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Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
61 - 80 of 164 results
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Direct Current (DC) Resistivity and Induced Polarization (IP) Monitoring of Active Layer Dynamics at High Temporal Resol
Authors J. Doetsch, G. Fiandaca, T. Ingeman-Nielsen, G. Fiandaca, A.V. Christiansen, E. Auken and B. ElberlingSummaryWith permafrost thawing and changes in active layer dynamics induced by climate change, interactions between biogeochemical and thermal processes in the ground are of great importance. Here, active layer dynamics have been monitored using direct current (DC) resistivity and induced polarization (IP) measurements at high temporal resolution at a heath tundra site on Disko Island on the west coast of Greenland (69°N). Borehole sediment characteristics and subsurface temperatures supplemented the DC-IP measurements. Data acquired during the freezing period of October 2013 – February 2014 clearly image the soil freezing as a strong increase in resistivity. While the freezing horizon generally moves deeper with time, some variations in the freezing depth are observed along the profile. Comparison with depth-specific soil temperature indicates an exponential relationship between resistivity and below-freezing temperature. Time-lapse inversions of the full-decay IP data indicate a decrease of normalized chargeability with freezing of the ground, which can be the result of a decrease in the total unfrozen water and thus a higher ion concentration in the pore-water. We conclude that DC-IP time-lapse measurements can non-intrusively and reliably image freezing patterns and their lateral variation on a 10–100 m scale that is difficult to sample by point measurements.
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Ambient Vibration Recordings Used to Map Glaciers Thickness - A First Study from Alpine Glaciers
Authors S. Picotti, M. Giorgi, R. Francese and F. PettenatiSummaryMicrotremor measurements and the Nakamura’s HVSR (Horizontal to vertical spectral ratio) technique, generally used for site-effect studies as well as to determine the thickness of soft sediment layers, can be effectively applied to map the thickness of glaciers. In this work the GPR (ground penetrating radar) and multichannel seismic methods, widely employed to image the earth interior, are applied to validate the HVSR technique on some alpine glaciers: the Pian di Neve glacier on the Adamello massif (Italy), the Forni glacier in the Ortles/Cevedale massif (Italy) and the Aletsch glacier (Switzerland). Comparing the results obtained from the different methods we show that the resonance frequency in the HVSR spectra can be well correlated with the ice thickness at the site, in a wide range from tens of meters to more than 800 m. The application of the passive seismic method on glaciers, requiring less economic and operational efforts, enables to avoid most of the logistical difficulties affecting the GPR and active seismic methods in these extreme environments.
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Seismic Analysis of a Pingo, Svalbard Islands, Arctic
Authors G. Rossi, F. Accaino, J. Boaga, L. Petronio, R. Romeo and W. WheelerSummaryPingos are oval dome-shaped hills that form in permafrost areas when the hydrostatic pressure of freezing groundwater causes the raising of frozen ground. One of the most famous and studied pingos is the Innerhytta pingo in Adventdalen, Svalbard Islands, with a giant icefall, testifying the significant fluid circulation in their correspondence. Its characteristics make it an ideal site for the seismic experiment of the Italy-Norway project IMPERVIA, aimed to define the inner structure of the pingo, and the ice-bound permafrost full thickness. Another task was to test the capabilities of near-surface seismic techniques in the severe Arctic environment, both in terms of sources and sensors, and of methodologies. The survey took place in spring 2014. We tested three sources (sledge-hammer, Seisgun, and firecrackers) evaluating their efficiency in terms of penetration and resolution, and 4.5 Hz and 14. Hz geophones and a 14 Hz mini-snow streamer. We acquired three intersecting lines, to enable an adequate tomographic analysis of the pingo structure, and we did two surface wave experiments along two of the lines. The data quality is good, allowing the recognition of several refracted and reflected events and a first topographic model of the velocity structure of the pingo.
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Laboratory Scale Resistivity Monitoring of Thermal Flows - Analogical and Numerical Simulations in Water Flux Condition
Authors N. Giordano, C. Comina and G. MandroneSummaryThe distribution of thermal plumes in porous media plays an important role in many geological and engineering applications related to closed and open loop low enthalpy geothermal systems. This is particularly true when natural water flows are present affecting the areal distribution of thermal plumes. The present paper present an example of several laboratory tests performed on an ad hoc designed apparatus for testing and calibrating a methodology for monitoring temperature changes caused by heat propagation. Resistivity and temperature measurements are compared with numerical simulations to estimate the reliability of electrical resistivity in evaluating the temperature changes within the medium. The outcomes of the tests highlighted the reliability of the time-lapse electrical measurements for qualitatively and quantitatively predicting the heat propagation within saturated porous media and induced water fluxes at the laboratory scale, to be promisingly extended also at the field scale.
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Monitoring of a Borehole Thermal Energy Storage System Using 2D and 3D Resistivity Surveys in an Highly Urbanized Area
Authors A. Arato, C. Comina, N. Giordano and G. MandroneSummaryA field scale Borehole Thermal Energy Storage (BTES) test site has been recently built up in Grugliasco (Torino, Italy), in the campus of the Scuola di Agraria e Medicina Veterinaria of the Torino University in an highly urbanized area. This paper is focused to report the results of 2D and 3D electrical resistivity surveys executed on site to evaluate the potentiality of electrical resistivity, to monitor thermal variations within the underground for similar energy storage applications which are boceming more and more diffuse. In this context correct monitoring strategies appear to be of major importance to better understand thermal exchange processes and their environmental effects, particularly into high populated areas. Classic monitoring strategies often rely on local and point-based measurements to detect changes in temperature. In this context, geophysics can bring complementary information which is spatially distributed and acquired directly from the ground surface. Correct acquisition, elaboration and interpretation of resistivity surveys is however very challenging, particularly in highly populated areas where these kind of applications are usually designed.
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Electrical Resistivity Structures and their Relation to Geological Features at the Larderello Geothermal Field (Italy)
Authors A. Santilano, A. Godio, A. Manzella and I. DiniSummaryIn this paper Magnetotelluric data were used to investigate the electrical resistivity structures of the Larderello-Travale geothermal system (LTGS) in southern Tuscany, Italy. MT data acquired by ENEL (Italian power company) in the ‘90 were re-analysed and two-dimensional inversions were performed along a E-W profile crossing the LTGS. The near surface inhomogeneity effects have been reduced by means of tensor decomposition and inversion for static shift. The integration of geological, geophysical and well data was performed in a three-dimensional environment (Petrel, Schlumberger) in order to improve the 2D MT inversion by setting a well-constrained a-priori model and to check inversion results supporting the interpretation. Despite the lithological features, it was clear that an important reduction of the electrical resistivity was required in the crystalline basement to fit experimental data. The results of the MT inversion along the profile have been compared with available public geophysical data.
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Use of Airborne TEM, Ground RMT and Borehole Data to Answer Hydrogeological Questions. A Case Study in Gotland, Sweden
Authors M. Bastani, L. Persson, C.A. Triumf, P. Dahlqvist, H.T. Olander and F.V. JørgensenSummaryAirborne Time Domain Electromagnetic (ATEM) measurements are conducted over four areas in the Island of Gotland in Sweden to identify and study underground water resources such as paleo-valleys and fracture zones in bedrock. Inversion of ATEM data contains new information about potential underground reservoirs. ATEM models are compared with more detailed ground RMT models and borehole data. The RMT and ATEM resistivity models demonstrate a considerable correlation in resolving layered models with lateral and vertical variations that reflect changes in geology and physical properties such as porosity and salinity. Borehole data collected close to the RMT profiles were used to calibrate the models against geology. The formation resistivity from borehole measurements was used as link between the borehole measurements and modeled ATEM and RMT data. Interpretation of ATEM models in areas 1 to 3 shows that towards the southeast the resistive limestone becomes thicker. Low resistivity zones, probably due to fractures within the limestone, are identified and can be counted as new potential underground water resources. Using a 40 m threshold, the saltwater surface found to be deeper towards the east in areas 1 to 3 and shallower in area 4.
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Groundwater Volume Estimation by Combining AEM, NMR and Petrophysical Data
Authors R.S. Brodie, K.P. Tan, L. Halas and K.C. LawrieSummaryEstimating groundwater storage volumes is important for assessing the potential capacity and recovery efficiencies of Managed Aquifer Recharge (MAR) strategies. However, such storage estimates are challenging, particularly in areas of poor data density. This challenge faced the Broken Hill Managed Aquifer Recharge (BHMAR) project tasked with assessing groundwater options in the Darling River floodplain, NSW, Australia. Within the ~7500 km2 area, 14 fresh groundwater zones were identified with AEM and a workflow was developed to estimate the groundwater volumes in these targets.
Pore fluid sampling from sonic core provided the opportunity to develop simple AEM conductivity thresholds to produce surrogate groundwater salinity maps for each AEM depth slice. Combining the AEM data with other project datasets such as downhole geophysics, detailed sonic-core logging and laboratory analyses facilitated the mapping of hydrostratigraphy, textural classes and saturation within AEM depth slices, which were then used to calculate bulk aquifer volumes. Nuclear magnetic resonance (NMR) free-water statistics were used in effective porosity estimates, to calculate groundwater volumes.
Although preliminary, the volume estimates were essential in the identifying, prioritising, interpreting and reporting of groundwater resource targets during the project and to make recommendations for future phases of investigation.
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What Exists Beneath the Place Where Conrad Schlumberger Achieved the First (1912) Electrical Prospection Experiment
Authors G. Hulin, C. Maneuvrier, J.B. Vincent and A. TabbaghSummaryThe first experimental demonstration of the voltage distribution at ground surface achieved by C. Schlumberger in 1912 was in fact located over the remains of the ancient Val Richer Cistercian abbey. The electrical survey of these remains allowed the location of the ancient church and cloister. It also permitted the deeper ground resistivity to be known thus a better understanding of the 1912 results.
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Geophysical Imaging of the WWI Archeological Site of Linke Peak (Forni Glacier, Italian Central Alps)
Authors R.G. Francese, A. Bondesan, M. Giorgi, C. Baroni, M.C. Salvatore, S. Picotti and F. NicolisSummaryThe centenary of World War I triggered several and relevant studies in the fields of Conflict Archeology. The research scenario becomes even more challenging when the study site is located at high altitude in the Alps were past battles and also the modern research should face extreme conditions.
A major research project was recently undertaken in the saddle between Mount Vioz and the Linke peak, in the Cevedale massif, where the progressive retreat of the Forni and of the Vedretta Rossa glaciers exposed barracks, tunnels, barbed wire, sledges, ammunitions and several other war remains. Geophysical imaging was the core of the project that was focused on a double objective: mapping possible WWI remains embedded in the ice and a obtaining some information on the glacier. Results from two different geophysical campaigns were encouraging as radar and seismic profiling provided detailed data about geometry and properties of the glacier as well as unexpected evidences of buried structures.
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Geophysical Investigations in Tomb Detection in Helsinki
More LessSummaryGeophysical investigations performed for detecting concealed tombs in Helsinki Orthodox Cemetery, in Finland. Since some heavy monuments on tombs have been collapsed and the church is lacking of land for graves the investigation project has been launched to map all unknown tombs in 2013 and it is still going on.
Ground Penetrating Radar (GPR), gravimeter, and other methods were used to map the locations of unknown tombs from the most potential areas of the graveyard. The whole study area was covered relative fast by GPR measurements. Due to numerous uncertain GPR reflections gravimeter and other investigation methods were included to the study to confirm tomb detection. As the result of studies less than 20 tombs were either recognized or confirmed with high confidence.
Non-invasive feature of gravity and GPR was one of the main reason they were chosen the main investigation methods in this project. According to studies they are appropriate tools and have a great potential for concealed tomb hunting if integrated with other methods.
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Integrated Geophysical Investigation for the Byzantine Church (12th Century) of the Kaisariani Monastery, Athens Greece
Authors G. Apostolopoulos, G. Amolochitis and M. PapadopoulouSummaryGreece often hosts many earthquakes, whilst a moderate or small magnitude earthquake is felt every 2–3 days on average. Greece is also full of historical monuments with the great need for their protection as well as for archaeological excavation. An integrated geophysical investigation, in and around the byzantine church of Kaisariani Monastery, is designed in order to detect the stratigraphy under the church, the seismic velocities VP and VS of the near surface layers and possible anthropogenic structures or other local features in the underground. EM, GPR ERT and seismic measurements and results, fulfilling the above targets, have provided useful information both to archaeologists and engineers.
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Cluster Analysis to Support Microzonation Studies
Authors P. Capizzi, R. Martorana, A. D’Alessandro, D. Luzio and R. SiragusaSummaryIn recent times the use of microtremor techniques for subsoil investigation increased significantly. The use of HVSR (Horizontal to Vertical Spectral Ratio)technique for seismic microzoning studies allows in many case to obtain detailed reconstruction of the roof of the seismic bedrock and to identify areas with similar seismic behaviour.
Two different algorithms of clusteringhave been tested on a HVSR datasets acquired for studies of seismic microzoning in various Sicilian urban centers.
HVSR data were previously properly processed to extract frequency and amplitude of peaks by a code based on clustering of HVSR curves determined in sliding time windows. To select an optimal set of time windows we have implemented a cluster procedure based on Agglomerative Hierarchical Clustering algorithms.After defining the average HVSR curves a second multi-parametric clustering procedure has been used to group peaks tobe attributed to the same origin (stratigraphic, tectonic, topographic, anthropogenic or other sources). A nonhierarchical centroid-based algorithm has been implemented. The comparison of the HVSR pattern with the information about outcropping formations allowed to assess the geological hypotheses on the heavily urbanized investigated areas.
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Calibration of a VS,30 Map for Shaking-map Production in Piedmont (Italy)
Authors R. De Ferrari, G. Ferretti, F. Bosco and A.R. GaetaSummaryIn this work, with the aim of producing shaking-maps which take into account the local amplification phenomena related to litho-stratigrafical condition, a raster map of Vs,30 values at a regional scale is derived for Piedmont, Italy. After the collection and the validation of all data available within the regional databases, a logic procedure in GIS is applied to assign a Vs,30 value to each of the zone with homogeneous seismic behavior already identified in a previous work. The result is a new Vs,30 map for the Piedmont region that allows a better and more reliable reconstruction of ground shaking including a more detailed evaluation of site effects.
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Soil Characterization of Jeddah City, Saudi Arabia
Authors M. Fnais, A. Al-Amri and K. AbdelrahmanSummary75 of geotechnical boreholes have been conducted in Jeddah City to identify the soil characteristics and their geotechnical parameters. The depth of boreholes extends up to 40 meters in some sites. The standard Penetration Test (SPT) was assigned at each 1.5 m depth interval. Water table fluctuated between 0.5m to 9m below the ground level.
In this study, 75 one-dimensional (1-D) MASW survey has been carried out and respective velocity profiles are obtained. Number of 2-D MASW profiles has been carried out through multiple shot gathers in a consecutive way along the survey line moving both source simultaneously by fixed distance after each shot. Shear velocity 1-D and 2-D profiles have been processed using surfseis software. The shear wave Velocity calculated at the mid-point of each survey line is comparable with the borehole location because the survey line mid-point is coinciding with the borehole location. The average shear wave velocity values up to 30m depth obtained from survey has been used to construct the Vs(30) map for Jeddah city According to Vs(30) values, the soil classes for Jeddah City are classified into three classes as “site class D”, and “site class C” and “site class B”.
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Assessment of the Reliability of the SPAC Method in Urban Areas
Authors A. D’Alessandro, R. Martorana, P. Capizzi and D. LuzioSummaryWe have carried out several long-term measures of seismic noise with triangular arrays of broad-band seismic sensors, to investigate the reliability of the SPAC method in urban areas. We investigate the stationary and anisotropy of the microtremor wave field. We have found that the anisotropy of the microtremor wave field can be evaluated analyzing the HVNSR as function the azimuth. Furthermore, we showed as strong anisotropics in the microtremors are often observable in particular at high frequencies (> 1 Hz). We have verified as microtremors can be difficult to considered as ergodic processes, and for this reason time averaging, also even over a long periods of time, cannot replace an appropriate azimuthal sampling.
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Application of SPAC and MASW Techniques to Earthquake-shaking Scenarios. The Case of the 1993–1994 Adra Earthquakes
Authors P. Martinez-Pagan, M. Navarro, J. Pérez-Cuevas, A. García-Jerez, F.J. Alcalá and F. VidalSummaryIn Adra town during 1993 and 1994 small earthquakes took place with a magnitude of 5.0 (Mw). Despite these earthquakes were moderate, several buildings suffered damage. The local soil conditions influenced the characteristics of the earthquake-shaking scenarios. In fact, these scenarios played a key role about the type of buildings damages reported and their distribution. This study deals with the combination of Spatial Autocorrelation (SPAC) and Multichannel Analysis of Surface Waves (MASW) techniques to obtain the detailed surface shear-wave velocity (Vs) structure of the town. These seismic results have been applied to simulated earthquake ground motion scenarios, which are given in terms of peak ground acceleration (PGA), peak ground velocity (PGV), Arias intensity (AI), and peak ground acceleration (PGA) amplification factor maps. It was found that most of the damaged buildings fall into a transition area comprised between a region classified as EC8 Site Class B2 and another region classified as EC8 Site Class C where higher PGA, PGV, AI, and amplification factor values were found. Besides, the southeastern part of the town, which is the part of the town where is planned to grow, was classified as Site Class C which is prone to generate seismic amplifications.
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Optimal Application of Geophysical Techniques for Subsoil Characterization of Seismic Stations
Authors A. Macau, B. Benjumea, A. Gabàs and S. FiguerasSummaryThe purpose of this work is the application of a dynamic methodology developed by the Institut Cartogràfic i Geològic de Catalunya which uses diverse geophysical techniques to study five seismic station of the Catalonia Seismic Network. This methodology changes depending on a priori geological knowledge and the dimensions of area. The method adapts to each site and considers different stages to characterize layers, geological structures or discontinuities. The H/V spectral ratio is the first technique applied and indicates if the station site is installed in soft soil or in rock. In any case it is important to perform an optimum geophysical characterization of the station. Surface wave analysis (MASW) or seismic noise array analysis are necessary to get the shear-wave velocity model of soil column. Seismic refraction tomography (SRT) allows obtaining a 2D P-wave velocity model to identify bedrock geometry and geomechanical characteristics. The electrical resistivity tomography (ERT) will be applied if exists some uncertainly in the interpretation like velocity inversion, or to improve the definition of interfaces, weathered bedrock or fresh rock. An optimum knowledge of the soil response of seismic station requires a good characterization. The proposed methodology accomplishes this aim and avoids errors in predicting ground motion in seismic hazard studies.
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Analysis of Site Amplification Effects at Four Installation Sites of the National Accelerometric Network (RAN) in Italy
Authors R. De Ferrari, S. Barani, G. Ferretti, M. Pavan and D. SpallarossaSummaryThe purpose of this work is to study seismic amplification effects at four sites hosting recording stations of the National Accelerometric Network (RAN) managed by the national Department of Civil Protection. At all sites, which are located in the Piedmont region (Northwestern Italy), both active and passive seismic prospecting methods are used, allowing identification of local amplification effects at three out of the four sites analysed. A ground type is then assigned to each site according to the national antiseismic code. Results obtained here will be fundamental to reduce the ground motion variability related to ground motion prediction equations and, consequently, to produce finer site-specific ground shaking hazard maps.
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New Geologic and Tectonic Insight in the Isonzo Plain through Seismic Imaging
Authors F. Accaino, G. Boehm, M. Busetti, L. Baradello and A. AffatatoSummaryA seismic line acquired near Gorizia (NE of Italy) and long about 3.3 km was reprocessed and interpreted with the aim to investigate the buried geological structures and the tectonic of the area.
The seismic profile well images the structures buried by the Quaternary alluvial deposits. In particular are well imaged the top of the flysch at about 100 m in depth and a dipping reflector from South to North that can be related to the limestone. The flysch outcrops in the south part of the seismic line in correspondence of the Farra hill., and is affected by compressional and transpressional tectonic deformation. Also tomographic inversion of refracted and diving waves was performed, allowing us to define the velocities of the alluvial deposits and of the flysh. Also the structural setting of the embricated thrusts are well imaged by the final velocity field obtained by tomographic inversion.
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