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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
151 - 190 of 190 results
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Experimental Evidence for the Dependence of Archie’s Law on Multiphase Flow Dynamics
Authors Zuolin Liu and Stephen MoyseyArchie’s law is a powerful tool for relating electrical measurements to fluid saturations in applications ranging from oil production to the migration of organic contaminants. However, the formation of channels or fingers during unstable multiphase flow is an issue that leads to preferential flow within the subsurface that is not typically considered in the analysis of electrical data. This study is directed toward understanding how the parameters of Archie’s law depend on multiphase flow instability. Flow experiments were conducted in a thin, two-dimensional tank (55cm x 55cm x 3.75cm) packed with 2 mm glass beads where mineral oil was displaced by Nigrosine dyed water. the light transmission method was used to provide time-lapse images of oil and water saturations in the tank over the course of the experiment while measurements of the bulk electrical resistivity of the tank are simultaneously completed. Different experiments were performed by varying the water application rate and orientation of the tank to control the generalized Bond number, which describes the balance between viscous, capillary, and gravity forces that affect flow instability. the experimental results show that overall the resistivity index gradually decreases as water saturation increases in the tank, but drops sharply as individual capillary fingering fronts reach the outlet to create a high-conductivity pathway that bridges the tank. the magnitude of this drop decreases as the displacement becomes increasingly unstable and disappears for highly unstable flow leading to the generation of many small fingers. At flow equilibrium, the saturation exponent in Archie’s law increases from 0.65 to 1.94 and is linearly dependent on the generalized Bond number. As the flow becomes increasingly unstable, the saturation exponent reaches a constant value of 1.94 for values of the generalized Bond number less than -0.106. these results document a dependence of Archie’s law on multiphase flow dynamics.
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Nuclear Magnetic Resonance Logging to Estimate Permeability in the High Plains Aquifer
Of interest for groundwater applications is logging the measurement of the proton Nuclear Magnetic Resonance (NMR) relaxation-time constant referred to as T2. in a water-saturated material, the distribution of T2 values can be related to aquifer permeability, essential for groundwater evaluation and management. In this study, the Schlumberger Magnetic Resonance Scanner was used to log a 152 m test hole in the High Plains aquifer in central Nebraska. the T2 distributions, obtained every 46 cm at a radial depth of 10 cm, were used with the Schlumberger-Doll-Research (SDR) and Timur-Coates (T-C) empirical equations to estimate permeability. these NMR permeability values were compared to preliminary hydraulic conductivity values obtained from aquifer tests and flowmeter measurements at the same site. the hydraulic conductivity values were converted to permeability assuming constant groundwater viscosity and density. Conversion of the aquifer test results provided single permeability values for the Quaternary alluvium and for the underlying Tertiary Ogallala Group. the flowmeter measurements were used to proportion the latter permeability estimate over 6-m intervals in the Ogallala.
in the alluvial sand and gravel, the NMR permeability appears to vary with the silt-clay content as indicated by gamma, neutron, and resistivity logs, reaching a maximum value near the center of the deposit where the NMR permeability matches the aquifer-test permeability. in the Ogallala, which consists of gravel, sand, silt, and clay, the NMR permeability also appears to vary with the silt-clay content and compares favorably with the trends seen in the flowmeter-proportioned permeability.
Outstanding research questions include the effect of measurement scales and detection limits on the permeability estimates, and the application of NMR-permeability empirical equations in different aquifer lithologies.
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Geophysical Investigations of Selected infrastructure Sites within the National Petroleum Reserve, Alaska
Authors Jared Abraham, Bethany Burton, Eric Anderson, Jeff Lucius and Brent LewisThe U.S. Geological Survey with cooperation of the U.S. Bureau of Land Management Alaskan Northern Field office conducted a study of the geophysical response of anthropogenic impacts in the National Petroleum Reserve-Alaska (NPRA), located on Alaska’s North Slope. the North Slope of Alaska presents new challenges in our ability to clearly identify and understand subsurface anomalies. the complexity of the tundra soil, which consists of intermixed ice lenses, permafrost, saline sediments, and air pockets, increases the difficulty of interpreting geophysical measurements. Rapid shoreline erosion over the last seven years along the Beaufort and Chukchi Seas is now impacting infrastructure within the NPRA. Erosion along the shoreline of Teshekpuk Lake and the Colville River is also of concern. in 2005, 38 NPRA infrastructure sites were identified as being in a high risk category for shoreline erosion. infrastructure sites include airstrips, current and former Distant Early Warning sites, gravel pads, road segments, old landfills, and legacy exploration/research wells with associated reserve pits, flare pits, and fuel pits. the geophysical Investigation of selected infrastructure sites within the NPRA involved the use of broadband EM instruments capacitively coupled resistivity systems, and total field magnetometers. the objective of this study was to geophysically characterize the subsurface at selected sites within the NPRA. Eleven NPRA infrastructure sites were studied during August 2005 using geophysical methods. Extent and volume of potential contamination and buried waste were determined. Inversion of the EM data provided 3-D distributions of conductive anomalies within the study areas. Additionally, indications on the extent and presence of permafrost below the sites were observed. these datasets where used for the evaluation of the risk of the selected sites and for planning and guiding remediation.
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High-Accuracy Inversion of Airborne Transient Electromagnetic Data for Hydrogeologic Framework Mapping Related to Climate Change
Authors Bas Peters and Paul BedrosianAddressing problems related to water quality and quantity (e.g. irrigation, contaminant transport and vulnerability to climate change), requires detailed Information about the hydrogeological framework over large areas. Airborne transient electromagnetic (TEM) is a geophysical technique capable of constraining subsurface resistivity to several hundred meters depth; the resulting sections can be interpreted in terms of the hydrogeologic framework which constrains groundwater flow.
the aim of this research is to define the hydrostratigraphy of the High Plains Aquifer over a range of different geological settings, in Nebraska, USA. A challenge is to accurately characterize the airborne TEM systems in order to recover subsurface resistivity models consistent with independent ground truth Information, especially in areas where targets exceed 100m depth. Several airborne TEM systems are being evaluated for their applicability to these problems.
Processed data are inverted in 1D with lateral constraints to obtain a pseudo-2D resistivity section. Recently developed model assessment techniques are applied and extended to investigate regions of the model that are most sensitive to the data. these Tools provide estimates of the depth of Investigation (DOI), the top of Investigation (toI) and determine the uncertainty of the resulting model parameters. the DOI, toI and start model selection problems are solved using a sensitivity-matrix analysis method.
DOI and toI Information is important because it is possible that the measured data do not have sensitivity to certain depth intervals within the target aquifer. One result is a map showing the configuration of the top- and base-of-aquifer surfaces, in relation to DOI and toI surfaces derived from the TEM data, together with estimates from model parameter uncertainty. Results are verified with ground-based TEM soundings and lithologic/geophysical bore hole logs.
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Evaluation of EM38 as a tool for Improving Irrigation Practices in Rural India
Authors Sudershan Gangrade and Stephen MoyseyWater scarcity is a major problem in Central India. Sustainable management of water is a particularly big challenge in rural areas where agriculture is the main occupation, an extended dry season limits water availability throughout a large part of the year, and technical and educational infrastructure for outreach efforts is limited. One issue in particular is that farmers practice flood irrigation over their fields with little ability to tailor water application based on varying soil conditions. This study examines whether electromagnetic induction (EMI) mapping of soils can be used as a practical field tool to help farmers in developing countries improve irrigation practices by better understanding field-specific conditions. Temporal variations in electrical conductivity during flood irrigation events are mapped over agricultural fields in the Salri watershed, Madhya Pradesh, India as well as near Clemson, South Carolina using the Geonics EM38-MK2. the range of conductivity values changed over time as a resulting of wetting and drying, but within each given field the spatial pattern of conductivity was observed to remain relatively consistent. We hypothesize that the different regions of the field identified using EMI can be zoned as distinct soil management units and that temporal variations in the data can be used to assist farmers in deciding when and where to irrigate. to test this hypothesis at our field sites, measurements of volumetric water content are being collected using 10inch capacitance probes inserted vertically from the soil surface and soil permeability measurements are being made using a disc infiltrometer. Statistical analysis of these data will allow us to evaluate whether the infiltration characteristics of the different soil management units identified using EMI can be used as a meaningful zonation for irrigation management by farmers at these sites.
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Application of the Multiaxial Perfectly Matched Layer to Near-Surface Seismic Modeling with Rayleigh Waves
Authors Chong Zeng, Jianghai Xia, Richard Miller and Georgios TsofliasPerfectly matched layer (PML) absorbing boundaries are widely used to suppress spurious edge reflections in seismic modeling. When modeling Rayleigh waves with the existence of the free surface, the classical PML algorithm becomes unstable when the Poisson’s ratio of the medium takes values greater than about 0.38. Numerical errors can accumulate exponentially and terminate the simulation due to computational overflows. Numerical tests show that the divergence speed of the classical PML has a non-linear relationship with the Poisson’s ratio. Generally, the higher the Poisson’s ratio, the faster the classical PML diverges. the multiaxial PML (M-PML) attenuates the waves in PMLs using different damping profiles in orthogonal directions. If the proportion coefficients of the damping profiles are set appropriately, the M-PML algorithm is stable for high Poisson’s ratio earth models. Through numerical tests of 40 models with Poisson’s ratios that varied from 0.10 to 0.49, we find that a constant proportion coefficient of 1.0 is sufficient to stabilize the M-PML for all isotropic elastic cases. Wavefield simulations indicate that the instability of the classical PML is strongly related to the wave phenomena near the free surface. When applying the multiaxial technique only in the corners of the PML near the free surface, the original M-PML technique can be simplified without loosing its stability. the simplified M-PML works efficiently for both homogeneous and heterogeneous earth models with high Poisson’s ratios. the analysis reported on here was based on 2-D finite difference modeling in the time domain that can easily be extended into the 3-D domain with other numerical methods.
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Mapping Disturbed Ground using Compressional and Shear Wave Reflection Sections
Authors Susan Pullan, André Pugin, James Hunter and Gregory BrooksAt some sites near Ottawa, Ontario, thick glaciomarine sediments deposited in the Champlain Sea after the final retreat of the Winsconsinan icesheet ~11,000 years ago show evidence of show evidence of failure by landsliding or severe “disturbance” by ground motions during a prehistoric earthquake(s). the Geological Survey of Canada has been investigating these areas to better understand landslide susceptibility, seismic site response and paleoseismicity. This paper presents results obtained with 3-component seismic reflection surveys over such failed or disturbed terrains.
Shallow seismic reflection surveys are used to provide subsurface architectural and stratigraphic Information. in Champlain Sea sediments, very high-resolution shear-wave reflection sections have been obtained using a vibratory source/landstreamer data acquisition system. However, where the large-scale depositional structure of the fine-grained marine sediments has been disrupted by deformation, fracturing, faulting and/or liquefaction, the shear wave reflection data quality can be highly compromised. This is interpreted to indicate that the disturbed ground is scattering shear wave energy at the wavelength scale (~ 1 metre). in contrast, compressional (P-) wave sections since the signal wavelengths are longer and the velocity is strongly influenced by interstitial porewater, are not degraded in quality to the same extent.
Comparison of compressional and shear wave data from areas of landsliding or disturbed ground can provide Information on the thickness of the soft fine-grained glaciomarine sediments and the underlying bedrock topography, as well as give new insights into the properties of the disturbed sediments. Such Information can lead to better hazard and risk assessments throughout the Ottawa Valley-St. Lawrence Lowlands region of eastern Canada where similar glaciomarine deposits are found.
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Buried Valley Imaging using 3-C Seismic Reflection, Electrical Resistivity and AEM Surveys
Authors André Pugin, Greg Oldenborger and Susan PullanIn the Canadian Prairies buried valleys are important sources of groundwater. Hydrological methods such as pumping tests provide very limited spatial Information to efficiently predict the sustainability of these aquifers. to obtain a full assessment in three dimensions of such complex reservoir geometry, geophysical Tools are an absolute necessity.
the Spiritwood valley in southwestern Manitoba, is a Canada-USA transborder buried-valley aquifer. in March 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AEM) survey (AeroTEM III) over a 1062 km2 area along the buried valley north of the US border. the results show multiple resistive elongated features which have been interpreted as coarse sediment filled channels inside a 15 km wide more conductive valley filled with finer sediments such as diamictons. the spatial distribution, directionality, and size of the channels is complex.
Follow up ground surveys were carried out during the summer and included a ground based, multi-electrode electrical resistivity survey to calibrate the resistivity of the various units seen in the AEM data, as well as a high-resolution seismic survey to obtain detailed architectural and depth Information. the seismic data were collected using a Minivib I in inline horizontal vibrating mode (20-240 Hz sweep) at a shot spacing of 6 m and a 3-component (3-C) landstreamer receiver array with 48 sleds spaced at 1.5 m. these data allow us to obtain both shear wave and compressive wave profiles. the younger, less compacted channels were better imaged with P-wave data, while some areas with shallow gas or organic peats were better imaged with S-wave data. the seismic images show detailed sedimentary sequences and permit some inferences on the relative ages of channels formed during multiple ice advances. the sections also showed the presence of other channels, which are interpreted to be infilled with finer sediments based on the seismic facies, and which are not associated with resistive features in the AEM data.
This combination of AEM, electric sounding and 3-C seismic profiling provides exceptional 3-D coverage which has highlighted key hydrological features such as buried channel aquifers and potential sub-surface hydraulic pathways or connections. Such Information is critical to groundwater prospecting and to the accurate assessment of recharge and discharge potentials associated with buried valley aquifers.
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Preliminary Results from an Airborne Electromagnetic Survey of Permafrost in the Area of fort Yukon, Ak
Authors Jared Abraham, Bruce Smith, Michelle Walvoord and James CanniaThe U.S. Geological Survey undertook a helicopter frequency domain electromagnetic (HFEM) survey in July 2010 of the area of fort Yukon, Alaska for the purpose of mapping the distribution of permafrost. Concerns over the impacts of climate change have recently energized research on permafrost and the potential impacts that thawing permafrost may have on groundwater flow, infrastructure, forests, and contaminant transport. there is typically little knowledge known about the three-dimensional distribution of permafrost, including thickness and where openings or taliks occur. This study maps the distribution of permafrost in selected areas near fortYukon in order to develop hydrogeologic Information and Interpretation for development of a groundwater model in the YukonBasin. An additional benefit is the collection of an anchor dataset of 3D permafrost distribution which can be compared to future similar data collections to determine a volumetric change over time which in turn can be correlated to climate change.the HFEM system used six frequencies from 140,000 hertz to 400 hertz, providing data for imaging electrical properties at varying depths. Approximately 1800 line kilometers of data were acquired in typical block style near Ft.Yukon and in long reconnaissance lines along the Yukon River flood plain and the Porcupine River tributary. in preliminary processing, the airborne data were converted to depth sections of electrical resistivity using a numerical transform. Electrical properties of earth materials are impacted by temperature and the presence of ice causing them to become more resisitive as the water freezes. A simplified conceptual model of areas of discontinuous permafrost is that high resistive areas have permafrost and low resistive areas are permafrost free. Results from the survey show continuous areas of high electrical resistivity indicating the presence of permafrost intermixed with areas of low electrical resistivity under the Yukon River and some lakes indicating areas that are not frozen. the area of the loess hills on the margins of the Yukon River has very high electrical resistivity indicating that they contain a relatively large amount of frozen water. A drill hole in the Ft.Yukon area drilled in the late 1990s found the thickness of permafrost was about 100 m. the HEM survey shows discontinuous high resistivities that extend to this depth. Drill hole electrical logging showed that the basement rocks in this area are conductive shale. Some shallow areas of low resistivity correlate with gravel pits and may offer insight into temporal changes in the character of the permafrost. the flood plain of the Porcupine River appears more resistive that the Yukon River flood plain which can indicate more permafrost and resistive bedrock. Further Interpretation and ground truth is required to verify the interpreted distribution of permafrost. However, this is a first look into the distribution of the permafrost in the areas around fortYukon and is a demonstration of the application of HFEM to permafrost mapping as part of an integrated climate change study. Such surveys conducted in other parts of the Arctic region will enable comparisons and contrasts of permafrost distribution on a world wide scale.
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Seismic Refraction Tomography using a Landstreamer for Estimating Void Volume in a Reclamation Project
Authors Curtis Link and Will GoldbergA large industrial facility that operated for almost ninety years was demolished and the 1200 acre site reclaimed. the site contained a number of below grade structures such as storage tanks, vaults and pipe chases. During reclamation these below grade areas were backfilled and a vegetated soil cover established. Recently, a number of voids and subsidence features have been observed. in an effort to quantify the magnitude and extent of these areas, a preliminary shallow seismic survey was performed in 2010 using refraction Tomography. We collected P-wave data for three profiles using a sledgehammer source and a 24 channel gimbaled geophone landstreamer with 1 m geophone spacing.
Overall data quality was low making first arrival time picking difficult. Picks were made only on traces exhibiting a reasonable signal-to-noise ratio. We hypothesize that poor data quality is related to the complicated shallow subsurface environment comprising a complex combination of cultural and geological features resulting in complicated scattering of seismic energy. Three P-wave velocity tomograms were produced showing a number of low velocity features that we associate with void zones. Subsidence features observed on the surface correlate well with low velocity zones on the tomograms. in addition, the tomograms show extensive lateral extent of similar low velocity zones with no surface expression that we interpret as void-like areas. Finally, we performed void volume calculations based on estimations of the in-line void area and an assumed cross-line extent. these void volume calculations will be used to estimate the amount of fill material required to complete remediation of the site.
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Improved Predictions of Groundwater Salinisation in the SE of South Australia – A Case Study On the Combined Role of Geology and Geophysics.
Authors Timothy Munday, Fred Leaney and Steve BarnettIn many areas of South Eastern Australia, saline soil-water in the unsaturated zone overlies fresher groundwater. in areas where remnant vegetation has been removed and has been replaced with pasture or cropping, enhanced drainage has resulted, which after a lag time, flushes the saline soil-water down into deeper unconfined aquifer systems that contain fresh groundwater. for an area near Tintinara in south-eastern South Australia, spatial simulations of groundwater salinisation under dryland and irrigation scenarios were developed for a 200 year timeframe using a distributed parameter recharge model. inputs to the model include spatial patterns of near surface clay materials determined from the constrained Inversion of helicopter frequency domain EM data. the AEM data were used to define the spatial distribution and thickness of a near surface clay-rich sedimentary unit. the presence of clay coupled with it’s thickness significantly affects the lag time for the downward movement of saline soil water and the salt flux to the aquifer. Under dryland agriculture leaching of the salt to the aquifer takes between 50 and 200 years, whereas under irrigation leaching takes between 20 and 50 years. Under irrigation the salinity of the groundwater is expected to increase from 1000 – 6000mg/L over the next 25-40 years. While there are no obvious economic measures available to prevent salinisation, the modelling suggests that efficient irrigation on appropriate soils could slow the process.
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Comparison of High Resolution Electrical Resistivity and Ground-Penetrating Radar Measurements in a Shallow Vadose Zone Environment
Authors Cameron Toy, Colby Steelman and Anthony EndresWhile previous studies have clearly demonstrated the capacity of both electrical resistivity Tomography (ERT) and ground-penetrating radar (GPR) reflection profiling as surface hydrogeophysical methods for characterizing soil moisture in the vadose zone, there have been very few direct comparisons between these two techniques. Further, the low resolution of those earlier data sets do not facilitate detailed analysis of moisture variations in the shallow (i.e., within the upper meter) vadose zone.
to address this issue, we have collected coincident sets of high resolution ERT and GPR images at a site near Waterloo, Ontario during an annual cycle of unfrozen conditions. GPR data acquisition included reflection profiles and common midpoint surveys using high-frequency (900 MHz) antennas. the ERT surveys included Dipole-Dipole and Wenner arrays at 0.25 m electrode spacing. the geology of our study site is a sequence of interbedded medium to coarse grained sand units ranging from a few centimeters to a few decimeters in thickness within the upper few meters of vadose zone; the local water table is located 15 m below ground surface. While the clean sand resulted in excellent depth of Investigation for the GPR profiling, the very high contact resistance during dry conditions restricted ERT data acquisition to periods when there was adequate near-surface soil moisture.
While analysis of data is ongoing, preliminary results show good correlation between depths of resistivity boundaries and reflection events. Seasonal variations due to moisture are evident from the analysis of resistivity data. We will be conducting further analyses on the spatial and temporal relationship between GPR interval velocities and ERT resistivity values as well as examining the affect of electrode array choice on the ERT-GPR comparison.
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Regional, Reconnaissance Scale AEM Surveys to Better Define Surface Water - Groundwater Processes Beneath Large Unregulated River Systems
Authors Andrew Fitzpatrick, Timothy Munday, Louise Stelfox and Glenn HarringtonThe groundwater resources in northern Australia offer the greatest potential for future irrigation development and as a source for water supply to the southwest of Western Australia. the Fitzroy River and its associated alluvial aquifer in northern Western Australia has been considered in planning for Perth’s future water supply and for agricultural development but its potential needs to be informed by detailed understanding of groundwater-surface water interactions occurring along its extent, and in particular must consider and account for the consequences that might arise when extracting groundwater from shallow and deep aquifers linked to this river system. While the Fitzroy is one of Australia’s largest unregulated rivers characterised by braiding channels within a wide floodplain, knowledge concerning the extent and significance of its floodplain storage is limited. This paper considers results from the analysis and Interpretation of a regional scale longitudinal transect (~274 line kms) of SkyTEM helicopter EM data to help elucidate river-bed processes occurring along its extent. the AEM data were acquired to provide an indication of the variation in groundwater quality and related aquifer characteristics associated with different parts of the river. Conductivity-depth sections derived from their Inversion using a 1D laterally constrained Inversion (LCI), were examined against available hydrochemical, environmental tracer (including 222Rn, 4He, 87Sr/86Sr and Cl-), and hydrostratigraphical/hydrogeological data sampled longitudinally. they provided further insight into the Fitzroy’s alluvial aquifer system and its links with the underlying sediments. the results demonstrate the value of regional, reconnaissance scale AEM surveys to better define groundwater processes beneath large unregulated river systems.
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Radon as an Indicator in Borehole Geophysics
Authors Leonid Anisimov and Irina VorontsovaRadon as an indicator in borehole geophysics Leonid Anisimov & Irina Vorontsova LUKOIL VolgogradNIPImorneft, Volgograd, Russia Radon indicator has been evaluated to have environmental benefit, because this tracer rapidly decays to safe levels. Other advantage of radon indicator is that it allows to quantify permeability profile of reservoir partitioned according to the level of incremented radioactivity. Geological analysis of reservoir based on core data is a principal instrument for reservoir modeling. the comparison between cross-plots of porosity and permeability enabled to derive permeability from core data and to calculate the parameter related to porosity obtained from geophysics and well logging. However, core data are often very limited to characterized reservoir as a whole, especially for fractured carbonate reservoirs. Large fractures are typically more widely spaced than the diameter of a borehole, therefore it is very difficult to get a core in such failure area and obtain reliable samples. Direct evidence of fracture spacing is lacking in well logs data due to small volume of fractures. On the other hand, long fractures having apertures 1-2 mm are much more conductive than matrix with high porosity and they make the main contribution to the permeability of the reservoir. to understand the reservoir quality, it is necessary to detect the real high permeability zones and their relationship with storage capacity of different parts of the reservoir. Pumping test technology is widely used by the oil industry for dynamic reservoir characterization. However, evaluation of available test data has low selectivity and tests do not cover the reservoir profile. Simultaneous analysis of pumping test and tracer test data would make it possible to receive the layer-to-layer variations in permeability. the radon tracer method was widely applied to show the permeability profiles of many reservoirs in the Volgograd Region. Modified special program can use radon to evaluate the residual oil saturation in-situ measurement. All aspects of these techniques have been elaborated in the Volgograd Research Petroleum institute - Lukoil-VolgogradNIPImorneft – and have been published.
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Fractional Diffusion Analysis of the Electromagnetic Fields Generated by a Straight Current Source over a Porous Geological Media
Authors Jianchao Ge, Mark E. Everett and Chester J. WeissAn Interpretation based on the Continuous Time Random Walk theory (CTRW) to the diffusion of electromagnetic fields generated by a transient straight current source over a porous geological media is presented here. the CTRW theory is demonstrated to be a powerful tool to concisely and more accurately model a transport process in a fractal medium with complex structures, comparing to the classical transport theory. in the controlled-source electromagnetic (EM) induction setting, the time dependent evolution of the EM field of some sources over a rough medium are governed by the fractional diffusion EM equation in a CTRW sense. the master equation can be solved for a uniform conducting half-space in the Laplace domain semi-analytically. We use 2D finite difference method to calculate the solution numerically for the assigned space and transform to time domain with Gaver-Stehfest algorithm. Here we adopt a spatially uniform roughness parameter β in the solution to characterize the complexity of the geoelectrical structure of the geological medium. to introduce the heterogeneity to our model, we set up the space as several 2 D blocks with different conductivities and βs. then we compare our results with the synthetic data we got from the high resolution numerical simulations. We are able to show that by introducing the heterogeneity to the fractional diffusion perspective, our approach is competent for tracing the diffusion process with less model parameters.
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Characterizing Fracture infill from its Seismic Response: Synthetic Modelling and Laboratory Experiments
Authors Ranajit Ghose, Carlos Almagro Vidal and Joost van der NeutThe presence of fractures and their characteristics determine the behavior of a rock mass, including its strength, stiffness, and all forms of conduction and diffusion – hydraulic, electrical, chemical and thermal. Fracture characteristics include orientation, spatial density, type and degree of saturation of the infill material, connectivity, fracture width and roughness. in this research we have looked at the seismic response of the fracture infill material while maintaining other fracture properties constant. in the context of near-surface geophysical applications, characterizing the fracture infill is important in geotechnical engineering, hydrogeology and contaminant transport management/mitigation.
the response of a single fracture in an elastic medium has been derived using the linear slip theory. This theory involving particle displacement or velocity discontinuity boundary condition has earlier been found useful to describe the transmission or reflection response at non-welded interfaces. Unlike the effective medium theories, this approach allows one to study the seismic response of a single fracture in terms of the fracture compliance (1/stiffness) even when the seismic wavelength is much larger than the individual fracture thickness. the goals of the present research are to look into the dependence of the fracture transmission response on the frequency and angle of incidence of the seismic wave, and to verify these theoretical results through careful laboratory experiments. We could resolve on laboratory data the frequency- and angle-dependence of the fracture compliance, and explain these results using the theoretical response derived for the full elastic case. Our results suggest the possibility of retrieval of fracture compliance as a function of fracture infill from the elastic transmission or reflection response.
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Simultanoeus Estimates of in-Situ Porosity and Permeabilty in the Near-Surface Soil from Seismic Dispersion at the Low Frequencies
Authors Alimzhan Zhubayev and Ranajit GhoseThe extraction of any meaningful subsoil Information from the velocity dispersion and attenuation Information contained in the low-frequency (10-100 Hz) field seismic data is generally conceived as an over-ambitious task. However, we have detected that even at these low frequencies the poroelastic behavior of different seismic wave attributes and wavetypes can differ in such a way that their Integration offers unique estimates of multiple soil properties at an undisturbed condition. We propose a new concept utilizing the underlying physical mechanism of frequency-dependent seismic wave propagation in the water-saturated near-surface soil.
We shall discuss the possible Integration of frequency-dependent velocity and attenuation of low-frequency compressional and shear waves derived from the seismic transmission data in various soil types e.g., sand, clay, peat and gravel. the choice of the poroelastic model is data-driven. the Integration leads to unique and stable estimates of in-situ porosity and permeability, simultaneously. This has obvious advantages compared with empirical approaches like those using Kozeny-Carman relation or multidisciplinary Integration performed at the Interpretation stage or joint Inversion based on only field observation/statistics. the uniqueness and stability of the estimates derive from the fact that we selectively use only those frequency-dependent wave attributes and the differences in their underlying physics that can exploit the convexity Inherent in the integrated property domain. Results will be illustrated using data from realistic synthetic tests and multiple field experiments.
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Magnetic Resonance Soundings and NMR Logging of a Limestone Aquifer- the Tamala Limestone, Western Australia.
Authors Andrew Fitzpatrick, Kevin Cahill and Timothy MundayMagnetic Resonance Sounding (MRS or Surface Nuclear Magnetic Resonance (SNMR)) is a geophysical technique used for direct groundwater detection and aquifer characterisation. the technique is based upon the nuclear magnetic resonance (NMR) method, most well known in medical imaging. the technique was developed in the early 1980’s but commercial systems only became available in the late 1990’s. the technique has been trialled in Australia previously, but the method has not been fully adopted into common use. the main impediment of the technique is in detecting a very small signal typically 8-10 orders of magnitude less than the background electromagnetic noise. with the advent of new system designs and modern processing technologies the latest developments in MRS have greatly improved the success of the technique. CSIRO is undertaking a study to test the latest in MRS instrument design- the Vista Clara GMR system in Australian settings. As part of the study a trial site over the Tamala Limestone in Western Australia was undertaken as part of a larger hydrogeological study of this particular limestone which provides a groundwater resource to the Perth area, Western Australia.
in this project MRS soundings were collected at 3 sites. Following the surface measurements, sonic drilling was performed, and each hole was logged with an NMR tool. in addition laboratory NMR and permeability tests were performed on recovered samples. This paper discusses the results of the three scales of measurements and demonstrates how MRS provides a non invasive method for characterising aquifers.
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Accurate Modeling of the System Transfer Function Is the Right Attitude: Using AEM to Map Clay Thickness and Extent for Groundwater Recharge Determination is a Case in Point!
Authors Andrew Fitzpatrick, Timothy Munday and Andrea ViezzoliThe pumping and disposal of saline groundwater from the margins of the River Murray in South Australia is an integral part of the State Government’s salinity management strategy. It is specifically aimed at reducing ground water levels and salt accession to the River Murray. Along the margins of the Murray, large volumes of saline water are typically disposed of at the land surface, in what are referred to as “saline-disposal basins”. their long-term efficacy and the potential for developing others whilst minimising the groundwater recharge requires an understanding of the extent and thickness of near surface aquitards present in the region. these clay-rich units are known to have a significant influence on the rates of groundwater recharge in the study area, and their accurate definition is a necessary input into groundwater recharge models, which in turn determine salt flux to the Murray River system from planned and extant disposal basins in the area. We have used RESOLVE frequency domain helicopter electromagnetic data acquired over the Stockyard Plains saline-water disposal basins and environs located southwest of Waikerie, South Australia to demonstrate their value in regional groundwater management. We have also set out to demonstrate that constrained Inversion with locally accurate 1D forward responses is critical to recovering accurate models from the AEM data. We have employed a constrained layered earth Inversion to generate Information on the depth, thickness and presence or absence of aquitards, specifically the Blanchetown Clay, and map variations in groundwater conductivity in the region. in this study we compare results before and after we take account of system attitude and altitude in the Inversion to highlight the importance of accurate modelling.
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Police Cars Spatial Movements Modeling, Analysis and Presentation
Authors Sarah Doze, Anahid Bassiri, Pouria Amirian and Ali bassiriMiniaturization of computing devices, and advances in wireless communication and sensor technology are some of the forces that are moving computing from the stationary desktop to the mobile outdoors. This new classes of applications has to deal with spatial objects whose position or extend changes continuously over time, for short, called Moving Objects. . in this regards, it is important to model the location of moving objects effectively and enable DBMS to omit the redundant and repeated data of a moving object by using efficient query manners. Trajectory simplification and compression are very fields since moving object’s recording their position in time produce a large amount of frequently redundant data. in order to reduce the amount of data to be stored in database it is very important not to store all the data. Some algorithms are proposed to reduce the data or to simplify the trajectory of a moving object. in this paper tracing and monitoring of police motorbikes and cars are considered to examine the proposed algorithm. in addition simplified trajectories are mapped onto the street network to understand which areas have not been supervised. Mapping the trajectories onto the street network enables us to navigate the police cars.
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Digital Soil Mapping with Depth using EM38 and EM31 Signal Data and a 1-D Laterally Constrained Inversion Model
Authors John Triantafilis and Fernando Acacio Monteiro SantosThe ability to map the spatial distribution of average soil properties using geophysical methods at the field level has been well described. This includes the use of electromagnetic (EM) instruments which measure bulk soil electrical conductivity (sa). However, soil is a three-dimensional medium. in order to better represent the spatial distribution of soil with depth, various methods of inverting EM instrument data have been employed. in this paper we use a 1-D Inversion algorithm with 2-D smoothness constraints to predict the true electrical conductivity (s) from sa data collected with an EM38 and EM31 in the vertical (v) and horizontal (h) dipole modes and at heights of 0.2 and 1.0 m, respectively. in addition, we collected EM38 sa at heights of 0.4 and 0.6 m. We compare and contrast the value of the various sa data by conducting individual and joint Inversions. We find that the values of s achieved represent the duplex nature of the soil. the EM38 data assists in resolving root zone variability of cation exchange capacity (CEC – cmol(+)/kg of soil solids) and the electrical conductivity of a saturated soil paste extract (ECe – dS/m), whilst the EM31 assists in characterising the likely location of a shallow perched-water table. in terms of identifying an optimal set of EM sa data for Inversion we found that a joint Inversion of the EM38 at a height of 0.6 m and EM31 signal data provided the best correlation with electrical conductivity of a saturated soil paste (ECp – dS/m) and ECe (respectively, 0.81 and 0.77) closely followed by a joint Inversion of all the EM38 and EM31 sa data (0.77 and 0.56).
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Compare and Contrast Three Geophysical instruments to Resolve the Location of a Municipal Landfill
More LessMunicipal landfills are potential areas of contamination for water resources. This is the case in the highly permeable Botany Sands Aquifer of southern Sydney, where a decommissioned landfill is the source of at least one known leachate plume. in order to monitor and manage groundwater pollution, piezometers have been installed. However, these methods can be expensive, time consuming and involve permanent installations that provide spatially discrete data. As a means to better characterise the leachate plume and sediments, surface geophysical methods have been used. the value of this approach is not in the output data obtained from the individual instruments, but in the Integration of all geophysical results, which combine to more accurately characterise the sub-surface conditions. the surface methods we have used include direct current resistivity (DCR) and electromagnetic induction (EM). in this paper we compare and contrast these methods in addition to a Capacitively Coupled Array (CCA) to assess the extent of leachate plume emanating from the municipal landfill in Astrolabe Park. in this study we detect, measure and map the location and spread of the leachate plume emanating from the landfill using the: ABEM automated resistivity system (DCR), a DualEM-421 (EM) and an OhmMapper (CCA).the results suggest each method has its advantages and disadvantages in terms of resolution, depth of Investigation and ease of use. Inversion results from all three methods are well correlated, with each having identified a highly conductive zone (>35mS/m) which is consistent with the location of a known leachate plume. the results suggest that of the three instruments that the portability and speed of data collection of the DUALEM-421 and Geometrics OhmMapper could be used to map the entire park, with a view to developing a 3D models.
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Mapping Soil and Regolith Properties in 3-Dimensions using Electromagnetic Imaging in the Lower Gwydir Valley, NSW, Australia
More LessThe clay alluvial plains of north-western New South Wales (NSW) are dominated by uniformly textured clay-rich Vertisols and are primarily used for irrigated cotton production. Unfortunately prior stream channels, characterised by Chromosols which are of a duplex nature, occupy 15 % of the landscape. in these channels, deep drainage leads to water use inefficiencies. in order to improve water management in these irrigated areas, these channels need to be mapped in terms of their areal extent and stratigraphy. in this paper we use legacy apparent electrical conductivity (σa) data, measured using an EM38 and EM34, from the Ashley area of the lower Gwydir valley, NSW. the EM data is modelled using a 1-dimensionally spatially constrained Inversion algorithm to develop a quasi 3-d true electrical conductivity (σ) model. We calibrate σ to map the distribution of clay content and electrical conductivity of a saturated paste extract (ECe) at different depths. the EM38 provides Information which assists in mapping clay content and CEC within the root-zone (0 to ~1.5 m) whilst the EM34 provides Information useful for mapping these soil properties in underlying strata (>7 m).
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Extension of the Poroelasticy theory to Address in-Situ Stress in Shallow Subsoil
Authors Ranajit Ghose and Alimzhan ZhubayevThe poroelasticity theory of Biot and its many modifications and extensions have so far been used in rock physics and soil physics to explain and exploit frequency-dependent propagation of seismic waves through a porous subsurface layer. the physical properties that have been associated are hydraulic/flow properties like porosity, permeability, viscosity and tortuosity. Poroelasticity has not been used to address the level of in-situ stress.
in the context of shallow subsoil characterization, we have performed laboratory tests on water-saturated sand where we have measured frequency-dependent seismic wave velocity (especially S-wave velocity because of its geotechnical importance) for various levels of in-situ horizontal and vertical effective stresses. in order to understand our experimental data, the poroelastic stress-strain relations and the wave equation in full anisotropic case have been revisited. On the other hand, we have employed micromechanical theories or stress-porosity-rigidity relations to introduce stress-dependence in the poroelastic material properties. This has resulted in, for the first time, varying levels of velocity dispersion for varying stress levels, as we indeed observe in our experiments. the theory has been developed for isotropic and transversely isotropic (VTI) media. Since spatial wavenumber is involved, the dependence of seismic dispersion on the direction (angle) of wave propagation can be described. Finally, when the porosity is known, we can invert the value of in-situ stress and permeability from the observed seismic dispersion and attenuation in the field seismic frequency range.
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Magnetic Resonance Soundings in the Central Platter River Basin for Groundwater Models
Authors Jared Abraham, James Cannia, Trevor Irons, Greg Steele, John Williams and Duane WoodwardIn 2007, the USGS in cooperation with the Central Platte Natural Resources District, central Nebraska, initiated a five year study to develop magnetic resonance sounding (MRS) techniques to gather Information on aquifer characteristics for input into groundwater models. Magnetic resonance sounding is a surface geophysical tool which has the potential to measure water content and hydraulic conductivity. This in turn will provide a low cost alternative to traditional aquifer tests. MRS also will allow for collection of large data sets of aquifer properties during short periods of time. the work is under way in Dawson County near Lexington, Nebraska to characterize the hydrogeology of the Quaternary-age alluvial and underlying Tertiary-age Ogallala Group aquifers that occur within the Platte River Valley. This study area was selected because it lies in an area of Nebraska that has major groundwater- surface water management issues which have stimulated the development of regional and local groundwater models. Data used to evaluate the MRS during this study were derived from traditional constant discharge aquifer tests, borehole flow meter tests, lithologic descriptions, borehole geophysics, and time-domain electromagnetic soundings. This study presents methods and Interpretation of MRS. the MRS-derived hydraulic conductivity data will be compared to hydraulic conductivity data from two constant discharge pumping tests of the alluvium and Ogallala Group aquifers at two sites within the study area. the MRS-derived hydraulic conductivity data will also be compared to conductivity estimates based on data from a borehole flow meter test. This Information can potentially be incorporated into groundwater models of the area to provide improved data sets of aquifer characteristics. the research will document an integrated MRS, surface geophysical, borehole geophysical, borehole flow meter and aquifer test approach in which the hydrostratigraphy of the Platte River alluvial aquifer and Ogallala aquifer can be described.
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The Use of Geophysical Data for Evidence-Based Groundwater Management
Authors James Cannia, Jared Abraham, Bruce Smith and Steven PetersonAirborne geophysical surveys provide high quality subsurface data not available from any other source for building the complex hydrogeologic frameworks needed by water resource managers. Airborne surveys have the ability to cover large areas quickly with low impacts to local activities and the environment. these data can be collected processed and inverted to provide Information on the structure of the geological and hydrogeological environment. often the data used in building hydrogeologic frameworks consists of borehole lithology and geophysical logs combined with surface geologic maps and occasional surface geophysical soundings. for regional studies this is adequate, but for the detailed studies being done for understanding local aquifers and surface water systems too much error is introduced into the three dimensional framework to accurately represent actual conditions. the addition of data collected by airborne geophysical surveys provides near continuous data throughout the area that can then be calibrated to the bore-hole logs and other data creating a very accurate and precise subsurface Three-dimensional map.
the three-dimensional maps provide powerful Tools for locating features of the aquifer system. these maps can be combined with water table maps to provide the geometry of the aquifer including locations of the most saturated thickness, heterogeneity of aquifer, barriers to groundwater flow and connections to the surface water system. they indicate preferential flow paths which is particularly important in understanding water quality. Examples of using this Information are; site well locations, recharge sites and facility construction.
Water Managers put this data into groundwater models for predictive analysis of management scenarios. Examples from a Nebraska groundwater model constructed with this data shows significantly improved model performance. All of this Information significantly benefits water resource managers to make decisions on what is most effective and cost efficient to manage water supplies.
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Inversion of Seismic Refraction Shootout First-Arrival Times using Simulated Annealing
Authors Khiem Tran and Dennis HiltunenThe authors have developed a technique to invert first-arrival times using simulated annealing, and this technique will be utilized to analyze the seismic refraction shootout travel time data. the scheme is based on an extremely fast finite-difference solution of the Eikonal equation to compute the first-arrival time through the velocity models by the multistencils fast marching method. the core of the simulated annealing, the Metropolis sampler, is applied in cascade with respect to shots to significantly reduce computer time. Although the technique requires more computer time than some commercial packages, the technique offers several advantages. First, the Inversion technique does not depend on the Initial model, and this becomes important in regions where prior Information about subsurface profiles is not available. Second, rather than just one final model, simulated annealing provides a suite of final models clustering around the global solution and having comparable least-squared error. This provides an Inversion result by averaging all of these accepted models to mitigate the influence of noise and the non-uniqueness of the Inversion solutions. Last, the technique also can determine the uncertainties associated with inverted results. in cases where the Inversion results of subsurface formations are used for the design of engineering structures such as foundations, the uncertainty can be particularly useful in implementing the new load and resistance factor design (LRFD) methodology that can explicitly account for spatial variability and uncertainty in design parameters. the capability of this Inversion technique has been tested with both synthetic and real experimental data sets. the Inversion results show that this technique successfully maps 2-D velocity profiles with high variation. the inverted wave velocity from the real data appears to be consistent with cone penetration test (CPT),geotechnical borings, and standard penetration test (SPT) results
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Application of GPR to the Delineation of Melt Regimes, Internal Stratigraphy, and Flow Dynamics at Three Potential Ice Core Drill Sites in the Alaska Range
Authors Seth Campbell, Karl Kreutz, Erich Osterberg, Steven Arcone and Cameron WakeEnvironmental impacts from climate change such as global sea level rise, continues to negatively impact millions of humans, worldwide. Sea level rise primarily results from sea water expansion and increased glacier melt from warming temperatures. Quantifying glacial melt is challenging because glaciers are located at a variety of elevations and latitudes, causing unique spatial and temporal melt variability. Defining boundary elevations between glacier melt regimes (wet, percolation, and dry snow zones) is a fundamental step towards quantifying spatial variability of melt, locally and regionally. Secondly, ice cores, a primary source of paleoclimate Information useful for predicting future climates change effects on melt, require glacier ice which has experienced minimal melting and deformation. Ground penetrating radar (GPR) is a useful tool for delineating melt regimes and appropriate drill site locations. Herein, we use GPR as evidence collected from three glaciers in the Alaska Range, to estimate regional melt regime boundary elevations and assess englacial stratigraphy and flow dynamics at each site to make recommendations for potential ice core drill sites.
Results show that the upper Yentna Glacier of Mount Russell (2652 masl) is inappropriate for paleoclimate studies because it is located in the wet snow zone which causes significant chemical diffusion through melt. Kahiltna Pass Basin (3100 masl) is in the percolation zone and exhibits shallow surface conformable strata overlying thick, complex, and significantly deformed strata making it also unsuitable for obtaining a long term climate record. A basin on Mount Hunter (3912 masl) is the best potential drill site yet found in the Alaska Range because it has deep surface conformable stratigraphy and no melt, suggesting it has experienced minimal deformation and is in the dry snow zone, respectively. Depth-age models estimate ~5000 years of ice in the deepest region of the Hunter basin.
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The Warrp Program Package
More LessThe WARRP program package from Geopro GmBH in Hamburg is used for the Inversion. the WARRP Inversion gives interface configurations and velocity distributions in layers (Ditmar et al. 1999, Geophysical Prospecting). in the present case I work with a one-layer model.
the program enables the display of calculated traveltime curves and a comparison of these to the picked traveltimes. the quality of the inverted model is thus established both by the RMS traveltime residual and by the inspection of traveltime curves on each gather relative to picked events. for a small scale experiment with a total offset of 300 m, a 100 Hz signal is considered to be a relatively low-frequent signal. Ray-theory is in principle not well suited to model relatively low-frequency seismic signals. However, in the case of inverting seismic data, the scales of heterogeneity in the model will be determined by the resolution of the seismic signal. the resulting model will thus describe correct ray-paths for the seismic signal while the heterogeneity of the model will have larger scales according to the frequency content of the seismic signal. the parameters for the Inversion are: the gridding of the model, the timesteps in the ray-tracing and the “velocity regularisation parameter” (VRP). the VRP is a factor determining the dependency on the neighbouring gridpoints. the ray-theory versus low-frequency seismic signal considerations leads to that the VRP has to be kept as high as possible, meaning high dependency on the neighbouring gridpoints, while still producing consistent traveltimes for all gathers and all offset ranges. in the present case, where we work with picked traveltimes and not full-waveform data, all Inversions that produce consistent travel times for all gathers, no matter method, are expected to produce similar models at least down to the first velocity Inversion as long as they obey the resolution of the 100 Hz wave.
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Application of Geophysical Methods to Locate Buried Tunnel Channels Beneath the Glacial Drift Deposits in Texas Township, Kalamazoo County, Mi
Authors Arthur Obiadazie and Sarah DozeThe objective of this research was to delineate and map the network of gravel and sand-filled channels beneath the glacial drift of Texas township in SW Kalamazoo County, MI. these channels are excellent local aquifers that are prized by water agencies. the delineation was done by mapping the depth to the interface between the glacial drift and the much older Mississippian Coldwater Shale, which has higher density. Tunnel valleys produce gravity anomalies on the order of -0.2 to -0.4mGal. the gravity method was used to create a Bouguer gravity anomaly map of the township. A total of 360 stations were surveyed, mostly along roads, but did include off-road profiles. Station elevations were obtained using differential GPS techniques, and the data were reduced using an average crustal density of 2.67g/cm3. A regional trends map from a much larger area was subtracted from the Bouguer anomaly map to produce a residual Bouguer gravity map. the residual anomaly contour map revealed a low gravity anomaly trending NE-SW and another set of anomalies trending in the N-S direction. in general, the low gravity anomalies showed a complex network of tunnel channels but for the most part, corresponded strongly with the occurrence of a linear alignment of small lakes and wetlands. A seismic survey was conducted in the axis of the valley based on access and proximity to water wells for depth-to-bedrock control. the deepest reflector obtained by interpreting the seismic reflection data was located 120 meters below the surface, while the deepest adjacent production well (B-3) was 104 meters deep and does not make contact with the shale bedrock. Careful mapping of the tunnel valleys is very important for future water supplies of the Kalamazoo-Portage urban area, as this area is totally dependent upon ground water.
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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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