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Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
61 - 80 of 139 results
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Increased Penetration Depth in TDEM Measurements Using a Segmented Receiver Coil Setup
Authors N.S. Nyboe and K.I. SørensenRecent investigations of the origin of noise in ground based TDEM datasets have demonstrated that instrumentation noise constitutes a considerable component of the total noise contribution. This is despite the fact that state-of-the-art low noise electronic components and instrumental designs are applied. This problem could be remedied by increasing the effective area of the receiver coil. However, physically small shielded receiver coils which are traditionally applied cannot fulfil the requirements of having a large effective area and a high bandwidth. Meeting this challenge, we introduce an enhanced TDEM system which uses a newly developed flexible and shielded receiver coil with significantly enlarged area as a complementary receiver in a segmented receiver coil setup. Investigations so far have demonstrated TDEM data sets with an enhanced signal/noise ratio of a factor of 10-20! The TDEM system is applied in a mapping campaign at the Rækker Mølle site in Denmark. The results further support the advantages of the segmented receiver coil instrument, as the increased penetration depth provides deep lithological information for integrated interpretation together with vibroseismic measurements from the area.
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Rapid and Objective Integration and Zonation of Partially Co-located Models
Authors H.C. Paasche, J. Tronicke and P. DietrichCluster analyses are powerful tools for rapidly exploring disparate geophysical models with unknown interrelations. Despite its high potential to objectively extract the dominant structural information from suites of disparate geophysical models, cluster analysis techniques are not commonly used, yet, when analyzing suites of geophysical models. This is due to the following limitations: (i) The considered model area must be fully covered by all models subjected to cluster analysis, (ii) cluster algorithms usually ignore spatial information present in the data bases and are therefore sensitive to high-frequent spatial noise (outliers), and (iii) cluster algorithms, such as fuzzy c-means (FCM), classify data in an unsupervised fashion, potentially ignoring expert knowledge additionally available to the experienced human interpreter and not present in the geophysical data base. In this study, we address all of these issues by modifying and regularizing the standard FCM cluster algorithm to tolerate incomplete data bases incorporating model areas not covered by all available data sets and to consider spatial information present in the data base. We evaluate this new regularized missing value fuzzy c-means (RMVFCM) algorithm in a synthetic study and apply it to a field data base comprising partially co-located crosshole tomographic velocity models.
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The Role of Geophysical Investigations in Physical Planning over Covered Karstic Areas in Southern Belgium
Authors O. Kaufmann, P. Lizin, T. Martin and J. DeceusterSince 1999, maps (at 1:10,000 scale) of zones of karstic constraints have been established in the Walloon Region of Belgium (the southern part). These maps sort areas in terms of karstic hazards (negligible, low, moderate and high). They were drawn based on geological settings, field observations, background knowledge and geostatistical forecasts. Nowadays, these maps are used as background documents in physical planning over covered karstic areas. Indeed, the local government now requires to conduct geophysical and/or geotechnical investigations previous to building construction over moderate and high karstic hazard areas. In this context, we conducted several 2D, quasi-3D and 3D resistivity tomography surveys using a total number of electrodes ranging from 100 to 1000. 3D resistivities were measured using a cross-diagonal survey with a combination of dipole-dipole, pole-dipole and equatorial dipole arrays. The inversions were computed within Res2DInv and Res3DInv. The results of these investigations allowed pointing out resistivity anomalies interpreted as effects of karstic features. When possible, boreholes are then drilled over and away from the detected anomalies to confront geophysical results. Finally, geophysical interpretations are used to adapt construction projects.
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Geophysical Survey in Lavrion City for the Detection of Water Resources and Underground Voids or Cavities
Authors G. Apostolopoulos, K. Pavlopoulos and S. StamatakiLavrio city with the great history in mining and metallurgical activity from ancient times has the interest to exploit water resources and detect possible tunnels or voids in certain places where historical data indicate. A geophysical survey with 2D resistivity soundings detected the overall geological-hydrogeological environment. A combination of electrical resistivity tomography and georadar profiles has given a detailed picture of the underground indicating areas of fracture zones or cavities. Finally two drillings are proposed, one for water exploitation and one both to detect the stratigraphy and the fracture zones and exploit water of low quantity and quality.
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Karst Mapping Using Airborne Electromagnetics – Multi – Layer Inversion to Derive Spatially Distributed Parameter
Authors R. Supper, D. Ottowitz, A. Ahl, K. Motschka, A. Römer, G. Merediz Alonso, P. Bauer-Gottwein and B. GondweKarst aquifers represent important but very vulnerable sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. However a flow model for karst aquifers requires detailed, spatially distributed information on the flow- relevant characteristics of the subsurface. Methods determining the distribution of the electrical resistivities within the subsurface could provide such information. To explore the potential of airborne electromagnetic mapping for providing such innovative input information, the international scientific research initiative XPLORE was initiated. Within this paper, successful approaches to derive the subsurface cave structure and to map the depth of the halocline using multi-layer 1D inversion of frequency domain electromagnetic data are presented.
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On the Use of the Mexican Hat Wavelet in the Inversion of Gravity Data
More LessThis paper describes a general simplified approach to inverting gravity profile data using generic wavelets. No initial assumptions about the causative body are made, and a Mexican Hat wavelet is used rather than a specific form derived from the analytic solution for the gravity anomaly in question. The manifestation of the causative bodies in the CWT sections produced are examined to draw some general conclusions on the utility of this simple practical approach in extracting useful information from processed gravity data.
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Large-scale Acoustic Classification of Multibeam Echosounder Datasets within the INFOMAR Programme
Authors D. Hardy, R. O'Toole, C. McKeon and X. MonteysModern multibeam sonar systems, in addition to measuring water-depth, also provide measurements of the intensity of the returned acoustic pulse, termed backscatter. While several factors influence backscatter strength, seafloor composition is often the most significant contributor. Once appropriate corrections compensating for geometric losses and system artefacts are applied to such data, images outlining seafloor areas of similar acoustic response are readily produced. Statistical image segmentation methods are used to subdivide these images into a number of discrete ‘seabed classes’. Correlation of these remotely-sensed ground-types with sediment samples and underwater-imagery allows the assignment of descriptive labels to such ‘seabed classes’, and provides a cost effective means of producing baseline maps delineating seafloor conditions. Such products have applications in the fields of coastal zone management, habitat and fisheries studies, marine research and offshore engineering projects.
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Shallow Geophysical Characterization and Fluid Flow Processes in Two Large Pockmarks on the Malin Shelf, NW Ireland
Authors X. Monteys, X. Garcia, R. Evans, M. Szpak, B. Kelleher and D. HardyShallow geophysical datasets and ground-truthing have been used in this research to characterize in detail two large gas related depressions in a recently discovered pockmark field on the Malin Shelf, northwest Ireland. Pockmarks are aligned to the main deep fault of the region, the SW-NE Skerryvore Fault. High resolution multibeam bathymetry and backscatter reveal the surface morphology of these seabed features to the meter scale. They appear as subcircular depressions and present several units within a generally smooth seabed. Shallow seismic and single-beam echosounder records reveal evidence of gas related activity within the subsurface strata. Electromagnetic results present a number of anomalies in conductivity associated with inhomogenities in the upper strata. These anomalies consist in a decrease in conductivity and in general, correlate well with acoustic anomalies.
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Inversion of Single-channel Seismic Data for Acoustic and Physical Properties of Unconsolidated Seabed Sediments
Authors K.C. Leurer and C. BrownWe present results of neural network inversions of normal-incidence, single-channel seismic profiling data for sediment physical and acoustic properties of unconsolidated seabed sediments. The inversion is based on a comprehensive Biot theory-derived acoustic model for a binary grain-size sphere pack sediment structural model.
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Characterization of Quaternary Sediments Using Geophysical Techniques in the Tullamore Region of Co. Offaly
Authors X.M. Pellicer and P.J. GibsonElectrical Resistivity (ER) and Ground Penetrating Radar (GPR) geophysical techniques have been used to characterize a number of glacial and postglacial geomorphological features occurring in the Tullamore region (Co. Offaly) located in the Irish Midlands. Lithological changes in the Quaternary sediments can be inferred from ER data. GPR has been used to classify and characterize the sedimentological and deformational structures within esker ridges, frontal moraines, glaciolacustrine sub-aqueous fans, lacustrine plains and glaciodeltaic sediments. A number of exposures on glacial sediments have been used as test sites to calibrate GPR data collected by a number of antenna frequencies, GPR surveys have been carried on top of the exposures and the results compared to the their sedimentological analysis. These surveys have facilitated the interpretation of the data gathered on areas where exposure was not available. The combination of both techniques (ER and GPR) on a survey line allows inferring the lithology and sedimentology of the subsurface. Furthermore, water table and lithological boundaries can be detected using these techniques.
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Mapping Thickness of Raised Peat Bog Deposits Using GPR
By A. TraffordThe Irish Midlands have a unique landscape resulting from the retreat of the last ice age and subsequent post glacial deposition, forming extensive areas of raised peat bog. These natural resources have been used extensively for the supply of energy from domestic peat cutting to the generation of electricity on a large scale. In order to manage these vast areas there is a need for an accurate method to assess the remaining reserves and to aid future production planning. To achieve this a LIDAR survey was carried out to provide elevation data over the entire infrastructure, and accurate peat thickness data is required for approximately 35,000Ha of production bog. GPR was used to provide this thickness information and the accuracy of the data was confirmed by peat probing. The survey showed three distinct GPR signatures associated with the presence of the three main sub-peat soils, namely gravels, plastic clays and shell marls, as well as from internal peat layers. This paper describes the work carried out at Blackwater Bog, an area of 1,890Ha of raised bog east of the River Shannon in County Offaly and presents examples of the different GPR responses.
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A Geophysical Investigation of Imaging Stability Features in Peat
More LessA georadar survey was performed during July 2004 around one of the 40 peat slides that occurred on Dooncarton Mountain near Pollatomish, Co. Mayo, Ireland on the 19th September 2003. Three survey lines were recorded parallel to the slope, while another three survey lines ran above the peat failure and were perpendicular to the first three. A Sensors and Software Pulse EKKO 100 ground penetrating radar was used with antennae frequencies of 100 and 200MHz. The processed radargrams imaged sub-surface layering both in the peat and beneath it. Whilst natural pipes and sub-terrain cracks were also detectable, they required truthing or prior knowledge in order to be accurately interpreted. In comparison, an attempted seismic refraction survey proved ineffective at imaging subsurface layering in the peat soils. The seismic survey did however provide information on the Poisson’s ratio and Young’s modulus of peat and the underlying weather rock layer. In summary, GPR proved to be an excellent tool for imaging both layering and stability features in peat and it can greatly assist stability analysis of peat land areas.
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The Role of Selected Geophysical Methodologies in Protection and Development Investigations in Irish Peat Areas
By T. LombardA range of shallow geophysical methods has become increasingly important in the investigation of peat areas in Ireland in recent years. This paper sets out to show the role a series of methods including Ground Penetrating Radar (GPR), Seismic Refraction Profiling and 2D-Resistivity profiles play in a range of studies. Such surveys are used to determine peat thickness, depth to underlying rock, peat stability, sub peat fault and karst feature detection and bedrock characteristics. This information is important for infrastructural developments on and protection of the natural peat and bog areas. Datasets will be presented here to demonstrate the usefulness of the afore mentioned methods in determining peat thickness and layer boundaries at a number of sites including the site of a vulnerability mapping exercise within the catchment area of a river system. The data shows the importance of such geophysical studies in the overall context of infrastructural development projects.
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A Multi-disciplinary Study of the Bar Hill-Whitchurch-Wrexham Moraine Complex with Emphasis on Geophysical Techniques
Authors A.A. Parkes, I.G. Stimpson, R.I. Waller and P.G. KnightThe Bar Hill-Whitchurch-Wrexham (BHWW) moraine complex is a major landform resulting from the incursion of a lobe of the former British-Irish ice-sheet into the Cheshire Basin. Several theories have been postulated for its origin, but the lack of exposure does not allow discrimination between these. A multi-disciplinary approach involving a suite of geophysical techniques including ground penetrating radar (GPR), electrical resistivity tomography (ERT) and seismic refraction, combined with more conventional landform mapping and sedimentological analyses has enabled constraint of the moraine's key characteristics. Combining in-situ measurements of resistivity for key glacio-sedimentary facies with a more extensive ERT survey has enabled mapping of the spatial distribution of the glacio-fluvial and diamicton facies. GPR and seismic refraction techniques have been applied in order to identify major interfaces between glacio-sedimentary units. GPR has successfully identified a thrust imbricate sequence, suggesting thrust deformation up to 400m distal of the former ice margin. This integrated application of geophysical techniques has enabled the identification of the BHWW moraine as large glacio-tectonic landform generated by ice push and it is hoped this study will stimulate further geophysical research in formerly glaciated regions with poor exposure.
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Characterization of Holocene Near-surface Geology Using DC, SIP and FDEM Methods, Cuxhaven Area, Germany
Authors M. Attwa, T. Günther, M. Grinat and F. Binotr aquifer. At Lüdingworth, a 2D imaging was conducted from DC and SIP methods. The inverted profile indicates that the Holocene sediments consist of silt and clay with thin parts of saltwater sand. The 2D resistivity profile yielded also considerable information revealing the existence of sheltered tidal flat deposits. A frequency domain electromagnetic profile was measured to discrete these electrical features. Thirty electrical soundings were measured using Schlumberger array to predict the hydraulic parameters, using Yadav’s (1995) equation. Börner’s model was used to calculate the transmissivity from SIP sounding point, derived from the 2D inverted profile. The estimated transmissivity varies between about 3 m2/day and 17 m2/day.
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Numerical and Experimental Mapping of Root Zone Using Surface and Borehole Resistivity Tomography
Authors T. Petersen and S.A. al HagreyThe global water shortage in terms of quality and quantity demands a sustainable management of water and land use which in turn demands a better mapping of roots and monitoring their process. Generally, roots can be divided in soft, fine branches that cause electrically conductive anomalies and thick, isolated branches that produce electrically resistive anomalies within the soil material. Electrical resistivity tomography at the ground surface and in boreholes allows mapping root zones. But the resolution generally increases by increasing number of measurements and accordingly the time and costs. We numerically tested different non-standard, standard and the new approach of optimised electrode configurations in different survey designs. We have studied the resolution of the different configurations to find the optimum dataset of practical size and high resolution. We have applied forward and inverse modelling of different setups on 2D models of the root zone with conductive/absorbing and resistive/transporting roots, respectively. The best resulting models are presented and discussed as a function of the different applied configurations and survey designs.
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Geophysical Investigation for a Proposed Landfill Site in NW Ireland
More LessThis case history summarises the geophysical survey carried out for the ground investigation of a proposed landfill site in NW Ireland. The methods employed were VLF-R, VLF-EM, Gauge Coring and Seismic Refraction for the first phase and 2D-Resistivity and additional Seismic Refraction for the second phase. The geophysical survey contributed significantly to determine the overburden and bedrock geology and made targeted drilling possible. The result of the survey determined the thickness of peat, glacial till and the depth to bedrock. A fault zone/dolerite intrusion was delineated. At the second phase, 2D-Resistivity measurements confirmed the VLF survey results and helped highlight more detail due to the higher resolution of the method.
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Geophysical Mapping of Quick Clay – A Case Study from Smørgrav, Norway
Authors S. Donohue, M. Long, P. O'Connor, T.E. Helle, A. Pfaffhuber and M. RømoenMarine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides. Quick clay may be described as highly sensitive marine clay, deposited in a marine environment during the last glaciation. In Norway some of the most densely inhabited areas, such as the areas around Oslo and Trondheim are located in potential quick clay areas and hence significant efforts are being taken to map its occurrence and extent. In this paper Electromagnetic (EM-31), Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested on a site known to contain quick clay. The site under investigation, Smørgrav, has a history of quick clay sliding, the most recent event occurring in 1984. A number of these approaches have proved promising, in particular ERT, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and borings.
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Hydrogeophysical Monitoring of Landslide Processes Using Automated Time-Lapse Electrical Resistivity Tomography (ALERT)
Authors J.E. Chambers, P.I. Meldrum, D.A. Gunn, P.B. Wilkinson, O. Kuras, A.L. Weller and R.D. OgilvyGeoelectrical techniques, such as resistivity and self-potential are being increasingly applied to study the hydraulics of landslide processes. The great strengths of these techniques are that they provide spatial or volumetric information at the site scale, and are sensitive to hydraulic changes in the subsurface. In this study we described the development and installation of an automated time-lapse electrical resistivity tomography (ALERT) system on an active landslide at a site near Malton, North Yorkshire, UK. The overarching objective of the research is to develop a 4D landslide monitoring system that can characterise the subsurface structure of the landslide, and reveal the hydraulic precursors to movement. In this paper we describe the installation of the ALERT system on an active landslide, and present initial results showing the 3D structure of the landslide and subsurface resistivity variations that occurred between static conditions and an active phase of slope failure.
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Correlation between Inverted Chargeabilities and Organic Compounds Concentrations in Soils – A Field Experiment
Authors J. Deceuster and O. KaufmannResistivity and induced polarization tomographies were performed along profiles on an industrial waste land to detect and delineate hydrocarbon-impacted areas. To confront geophysical results and to conduct cross-hole measurements, 4 boreholes were drilled. Soil samples were taken and analysed. Concentrations of organic compounds in soils were then compared to inversed chargeabilities obtained from surface measurements. This comparison shows that chargeabilities are linked to contaminant concentrations in soil samples except from areas where clays were found. A filtering technique based on normalized chargeability values was implemented and validated in order to discriminate chargeabilities linked to the presence of clayey sediments and contaminant concentrations in soil samples.
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