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24rd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 10 Apr 2011 - 14 Apr 2011
- Location: Charleston, USA
- Published: 10 April 2011
61 - 80 of 190 results
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Application of the Automatic Optimal Spline Smoothing Method to Optimizing Edges of Moroccan Bouguer Gravity Anomaly Map
More LessWe present a singular method that is capable to filter out noise as well as suppress outliers of sampled real functions under fairly general conditions. from an a priori selection of the number of points that define the adjusting spline, but not their location in that curve, the automatic optimal spline smoothing method automatically determines the adjusting cubic spline in a least-squares optimal sense. the method is fast and easily allows for selection of various possible number of knots, adding a desirable flexibility to the procedure. As an illustration, we apply the AOSS method to Moroccan Bouguer gravity data map. the AOSS smoothing technique is an efficient tool in the Interpretation of geophysical potential field data particularly suitable in denoising, filtering and analyzing gravity data singularities. the AOSS smoothing and filtering technique was found to be consistently useful for optimizing edges and contours of geophysical data maps as Moroccan Bouguer gravity anomaly data map.
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Modelling Phosphates “Disturbances” Depth using Analytical Signal Responses of Geoelectrical Resistivity Data (Sidi Chennane, Morocco)
By Saad BakkaliExploitation of the phosphate layers in Sidi Chennane deposit (Morocco) collides frequently with problems bound to the existence, in the phosphate series, of sterile bodies qualified as derangements. Our study shows that these bodies, masked by the Quaternary cover can be mapped using the geoelectrical prospecting survey. A Schlumberger resistivity survey over an area of 50 hectares was carried out. A new field procedure based on analytic signal response of resistivity data was tested to deal with the presence of phosphate deposit disturbances. Models of the geology were successfully obtained from surface modelling of 2D peaks of resistivity data. Image processing optimization was based on surface optimization Tools. Downward analytical continuation of the surface modelling along 30 meters depth was used for modelling Sidi Chennane phosphates “disturbances” distribution. Analytical procedures were found to be consistently useful. Optimization of phosphate reserves were improved and better constrained.
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Instrument Assisted Criteria for Freezing Damage Prevention
More LessFreeze-thaw damage is among the most detrimental factors on concrete durability. Cost of thermal heating also leads to high cost and delay during winter construction activities. This paper describes the development of mechanistic criteria assisted by instrument to prevent freezing damages for curing of early stage concrete. It is based on the mechanical physical and measurement criteria involving the determination of the free water content and the volume of air void. Preliminary results are provided.
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Noise Cancellation for Surface NMR: A Comparison of Time and Frequency Domain Approaches
Authors Mike Mueller-Petke and Ugur YaramanciEven though surface NMR is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffer from bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise.
to overcome these restriction some approaches using a reference loop based system were developed during the last years (Mueller-Petke and Yaramanci (2010), Radic (2006), Walsh (2008)). All approaches have demonstrated useful capability to improve S/N. But comparison that allows for determining properties, i.e., pro and contra of each approach is missing.
the scheme of Mueller-Petke and Yaramanci (2010) calculates a transfer function between detection loop and reference loop based on an optimal filter, i.e., working in the time domain. However, calculating transfer functions in the frequency domain (as proposed by Radic (2006) but not published as replicable and verifible) is close to the time domain approach. in order to test the frequency approach explicetely, we developed our own frequency domain code.
This now allows for evaluating and comparing important properties of the schemes such as (i) factor of improving S/N, (ii) appropriate filter length, (iii) accuracy and stability of the transfer function and (iv) numerical costs. Thus, for the first time we present a comprehensive comparison of the mathematical basis of the schemes and application of the time and frequency domain approach on synthetic and field data.
Finally we conclude that the frequency domain approach is to be preferred usually provided that the noise is stationary.
Mueller-Petke, M. and Yaramanci, U. 2010. Improving the signal-to-noise ratio of surface-NMR measurements by reference channel based noise cancellation. Proceedings of NearSurface 2010, Zurich.
Radic, T. 2006. Improving the signal-to-noise ratio of surface-NMR data due to the remote reference technique. Proceedings of NearSurface 2006, Helsinki.
Walsh, D.O. 2008. Multi-channel surface-NMR instrumentation and software for 1D/2D groundwater Investigations. Journal of Applied Geophysics, Vol. 66, 3-4, 2008.
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A Global Measure for Depth of Investigation
Authors Anders Vest Christiansen and Esben AukenDepth of Investigation (DOI) is a commonly requested parameter in geophysical surveys. for diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no Information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model.
the method is based on the actual model output from the Inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points are integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated.
Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. the threshold limit applies to all 1D inverted data and will thus produce comparable numbers of DOI.
in the presentation we will show examples from systems spanning from the ground conductivity scale over DC-equipment to large airborne EM systems. the DOI will be calculated using a global threshold enabling detailed comparison of resolution capabilities of very different systems and methods.
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Creating 3D Models of Clay Content - An Inversion Approach using Geophysics and Borehole Information
More LessWe present a concept to produce 3D-voxel models of clay content Integrating geophysical resistivity models and lithological Information from boreholes. the concept minimizes the difference between reported clay volumes in boreholes and calculated clay volumes based on geophysical resistivity models.
For sedimentary areas the clay content is the most important factor for the water infiltration speed and thereby the vulnerability of aquifers to pesticides, nitrate etc. with the methodology presented we produce 3D models of clay content in a manner that can be fed directly into hydrogeological models.
Borehole Information contains detailed Information on the clay contents, but borehole Information is too sparse for the detail level required in actual mapping situations. However, the clay contents are also reflected in the resistivity of the sediments and thereby in geophysical data measuring sensitive to resistivity.
in short, the concept estimates clay content in a 3D voxel model based on:
1. Clay distribution identified in boreholes and accompanying uncertainties.
2. Layered models obtained from Inversion of geophysical data including the parameter uncertainties.
3. A non-linear Inversion scheme minimizing the difference between observed clay contents (from boreholes) and calculated clay contents (from geophysics) using a spatial interpolation (kriging) to match the observation points in space.
the model which is the output of the Inversion scheme not only gives the best model for clay content, but it also describes the uncertainty of the model. the model greatly improves the overview of the huge data sets and it improves the basis for decisions regarding the area planning. The concept has been employed in a groundwater mapping campaigns in Denmark with promising results.
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Integration of Ground Penetrating Radar with Real Time Kinematic - Global Positioning System Receivers for Efficient Mapping of Drainage Pipe Systems Beneath Golf Course Greens
Authors Barry Allred and Robert FreelandThe U.S. alone has over 16,000 golf course facilities. the upkeep of these facilities requires continual maintenance and occasional remodeling. the superintendents and architects responsible for golf course maintenance and remodeling efforts need non-destructive Tools for obtaining shallow subsurface Information, particularly with regard to determining drainage pipe locations beneath golf course greens. Ground penetrating radar (GPR) has already been proven capable of finding buried drainage pipes under golf course greens; however, the use of this technology needs to be streamlined in order to map these drainage pipes in a timely manner with minimal disruption of golf course activities.
A case study was therefore initiated to evaluate the efficiency of Integrating GPR with real time kinematic - global positioning system (RTK-GPS) receivers to map drainage pipe systems beneath golf course greens. the Integration of GPR (using 400 MHz antennas) with RTK-GPS for drainage pipe mapping was tested at six different golf course green sites in central Ohio. Integration of GPR and RTK-GPS allowed data to be collected without a grid set-up of well defined measurement transects, and as a result, the time needed to conduct a golf course green GPR survey was reduced by half with the added benefit of no disruption to golf course activities. Furthermore, based on knowledge of typical golf course green drainage pipe installation practices, extremely simple and easy to use processing techniques can then be employed using the GPR/RTK-GPS data to generate accurate golf course green drainage pipe maps. Consequently, this case study provides strong evidence that Integration of GPR with RTK-GPS is a valuable tool that can be used by the golf course industry for efficient mapping golf course green drainage pipe systems.
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Employing Airborne Electromagnetics for Spatial and Temporal Hydrogeophysical Monitoring: A View from Opposite Ends of the Globe
Authors Andrea Viezzoli, Timothy Munday and anders Vest ChristiansenAlthough the notion of hydrogeophysical monitoring of natural landscapes using multi-temporal airborne EM data sets, has been around for some time, translating this into practice has been limited by several factors, including the availability of fully characterised systems and by accurate Interpretation procedures. there is also the issue of what we term “a will” – specifically a willingness to employ hydrogeophysical methods because they are trusted to yield quantitative Information on, for example, groundwater quality. This paper examines two contrasting cultures, namely an Australian and Danish one, and considers how hydrogeophysical monitoring is being developed and deployed there. in Denmark, over a period of nearly 5 years, large portions of the country have been surveyed with AEM. Derived datasets complement a large suite of ground- based TEM measurements (> 40000 soundings), dating back to the early 1990’s. Both data sets, after Interpretation, have been used to produce a seamless output covering acquisition with different instruments and configurations, often many years apart. This was made possible through the use of a national TEM test site, against which all instrumentation, had to be calibrated prior to use. This systematic approach provides the framework for hydrogeophysical monitoring, ensuring that the long term, quantitative measurement of change is a reality. By virtue of this national approach to groundwater management Denmark is uniquely placed to employ hydrogeophysical methods in spatio-temporal Investigations. in contrast, other countries such as Australia, who have a more fragmented, regional approach to groundwater management, with jurisdiction commonly the responsibility of individual States, have seen hydrogeophysical monitoring options confined to small areas and targeted, high value, assets. An example of this is the floodplains of the Murray River, where existing knowledge regarding floodplain behaviour during and after floods is limited and hydrogeophysical methods are seen as offering the promise of elucidating process.
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Automated Leak Detection of Buried Tanks using Geophysical Methods At the Hanford Nuclear Site
Authors Shawn Calendine, Dale Rucker, James Fink and Marc LevittAt the Hanford Nuclear Site in Washington State, the Department of Energy oversees the containment, treatment, and retrieval of high-level mixed radioactive waste. Much of the waste is currently stored in single-shelled tanks (SSTs) that do not meet current State criteria for waste storage. Currently, the waste is being retrieved from the SSTs and transferred into new double-shelled tanks (DSTs) for temporary storage before final treatment. Monitoring the tanks during the retrieval processes is critical in indentifying leaks and creating a warning system for the older SSTs.
An electrical geophysics monitoring program for Leak Detection and Monitoring (LDM) has been successfully deployed on several SSTs at the Hanford site since May of 2004. the monitoring program takes advantage of changes in resistivity that will occur when conductive tank liquid leaks into soil contiguous to the subsurface tank shell. During monitoring, voltages are measured and recorded between steel cased wells, surface electrodes, and the tanks themselves. Data acquisition hardware and software allow for continuous real-time monitoring of the received voltages through time-series data analysis, creating a highly sensitive method of leak detection and complementing drywell logging as a means to detect and quantify leaks. Working in an industrial environment such as the Hanford site involves many challenges for electrical monitoring, such as cathodic protection, grounded electrical infrastructure, lightning strikes, diurnal and seasonal temperature trends, and precipitation, to create a complex environment for leak detection analysis. in this discussion we present solutions to the complicated challenges of working in the tank farms of the Hanford site.
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Spectral Induced Polarization Monitoring during Microbial Enhanced Oil Recovery
Authors Jeffrey Heenan, Dimitrios Ntarlagiannis and Lee SlaterMicrobial Enhanced Oil Recovery (MEOR) has been established as a cost effective method for enhancing tertiary oil recovery. Although not commonly used for shallow heavy oils, it could be a viable alternative since it can offer sustainable economic recovery and minimal environmental impact. Successful MEOR treatments require accurate, real time monitoring of the biodegradation processes resulting from the Injection of microbial communities into the formation. Results of recent biogeophysical research suggest that minimally-invasive geophysical methods could significantly contribute to such monitoring efforts.
Here we present results of laboratory experiments to assess the sensitivity of the spectral Induced polarization method (SIP) to MEOR treatments. We used heavy oil from a shallow oilfield in SW Missouri to saturate three sand columns. We then followed common industry procedures and used a commercially available microbial consortia (Para-Bac/STM) to treat the oil columns. the active MEOR experiment was performed in duplicate while a control column maintained similar conditions, without promoting microbial activity. We monitored the SIP signatures, between 0.001 Hz and 1000 Hz, for a period of six months. to support the geophysical measurements we also monitored geochemical parameters, including pH, Eh and fluid conductivity, and collected weekly fluid samples from the outflow and inflow which were analyzed to confirm that microbes actively degraded the heavy oils in the column. Destructive analysis of the solid materials was performed upon completion of the experiment,
Preliminary analysis of the results suggests that SIP is sensitive to MEOR processes. in both inoculated columns, we recorded an increase in the low frequency polarization with time, where there were measurable changes up to 3.5 mrads in the phase shift recorded for both active columns, while no change was observed in the control column for the duration of the experiment. these results may indicate that remote geophysical methods could successfully complement current MEOR monitoring schemes for promoting sustainable oil recovery.
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Integration of Geological and Geophysical Data to Improve the Understanding of A Deep-Seated Landslide
Authors Diego Arosio, Davide Brambilla, Laura Longoni, Monica Papini, Giovanni Savazzi and Luigi ZanziHeavy rainfalls in late 2002 reactivated an ancient deep-seated landslide in the Varrone valley (Northern Italy). the unstable body threatens a hydroelectric power plant as well as a new industrial district placed along the valley line. Though the sliding surface is believed to be deeper than 100m, landslide reactivation concerned just a shallow portion of the slope close to the landslide toe. A detailed geological survey was undertaken after the events in 2002. inclinometers and piezometers were deployed in some boreholes, which were also used to assess rock layering and quality, along with the presence of discontinuities and of groundwater. to this final aim a preliminary Electrical Resistivity Tomography survey was also carried out. Recently, geophysical surveys were performed in order to validate geological assumptions and to assess the condition of the slope over a wider area. Ground penetrating radar profiles helped to determine the dip as well as the persistence of the discontinuities that are considered preferential paths for groundwater flow. A combined P- and S-wave refraction survey delineated the interface between shallow glacial sediments and underlying gneiss layer, and was used to infer elastic parameters of the subsurface. the outputs of the geophysical surveys were used to run a numerical model with the aim of evaluating the kinematics of the slide, possible future scenarios and the associated mitigation measures.
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Field Demonstration of Nuclear Magnetic Resonance (NMR) Logging Tools for Groundwater and Environmental Investigations
Nuclear magnetic resonance (NMR) measurements provide direct sensitivity to pore fluids enabling estimation of key petrophysical properties including porosity, bound/free fluid content, and permeability. NMR logging instruments have been used for decades in the oil and gas applications, but until recently, the size and high cost of oil-field Tools have limited the availability of this powerful technology for groundwater and environmental Investigations. We present results from field tests of a newly-developed suite of economical, small-diameter NMR logging Tools, specifically designed to meet the unique needs of near-surface Investigations. Principal field tests of two NMR probes were conducted at the Geohydrologic Experimental and Monitoring Site near Lawrence Kansas. One of the probes, is optimized for logging measurements in very small-diameter holes that are commonly available for environmental studies. with an outer diameter of 43 mm, this probe operates in small PVC or open-cased holes ranging from 50 to 100 mm. the second probe has a diameter of 64 mm and is designed for dynamic and minimally-invasive operation with a Geoprobe ® direct push machine. Both probes were operated with a minimum echo time spacing of 2.0 ms; the depth resolution is1.0 m for the 45 mm probe and 0.5 m for the direct-push probe. the NMR logs provide continuous distributions of the transverse (T2) relaxation time versus water content at each depth interval, which are used to derive estimates of porosity, bound/free water content, and hydraulic conductivity. Results from the two Tools were found to be consistent with one another and with auxiliary subsurface Information available at the extensively characterized site. the availability of these newly-developed and unique Tools open vast opportunities for utilizing NMR to enhance groundwater and environmental Investigations.
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Applications and Advantages of a Short Dead-Time for Surface Nuclear Magnetic Resonance (SNMR) Measurements
Authors David Walsh, Elliot Grunewald and Peter TurnerSurface nuclear magnetic resonance (SNMR) measurements provide non-invasive detection and characterization of groundwater by measuring the relaxation decay of hydrogen nuclei following excitation by a surface loop. A key factor controlling the sensitivity of the SNMR measurement to groundwater is the so-called dead-time, which describes the delay between the end of the excitation pulse and the first reliable recording of the NMR signal. the dead-time will fundamentally limit the shortest signals that can be detected and thus effectively controls the range of environments in which SNMR can be successfully used. We describe the recent development of very short dead-time SNMR instrumentation and application of these measurements in a series of case studies. We demonstrate the achievement of dramatically reduced dead-times, as short as 4 milliseconds, which enable detection of groundwater in environments previously considered inaccessible by SNMR. Specifically, we illustrate detection of very short NMR decay signals associated with water in magnetic geology, bound water in silts and clays, and capillary water in the vadose zone. the availability of a new small diameter NMR logging tool provides an opportunity to ground-truth the surface NMR data and confirms that these short signals are reliably detected using short dead-time surface NMR capabilities. Based on our field results, we discuss the advantages and implications of a short-dead time SNMR instrument for quantifying short signals, improving signal to noise ratios, and characterizing aquifer properties.
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A New Marine Acoustics Prospecting instrument with the Advantage of Sub-Bottom Profiler and Shallow Seismic Reflection Method On River and Sea
More LessIt is required for prospecting of subaqueous to explore the thickness fluctuation of subaqueous silt seam, sand layer, pebble bed, and riprap layer, fluctuation of subaqueous bedrock surface, burial depth of bedrock, fault in the bedrock and fracture zone. Sub-bottom profiler and seismic reflection method are the most common. Generally, the Sub-bottom profiler is adopted for exploration of thinks of silt seam, sand layer of bottom with high resolution from the high frequency band however the relatively poor penetrating power; while the low frequency band is adopted for the water shallow seismic reflection method, resulting high penetrating ability but low resolution, which makes it suitable for exploration of rock stratum and geological structure in straight measuring line. Is there any method or instrument combining the advantages of there two methods above? That is the very hope of the workers engaged in geophysical prospecting. KS marine acoustics prospecting instrument is designed to address such problems on the base of the aforementioned ideal, and was awarded the patent of invention. It is basically aimed at penetrating pebble and riprap layer, with enough sensitivity to measure the layer thickness above 1m, and replacing in whole or in part the shallow seismic reflection method on water to penetrate underwater rock strata. the author is striving to achieve breakthrough which are adopting the very wide frequency-band based receiving system to receive all reflection signals between tens of Hertz to 10kHz, adopting receiving detector with strong directing property and adopting the spark source that can control emission energy. It is possible for the instrument to explore silt seam, sand layer, pebble bed, top and bottom surface of riprap layer which are thicker than 1m, and deep stratum through collecting the reflection wave with a reflex time above 150ms.
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A New Version of Seismic Wave Reflection Method Fit for Urban Exploration —Landsonar
More LessThere are two problems on shallow exploration in urban:(a) frequent exposure to vibration noise and terrestrial electricity peculiar to city. Seismic methods are usually highly embarrassed by the vibration from the electric cars, pedestrians, engineering construction and operating metro lines; (b) Many geophysical exploration methods cannot meet the requirements on desired exploration depth and resolution. Landsonar method is the shortened form of an elastic reflection continues profiling with extremely small offset and very wide frequency-band, bringing the characteristics of zero-offset into full play. Adopting single-channel collection with almost zero-offset; increasing signal-to-noise ratio through vertical stacking ; Through combination of wave detector with very wide frequency-band and main unit of receiver, it is able to stimulate and receive the wave with 5Hz to 4000Hz without any frequency distortion (no compressing or magnifying wave with any frequency). with these features Lansonar can fit for urban shallow high resolution exploration , and several applications have got the correct feedback over deep of 150 m in China.
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Recent Developments in the Skytem System
Authors Kurt Sorensen, Esben Auken and Max HalkjaerThe SkyTEM system has originally been developed for groundwater mapping and is capable of measuring in a wide time range from very early decay times to late times without any bias correction applying high altitude data sets. the system has been extensively used since its introduction in 2002 and have proven its superior performance in ground water mapping as well as mineral exploration when near surface as well as deeper resolution is required. Further developments including higher transmitting moment by segmented transmitter techniques, higher signal to noise ration by motion noise suppression and adaptive signal processing and reduction of coil responses at very early times by on-time measurements have expanded the measurable time decay interval, the deep of penetration and the accuracy of the measurements, thus meeting the increasing demands for detailed and accurate Interpretation of data sets. Selected topics from the developments will be presented together with examples of applications.
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Integrating Hydrology and Geophysics to Evaluate the Impact of Artificial Recharge on Groundwater in Rural India
The monsoonal climate of India coupled with the complex geology and low storage capacity of the Deccan basalts contribute to water scarcity in central India during the dry season. One of the primary Tools proposed to manage this problem is the artificial recharge of runoff captured during the monsoon to enhance groundwater availability throughout the year. One common approach for artificial recharge is the construction of small dams to generate percolation ponds, as exemplified by a small reservoir in the Salri watershed of Mahdya Pradesh, India. We use this specific example to illustrate how the Integration geophysical and hydrologic data can be used to understand the influence of the dam on groundwater in the watershed. Electrical resistivity and electromagnetic induction surveys are used to assist in developing a geologic conceptual model for the watershed consisting of a thick sequence of basalt flows overlain in the lowland portion of the watershed by weathered basalt and alluvium for a depth of up to 10m. This geologic model has guided our understanding of the local flow system. A shallow flow system in the near-surface weathered basalts and alluvium is the primary source of water for agriculture. in contrast, vertical variability in the competent basalt flows is expected to create a highly anisotropic flow system with high horizontal permeability and low vertical permeability. As a result, the geophysical data help to form a conceptual model where the dam primarily impacts the shallow aquifer and has limited impact on deeper regional flow systems. to assess this hypothesis and quantify the impact of the dam on the overall hydrology of the watershed a hydrologic monitoring program was implemented. By Integrating the geophysically-based conceptual model with this hydrologic data we are able to provide a quantitative assessment of the role of the dam within the watershed.
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Estimation of Aquifer Transport Parameters from Resistivity Monitoring Data within a Coupled Inversion Framework
Authors Dylan Fowler and Stephen MoyseyDespite the fact that electrical resistivity surveys are being increasingly used to image solute migration in porous media, there is limited experience in how these data can be used to characterize aquifer transport parameters. in this study we evaluate whether electrical resistivity monitoring data can constrain the hydraulic conductivity, porosity, and dispersivity of a homogeneous aquifer within the framework of coupled Inversion. We focus on responses from a single set of current electrodes to provide insights into the Information content of these measurements and motivate future work in designing optimal surveys for monitoring groundwater transport processes. We have found that even in this simple system two distinct types of tradeoffs exist that could confound the direct estimation of transport parameters. First, different values of porosity and hydraulic conductivity can produce the same plume velocity, thereby leading to identical concentration distributions in the subsurface. in ideal cases this hydrologic non-uniqueness can be resolved by electrical data because electrical resistivity is dependent on both solute concentration and porosity, but this discrimination will only possible when the rock physics relationship between concentration and resistivity can be accurately determined. the second type of non-uniqueness occurs when different concentration plumes produce equivalent electrical responses. in this case, tradeoffs between plume velocity and dispersivity cause shifts in the position and mass of solute relative to the electrodes that result in equivalent values of subsurface apparent resistivity. This non-uniqueness can be overcome by explicitly recognizing that different Information about transport processes is captured by the timing versus magnitude of the electrical response in monitoring data. Though we have investigated a homogeneous system, we argue that the insights here can also be applied to the estimation of effective transport parameters for heterogeneous systems.
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Time-Lapse Imaging of Dynamic Systems using Multi-offset GPR Reflection Data
Authors Adam Mangel and Stephen MoyseyGround-penetrating radar is widely recognized as a powerful tool for mapping the subsurface to image structures and locate targets. However, velocity variations resulting from heterogeneity in soil texture and water content can greatly degrade the coherency of radar responses. This issue is particularly problematic in dynamic environments where the background environment may change over time, e.g., as a result of rainwater infiltration. to address this problem we investigate the radar signature of reflectors embedded in homogenous and layered soils during the course of a simulated infiltration event. the data are collected using multiple antenna offsets to allow for evaluation of traveltime and amplitude changes with offset. the result of this survey design is a rich data cube with dimensions of GPR traveltime, experiment time, and antenna offset. We demonstrate the value of this data by performing normal moveout velocity analysis for arrivals obtained over the course of the experiment. We show that we are able to perform time to depth corrections of the GPR data that provide a consistent image of the subsurface over the course of the experiment as moisture conditions change. for example, in experiments performed in a homogenous tank we show that the depth to the reflection produced at the bottom of the tank can be resolved with an error of ±5cm (±10%), where the error is mainly associated with picking errors. While this error can be somewhat reduced by improved traveltime picking, we have also found that hydrologic processes, e.g., the buildup of water at a seepage face formed at the bottom of the tank, contribute to complexity in the GPR data that requires analysis of wave interference for improved Interpretation. We suggest that multi-offset reflection GPR shows promise for both estimating velocity variations in the shallow subsurface and improving subsurface imaging.
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Integrating Geophysical Surveys with Geotechnical Investigations at offshore Wind Facilities
By John MadsenThree-dimensional variation in sediments due to their geologic setting is a common occurrence in marine environments. Adequate geophysical data, including that from acoustic, side-scan sonar and high-resolution seismic reflection surveys constrained by grab sampling, coring and video ground-truth, must be available to define the types and distribution of these sediments. Due to their having differing geotechnical properties, the three-dimensional variable distribution of sediments has significant implications for the siting and construction of offshore wind facilities along the East Coast of the US.
Geophysical surveys need to be integrated with geotechnical requirements at regional, local and site specific scales when developing offshore wind facilities. At the regional scale, geophysical, coupled with geological, data provide Information that along with first-order considerations such as wind, wave, and/or tidal conditions, water depths, access to onshore grid infrastructure, and ecological and human impacts, must be considered in the selection of a location for an offshore wind facility. the geologic setting is relevant because coastal marine areas have evolved under conditions of sea level rise and fall during the past several hundred thousand years. these conditions have determined the present and past sedimentary environments where facilities will be sited. At the local scale, geophysical data can be used to determine the distribution of surficial and sub-surface sediments. for example, paleochannels, ancient river valleys formed during periods of lower sea-level, are in-filled with coarser- to finer-grained sediments and preserved in the sub-surface. Shallow, high-resolution, seismic surveys can image networks of these paleochannels thus providing constraints on grain size and spatial variations of sediments. At the site specific scale, bottom and sub-bottom sediment types and their distribution play a role in selection of the type of wind turbine foundation (e.g., monopile, tripod pile structure, or gravity base), and in the subsequent location and installation of these foundations.
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