth. Additionally, the spatial resolution of the sensor arrays determines the accuracy with which TOI can be localized, potentially saving time in the UXO extraction phase. In this project, we collected data over UXO test lanes with two commercially available EMI sensors - the Geonics EM61-MK2 and the Minelab Single-Transmit Multiple Receive array. Interrogated targets range from 40mm projectiles to 81mm mortars buried at multiple depths and different orientations. These tests were conducted using the Scout UXO detection system, a trailer unit originally developed by Applied Research Associates, Inc. for the U.S. Army Night Vision Electronic Sensors Directorate Humanitarian Demining division to perform DGM on live-site, overseas clearance areas . The performance of these sensors are presented and compared based on the criteria of SNR, rate of coverage, TOI localization capability, and the ability to detect “difficult” (small) targets.
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MAN-PORTABLE TEMTADS 2X2 TIME-DOMAIN EMI SENSOR ARRAY FOR UXO CLASSIFICATION
Authors Daniel Steinhurst, Thomas Bell, James Kingdon and Glenn HarbaughUnexploded ordnance (UXO) contamination at former and current Department of Defense sites is an extensive problem. Site characterization and remediation activities conducted with the current state-of-the-art technologies at these sites often yield unsatisfactory results and are extremely expensive to implement. This is due in part to the inability of current technology to distinguish between UXO and nonhazardous items. Newly emerging electromagnetic induction (EMI) sensor technologies offer the ability to robustly distinguish between these two classes of objects. Early versions of these systems have tended to be large and designed for towed operation on open fields with good sky view for geolocation.
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APPLICATION OF ELECTRICAL RESISTIVITY TOMOGRAPHY IN MAPPING SUBSURFACE HYDROCARBON CONTAMINATION
Authors Elijah Ayolabi, Adetayo Folorunso and Samuel Idemseasonal variation in the water table which is usually close to the surface during rainy season) to the surface and downwardly at greater depth into the subsurface, through a porous medium – sandy layer. This may probably accounts for the reason while most of the handdug wells in the area are contaminated with hydrocarbon products.
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INCORPORATING DEPHASING DYNAMICS INTO SNMR FID INVERSIONS IN THE HIGH PLAINS AQUIFER
Authors Trevor Irons, Jared Abraham, James Cannia, Yaoguo Li and Jason McKennaWhile it is accepted that T2 and T2 * diverge under most surface NMR (SNMR) conditions, the mechanism for this difference is usually not accounted for in the forward modeling kernels. In SNMR T2 * is often used as a proxy for T2 in the equations used to map NMR to aquifer characteristics. The reason that T2 and T2 * are not equivalent is due to inhomogeneities in the static magnetic field. As the Larmor frequency is a function of the static field, any inhomogeneity causes the spins to dephase and lose coherence. This dephasing causes non-exponential decay in the recorded FID time series.
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JOINT USE OF SEISMIC REFLECTION AND TIME-DOMAIN ELECTROMAGNETIC METHODS TO AID GROUND WATER MODELING
Authors Jacob Sheehan, Phil Sirles, Nicole Pendrigh and Mayo ThompsonHigh resolution seismic reflection and time-domain electromagnetic geophysical surveys were conducted in support of subsurface characterization and ground water modeling at a site in Wyoming contaminated with TCE. Two kilometer-long seismic profiles reveal several discontinuous cemented layers within the overburden sediments, and also show evidence of several faults that could be correlated between the lines.
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