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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
21 - 40 of 190 results
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Long Term ERT Monitoring of Remediation at an LNAPL Site, Enid, Ok
Authors Shannon Jeffries and Todd HalihanMonitoring of the subsurface changes that take place during contamination remediation is difficult. One method that allows for stable long term monitoring is Electrical Resistivity Tomography (ERT). A contaminated site in Enid, Oklahoma, a saturated sand aquifer with overlying unsaturated clay, was evaluated to determine the distribution and migration of LNAPL. Fifteen direct push boreholes were drilled at ten meter spacing and 27 electrodes were grouted into place at a vertical spacing of 1.5 feet (0.46 meters) giving a total depth of about 40.2 feet (12.26 meters). Data were collected prior to remediation, during remediation and eight years after remediation began. the time lapse data shows the difference in resistivity values from one data set to the next and allows for the determination of background resistivity values and the movement of the contamination across the site. the data indicate that the LNAPL can be quantitatively evaluated and that the degradation over time appears to impact the unsaturated clay layer more strongly than the saturated sand. the clay layer is becoming more conductive with time suggestive of biological impacts in the clay.
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Time-Lapse Seismic Tomography of a Small Embankment Dam with Possible Zones of Weakness.
Authors Leti Teklu Wodajo, Craig Hickey, Gregory J. Hanson and Chung R. SongEarthen dam or levee failure can occur with little warning. Internal erosion, seepage and piping are among the major causes of failure in earthen embankment dams and levees. these Internal problems can only be detected visually after the damage has progressed significantly. Identifying such problems at an early stage requires a method of imaging the progressive change in the Internal structure of the dam. Seismic velocities depend on the elastic properties of the material; therefore, time-lapse seismic measurements have promise for monitoring changes in the Internal dam structure caused by piping, seepage or erosion. in this paper, results of time-lapse seismic measurements conducted on two small earthen embankments will be discussed. these dams were constructed at the USDA-ARS Hydraulic Engineering Research Unit; Stillwater, OK having two artificially built weak zones susceptible to Internal erosion. Seismic measurements were conducted on these dams at different stages from the start of construction up to failure. Time-lapse surveying of the experimental dams will provide insight on how the changes in the distribution of elastic properties manifest in the seismic Tomography. Comparing seismic Tomography images, an area of increased P-wave or lowered S-wave velocity could be used as an indication of possible seepage or wet area. While the increase of the P-wave velocity is due to the incompressibility of the water, the slight decrease in the S-wave velocity is associated with the increase in total density. A lower P and S wave velocity correlates with the weakening of the surrounding soil structure due to void formation caused by the erosion of fine materials from the dam. [This research was funded by the department of Homeland Security- sponsored Southeast Region Research initiative (SERRI) at the Department of Energy’s Oak Ridge National Laboratory.]
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Application of Geophysics for Environmental Characterization, Remedial Design, and Remedial Implementation at Three Fractured Bedrock Sites Impacted by Chlorinated Solvents in New England
Authors Peter Thompson, Kerry Tull and Scott CalkinOver the last sixteen years, geophysical methods have been used at multiple sites to characterize fractured bedrock impacted with chlorinated solvents and dense non-aqueous phase liquids (DNAPLs). Borehole geophysical data is essential to develop accurate site conceptual models (CSM) for bedrock contaminant source areas and plumes. these data must be integrated with other hydrophysical and chemical data in order for the CSM to support remedial design decisions. the importance of this integrated characterization approach lies in the recognition that at many older DNAPL impacted fractured bedrock sites, the original DNAPL mass has been transferred from the fracture network to the rock matrix through DNAPL dissolution and matrix diffusion. Three case studies are presented to illustrate how geophysical data including 1) borehole geophysics (caliper, temperature, electric logs, acoustic televiewer (ATV), digital borehole image processing systems (BIPs), heat pulse flow meter (HPFM) logging), 2) interwell test data (HPFM interference testing, electrical resistivity Tomography (ERT)), and 3) surface methods (azimuthal resistivity) have been successfully integrated with other primary data, including: straddle packer groundwater data, methanol extracted rock matrix data, conservative interwell tracer tests (CITT), and partitioning interwell tracer tests (PITT). the platform for the CSM is a dynamic three dimensional (3-D) visualization tool that allows a comprehensive Interpretation approach by Integrating these independent, multi-characteristic data. the visualizations relate the distribution of contaminants in specific fractures and rock matrix porosity to the hydraulic characteristics of the fracture network.
the three sites to be presented include two sites in Maine and one in Massachusetts. Geophysical data in one of these case studies was used to support regional scale fractured flow contaminant fate and transport models and two Technical Impracticality (TI) waiver evaluations. the geophysical data at the other two sites supported detailed source area characterizations for evaluating and implementing remedial actions supported by tracer studies.
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Determination of the Near-Surface Structure using Multimodal Ground-Roll Inversion
Authors Soumya Roy and Robert StewartThe near-surface often consists of unconsolidated, low-velocity layers which can cause time delays in passing seismic waves, especially S-waves. the high resolution near-surface S-wave velocity structure can be useful for the S-wave static corrections (adjustments required for those time delays) in multicomponent seismic analysis. the near-surface S-wave velocity structure can be obtained using the surface-wave Inversion method. in this paper, the Multichannel Analysis of Surface Waves (MASW) method has been used. This method uses the dispersion properties of ground-roll (Rayleigh wave) to create dispersion curves (phase velocity versus frequency plots). then, the Inversion of these dispersion curves for the fundamental (and/or multi) modes provide the near-surface S-wave velocity structure. A detailed study of the near-surface S-wave velocity structure has been performed using seismic data sets acquired at La Marque Geophysical Observatory, Texas. the site is flat and undisturbed by cultural activity. the sediments are from the Quaternary Beaumont formation and consist of clay and silty clay. Different source-receiver combinations have been used. for receivers, planted vertical geophones provide better results and better separation of modes compared to vertical land streamer. for sources, accelerated weight drop provides greater depth of Investigation compared to 10 lb sledgehammer. the near-surface S-wave velocity structure (obtained using only fundamental mode) varies 100-300 m/s for top 17 m and going up to 410 m/s at 23.5 m. One of the recent advances is the use of higher modes which have higher velocities than fundamental mode at a particular frequency. Thus, they can ‘see’ deeper for that particular frequency. Also, as higher modes appear more at higher frequencies, they can resolve the shallow near-surface better. Improvements in velocity resolution for first few meters have been observed when Inversion for multiple modes has been performed. An overall increase in the depth of Investigation (up to 5 m) and velocity (up to 50 m/s) have also been observed. the near-surface stratigraphy of the area obtained from a hydraulic conductivity study shows an embedded silty clay layer at about 5 m depth within the surrounding clay formation. This bed is better resolved when multimodal Inversion is applied.
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Estimation of Near-Surface Quality Factors By Constrained Inversion of Rayleigh-Wave Attenuation Coefficients
Authors Jianghai Xia, Richard Miller, Julian Ivanov and Shelby PeterieQuality factors of near-surface materials are as important as velocities of the materials in many applications. High-frequency (≥ 2 Hz) surface-wave data are generally inverted to determine near-surface shear (S)-wave velocities, in which only phase Information of surface-wave data is utilized. Amplitude Information of high-frequency surface-wave data can be used to determine quality factors of near-surface materials. Given S-wave velocity, compressional (P)-wave velocity, and Rayleigh-wave phase velocities, it is feasible to solve for S-wave quality factor QS and P-wave quality factor QP (for some specific velocity models) by inverting high-frequency Rayleigh-wave attenuation coefficients in a layered earth model down to 30 meters below the ground surface in many settings. Amplitude is an exponential function of attenuation coefficients. When calculating attenuation coefficients from changes in amplitude, this nonlinear nature would result in that small variations in amplitude cause huge changes in attenuation coefficients, which suggests data (attenuation coefficients) normally possess large errors that eventually transfer to a model (quality factors). therefore, constraints (a priori Information) on models become necessary. Because an Inversion system of this problem itself is unstable, a regularization parameter must be introduced into an Inversion algorithm to stabilize Inversion procedure. these characteristics of the Inversion problem lead us to solve a constrained and regularized linear system. Usually, a set of models that meet the constraints can be obtained by solving the system. Based on the linear nature of the Inversion system, we can pick a smooth model as a solution of the Inversion using the L-curve method, which is a trade-off solution between data misfit and model length. Several real-world examples demonstrate importance of constraints in finding acceptable quality factors from real data.
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Application of an Airborne HEM System on a Freshwater Lake
Authors David Smith, Maria Deszcz-Pan and Bruce SmithAirborne electromagnetic systems are superior to ground-based geophysical techniques in many respects. they afford rapid reconnaissance of large areas for producing images of earth electrical resistivity as two-dimensional maps and as pseudosections. Systems today are powerful, stable and precise. Adapting such a system for ground-based operation can provide increased sensitivity to the subtle variations in the resistivity of the ground and increase lateral resolution. We analyze the limits on a helicopter electromagnetic (HEM) system owned by the USGS using forward model analysis, and we present the results of a proof-of-principle test on Lake of the Arbuckles in Chicksaw National Recreation Area in south-central Oklahoma. the HEM system was mounted on a floating platform and towed behind a motorboat which carried the data acquisition electronics and a bathymeter.
Preliminary results show that the waterborne system is capable of mapping geology under shallow cover of water.
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Practical 3D Inversion of AEM Data for Environmental Applications in Complex Regolith Settings
Authors Glenn Wilson, Timothy Munday, Andrew Fitzpatrick, Leif Cox and Michael ZhdanovThis paper presents the application of an innovative approach to 3D Inversion of airborne electromagnetic (AEM) data which renders the routine Inversion of entire surveys realistic from the computational perspective. the method uses a moving footprint, and is based on the 3D integral equation method for computing data and sensitivities, and the re-weighted regularized conjugate gradient method for minimizing the objective functional. Specifically, we examine results from several well constrained AEM surveys that target the geometry, distribution and water quality associated with palaeochannel aquifers which are developed in contrasting, variably complex regolith settings. the TEMPEST and SkyTEM surveys targeted anastomozing coarse sands and gravels beds in a sandy clay background. the surveys also targeted the abruptly changing and varying basement (due to changes in lithology and structure) which defines the base of the alluvial aquifer and/or the weathered regolith. Results from the 3D Inversions are compared against drill hole geological and other geophysical data. the integrated Interpretations are able to define the location and groundwater salinity within the palaeochannels. As a result of the surveys and drilling, local farm water and irrigation systems are being developed.
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MRS as a Groundwater Investigation tool
By Jean RoyDuring the past 75 years, indirect measurements, e.g. surface geophysical techniques, contributed to groundwater Investigations. Such non-invasive techniques permit lowering costs and delays in acquiring subsurface Information while preventing invasion or interconnection between hydrostratigraphic units. However, at results exploitation step, some ambiguity is noticed. During the successive developments of the different indirect techniques, two goals have been targeted: (1) lower this level of ambiguity and (2) supply results directly useful for groundwater resource characterization. MRS (Magnetic Resonance Sounding) is a significant step in this direction.
MRS is NMR-based using the Earth's magnetic field as static field and provides its own excitation field which, contrary to usual laboratory case, is not an RF field but an audio-frequency field i.e. the hydrogen nuclei (1H+) Larmor frequency. the excitation field moment is used as sounding parameter i.e. as the controlled parameter for depth discrimination.
MRS supplies, as a function of depth, the in-situ water content and the NMR signal decay rate. This decay rate is linked to pore-size and therefore to the investigated medium water flow properties. Below the water table, the MRS-supplied water content is roughly equivalent to effective porosity under suitable conditions; bound water is usually not quantified with current generation MRS Tools. As any NMR technique, MRS is sensitive to the static field homogeneity which is the topic of another contribution.
MRS 'delivers the goods' on a commercial basis since 1996 in most places. It performs non-invasively down to about 150 m in a resistive medium; this bottom limit is reduced as the medium gets more electrically conductive.
the support of ITC and the collaboration of the following institutions and their personnel are greatly appreciated: AGEOS, BRGM, CSIR-Envirotek, DWA-B, DWA-N, École Polytechnique, GSD GSN, IGM, inRS-ETE, IRD, IRIS, MBG, QUALITAS, UQAC, UQAT, WCS, WRC.
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The Use of Microtremors to Map Subsurface Structure of Shear Wave Seismic Impedance Boundaries in Unconsolidated Overburden: Examples from Eastern Canada
Authors James Hunter, André Pugin, Susan Pullan, Didier Perret, Karl Butler, Heather Crow and Gregory BrooksIn overburden areas of the Eastern Canada, shear wave seismic impedance boundaries between soft and firm soils can exceed 20:1; such boundaries are commonly associated with Holocene post-glacial sediments overlying Pleistocene glacial sediments. in some locations, where post-glacial sediments overlie firm bedrock the impedance contrasts can be even larger. in all such cases horizontal-to-vertical spectral ratios (HVSRs) of microtremor ambient seismic noise yield narrow well-defined spectral peaks associated with the fundamental resonant frequency of these sites as governed by the thickness and shear wave velocity-depth function of the soft upper layer. Microtremor instruments using broad-band (0.2 – 60 Hz) 3-component velocity sensors have been utilized in a reconnaissance mode to estimate thethickness of soft soil at survey areas in Eastern Canada where groundwater Investigations or earthquake seismic vulnerability studies are being conducted. Many of these studies have been “ground-truthed” with multi-component Landstreamer reflection seismic lines or with shear wave refraction/reflection/downhole site Investigations. Regional site period-versus-thickness curves have been developed for isopach mapping and cross-section construction. Examples of microtremor applications include detection and delineation of buried valleys in soft Champlain Sea sediments of the Ottawa Valley-St. Lawrence Lowlands. these include: -areas where the subsurface topography of firm ground may have influenced earthquake shaking amplification with subsequent landsliding and other forms of ground deformation, -delineation of buried valleys for positioning of long-term earthquake monitoring facilities, and -mapping the presence of buried valleys for groundwater aquifer determination. Other similar studies have been conducted in the St. John River valley near Fredericton N.B. as an aid in the assessment of earthquake hazards.
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Correlation of High Resolution Eri Results to Geochemical Zonation at a Degraded LNAPL Site in Grand Junction, Colorado.
Authors Marcy Stonecipher and Stuart McDonaldGeochemical zonation in the subsurface has been imaged successfully at several environmental sites impacted by LNAPL using GeoTrax Survey™ technology (a high resolution form of electrical resistivity imaging). One of these sites is located in Grand Junction, Colorado, and shows a zone of decreased sulfate that is electrically distinct from zones containing oxygen and nitrate in the subsurface.
Overall, the site presents as electrically conductive (<8 ohm-m), with extremely conductive zones (< 1 ohm-m) in the eastern portion of the site, and very conductive zones (1 – 3 ohm-m) in the central and western portions of the site. RockWorks ™ visualization software was used to create a 3-D model of site, which integrated GeoTrax Survey™ images, groundwater analytical data, monitoring well locations, and groundwater elevation data. Bio-parameter data was also included in the 3-D model, and consisted of dissolved oxygen, sulfate, nitrate, and redox potential. the 3-D model demonstrates a strong correlation between the conductivity of the site and geochemical zones which are expected to indicate the type of bacteria present. Data in the 3-D model show oxygen and/or nitrogen zones near the eastern portion of the site that correlate well with extremely conductive zones (<1 ohm-m), and sulfate areas in the central and western portions of the site correlated well to the very conductive zones (1-3 ohm-m). Data obtained at this site indicate that it is possible to use GeoTrax Survey™ technology to identify geochemical zones that correlate to various nutrient reducing bacteria. these findings are significant to environmental professionals investigating and/or remediating LNAPL impacted sites. Specifically, this approach can be used to support natural attenuation arguments and/or identify areas where bioactivity is nutrient limited, thereby allowing efficient and focused deployment of bioamendment technologies to enhance existing levels of natural attenuation.
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Using Data from Multiple Loop Sizes Simultaneously in a 1D Surface Nuclear Magnetic Resonance Inversion
Authors Trevor Irons, Yaoguo Li and Jared AbrahamWhile the surface nuclear magnetic resonance (SNMR) method holds enormous promise, the technique has not been widely adopted due to its challenges. to be useful to hydrologists reliable Inversions for both porosity and decay parameters must be developed, and then relationships between these decay parameters and hydraulic permeability established. using field data collected around Lexington, Nebraska we first illustrate that the variability of the SNMR data is sufficient, even over short time intervals, to lead to dramatically different Inversion results with currently available Inversion schemes. these field sites are unique because a battery of geophysical and hydro-geologic tests have been done in the area, providing insight into the true aquifer characteristics. the same sites were returned to for SNMR data collection several times a year between 2007 and 2010. Most of the variability in Inversion results can be attributed to changing noise levels. often smaller diameter figure-eight loops were deployed to suppress noise and better illuminate the very near surface. However, these data are currently inverted independently of the deeper sensing large diameter loop data. We present a new 1D Inversion scheme that uses the complete dataset to simultaneously invert for T2* and partial water content in the Fourier domain. Electrical conductivity effects are taken into account and arbitrarily shaped transmitter and receiver wires are incorporated. the scheme has several advantages over existing Inversion algorithms in that the NMR signal is demodulated in this domain, and is narrow band. As such a generalized Inversion in this domain delivers significant compression compared to a time-domain formulation. Due in part to this compression, it becomes practical to combine multiple SNMR datasets into a single Inversion. By combining datasets from multiple loop sizes, more consistent results are realized as the sensitivity of each loop configuration contributes to the solution.
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Noise Reduction in a Multi-Channel SP Monitoring System
Authors Karl Butler, Aaron DesRoches and Peter SimpkinSelf-potential (SP) monitoring is attracting growing interest as a tool for inferring groundwater flow, both in saturated media and in the vadose zone. Multi-channel recording systems are being used to acquire data from large arrays of electrodes with the simultaneous temporal sampling desirable for data processing and Interpretation. We present an analysis of the noise performance of one relatively common commercially available datalogger (Campbell Scientific CR1000, with AM32B multiplexer) in SP monitoring applications and illustrate the improvements realized by incorporation of passive RC filters. Our filters, consisting of a 10 kOhm resistor (Rf) in series with each input and a 10 microfarad capacitor (Cf) across each differential input pair, provide a lowpass filter with a cut-off frequency of 0.8 Hz. This corner frequency is lowered further by the source impedance Rs (sum of the electrode contact resistances) giving a lowpass corner of approximately 1/(2pi(2Rf + Rs)Cf), or 0.5 Hz for Rs = 10 kOhm. We suspect that the principal improvement arises from the attenuation of powerline noise at 60 Hz and its harmonics prior to sampling. Such noise can commonly be hundreds of times stronger than the SP signals of interest. Although the datalogger uses analog Integration over 60 Hz periods to effectively attenuate stable powerline harmonics, it remains vulnerable to instabilities in harmonic amplitude or phase, and to settling time artifacts that can arise on switching between channels with vastly differing voltages. Our limited experience to date indicates that the benefits of pre-filtering are somewhat site dependent. the filters proved critical during recent SP monitoring of water well pumping where they reduced noise levels by at least a factor of 10, allowing the identification of coherent sub-milliVolt temporal variations in SP over an array of 34 electrodes distributed around the pumping well on surface.
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Multiple Receiver Loop SNMR Surveys Applied to Geo-Technical and Civil Engineering Problems
Authors Trevor Irons, Yaoguo Li and Jason McKennaWhile surface nuclear magnetic resonance (SNMR) has primarily been applied to hydrology problems -- particularity in 1D -- there exist myriad other applications for this technology. there is a large number of problems where directly detecting and/or imaging water is of primary concern. A few examples include geo-technical and civil engineering problems such as locating voids in roads that are potential sinkholes, monitoring or exploring for leaky dams, landslide hazard assessment and permafrost monitoring. Many of these problems are Inherently 3D in nature. Although the latest generation of multi-channel SNMR instruments has opened the door to 3D surveys, little work has gone into this area. Questions such as feasibility and survey design require 3D modeling capabilities.
We have developed a general forward modeling algorithm that supports arbitrarily shaped loops, with surface topography, as well as decoupled transmitter and receiver loops. the algorithm makes all calculations in the frequency domain and models the entire dataset. Three dimensional electrical conductivity and water distributions are supported and the loops may be embedded in or at the surface of the earth.
using our new algorithm, we investigate the feasibility of several novel SNMR applications. We consider the case of a leaky levee or dam structure with Internal erosion and piping. We seek to determine whether SNMR loops can be used to monitor the presence and progression of the erosion. We consider loops both on on the surface and embedded within the structure. We also examine the problem of using SNMR to image water filled voids that can turn into sinkholes.
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Important New Archaeological Finds Resulting from Shallow Seismic Anomalies at the Ancient Hierakonpolis Temple-town Site in Upper Egypt
Authors Shelton Alexander, Elizabeth Walters and Recep CakirProminent high-velocity, high-frequency signals from very shallow depths observed on contiguous and crossing seismic refraction profiles have been used to map anomalous areas within the perimeter wall of the ancient Hierakonpolis Temple-town in Upper Egypt, a site continuously occupied from at least 3800 BCE through Roman times. these shallow, high-velocity anomalies are characterized by laterally continuous, high-frequency (200-300 Hz) arrivals with velocities comparable to or exceeding the deeper water table refraction velocities. these anomalous areas are within a 1 to1.5 m-thick upper layer of unconsolidated, air-filled sediments that have extremely low velocities and very low Q.
Several localized excavations in the anomalous area in the western portion of the site have revealed the presence of a zone of closely spaced artifacts (dense in potsherds and stone fragments) that provide new evidence of human occupation in the ancient town as early as Dynasty I, c. 3200 BCE. in the northwest excavation an ‘in situ’ deposit of special pottery lies next to a bench, a large block of dressed limestone. Further to the north, layered occupation suggests a secular context with pottery of Dynasty II, 2900 BCE preserved ‘in situ.’ Recent field work in 2008-2010 has clearly designated these special finds as part of a site for offerings, dating to 3200 BCE, the time of the early rulers. Most exceptional is an ebony statuette leg, a masterpiece and the first preserved evidence of an ebony statue for this early time. these and additional findings from further planned excavations in early 2011 will be presented.
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An Integrated Geophysical Study to Characterize the Near-Surface of Barringer Meteorite Crater
Authors Soumya Roy, Robert Stewart, Arkadiusz Turolski, Susan Green and Li ChangAn integrated geophysical survey was conducted by University of Houston, University of Texas (Austin) and NASA in May, 2010 at Barringer (also known as Meteor) Crater, Arizona. the crater was excavated some 49,000 years ago by the impact of an iron-nickel meteorite. the crater has a diameter of 1.2 km with a rim rising some 30–60 m above the surrounding plain and 180 m deep bowl-shaped depression. the startigraphy of the area consists of Coconino sandstone overlain by the toroweap (sandstone and dolomite), followed by the Kaibab (dolomite and dolomitic limestone) and then the red Moenkopi (calcareous siltstone with iron-rich matrix and sandstone). An overturned sequence (Coconino underlain by Kaibab and then the Moenkopi above the bedrock Moenkopi) is observed at the rim and beyond as a result of the impact. the excavated debris also consists of this overturned sequence (collectively known as the ejecta blanket) and is found on the flank of the crater. the ejecta tapers as a function of distance from the crater rim. the aims of this survey were to unravel some of the existing mysteries related to the asymmetry of the crater, thickness of the ejecta blanket and its variation, depth and orientation of fractures. Different geophysical surveys (seismic, ground penetrating radar (GPR), gravity, magnetic) were conducted on the gently dipping flanks beyond the southern crater rim. A near-surface S-wave velocity structure has been obtained using the ground-roll Inversion technique. the near-surface S-wave velocity varies from 200-600 m/s within top 15 m. A change in the S-wave velocity at a depth of 10 m is observed from a seismic line at the southeast flank, which is interpreted as the transition from the ejecta blanket to the underlying Moenkopi sandstone. Ultrasonic measurements of Moenkopi hand samples indicate P-wave velocities up to 1600 m/s and Initial first-break picks show a near-surface P-wave velocity of 1700 m/s. Thus, S-wave velocities in the neighborhood of 600 m/s are reasonable. GPR surveys were also conducted at the southeast crater rim to probe the very shallow subsurface (3-5 m). Several anomalies in the GPR data were found indicating objects with a strong contrast in electrical properties with respect to the host material, possibly an iron concretion formed from the impact.
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Advances in GPR Imaging with Multi-Channel Radar Systems from Engineering to Archaeological Sites
Authors Dean Goodman, Alexandre Novo, Gianfranco Morreli, Doria Kutrubes and Henrique LorenzoAdvances in ground penetrating radar imaging with multi-channel systems have greatly improved the speed and areal coverage of the ground. Along with improved imaging software, datasets recorded with multi-channel systems can be processed at similar speeds to coarsely spaced single channel data that would normally require additional time for interpolation processes to fill in the gaps between lines. with the crossline spacing approaching a 1/4 wavelength or less of the transmitted microwaves into the ground, multi-channel systems have the advantage of complete coverage of a site with no need for interpolation in most cases except to fill in the gaps between adjacent tracks if so desired. Multi-channel systems do require additional RSP (radagram signal processes) in order to balance the channels and to condition the data prior to imaging. Spectral whitening and several other RSP methods are shown with their application to imaging of sites from bridgedecks for deterioration to the discovery of subsurface archaeological remains. Data processed from several different multi-channel GPR systems are shown.
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Prospecting for a Municipal Water Supply using Multiple Geophysical Methods, Long Lake, NY
Authors Doria Kutrubes and John F. KickAs part of a continuing effort to find a potable water supply that meets both hydrogeologic, legal and political criteria, a geophysical Investigation was conducted on the Stanton Hill property east of Long Lake, New York. Seismic refraction, GPR, gravity, VLF and magnetic surveys were conducted to characterize the properties of both unconsolidated materials and bedrock Two seismic lines, each 440 feet in length were established along gravel roads. Explosive energy sources were placed at five locations along each line and at 200 feet off-end locations. the 24 channel data was interpreted using “critical distance” and non-linear refraction travel time Tomography methods. GPR, gravity, VLF and magnetic surveys were completed along the seismic lines and over much of the area of Investigation. A major discontinuity was detected near the midpoint of Line 1. Northwest of the discontinuity, bedrock, with a velocity of 9000 ft/s, is detected; its surface slopes downward to the southeast. South east of the discontinuity bedrock with a velocity of 15,000 ft/s is found with surface sloping upwards to the southeast A third segment with velocity of 13,500 ft/s and surface parallel to the ground surface makes up the southeast end of the profile. the overburden has a velocity of 6000 ft/s which is likely a dense till, and appears thickest over the bedrock contact. Coincident with this interpreted contact. GPR depth-slice imaging indicates a zone of high-amplitude reflectors, caused by an increase in water content or change in mineralogy, 35 to 55 feet below grade. Bouguer gravity values show an increase of over 35 microgals coincident with the bedrock profile. An increase in conductivity was also observed in the VLF data coincident with the deep pocket of unconsolidated material and bedrock contact. Magnetic data were inconclusive, biased by iron-rich rock placed as a cover on the gravel road.
A bedrock well, drilled in proximity to, but unfortunately up-dip from this contact and tens of feet from the thickest low velocity area determined by the seismic, confirmed the presence of a highly weathered rusty gneiss overlying a highly competent black, iron rich rock. However, it did not yield more than 10 GPM. Information from the Geophysics did, however, enable our client to situate two other wells in thicker sections of the highly weathered gneiss to produce yields sufficient to meet the town's water supply requirements.
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An Improved Early-Channel Vtem Helicopter System for Near-Surface Applications
Authors Jean Legault, Pavel Tishin, Alexander Prikhodko and Harish KumarThe helicopter-borne Versatile Time Domain Electromagnetic System (VTEM) is a geophysical instrument which has been in continuous development, utilizing the most recent advances in digital electronics and signal processing for deeper penetration; higher spatial resolution; better resistivity discrimination; and increased detection of a broad variety of conductive targets. Since its inception in 2002, more than 1 300 000 line kilometers have been flown around the world, in a wide variety of geological environments, primarily involving mineral exploration, but also including near-surface groundwater applications. However, although the VTEM system has progressively achieved marked improvements in its deep penetration characteristics, at the same time its near-surface imaging capability has been limited by its early time data. This is significant because early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas. Yet many time-domain AEM system have problems obtaining quantitative early-time data due to a variety of issues, namely system bandwidth. Recently, workers and researchers in shallow electromagnetic sounding have confirmed the calibration accuracy of VTEM mid to late delay time data (>100μsec to 10msec) while at the same time highlighting the quantitative incompatibility issues with its early channel data and near-surface conductivity layering. Processing strategies, such as deconvolution, aimed at correcting system imperfections, such as bandwidth, have been proposed with promising results but are still not routinely implemented.
in an effort to address this issue, Geotech has embarked on a system design strategy aimed at improving the early-channel VTEM data and achieving fully calibrated, quantitative measurements closer to the transmitter current turn-off. This development has led to new model of VTEM system designed specifically for near-surface, high resolution applications, while maintaining reasonably optimal deep penetration characteristics. Results have shown a significant improvement in quantitative VTEM data at earlier times than previously achieved, approaching 5μseconds after the current turn-off.
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Geoelectrical and Hydrogeological Modeling of the Fresh Water / Saline Water Interface in the Lower Florida Keys.
Authors Albert Yeboah-forson, Dean Whitman, Danielle Ogurrcak and Mike SukopSalt water intrusion is a critical challenge for the management of fresh water resources on small oceanic islands like those in the lower Florida Keys, which have extensive coastal development including construction of canals. We present the use of the Direct Current Resistivity (DCR) method for investigating the location of the fresh water, saline water (FW/SW) interface. A Super-Sting R1 28-electrode resistivity imaging system was used to investigate the subsurface geology and the FW/SW interface on Big Pine Key. the survey consisted 54m long profile in the center of the island and a 250m long roll along profile extending inland from the western shore.
in the center of the island, resistivities range from 40 – 250 ohm-m near the surface and correspond to the freshwater lens which extends to a depth of 6 m below sea level. Below this depth, values of 20-30 ohm-m indicate brackish groundwater. On the western shore of the island, resistivities of 1-3 ohm-m corresponding to sea water are found at the surface and extend 25 m inland. Farther inland, resistivities increase to 50 ohm-m indicating a 2-3 m thick fresh water lens. the FW/SW interface slopes landward and 250 m inland from the shore, brackish resistivity of ~20 ohm-m extend to 4 m below sea level and saline found at 8 m depth.
Observed geoelectrical data were compared with a 2-D salinity profile simulated from the USGS SEAWAT model .Analysis of the data show that the largest depth to FW/SW interface occurs at 8m depth in the middle of the island with a gradual decrease in the FW towards the saline zone on the shore of the island and this is generally consistent with the geophysics. Ultimately, both geophysical methods and hydrogeological model Tools like SEAWAT can predict island sea water intrusion with reasonable accuracy.
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Tomographic Imaging of the Common Dataset
By Don ZhaoTomographic Imaging of the Common Dataset Don Zhao, Geogiga Technology Corp., Alberta, Canada Abstract A set of P-wave first-arrival times for a synthetic common dataset is analyzed with refraction Tomography to determine the two-dimensional seismic velocity structure. the refraction Tomography employs a hybrid ray-tracing scheme based on the shorted path method and the ray-bending method to efficiently obtain an accurate forward solution, and regularizes the non-linear iterative Inversion with vertical and horizontal smoothing constraints, damping contraints, and first-arrival picking errors. A vertcial gradient velocity model with lateral homogeneity is used as an Initial model to generate the velocity structure from the first-arrival times. the final result shows a non-flat bedrock structure overlaid with lower velocity layers and identifies two lower velocity zones within the structure.
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