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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
181 - 190 of 190 results
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Accurate AEM Data Processing in Hydrogeological Projects… Is It Worth the Time and $ ?
Authors Andrea Viezzoli and Camilla SoerensenAirborne Electromagnetics is used in many countries as a major tool for groundwater management, and environmental management, including parts of biosphere. in Denmark and Australia, for example, the geological surveys have, and continue to do so, acquired large datasets covering relevant portions of their sensitive areas, for this kind of applications. in both countries there is a high level of attention to the quality of the source data, of their processing and modeling, of the Integration of results with ancillary Information in order to produce derived products. in this paper we assess the effects that different levels of processing of Helicopter TEM data have on the resulting electrical models and then on the hydrogeological models. We use a SkyTEM dataset from XXX in Denmark, acquired in 2009. We focus on different approaches in the processing of the raw data found in the industry, whilst Inversion, which is necessary to show the effect in the model space, is undertaken in rather standard way. We show how the electrical models, and then the hydrogeology, change depending on the level of decoupling of artifacts, of lateral averaging, of late time noise assessment, and compare it with available borehole Information. Each one of these processing steps alters the output, and therefore the derived models. with respect to e.g., under/overextimating the depth to bedrock by several tens of metres, or the absolute resistivities by hundreds of Ohm m, and therefore potentially assigning the wrong hydrogeological unit to a given electrical layer, the extra time, effort and monetary investment involved in accurate detailed processing is probably worthwhile.
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Fast and Effective Groundwater Mapping from 10 to 300 m Depth with Accurate Processing and Inversion of Skytem Data.
Authors Andrea Viezzoli, Jared Abraham, Paul Bedrosian, James Cannia, Burke Minsley and Bill BrownWe present results from an airborne electromagnetic survey for groundwater mapping near Sidney, Nebraska, commissioned by the US Geological Survey in cooperation with the North Platte, South Platte, and Twin Platte Natural Resource Districts to SkyTEM Aps and Aarhus Geophysics who did processing and Inversion. An important innovation of this project is near real-time processing and Inversion performed daily in the field using a demo area, surveyed prior to collecting data in the actual project area, to illustrate the effectiveness of the system in resolving the target which contains a series of aquifers of varying thicknesses. Because SkyTEM data require no leveling and complex bias removal procedures, the data can be used immediately after each flight. A fast, first-pass processing (decoupling etc) can immediately start and, upon completion, the Inversion is carried out. with a quad-core desktop it is possible to process and invert overnight, with full non-linear Inversion, an average day’s worth of EM data acquisition. Although the results are not as accurate as obtained from more thorough processing and multiple Inversion runs, they are sufficient to evaluate the systems resolution and accuracy, and allow for informed decisions to move forward with production flights in the project areas. the data from these production areas were processed in the office to eliminate large and dense coupling effects due to power lines, pipelines, and irrigation infrastructure, and then inverted with a spatially-constrained Inversion, which incorporates prior Information to produce more robust results. from a hydrogeological standpoint, the results obtained greatly improved the understanding of the groundwater system, mapping aquifer thickness over ranges of 10 m to 300 meters. We present a comparisons with ancillary Information, including ground-based electromagnetic measurements, borehole lithologic and geophysical logs. using selected prior Information to constrain the Inversion can further improve the resolution of some model parameters.
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Multi-Elevation Calibration of Frequency Domain Electromagnetic Data
Authors Burke Minsley, Greg Hodges, Bruce Smith and Jared AbrahamThe ability to make quantitative inferences about subsurface properties is an important component of interpreting frequency domain electromagnetic (FDEM) data. Systematic data errors caused by imperfect instrument calibration can lead to Inversion artifacts or, in some cases, best-fit models that are inconsistent with the measured data. Factory and in-flight Internal system calibrations have helped to reduce, though not always eliminate, calibration errors in modern FDEM systems. A number of methods have been developed to calibrate data after it has been acquired, but these are primarily based on having auxiliary Information about subsurface properties from well logs or ground-based geophysical surveys, which are not always available and may have inaccuracies of their own.
in this work, we propose a new strategy for calibrating FDEM data that does not rely on prior knowledge of the subsurface structure. This calibration procedure involves acquiring multiple datasets along a single calibration line at several different survey elevations at the beginning of a survey. Calibration parameters, consisting of gain, phase, and bias correction factors for each frequency, are derived by requiring that data from the multiple survey elevations must be consistent with the same earth model at each location along the line. This is accomplished by simultaneously inverting the multi-elevation data for an earth model at each location along the profile along with a single set of calibration parameters. This joint Inversion strategy recovers the combination of earth models and calibration parameters that are optimally consistent with the multi-elevation data. the derived calibration parameters are then applied to the survey data, and the calibration procedure can be repeated as necessary to correct for system drift.
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Application of Airborne Electromagnetics for Hydrogeological Modeling Below Internal Surface Waters
Authors Andrea Viezzoli, Timothy Munday and Andrew FitzpatrickAirborne ElectroMagnetics (AEM) is been used more and more often has a great potential in groundwater applications by geological surveys worldwide. However, its full potential over surface waters has not been fully explored yet, with the most documented stream of publications describing the use of AEM over surface water mainly to recovery bathymetric data rather than Information about groundwater and its interaction with surface water. We believe AEM can also greatly improve the data quality and coverage in tidal and coastal areas, together with lagoons, esturaries, and river deltas while cutting significantly the acquisition costs. integrated with ancillary Information, it can provide a very flexible and powerful tool for the management of these areas.
Here we present results from AEM surveys over the Venice lagoon in Italy and over the Murray river in Australia, flown respectively with SkyTEM and RESOLVE, touching on the main technical aspects of the data processing and modeling when applied over surface water.
the Venice lagoon dataset shows interesting paleostructures in the sediments, large fresh water aquifers, and delineates the interface between different geological units with different permeabilities. the comparison with sub bottom profiling (seismic) data is also very encouraging, in the sense that technique measuring different physical parameters, from different platforms, produce comparable and complementary results. incorporating prior Information from, e.g., bathymetry data in the Inversion can help resolving otherwise poorly resolved model parameters.
in the Murray river case, the AEM shows very clearly the interaction between surface and groundwater, the areas of recharge and discharge. This Information is extremely valuable when managing the river and riverine system in a broader sense.
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Towards an integral Approach of Levee Safety: Smart Levees Combining Geophysics and in-Situ instrumentation
Authors Andre Koelewijn and Robert MeijerIn the Netherlands several destructive levee tests have been carried out employing an abundance of various in-situ instruments and a variety of geophysical methods within the IJkdijk. these test focused on slope stability and backward erosion piping.
We are now entering the next stage, deploying both geophysical methods and in-situ measurements to observe levees across the Netherlands and abroad (e.g. Boston, UK). these so-called ‘smart levees’ vary from huge sea defences to small polder dikes yet protecting significant urban areas and valuable assets.
When observing a wider range of levee types, with different types of sensor systems supporting each other, ranging from inSAR to local seepage flow detection, the need for a proper, flexible and reliable data collection and data processing system becomes more pronounced. We address this need in a (virtual) Levee Data Service Centre, capable of handling various types of data and levees and able to issue early warnings when needed, employing Artificial intelligence and cloud computing.
the aim is to arrive at a global system on levee safety, using both remote sensing to detect anomalies on a larger scale and a combination of geophysical methods and in-situ instrumentation.
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Full Waveform Modeling of Time Domain Induced Polarization Data and Inversion
Authors Gianluca Fiandaca, Esben Auken, Aurélie Gazoty and Anders Vest ChristiansenDuring the last decades the scope of Time Domain Induced Polarization (TDIP) has considerably broadened from mineral exploration to environmental geophysics, mainly to clay identification and landfill characterization. Furthermore modern instruments allow multi-channel acquisition of IP data with multi-core cables and steel electrodes. for TDIP surveys the acquisition time is shorter and the logistic simpler in comparison with Spectral Induced polarization (SIP) surveys.
Despite this the Inversion techniques have not undertaken consequential enhancements: TDIP data are usually inverted using only integral chargeability, without considering the effective shape of the transmitter pulse or the system transfer function of the receiver. for these reasons a new Inversion algorithm has been developed using the full time decay of the IP response to reconstruct the distribution of the Cole-Cole parameters of the Earth.
the forward response is computed modeling the full current waveform (in terms of sequence of positive and negative pulses) and instrumental low-pass filters. Even when stacking, needed for noise reduction and self potential removal, the signal changes significantly during the process and the finite numbers of pulses have to be modeled to obtain accurate time decays.
the new approach allows moving from a qualitative Interpretation of TDIP data for recognition of anomaly patterns to a quantitative analysis, able to discriminate soil lithotypes and, if present, the type/grade of contamination: a case history from a landfill-stream interaction supports this conclusion, correlating pollution and Inversion parameters.
the forward response has been implemented in the 1D Laterally Constrained Inversion scheme (1D-LCI), producing layered sections of the subsurface with lateral smooth variations. Currently a 2D and 3D implementation are being developed.
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Reliability of Time Domain Induced Polarization Data
The Time Domain Induced Polarization technique can be really useful in environmental geophysics, e.g. in study of aquifers for clay identification or in the characterization of landfills, but often the noise content and the consequent difficulty to assess data reliability completely overcome its potential benefits.
therefore a time lapse DC/IP experiment has been curried out at Hørløkke landfill (Denmark), in order to discriminate among the factors that influence data repeatability and to recognize critical aspects in DC/IP survey design.
Two profiles crossing the landfill (410 m long, electrode spacing being 5 m) have been acquired, the time lapse test being conducted with one, two and thirty days of delay. Furthermore different arrays have been tested, to compare the effect of varying quadrapoles sequences on noise content, electrode polarization, gate sampling and stacking. Moreover the effect of the current waveform settings (On and off time length and stacking) has been investigated, in conjunction to IP time gates selection.
the Syscal Pro equipment from Iris instruments has been used to acquire data, but a comparison with the Terrameter SAS 4000 (Abem instrument AB) has also been carried out.
the results of the study suggest that the driving factor in data repeatability is the voltage level of the IP signal, electrode polarization being a second order effect: from the acquired data set it has also been possible to extract safe thresholds for the signal level. We conclude the presentation by showing the results form the comprehensive Investigation of the landfill.
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Opportunities for Improved Collaboration Between Hydrologists and Geophysicists
By Ty FerreThere is general agreement that geophysics can add invaluable Information regarding the structure of the subsurface. there is also increasing evidence of the ability of geophysics to monitor changes in hydrologically relevant subsurface properties with time. However, too often, geophysical surveys are conducted in partial or complete isolation from hydrologic Investigations. We present an example of the improvement in the efficiency and accuracy of a geophysical survey than can be achieved through early and direct collaboration between a hydrologist and a geophysicist. We also provide Information about a funding opportunity to support increased interaction between hydrologists and geophysicists through the Consortium of Universities for the Advancment of Hydrologic Sciences, inc. This NSF-sponsored program provides funds, on a competitive basis, for a geophysicist to visit a hydrologic research site with the objective of establishing or enhancing hydrogeophysical collaboration.
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Interpretation of First-Arrival Travel Times with Wavepath Eikonal Traveltime Inversion and Wavefront Refraction
More LessInterpretation of first-arrival travel times with Wavepath Eikonal Traveltime Inversion and Wavefront refraction method.
We describe Interpretation of the synthetic traveltime dataset, made available by Colin Zelt. A 1D-gradient Initial model (with horizontal layering, parallel to smoothed topography) is obtained automatically from the traveltimes, without requiring the user to assign first breaks to assumed refractors. This Initial model is then iteratively refined with WET (Wavepath Eikonal Traveltime Inversion : forward model synthetic traveltimes with Eikonal solver, back-project misfit along wavepaths aka Fresnel volumes or fat rays, in a SIRT-like algorithm).
Alternatively, traveltimes are interpreted with WR (Wavefront Refraction, layer-based ray Inversion) method. for WR, first breaks are assigned to refractors interactively. WR has problems with imaging faults, pinchouts, outcrops and other velocity anomalies, which violate the WR assumption of laterally continuous layers. and the assignment of first breaks to hypothetical refractors is subjective and non-unique. But WR is still useful to detect lateral change of velocity, independent of WET.
Artefacts of the 1D-gradient Initial model (horizontal layering in basement) are progressively removed, with increasing number of WET iterations. Fit of WET model to WR Interpretation (fault in basement) improves with increasing iteration count, even after the RMS error stops decreasing. This demonstrates that using solely the RMS error as a criterion for determining the optimum number of WET iterations is unreliable, and may stop WET prematurely. We propose the following criteria, to determine the optimum number of WET iterations : I. explain traveltimes with smooth minimum-structure model, II. minimum correlation with layering of Initial model, III. reasonable fit with WR Interpretation, and IV. small RMS error.
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Aquifer Vulnerability Mapping with An Airborne Transient Electromagnetic System - Skytem
Very often the protection of aquifers against pollution from the surface is controlled by near- surface clay layers. While airborne electromagnetic methods have proven to be superior for 3-D mapping of aquifers, so far only frequency domain methods have had the ability to map the very shallow geology.
in this study we present recent developments of the SkyTEM system, which is possible to map the very near surface geology. the resolution is proven in a large-scale verification program in which four transects of 5 kilometres of SkyTEM profile line were remeasured by DC (PACES system), and subsequent the connection to geology was made by 5 double drill holes. the first hole cored to a depth of 15 and 20 m, while the second hole was measured with undisturbed el-log and gamma log. the four transects were carefully selected to represent various geological settings typical for glacial landscapes.
Prior to the groundbased field program substantial development had been done of the SkyTEM system and the laterally constrained Inversion algorithm. these developments make it possible to measure early-time data from as early as 8 microseconds (from begin of ramp), even with a 500 m2 transmitter frame and a maximum magnetic moment of approx. 170,000 Am2. the system itself provides unbiased data from around 12 - 14 microseconds, but because the bias response at earlier times is 1) dependent of the transmitter coil geometry only, 2) is slowly varying during flight operation, and 3) can be estimated from high-altitude measurements, the amplitude of the bias response can be determined during the Inversion process. Thus the very early time data can be used in the Inversion.
the outcome of the comprehensive field verification is that SkyTEM with bias-correction yields an accurate resolution of even thin clay layers in the upper 10 metres. Thus the method is perfectly fitted for resolving the entire geological model from the surface to a depth of 200 – 300 m.
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