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Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 14-18, 2014
- Location: Athens, Greece
- Published: 08 September 2014
41 - 60 of 185 results
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A Multidisciplinary Approach for the Determination of Subsurface Structure of Leonessa Plain (Central Apennines, Italy)
Authors K. Skrame, M. Di Filippo and M. Di NezzaSummaryIn this study, detailed gravity survey and microtremor observations were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits of the Leonessa plain.
The study involved a test area of 50 km2 occupied by 300 gravity stations and by 30 sites of 3-component single-site observations. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. The microtremor data were analyzed by means of the horizontal-to-vertical spectral ratio (H/V). A realistic density of the unconsolidated Quaternary deposits (1.75–2,00 g/cm3) determined on core samples and a density of 2.60 g/cm3 for the basement limestone were used to constrain the 2D gravimetric models.
The gravity and microtremor data were compared with a collection of existing well logs and geophysical data. The 2D models match quite well with the information determined from the previous works. The data obtained with these different methodological approaches allowed us to reconstruct the geometry of the depression and to evaluate the thickness of the Quaternary sedimentary infilling.
Finally, on the basis of the obtained morphology, we were able to reproduce and to define the 3D model and the Quaternary evolution of the Leonessa plain.
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Edge Detection of Magnetic Sources Related with the Philippi Granitoid in N. Greece
Authors A. Stampolidis, G.N. Tsokas, P.I. Tsourlos and G. VargemezisSummaryMagnetization differences in rocks are related with differences in the content of magnetic minerals within rock units. Assuming that magnetic properties (susceptibility and/or remanence magnetization) are discernible between the various rock units, magnetic field anomalies could provide valuable information for the texture of lithological units and even be correlated with distinct lithologies. Delineating edges of magnetized structures is a common application of magnetic data to geological interpretation. We evaluate a number of edge mapping techniques in order to study the magnetic anomaly caused by Philippi granitoid and to estimate its lateral extension. Edge mapping methods can definitely improve the interpretation of magnetic data. They can delineate the locations of the edges of the subsurface magnetic sources. Applied on the magnetic data of the Philippi granitoid edge mappers succeed to map the edges of the granitoid. The PSG-HGM mapper is a robust method that can detect contacts even in the presence of noise. TDXAS filter was able to detect signal from shallow and deeper contacts.
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Keynote Speech - Near-Surface Geophysics and Australia’s Challenge
By K. SutoSummaryThe definition of “near-surface” in geophysics has not reached a consensus. There are two classes of geophysicists who work on the “near-surface” problems: one interested in physical properties of near surface material and the other trying to understand near surface to improve deeper image. The former is typically for engineering and environmental applications and the latter is for resource exploration.
Engineering geophysics is slowly penetrating to geotechnical community in Australia but many engineers are not aware of its effectiveness and geophysical techniques are not utilised to their full potential. Geophysics is only a minor part of geotechnical engineering curriculum. In the field of environmental geophysics, airborne electromagnetic surveys are used to address Australia’s salinity problem.
Declining discovery rate of minerals in Australia prompted a new initiative to collaborate the governments, research organisations, academia and industry. Geophysics is expected to be a major contributor in mapping out the thickness and properties of the cover material in the area as large as Europe. We are facing a big challenge in providing accurate information over a large area in a timely manner.
While the challenges are within Australia, the issues addressed here are universal, and the result achieved may be useful globally.
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Quantitative Depth to Bedrock Extraction from AEM Data
Authors H. Anschütz, C. Christensen and A.A. PfaffhuberSummaryA new road segment is being planned northeast of Norway’s capital city, Oslo. In this context, knowledge of sediment thickness is vital, as is information about occurrence and extent of highly sensitive marine clay (so-called quick clay).
Airborne EM measurements were conducted to provide information of depth to bedrock/sediment thickness between drilling sites and guide the further drilling program. AEM data indicate a variable bedrock depth with a general trend towards shallower bedrock in the northeastern part of the investigation area. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings.
Various methods for estimating depth to bedrock were compared: (1) Using a constant resistivity threshold either survey-wide or for each profile line; or (2) by using an appropriate resistivity value as a function of position. A contouring algorithm was developed to for the second method to incorporate both borehole and AEM data and hence account for apparent variations in resistivity at the sediment-bedrock boundary. Though both exhibit the same general trends, predictions for some locations differed significantly. Based on the AEM results recommendations for further drillings were given, thus reducing the overall costs of the project
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Assessing Climate Effects on Railway Earthworks Using MASW
Authors S. Donohue, D.A. Gunn, P. Bergamo, E. Hughes, B. Dashwood, S. Uhlemann, J.E. Chambers and D. WardSummaryMany parts of the UK’s rail network were constructed in the mid-19th century long before the advent of modern construction standards. Historic levels of low investment, poor maintenance strategies and the deleterious effects of climate change have resulted in critical elements of the rail network being at significant risk of failure. The majority of failures which have occurred over recent years have been triggered by extreme weather events.
Advance assessment and remediation of earthworks is, however, significantly less costly than dealing with failures reactively. It is therefore crucial that appropriate approaches for assessment of the stability of earthworks are developed, so that repair work can be better targeted and failures avoided wherever possible. This extended abstract briefly discusses some preliminary results from an ongoing geophysical research project being carried out in order to study the impact of climate or seasonal weather variations on the stability of a century old railway embankment on the Gloucestershire Warwickshire steam railway line in Southern England.
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Do We Get the Right Position?
Authors J. Endom, C. Holst and M. StrickerSummaryLiterature for applied geophysics present rules of good practice for field work and descriptions on the different methods. Parameters like depth, frequency, resolution and further conditions are discussed in detail. The fundamental requirement that geophysical data have to be linked to a position seems to be self evident in times of GNSS and laser total stations.
Whereas surveying is an engineering discipline, the today positioning equipment implies that everyone can produce exact positioning data. Many geophysical experts perform their positioning with quite rudimentary surveying knowledge and blind faith into GPS & Co.
In civil engineering applications we observe rectangular shaped inspection areas as well as long linear structures like railway tracks or roads. Surveying a soccer field sized site requires different positioning technologies than surveying along highways. Standard positioning methods do not longer apply to this problems.
Focussing on GPR in civil engineering a survey speed of 80 km/h is required today. With velocities of more than 22 m/sec latency of surveying systems gets quite relevant.
Different available technologies of surveying are discussed in this presentation. Manufacturers as well as users are addressed to work on a unified methodology that could be implemented as a standard in future.
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Void Detection at Chania Airport by Combining ERT and Borehole Data
SummaryA detailed geophysical survey was carried out at Chania airport located at the north-west part of Crete, Greece in order to detect cavities in carbonate rocks. This survey combines electrical tomography and borehole data from Chania airport where new buildings are planned to be constructed. The site mostly consists of recrystallized carbonate rocks belonging to Trypali unit. Electrical resistivity tomography applied in dense grids at Chania airport provided images of the subsurface which conducted a borehole program for the detection of karstic forms in the carbonates. At the area under investigation according to resistivity imaging as well as borehole and surface geological data, the formations are mostly characterized as highly fractured. The size of the existing voids ranges from 0.5m to 6m.
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The Use of GPR in the Rehabilitation of Built Heritage
Authors N. Barraca, M. Almeida, H. Varum and M.J. Senos MatiasSummaryRehabilitation of city centres and of buildings of architectural interest is an increasing activity in Civil Engineering works. A thorough, economic and careful project demands à priori information about the original construction materials and techniques, changes to the original structure, infrastructures (water, electricity, sewage, etc) defects and foundations. Direct physical intervention on the buildings is not advisable and, therefore, non intrusive and non destructive techniques must be used.
Herein it is proposed to give an account, using selected data, from a larger on going GPR survey in the study and characterization of an Art Deco building (from early XXth century). Data were collected with shielded antennas and are discussed both in 2D and 3D and, finally, the benefits of the survey are appraised confirming the potential of GPR in the investigation to support conservation interventions.
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Soil Classification and Looseness Evaluation by Rock Physics Based Cross-plot Analysis for Unsaturated Soils
Authors C. Konishi, K. Hayashi, T. Tanaka and T. AbeSummaryRock physics-based methodology for cross-plot analysis in the interpretation of the integrated geophysical survey is proposed to overcome an issue of the current cross-plot analysis. By using the rock physics models, we can avoid ambiguity or subjectivity associated with the determination of threshold values for discriminating the data in the cross-plot, which is the critical part to identify the vulnerable location in the survey area. In addition to the vulnerability section, the method provides more detailed soil type classification and looseness evaluation profiles by using theoretical cross-plots individually. We applied the methodology to actual field data obtained at the river levee where the levee body is supposed to be replaced with new one. After the survey, several points of the levee body are exposed so that we can observe actual soil types and conditions and verify our interpretation results. As a result, the estimated soil type and looseness are fairly consistent with actual soil type and conditions. Therefore, it is considered that the methodology is applicable and beneficial for the interpretation of the integrated geophysical survey.
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Mapping Overpressure Zones Using Acoustic and Seismic Techniques
Authors J.N. Makris, L. Marechal and J.E. PapouliaSummaryA combination of acoustic and seismic methods was applied in order to define hazard on the sea floor and overpressure zones for planning drilling activities in the Caspain Sea. The following methods were used:
- Multibeam Echosounder Swath mapping (MES), in order to produce accurate bathymetry and map seafloor irregularities, faults and obstructions that could influence drilling operations.
- Side-Scan Sonar mapping (SSS), needed to identify obstacles on the seafloor with a very high resolution and particularly to locate positions of abandoned wells.
- Sub-Bottom Profiling and multichannel high-resolution seismic mapping (MCS) with the intention to resolve the stratigraphy of the area, identify faults, map high pressure zones and provide the link between the very shallow structures and those dominating at depth.
The obtained results proved the efficiency of the methods proposed in satisfying engineering requirements.
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Geophysics in the Context of Disaster Mitigation -Results from the Landslide at Pechgraben, Austria
Authors R. Supper, D. Ottowitz, J.H. Kim, B. Jochum, S. Gruber and S. PfeilerSummaryIn early June 2013 a large landslide was triggered at the area of Pechgraben, situated in a mountainous area of Upper Austria. Only several hours after the onset of the first movements, a geophysical investigation program was started to support the planning of the disaster response actions as well as the design of a follow up early warning system. The geophysical methods included geoelectric profiling, geoelectrical monitoring, borehole logging, GPS, inclinometric, piezometric, soil humidity and optical monitoring as well as airborne geophysics. Within this paper results of different methods are given and evaluated on their role within disaster mitigation.
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Time-lapse VSP Monitoring of Shallow Gas Injection at the Maguelone Experimental Site (Languedoc, France)
Authors H. Perroud, O. Jaafar, P. Pezard and J. LofiSummaryThe Maguelone experimental site has been the subject of surface and downhole geophysical monitoring of gas injection experiments in a shallow subsurface reservoir. In particular, vertical seismic profiles (VSP) were conducted in two holes located within meters of the gas injection hole, before, during and after a series of injections. The times of first-arrival were obtained by picking and interpreted in terms of seismic velocity models. Before injection, the lateral variability of seismic velocities at the site is underlined. The comparison of seismic velocities between experiments for a single hole brings out the impact of gas injection on the elastic properties of the porous clastic sediments at the injection site.
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Statistical Analysis of Three-years Time-lapse Electrical resistivity Imaging for the Study of the Petacciato Landslide
Authors G. Calamita, R. Luongo, A. Perrone, S. Piscitelli, T.A. Stabile and V. LapennaSummaryIn the last ten years, time-lapse electrical resistivity tomography technique has been applied to monitor the temporal and spatial changes of resistivity values in different environmental and natural processes like saline intrusion, aquifer recharge, water content change, etc. The growing use of this technique is mainly due to the significant technological improvements occurred in recent years. Indeed, the introduction of new static multi-channel systems has allowed the reduction of acquisition time, the remote control of system and the monitoring of temporal variation of resistivity also in areas where the accessibility can be complicated.
This paper reports the results of a statistical analysis of time-lapse ERT data acquired in a landslide area located in Molise region (Italy). A prototype geoelectrical system was installed in the area with the aim to indirectly estimate the water content change in the subsoil by continuously measuring the resistivity. The analysis was performed on apparent resistivity values without applying any inversion software. In this way no artefacts or errors due to the inversion process were introduced.
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Evaluation of Time Lapse Acoustic Monitoring of Immiscible Fluid Flows in Near Surface by Attenuation Examination Method
Authors B. Hassan, S.D. Butt and C.A. HurichSummaryVelocity information alone, in acoustic monitoring of fluid fronts and interfaces when non-aqueous spills occur in the near surface unconsolidated sediments or water table/aquifers, and of those associated with consequent remediation, may not describe such immiscible flows and displacement fronts adequately. Attenuation characteristics examination, for its greater sensitivity to subsurface fluids, can offer better resolution in describing such subsurface processes. Characterization of controlled immiscible fluid displacement process signifying examination of attenuation characteristics, by comparing spectral curves, so, is demonstrated as extended results of an experimental study. In the experiment oil is displaced with brine, flowed through a saturated pack of ceramic beads against gravity under controlled constant head flow conditions. P-wave data were acquired employing pulse transmission with 1MHz sensors. Pertinent concise review of well studied wave attenuation mechanisms and associated anelastic and viscoelastic models is provided for connective lucidity of presented results and interpretation. Application of the method following a benchmarked time-lapse type workflow for data analyses, results presentation and plausible inferring/interpretation is effectively evaluated. Cited method unambiguously resolved, oil from brine and the interfacial mixing and evolution. Subtle insight instilling and interpretation enhancing frequency dependent combined effects related to fluid density, viscosity and flow rate were observed, too.
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Optimized Arrays for Surface-to-tunnel ERT Measurements
Authors K. Simyrdanis, P. Tsourlos, N. Papadopoulos and P. SoupiosSummaryIn this work optimum protocols using the criterion of Jacobian matrix were tested to increase the resolvable power of surface-to-tunnel electrical resistivity tomography measurements. The problem was approached through synthetic numerical modeling, experimental data collected in a controlled tank environment that was constructed for this purpose and data that were retrieved from a real case scenario in Crete, where an aqueduct tunnel exists in the archaeological site of Eleftherna. The resistivity inversion images were reconstructed through a 2-D inversion algorithm that was modified to account for the special surface-to-tunnel ERT measurements. The optimized data sets contained fewer measurements with respect to the original data sets (∼30%) but the optimized inversion images show comparable resolvable capabilities with the original inversion models.
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Geoelectrical and Geotechnical Investigations at Tenth of Ramadan City, Egypt - A Structure-Based (SB) Model Application
Authors M. Attwa and A. El ShinawiSummaryGeoelectrical measurements were used in association with soil engineering tests in environmental engineering investigations at Tenth of Ramadan city, east Nile Delta, Egypt. The genetic algorithm (GA) approach with structure-based (SB) model is applied towards an optimal interpretation of DC resistivity measurements. The inverted models together with borehole data and soil classification exhibit two soil types, gravelly sand and clayey sand with free lime. The survey demonstrates that the combined use of the GA with SB model provides valuable information to constraint the number of interfaces. The reliable resistivity inversion results are considered as a key role in studying the geotechnical parameters such as Atterberg limits unconfined compressive strength and swelling tests. The clay sediments are described by medium plasticity and sensitivity for swelling. Further, the free lime with water makes gill action with decreasing the compressive strength. Therefore, such soil, in turn, may cause soil water appearance on the ground surface and geotechnical problems during or after the process of any construction. In this respect, the integration of the geoelectrical survey with soil engineering properties is efficient and less time consuming for characterizing the engineering-geologic zone.
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Hydrogeophysical Tools for Investigating Groundwater Storage in the Subsurface of a Karst System
Authors A. Watlet, K. Van Noten, T. Lecocq, J. Chambers, P. Meldrum, O. Francis, M. Van Camp and O. KaufmannSummaryThrough the use of three different geophysical techniques, we highlight structures and groundwater storage changes in the subsurface of a karst system, in the so-called epikarst layer. An Electrical Resistivity Tomography (ERT) has been installed onsite informing on conductivity variations directly related to changes in groundwater content within the unsaturated zone. Both relative and absolute gravimetric measurements monitor the charges and the discharges of the groundwater storage within the epikarst. Analysis of Horizontal to Vertical spectral ratio of seismic ambient noise allows investigating the structure of the epikarst and validating the robustness of ERT data.
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Geophysical and Geochemical Study of Geropotamos Aquifer in the North-central Coast of Crete, Greece
Authors D. Kalisperi, P. Soupios, S. Kershaw, M. Kouli, P. Barsukov, F. Vallianatos and N. Lydakis-SimantirisSummaryThe geological setting of Geropotamos aquifer on the north-central coast of Crete, Greece, is considered complex, while the local tectonic regime of the study area is characterized by two sets of faults orientated NW-SE and NE-SW. Investigation of the aquifer using the Transient ElectroMagnetic method (TEM) has resulted initially in 1D models of geoelectric structures and a final 3D geoelectrical model was constructed, depicting the zones of salination of groundwater in the aquifer. Groundwater samples were analysed and the most important chemical parameters were determined to provide an independent dataset for comparison with the TEM results, while Groundwater Quality Maps were produced. TEM and geochemical data correspond and provide verification of the TEM approach. As a result, saline intrusion is likely to occur along fractures in a fault zone through bedrock, and this work emphasises the critical role of fracture pathways in salination problems of coastal aquifers.
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Magnetic Resonance Sounding Signals Retrieval Based on Empirical Mode Decomposition and a Statistical Method
Authors M. Fallah-safari, R. Ghanati and M.K. HafiziSummaryThe technique of magnetic resonance sounding (MRS) is a non-invasive hydro-geophysical tool for estimating the distribution of water content and the pore size distribution with depth in the subsurface. It is well-know that the quality of MRS measurements is strongly affected by even very low noise. In the present study, in order to enhance the performance in the noisy environments, a two-step noise cancellation approach based on the empirical mode decomposition and a statistical method is proposed. In the first stage, a significant part of the signal noise is eliminated using the decomposition of the signal by the empirical mode decomposition algorithm and based on the detection and removal of the noisy IMFs. Then the de-noised signal is reconstructed through the no-noise information. In the second stage, the signal obtained from the initial section enters an optimization process to cancel the remnant noise, and consequently, estimate the signal parameters. The strategy is tested on a synthetic MRS signal contaminated with Gaussian noise, spiky events and harmonic noise. By applying successively the proposed steps, we can remove the noise from the signal to a high extent and the performance indexes, particularly signal to noise ratio, will increase significantly.
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3D Hydrogeological Model Building Using Airborne Electromagnetic Data
Authors G.A. Oldenborger, C.E. Logan, M.J. Hinton, V. Sapia, A.J.M. Pugin, D.R. Sharpe, A.I. Calderhead and H.A.J. RussellSummaryWe develop a 3D geological modelling procedure supported by the combination of helicopter time-domain electromagnetic data, seismic reflection data, and water well records for the Spiritwood buried valley aquifer system in Manitoba, Canada. Our procedure is an innovative hybrid of knowledge-driven and data-driven schemes that provides a clear protocol for incorporating different types of geophysical data into a 3D stratigraphic model framework. The limited spatial density of water well bedrock observations precludes detection of the buried valley bedrock topography and renders the water well records alone inadequate for accurate hydrogeological model building. The expert interpretation of the geophysical data allows for leveraging of a spatially extensive dataset with rich information content that would be otherwise difficult to utilize for lithostratigraphic classification.
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