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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
1 - 100 of 164 results
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A Multidisciplinary Hydrogeophysical Approach Applied to the Chalk Aquifer Using MRS (North of France)
More LessFor several years LaSalle Beauvais has regularly carried out hydrogeological and hydrogeophysical measurements in order to characterize the chalk aquifer in the vicinity of Beauvais (Picardie, France), in terms of geometry, behavior and development (Zouhri and Lutz, 2010). A multidisciplinary approach has been chosen: piezometric and meteorological measurements, near surface geophysics measurements (electrical resistivity tomography, seismic refraction…), logging (gamma-ray, water conductivity, temperature), and since June 2014: Magnetic Resonance Sounding. The main hydrogeophysical results obtained on the experimental site of LaSalle Beauvais are displayed in order to show how geophysics methods can improve the hydrogeological knowledge. We insist in particular on the results of Magnetic Resonance Soundings. This method, little used in France because of its susceptibility to electromagnetic noise, allows us to estimate the water content, the permeability and the transmissivity. All of these results, which will be complemented by pumping tests and water samples study in the future, will be of great interest for the water management in the Beauvais area.
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Density Estimation Using Muon Imaging
Authors L.A. Preston, N.D. Bonal, D.J. Dorsey, D. Schwellenbach, W. Dreesen and J.A. GreenSummaryMuons are subatomic particles capable of penetrating the earth’s crust several kilometers. Muons have been used to image the Pyramid of Khafre of Giza, various volcanoes, and smaller targets like cargo. For objects like a volcano, the detector is placed at the volcano’s base and muon fluxes for paths through the volcano are recorded for many days to weeks.
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Geophysical Imaging at the U.S. Critical Zone Observatories
Authors W.S. Holbrook, B. Carr, J. St. Clair and J. HayesSummaryOver the past two years, the Wyoming Center for Environmental Hydrology and Geophysics (WyCEHG) has imaged the subsurface at five CZO’s: Calhoun, Boulder Creek, Eel River, Reynolds Creek, and Southern Sierra. Techniques applied include seismic refraction, electrical resistivity, downhole logging, ground-penetrating radar, magnetic gradiometry, EMI, and surface NMR. We will present results from these sites.
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Advances in the Application of Thermal Logging Techniques for Hydro-physical Characterization of Flow through Fractured Rock
More LessSummaryThe use of thermal techniques for identifying groundwater flow in fractured rock is experiencing a revitalization as a result of both advances in sensors and synergistic developments of other technologies. Multiple sensors with resolution approaching 0.001 C8 are combined to measure an orientated temperature field in the thermal vector probe (TVP).
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Chargeability from Airborne TDEM Data - Model Studies and Field Examples
More LessSummaryElectrical chargeability has been observed in time-domain EM data for many years, but mostly as a curiosity or considered to be interference, rather than a useful geophysical measurement.
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Spectral Analysis and Correlation of Ambient Seismic Noise. The Case Study of Madonna del Sasso (NW Italy)
Authors C. Colombero, L. Baillet, C. Comina, A. Helmstetter, D. Jongmans, E. Larose, J. Valentin and S. VinciguerraSummaryAmbient vibrations recorded on potentially-unstable rock slopes show that the temporal variations in the spectral content and in the correlation of seismic noise can be related to both reversible and irreversible changes within the rock mass. In this work, we analyzed the seismic recordings acquired at the potentiallyunstable granitic cliff of Madonna del Sasso (NW Italy) from October 2013 to November 2014.
The spectral content of noise systematically highlighted clear energy peaks at specific frequencies on the most unstable sector, interpreted as resonant frequencies of the investigated volume. Horizontal ground motion at the fundamental frequency was moreover found to be orthogonal to the main fractures observed at the site and consequently parallel to the potential direction of collapse. Cross-correlation was computed between the recordings of the sensors placed in the prone-to-fall compartment and a stable reference station.
Both the temporal variations of the resonant frequencies and the results of cross-correlation showed seasonal reversible variations related to temperature fluctuations. No irreversible changes, resulting from damage processes within the rock mass, were detected during the monitored period.
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Microseismic Monitoring of an Unstable Rock Face - Preliminary Signal Classification
Authors D. Arosio, L. Zanzi, L. Longoni and M. PapiniSummaryWe analyse signals collected by a microseismic monitoring network installed on an unstable rock face threatening the city of Lecco, in the North of Italy. We propose a classification process based on parameters computed in both time and frequency domains able to identify events possibly related to microseismic activity within the unstable rock mass and discard unwanted signals caused by meteorological conditions or other sources of noise. Our classification routine seems to be robust and the preliminary outcomes are in agreement with the findings reported in previous studies. We believe that further investigations together with the integration with a localization process of the significant microseismic events will improve our approach.
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Seismic and Resistivity Monitoring of Pont Bourquin Landslide in Switzerland
Authors S.R Carrière, G. Bièvre, E. Larose, D. Jongmans, T. Lebourg, L. Baron and M. JaboyedoffSummaryLandslides are natural and complex phenomena which can be encountered all over the world. They affect all types of geological formations and present a large variety of size, morphology and displacements rates. Among these phenomena, landslides in clay-rich formations are particularly complex due to the still unpredictable acceleration and fluidization that characterize them. In order to better understand this solid-fluid transition, the Pont Bourquin landslide in Switzerland has been continuously monitored with at least two vertical seismic sensors since the beginning of 2010, and 36 electrodes since the beginning of 2014 to assess the resistivity changes at different locations of the landslide. Relative velocities changes processed from ambient noise correlation show seasonal fluctuations between −2 and +2 % along the years. Only one drop of more than 7% was observed in summer 2010 preceding a major earthflow, showing that daily dV/V processing could be used as a precursor signal for landslide activation. Resistivity measurements show that certain parts of the landslide undergo strong electrical changes for which the causes are not yet determined. Because of the structural complexity of the landslide, further studies are requested to better understand the coupling between geophysical soil properties and environmental parameters.
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Role of Fracturing on Seismic Noise Measurements - The Case of the Bory Crater (La Réunion Island)
Authors P. Bottelin, J. Valentin, D. Jongmans, L. Baillet, F. Donze, O. Brenguier and A. MangeneySummaryIn recent years, ambient seismic noise measurements have significantly developed in gravitational hazard assessment for both investigation and monitoring purposes. The wide frequency range allows ambient vibrations to be applied for investigating geological and civil engineering structures in a great variety of sizes, from the lithospheric or crust scale to a few m-thick landslide and rock column or buildings. On landslide sites, several studies revealed significant spectral amplification at given frequency and polarization of the wave-field in the direction of maximum slope displacement. They show that different characteristics (resonant frequencies, polarization, and spectral amplification) could be used from the spectral analysis of the ambient motion or from spectral ratios for characterizing landslides.
This study aims to study a prone-to-fall column located at the top of a basaltic cliff in the Bory Crater (Réunion Island). Seismic prospecting profiles were performed on the plateau to characterize the basaltic lava deposits and the ambient seismic noise was measured using 3C sensors located on the column and at several distances from the cliff. The studied showed a clear resonance effect due to the column vibration, as well as larger site effect which could be related to the fracturing in the Bory crater.
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On the Use of ERT and AEM Resistivities for a Tunnel Pre-investigation
Authors S. Bazin, H. Anschütz, A.K. Lysdahl, J. Scheibz and A. A. PfaffhuberSummaryWe have investigated the applicability of resistivity derived either from surface electrical resistivity tomography (ERT) or airborne electromagnetic (AEM) surveys to a tunnel pre-investigation. We have exploited resistivity models to map the extent of toxic black shale along a tunnel alignment in central Norway. The resistivity models acquired before excavation and supported by laboratory measurements enabled us to map geological layers that are in agreement with the rock types subsequently observed from drillings and geological logging during tunneling. Our results highlight the potential of AEM surveys for such tunnel pre-investigations.
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A Study of Misfit Topography towards Waveform Inversion for Prediction Ahead of Underground Tunnel
Authors L.T. Nguyen and T. NestorovicSummaryThis work presents a synthetic study of misfit topography towards identification of geological structure ahead of the underground tunnel by waveform inversion. Simulation of viscoelastic waves is performed by the spectral element method implemented in SPECFEM2D package. By modeling a simple 2-dimensional underground tunnel model with a dip geological interface ahead of the tunnel face, we construct the misfit error landscapes with respect to geometric parameters defining the dip layer interface. As a result it is found that the misfit topography is very multimodal and therefore finding the true model by waveform inversion is likely to be very challenging. However, the choice of misfit definition, for example by using the envelope misfit, can help ease the toughness of misfit topography and facilitate the inversion task.
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Statistical Soil Type Estimation Based on Cross-plots of S-wave Velocity and Resistivity in Japanese Levees
Authors K. Hayashi, T. Inazaki, K. Kitao and T. KitaSummarySoil type of levee body and foundation are statistically estimated using cross-plots of S-wave velocity (Vs) and resistivity in Japanese levees. The Vs and the resistivity are collected from surface wave methods and resistivity methods. Total survey line length of the geophysical methods is about 600 km on 37 rivers in Japan. The blow counts and soil types are collected from about 400 boring logs carried out on geophysical survey lines. Vs and resistivity at the depth of the blow counts were extracted from geophysical sections. The total number of extracted data is about 4000. The data is grouped in levee body and foundation. A polynomial approximation was used to estimate the soil type from Vs and resistivity. Coefficients of equations are optimized by a least squares method. Accuracy of estimation was statistically evaluated by comparing estimated and actual soil parameters. Soil type sections were calculated from Vs and resistivity sections using the polynomial approximations.
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Ambiesta Dam - Geophysical Survey to Define the Cracks Patterns inside the Structure
Authors A. Masera, D. Donnaruma and D. MilaniSummaryThe paper describes the investigation using sonic tomography, performed on the Ambiesta dam (Italy) and aimed to characterize and identify the concrete cracking patterns inside the dam body, starting from the surevy of cracks visible on the downstream face.
The investigation had to define the depth of the cracks patterns and define the concrete caractheristics in the area affected by the cracks compared to healthy areas or free from cracks.
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GPR Assessment of Columns in the Pavilions and the Church of the Modernista Building Sant Pau Hospital, in Barcelona
Authors S. Santos-Assunçao, R. Gonzalez-Drigo and V. Perez GraciaSummaryThe work presents a case study applied to the analysis of columns in cultural heritage. The GPR assessment of columns in three different places of a Modernista set of buildings allows determining the differences in constructive solutions. The study of the church denotes that the columns are masonry structures with four metallic reinforcements along the shaft. The analysis carried out in the pavilions highlights differences between two types of masonry columns: some of them are built with regular arrangement of bricks and seem to be solid structures; others have a metallic pipe (a rain drainage tube) in the centre of the structure, and the inner elements are most likely portions of bricks with a shape similar to a triangles.
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Photogrammetry and 3-D Ultrasonic Tomography to Estimate the Integrity of Two Sculptures of the Egyptian Museum of Turin
Authors L. Sambuelli, G. Böhm, C. Colombero and A. FilipelloSummaryWe present a fruitful combination of geophysical tests (Ultrasonic Measurements) and photogrammetric processing (Structure from Motion) for the analysis of the integrity of a couple of statues from the Egyptian Museum of Turin. Aim of the study was to investigate the persistence of the exterior widespread fractures within the sculptures. More than one hundred ultrasonic measurements were acquired on selected travel-paths across each statue, using an ultrasonic pulse velocity instrument. Dealing with complex-shape objects of restrained dimensions, it was very important to accurately define the three-dimensional coordinates of sources and receivers, in order to precisely measure their distances. A 3-D model of the statues was obtained from photogrammetric techniques. The acquired data were analyzed with both a statistical approach and tomographic processing, comparing the use of classical and staggered grids, in order to obtain the best fit of the local resolution. The final results revealed a valuable tool to guide the procedures for the mobilization, transport and restoration of the sculptures.
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High Resolution Geophysical and Topographical Surveys for the Characterisation of Fumane Cave Prehistoric Site, Italy
Authors M. Obradovic, N. Abu Zeid, S. Bignardi, M. Bolognesi, M. Peresani, P. Russo and G. SantaratoSummarySmall scale prehistoric archaeological sites, such as the Fumane cave, located north of Verona city in north Italy, poses a lot of challenges both to archaeologists and geophysicists. The cave is considered as one of the most important Middle and upper Prehistoric sites of Europe, discovered in 1884. It contains a well-preserved and impressive stratigraphic sequence of more than 12m thick where human activity remains are accumulated. In this work, we applied high resolution geoelectrical resistivity tomography and seismic passive geophysical methods to investigate, on one hand, the electrical properties of the shallowest part of the deposit, where, a number of medium to low resistivity anomalies were localised. On the other, a test using microtremors provide evidences on the probable maximum thickness of the deposit (4–5 m). In addition a high resolution and georeferenced laser scanner survey was carried out so as to have an high resolution topographic model that shall be used as a container to allocate in 3D the position of the already achieved findings as well as the geophysical results. This shall constitute the first step for the planning of a smart protocol for results conservation and dissemination both for cultural and scientific interests.
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Archaeogeophysical Experiments in the Large Scale Laboratory
Authors F. Perciante, L. Capozzoli, V. Giampaolo, R. Luongo and E. RizzoSummaryThe ArchaeoGeophysical techniques are based on the investigation of buried cultural structures and surrounding materials in the archaeological framework. Unfortunately, the geophysical contrast between archaeological features and surrounding soils sometimes are difficult to define due to problems of sensitivity and resolution both related on the characteristic of the subsoil and the geophysical methods. An archaeological experimental activity has been performed in the Hydrogeosite laboratory, a full large scale controlled site consisting on a pool shape structures of 230m3 where archaeological remains have been installed. The laboratory experiment was performed to assess of the capability of EM geophysical techniques to detect archaeological remains placed in the humid/saturated subsoil. The archaeological test site was characterized by remains of Roman times (walls, tombs, roads, harbour, etc.) covered by sediments. In order to simulate lacustrine and wetland condition and to simulate extreme events (for example underwater landslide, fast natural erosion coast, etc.) the phreatic level was varied and various acquisitions for the different scenarios were performed. The integration of electric and electromagnetic data in that conditions allowed us to overcome the limits of each technique in terms of resolution and depth. Moreover, the experiment highlight the uncertainties related on the interpretation of geophysical analysis.
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Quality Assurance of Jet Grouting Columns Using Seismic Methods
Authors E. Niederleithinger, J.C. Galindo Guerreros, S. Mackens and T. FechnerSummarySealing and strengthening of the subsoil by injection is a major issue in the field of geotechnical engineering. This involves also jet grouting, which allows creating columns of grouted soil by eroding and mixing the in-situ soil with a thin cement suspension. A general difficulty of this method is to predict the column diameter and its material strength. Here, we present two-dimensional finite-difference numerical simulation results of a promising non-destructive quality assurance testing tool to evaluate the diameter of jet grout columns. This approach incorporates crosshole and downhole seismic measurements. Preliminary tests showed that this tool is applicable under real site conditions.
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Sensitivity of Seismo-electric Waveforms and Electric Noise to Electrode Configuration
Authors M.S. Devi, D.R. Dietrich and D. R. GaramboisSummaryThe seismo-electric exploration method aims to analyse the electromagnetic signals generated when seismic waves excite, and then propagate through a fluid-filled porous or fractured medium. Seismo-electric signals are obtained by firing a seismic source and recording the resulting electric fields. Such experiments usually allow one to detect the dominant contributions of the seismo-electric response, namely the electric signature of seismic surface waves and guided waves propagating in the near surface. Of more interest are the electromagnetic waves generated at depth when seismic waves cross an interface. These signals are much more difficult to discern due to their very weak amplitudes and because of the ubiquitous man-made electric noise that dominates the records. As a workaround, we investigate here the possibilities offered by multi-electrode arrays to enhance the EM interface response and mitigate the power line noise at the same time.
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Drones — New Technologies for Geophysics?
Authors A. D’Alessandro, F. Bucalo, M. Coltelli and R. MartoranaSummaryDrones, also referred as Unmanned Aircraft Systems (UAS), Unmanned Aerial Vehicles (UAV) or Remotely Piloted Aircraft (RPA), are aircraft without pilot on board. USA is today the leading country in the field of UAS but recently, seven European countries have pledged to launch joint programs for drones, with the aim of producing the next generation of UAS before 2020. According to some studies on the potential market of UAS in Europe, the geoscience sector is definitely one of the most promising. In this paper, we bring a short description and classification of drones, according to their sized and shaped, with particular attention to the advantages and drawbacks of each type in reference to the possible employment in geosciences applications. Currently the drones are used for geological and topographic mapping, coastal control, landslide inspections, etc., and are capable to integrate geophysical sensors like magnetic, electromagnetic, infrared, radar, natural gamma ray sensors and etc.
UAS technology have several advantage over conventional airborne geophysics like resolution, accurancy, cost, etc. We are witnessing the birth of a new branch of aviation, which could be profitably applied to geophysics measurements.
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The Use of Mathematical Models in the Control Environment of Underground Gas Storage
SummaryThe operation of underground gas storages may be accompanied by deterioration of the technical condition of wells. This is due primarily to the variables of temperature and pressure loads of the element in the wellbore during injection and extraction of gas, which most from the earth’s surface and to a depth of 500–700 m. Under the influence of these factors disrupted the adhesion of cement to casing and formation, occur the leakage of columns, gas leakage from the wellbore, the accumulation of gas behind the columns and headers, behind-the-casing flows from the storage reservoir into the overlying sediments. All this results in considerable industrial gas accumulations, the complication safe operation of UGS and disturbance of the ecological situation in the area. Therefore, the development of the technology search wells — sources of technogenesis, elimination of leaks and cross-flows of gas in them is an urgent task for underground gas storage. The work deals with the identification of cross-flows in the wells of underground gas storages. We consider mathematical models and algorithms for identifying cross-flows according to the thermometer wells. Examples of the interpretation and processing of data, previous studies of wells, underground gas storage.
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Fast Multi-channel Data Transfer System for Ultrasonic Borehole Tools and Others
By T. RadicSummaryReliable and high resolution information about the current state of radioactive waste repositories and their casing can only be gained on site with borehole tools. Ultrasonic sensors are such a tool, making it possible to characterise the surrounding ground (salt, clay, granite) by measuring the elapsed times of high frequency seismic signals. The arising data must be transmitted to an above ground or to a subterranean observation point. There, the data (mostly still analogue signals) are collected, plotted and quantitative evaluated. The quality of the transmitted analogue signal is, however, reduced through, amongst others, the excitation signal (which is usually transmitted through analogue means). This problem becomes more severe as the distance of transfer increases. We are currently developing a digital data transfer system, which will span a distance of up to 2 km. This system allows 7 analogue signals, with a bandwidth of 250 kHz each, to be transmitted within a single optical fibre at the same time. The same single fibre is used in the opposite direction to remote control gain settings and others. Further areas of use lie where rapidly changing analogue signals must be transmitted in real time over large distances.
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Comparison of Different Acquisition Patterns for 2D Tomographic Resistivity Surveys
Authors R. Martorana, P. Capizzi, A. D’Alessandro and D. LuzioSummaryA systematic comparison is presented between some 2D resistivity models and their images by the inversion of synthetic datasets relating to three different arrays, suitable for multichannel data acquisitions (dipole-dipole (DD), Wenner-Schlumberger (WS) and multiple gradient (MG)). The goal is to study how the measurement errors affects the resolution of the tomographic models and the ability to retrieve correct information on buried targets. We considered different data acquisition patterns, gradually increasing the complexity of the combinations of potential spacing and dipolar distance. To this end we increased the number of current dipoles to obtain approximately the same amount of measures, increasing the investigation time. Results from noise-free and noisy data are discussed and compared with those from field data. The results show that: the quality of the inversion models, for a fixed noise level, depends significantly on the data acquisition pattern; the information recovery and the resolution, being equal the number of measurements, is overall better for WS and worse for DD; the decrease of sensitivity with depth is lower for particular acquisition patterns that allow to better resolve deeper targets; the MG array can be preferred because it provides comparable results, using a smaller number of current electrodes.
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Site Investigation of Twin Tunnels by Joint Application of Seismic and Engineering Geophysical Soundings (CPTe)
Authors É.B. Bujdosó, E.H. Hegedűs, A.C.K. Kovács, I.T. Török, R.C. Csabafi and J.S. StickelSummaryThe near-surface geophysical data have become key components in the development of the engineering of major underground structures. Geological and Geophysical Institute of Hungary (GGI of Hungary) carried out a detailed geophysical survey along the twin tunnels of a new Metro line in Budapest, Hungary. Following the tunnel boring activity several sinkholes were detected at the surface along the path of the twin tunnels. GGI of Hungary carried out engineering geophysical soundings (CPTe) and non-invasive seismic technologies (3D seismic tomography) in order to mitigate the potential risk of any material damages in the future. The aim was to get to know the physical parameters of the soil layers. The joint application of the two methods was able to detect the known and the hidden potential sinkholes using state-of-the-art technology.
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MASW 2D Seismic Survey in Urban Areas - The Case of the Turin Metro 1 Line
Authors C. Alessio, R. Crova, E. Duò and M. NaldiSummaryThe present article discuss the results of seismic investigation conducted in Turin, Italy, to assess the geological and geotechnical modeling related to the planned extension of the underground Metro Line 1 from Collegno to Cascine Vica (about 4 km of length, from Collegno to Cascine Vica). The specific objectives were to map the thickness of the man-made ground lying upon alluvial soil made of gravel and sand and evaluate the stiffness property of the subsoil. To overcome the typical obstacles of the urban ambient (electromagnetic and vibrational noise, hard traffic management, high energy source required) we applied the shallow wave technique with a continuous profiling (MASW 2D) by means of a customized system of geophones array on a land streamer and a controlled source of energy mounted on a vehicle. The surveys were performed overnight in two sessions (two days), with a fast data acquisition for a total length 2000 m. The results of the survey, calibrated with boreholes positioned all along the survey line, has precisely mapped the contact between the shallow reworked ground and the alluvial deposits. Moreover, the survey results have highlighted many lenses of cemented gravel (conglomerate bodies) inside the alluvial deposits.
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Geophysics and Tunneling - The How and the Why - A Focus on the Why
Authors L. Sambuelli, P. Fargione, E. De Cillis and M. PatruccoSummaryTunnels should be constructed, maintained and used without neglecting the basic principia of Occupational Safety and Health (OS&H), environmental protection and economic efficiency: this involves a Prevention through Design approach both from the very first steps of the project, and during the tunneling operations. At the purpose, the identification and careful evaluation of a number of parameters are essential for a correct Risk Assessment and Management (RAM) during the various phases of the analysis. However, many of these parameters are “hidden into the mountain” and difficult to identify with the desired detail. This problem can affect the quality of the result, and cause accidents and work stoppages, with consequent dramatic economic losses.
In the Authors’ opinion, only a multidisciplinary approach can contribute to solve the problem. The already consolidated contribution of geophysics plays an important role, and represents an efficient means for a more efficient definition of the design parameters. Future developments in the research work in geophysics are advisable and precious for a more efficient predictive RAM, both at the preliminary phase, and during the tunneling operations, particularly if a fast response will become achievable.
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Geophysical Time-lapse Monitoring of Leachate Recirculation in a Bioreactor Landfill
Authors A. Arato, G. Cappato, F. Manfredi, M. Fischetti, E. Perissinotto and A. GodioSummaryThis work deals with the time-lapse geoelectrical monitoring of leachate re-circulation inside a municipal solid waste landfill. The studied landfill is exploited as an anaerobic bioreactor, and leachate is recirculated in order to maintain constant and high moisture content within the waste. The goal of leachate injection is to ensure optimal conditions for the anaerobic bacteria to rapidly degrade the organic fraction of the waste. The main objective of this study is to evaluate the electrical behavior of the wastes, for estimating the moisture content dynamics during time.
Electrical resistivity tomography (ERT) and self potential (SP) methods have been applied both on surface electrodes installed on the top of the landfill and along borehole which have been installed for cross-hole measurements.
Both the methods helped in locating the zones affected by the leachate flow, but ERT measurements suffered from anomalous long-term polarization phenomena. On the other hand, SP measurements were in good accordance with the injected leachate volumes, and they can be useful for helping numerical modeling of the leachate flow within the waste mass.
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Investigation Concrete Quality of Zigana and Torul Tunnels by Using GPR Method
Authors I. Saricicek and A. SerenSummaryThe GPR method which can display the shallow depth of the investigated surface with high reslution, is a widely- used method to investigate the concrete structure without any damage. In this study it is aim to investigate the leaking water risk areas and accordingly the evaluation of the content of the concrete structure of the Zigana Tunnel which located on the highway of Trabzon-Gümüshane and faracture-joint system, deterioration and stratigraphy of the Torul Tunnel walls. During examinations, GPR data were collected along totally 20 profiles at the entrance of the both sides of tunnels by using 500 MHz and 800 MHz shielded antennas. After applying essential data processing stages to the collected data, 2D and 3D images with ReflexW3DScan software were acquired. When analyzed amplitude-slice maps at various depths into the wall from these images; while has impaired in places of concrete structures in belonging to the Zigana Tunnel, those obtained from the Torul tunnel have been identified quite durable and high quality of concrete internal structure.
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Integrated Geophysical Investigations for Calcione Dam Safety Evaluation
Authors V. Giampaolo, L. Capozzoli, A. Caputi, C. Corradini, G. Zucco, T. Moramarco and E. RizzoSummaryAn integrated geophysical field investigations have been carried out at the Calcione earthen dam near Lucignano town (Arezzo, Italy), for the purpose of better understanding the hydrogeological conditions at the site. In winter 2002, the Calcione dam embankment has been affected by some landslides and a gabions drainage system was installed to mitigate the phenomena. Therefore, the purpose of the geophysical investigation has been to identify the presence of eventually seepage areas through the embankment materials and provide information about subsurface water circulation. These tasks have been accomplished through the use of self-potentials (SP), electrical resistivity tomography (ERT) and ground penetrating radar (GPR). The results obtained allowed to identify an area of interest in correspondence of the central area of the dam above the installed gabions drainage system. Moreover, future efforts will focus on an integrated hydro geophysical interpretation, where geophysical data will be directly incorporated into the dynamic estimation of hydrogeological parameters.
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S-wave Velocity Monitoring During an Artificial Rainfall Experiment Using Large Scale Rainfall Simulator
Authors C. Konishi, T. Ishizawa, T. Danjo and N. SakaiSummaryWe conduct the rainfall experiment using a large scale rainfall simulator and an artificial embankment consists of sand and silt in order to evaluate S-wave velocity monitoring for a prediction of a slope failure. The intensity of the rainfall is controlled to maintain 15 to 200 mm/hour for a certain time frame. S-wave velocity cross sections are acquired by MASW survey before, during, and after the controlled rainfall. The obtained cross sections show little change in the S-wave velocity and that is considered to reflect water filtration process in subsurface. The difference is not so significant, but it is confirmed by both waveforms and the dispersion curves calculated from common shot gathers at a fixed shot point. The dispersion curves are included various higher modes and that makes it difficult to evaluate the result; however, the complex higher mode would be useful to derive more information in the future. Compared to the resistivity monitoring, the S-wave velocity monitoring is not so sensitive to the water content; instead, it is relatively easy to be obtained by surface wave method. Therefore, it will be an effective monitoring tool to assess vulnerability of slopes and mitigate damage by natural disasters.
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Electrical Resistivity Tomography in Support of Geological Surveys of Landslides Involving Clay Slopes in Asti Reliefs
Authors D. Barbero, M.G. Forno, M. Naldi and A. TissoniSummaryWe report on the results of geological, geotechnical and geophysical surveys for the characterization of a landslide, occurred in the Asti Reliefs (NW Italy) during the first ten days of March 2011, just after an exceptional meteoric event. The investigated landslide is located in San Damiano d’Asti (15 km E of Asti) and involved a hilly side, gently sloping. The hazard situation in this area regards a landslide that involved the back of a building. This landslide was caused by excavations made in the time at the toe of the slope with the aim to extend the narrow flat spaces. The contribution given by Electrical Resistivity Tomography (ERT) was essential, in the first phase of the investigation, to recognize deep and localized water seepage, otherwise undetectable. The identification of saturated clays into the slope allowed us to design the correct re-profiling of the scarps and make digging operations in safely. The use of these methods of investigation has a significant economic savings in the choice of interventions for slope stabilization.
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Landslide Geophysical Imaging - Kapušany Landslide Case Study
Authors R. Putiska, I. Dostál, D. Kušnirák and M. BednárikSummaryGeophysical survey is very useful and popular tool used by engineering geologists to examine landslides. We present a case study from the Kapušany landslide, eastern Slovakia, where a broad spectrum of geophysical methods were applied on several profiles in order to compare the ability of the methods to detect as many landslide’s structural features as possible. The 2D Electrical Resistivity Tomography inverse model was capable to define the geological structure of the landslide and to define the shear zone, however the resolution of the inverse model does not allows us to identify cracks or other minor landslide’s features. These, however, were nicely recorded in the results of Dipole Electromagnetic Profiling and Self Potential method. In addition microgravity was employed to validate the results obtained from electrical methods and afterwards a final geological model, based on the integrated interpretation of all involved methods was constructed. This model will serve as a reference model for long term landslide monitoring using modern geophysical and engineering geological approaches.
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Detection of Landslide Plane by Using Refraction Seismic Tomography Method
More LessSummaryAn application of refraction tomographic method in order to identify a predicted landslide’s plane is presented. This method is applied to detect the existence and the geometry of possible landslide’s plane that is situated in a Seling Village, Kebumen City — Indonesia. This area experienced several episodes of landslide type of creeping several years ago. Although it has been observed that the displacement of the most upper part of surface is only several centimeters per years, but there is a tendency that bigger landslide could be occurred in the future.
The seismic data was acquired on the surface that has a total length of about 150 m. Twelve shots and 24 geophones are planted on the seismic line that is perpendicular to the predicted strike of landslide’s plane.
The result shows that the most upper part of surface has low P-wave velocity values. Higher P-wave velocity layer is situated below this layer. The plane between those layers is then interpreted as possible landslide’s plane. The result of this investigation is a P-wave velocity section, which correlates well with almost all available a priori data. Several resolution tests are conducted in order to increase the certainty degree of tomographic results.
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Geotechnical Risk Assessment at South Eastern Suburb of Cairo, Egypt Using Integrated Geophysical Techniques
Authors N.I. Medhat, M. Atya, E.A. Ragab, A. Ali and M. AbdelzaherSummaryThe reason for implementation of this research is the prevalent of incidences of structural damage around the area of investigation causing large number of casualties and loose of souls.
The location of the district is very close to the quarry blasting operations that can intensely produce stresses in the ground due to continuous artificial explosions and ultimately result in the instability of the area.
Geophysical reconnaissance surveys using seismic refraction, electrical resistivity and ground penetrating radar were carried out to measure near surface structure setting and follow up its activity upon time by delineating the major fault and other minor features such as cracks, fissures and water saturated zone.
Integration of the geophysical data shows that the main reason of fractures in the study area is the swelling of marly clay layer due to retention of the drainage water. Also, there is a near surface major fault cuts the southern part of the study area causing variation in thickness of the marly clay layer and consequently affect on the inhabitant buildings.
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Estimation of Alluvial Sediments Properties with a Multilevel Cluster Analyses of Spectral Induced Polarization Data
Authors S. Inzoli, M. Giudici and J.A. HuismanSummaryIn order to infer the sedimentological properties of saturated samples of alluvial deposits, a cluster analysis is applied on spectral induced polarization data (amplitude and phase resistivity spectra). The hierarchical tree of clusters is dissected and analyzed at different aggregation levels, comparing the electrical and sedimentological intra-cluster variability. A low number of clusters densely populated guarantees a perfect allocation into cluster on the base of a few electrical parameters but permits to obtain only a rough textural characterization of the sample, while increasing the number of clusters yields to reduce the variability intervals of the textural parameters and of the chemical properties of the saturation water within each cluster, thus reducing the uncertainty in their estimation. An excessively large number of clusters is ineffective since the uncertainly in the assignment of a sample to a specific cluster is too high. These results can be used as an indirect tool for sediments characterization and have been successfully tested on an independent sample. The cluster analysis allowed to use the whole spectral information to establish a link between geological and geophysical parameters, avoiding the use of one-to-one correlations, whose validity has not yet been confirmed for natural unconsolidated materials.
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Using Borehole NMR Data to Determine the Effective Porosity of Sediment for Groundwater Resource Estimation, Australia
Authors K.P. Tan, K.L. Lawrie, J. Abraham and R.S. BrodieSummaryEstimating groundwater storage volumes is important for assessing the potential capacity and recovery efficiencies of Managed Aquifer Recharge (MAR) strategies. However such volume estimates are vexed by poor characterisation of factors such as the effective porosity of the aquifer. These issues were faced by the Broken Hill Managed Aquifer Recharge (BHMAR) project which aimed to define key groundwater resources and aquifer storage options in the lower Darling River floodplain of western New South Wales, Australia. The aim of this study is to determine if the NMR free-water data depicts the effective porosity of the sediment and could be used in a workflow to estimate groundwater storage volumes in the target aquifer. 26 sonic drilled bores were logged using the Vista Clara Javelin system. An investigation ensued to validate these NMR results. This included examination of the NMR data processing parameters and optimal regularization factor, additional matric potential and gravimetric water experiments to determine the fractional water and total porosities of sediment. The laboratory experiments showed comparable results to the NMR free-water and total water for sand and muddy sand, but the NMR underestimated the total water in mud.
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Monitoring of Injection of Hydrogen Release Compounds in a Contaminated Site
Authors A. Arato, S. Fiore, B. Ruffino, A. Godio, M.C. Zanetti and C. RoatiSummaryWe present the set-up and results of a pilot-scale field test concerning the remediation of a site polluted by chlorinated solvents. The hydogeological setting is mainly compound by a first aquifer in silty and sandy material and a second fissured aquifer made of fractured grey marl and gypsum/sulphur formations. A preliminary characterisation of the area have pointed out the groundwater contamination by chlorinated solvents, at a depth of about 15–20 m b.g.l. Vinyl chloride was identified as the secondary pollutant, proving the occurrence of natural degradation phenomena. Hydrogen Release Compound (HRC-X, Regenesis) was selected to support the on-site biological activity. In a selected test site, we injected about 250 kg of reagent in three injection points at the depth of about 15–18 m.
The chemical and physical monitoring of the clean up activity was performed for about 6 month after the injection by groundwater sampling in the three observation boreholes (downstream with respect the injection points) and by time lapse cross-hole resistivity tomography.
The enhancement of natural attenuation phenomena is testified by the decrease of halogenated pollutants with higher number of chlorines, and by an enhancement of the amount of the reductive dechlorination secondary products.
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Detecting Contaminants in Aquifers through Modeling of Surface-downhole Geoelectrical Data
Authors M.V. Bongiovanni, V. Grünhut and A.M. OsellaSummaryMost of the world’s reservoirs are currently in the advanced primary or secondary recovery stage. Even though sinks wells used by oil companies are located much deeper than aquifers, they may eventually cause environmental damages contaminating the water with brine leakage, oil spill or other polluting agent. This is why it is extremely important to do vulnerability studies.
Since contaminants have different electrical conductivity than water, geoelectrical methods can be useful to detect and even monitor flow evolution of the contamination plume.
We want to analyze if it is possible to detect contaminations of an aquifer that lies above an oil reservoir, approximately at 300 m depth, with current injections on surface and voltages sensors in a well. In order to achieve this we have designed an electrical model of the reservoir including the contaminated aquifer with, either a conductive contamination such as saline brine or a resistive contamination such as oil spill, and numerically simulated the forward geoelectrical responses.
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High Amplitude SP Anomalies in Mountains of Electrokinetic Origin
Authors V.A. Shevnin, A.A. Ryjov and D.A. KvonSummaryIn different countries after 1967 were discovered local self potential anomalies above 1200 mV that didn’t related with redox processes in rocks. Such anomalies were found in mountains, they had inverse correlation with relief, and underground water had low mineralization and high electrical resistivity. Possible cause of these anomalies can be electrokinetic or streaming potentials due to water filtration. Calculation of electric field intensity on Helmholtz formula shows that these anomalies could appear at water resistivity above 100 Ohmm at filtration in loose sand-gravel- soil.
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Electrical Geophysical Monitoring of Organic Waste Contamination
Authors D. Ntarlagiannis, J. Robinson, P. Kirmizakis, P. Soupios and L.D. SlaterSummaryOlive oil production in the Mediterranean area is a major industry, with significant impact in the economy of producer countries. During the production of extra virgin olive oil, a substantial amount of waste is produced and is improperly disposed. since no regulation for the environmental deposition of olive oil mill waste exists. Thus, there is an urgent need for the development of methods that will allow efficient monitor waste management processes, and enable rapid recognition of environmental degradation incidents.
The most common disposal practice involves unrestricted damping of the olive oil mill waste in open evaporation ponds, leading to significant degradation of the environment including groundwater and surface waters. Geophysical methods could be used to monitor olive oil waste management processes, and identify olive oil waste plumes right at their inception times. We applied a series of electrical geophysical measurements at an olive oil mill waste (OOMW) site in western Crete to test their applicability as monitoring aids. Our results suggest that electrical methods are very efficient in accurately delineating young OOMW plumes due to their conductive signature.
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Modeling of Electromagnetic Soundings to Detect Groundwater Contamination by Intensive Livestock Farming
Authors C.M. Sainato, R.A. Espada, B.N. Losinno and J.J. Márquez MolinaSummaryThe feedlots, farms for feeding animals in corrals, are spreading at Buenos Aires Province, Argentina. It is important to evaluate the contamination by mineralization of organic matter. Geophysical exploratory methods give an important tool for monitoring groundwater and soil quality. The objective of this work was to perform electromagnetic soundings and to do a modeling of these data by an inversion procedure for different frequencies at a corral of a feedlot and the surroundings. The survey was repeated the following year. Bellow the corral, the unsaturated zone has lower resistivity (between 8 and 17 ohm m, more conductive) than the test zone. The second year, the test zone showed greater range of values for resistivity (up to 54 ohm m) but the layers bellow the corral showed an important decrease of resistivity. Groundwater samples were taken along four years. An increase of water conductivity was found at the lower well along the time.
The EMI modelling allowed characterizing the distribution of resistivity bellow the corral and the background, reaching the groundwater, detecting an increase of salinity bellow the corral which may be mainly due to leaching of nitrates.
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TEM Profile Using Fixed-loop Array in the Paraná Basin, Brazil: Preliminary Results
Authors J.D.R. Campana, J.L. Porsani and C.A. BortolozoSummaryIn this work the authors shows preliminary results of TEM profile by using fixed transmitter loop and a 3D coil receiver mobile in the Paraná sedimentary basin, Brazil. The objectives were to map the sedimentary and crystalline aquifers. The sedimentary aquifer is characterized by Adamantina Formation and the crystalline aquifer is characterized by fractures zones inside of the basalt layers of the Serra Geral Formation. 1D inversion results for component-Z for each sounding are presented as an interpolated geoelectric profile of 1000m long. Two low resistivity zones were observed: the first between 40 – 60m depth, being related probably to the Adamantina Formation (sedimentary aquifer), and the second between 150m and 300m depth, being interpreted as a probable fractured zone filled with water inside of the basalt layer of the Serra Geral Formation (crystalline aquifer). These results are in accordance with the local geological information.
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Electric and Electromagnetic Investigation of a Karst System
Authors M.P. Miensopust, J. Igel, T. Günther, R. Dlugosch and S. HupferSummaryThe limestone of the western flank of the Innerste Mulde in Lower Saxony, Germany, host a 27 km long karst water system which is surface-mapped by sinkhole and collapse structures. The karst water emerges at springs — one of them is a spring called Kirschensoog near the village of Altwallmoden which only temporarily yields water. From early March to mid March 2015 the water level fell visibly. During that time electric and electromagnetic methods were applied in the field and soil and rock samples were taken for laboratory studies. Field measurements comprise electrical resistivity tomography (ERT), ground penetrating radar (GPR) and 2D nuclear magnetic resonance (NMR) surveys. The laboratory measurements (e.g. NMR and spectral induced polarisation measurements, petrophysical studies, dielectric spectroscopy) are applied to support the interpretation of the field data. The preliminary results show that the covering, clay-rich soil has a strong attenuation restricting the investigation depth of GPR. The ERT models indicate a 3D subsurface structure and — as well as GPR — suggest dipping structures possibly related to the bedding of the limestone or fissures/ fractures. The rather low resistivity of the limestone might be an indication of disruption and presence of fluids.
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Seasonal Monitoring of Saline Intrusion in an Unconfined Coastal Aquifer in New Zealand Using DC Resistivity Traversing
More LessSummaryLong term time-lapse DC resistivity measurements along six 155 to 275 meter long traverses at the Kapiti Coast in New Zealand have been made to image a seasonal cycle in the saline interface movement. Two-monthly repetition of the measurements showed a continuous decrease of the bulk resistivity at depth and over an extended horizontal area when coming into the drier summer season. This observation is somewhat contradictory to the expectations and has its origin most possibly in a decrease of the fluid resistivity, likely due to a lower fresh water flow coupled with saltwater intrusion. Although, the changes from time step to time step are not always major on all profiles, ERT time-lapse monitoring proved itself a suitable tool to image the saline interface shape and movement along with changes in the subsurface resistivity distribution in an unconfined coastal sand aquifer and in a seasonal time frame.
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Prestack Depth Migration of Ultrashallow Seismic Data. A Critical Analysis
Authors G.P. Deidda, G. Satta, E. Battaglia, C. Strobbia and Z. HeilmannSummaryPSDM is increasingly becoming rule rather than exception in near surface seismic imaging when the goal is to get detailed sub-surface images and accurate velocity field. PSDM produces the seismic image of an earth model in depth converting the seismic reflection data into a depth section by means of a velocity field. Although different implementations of velocity model building tools exist, they usually start with an estimation of an initial velocity model using moveouts in the time domain and proceed with an iterative model refinement using residual moveouts in the depth domain and seismic reflection tomography. In addition, if available, a priori velocity/depth information, is also considered to improve the accuracy of the velocity model. In shallow seismic reflection imaging, instead, refraction tomography has been used not only to improve the velocity model built with PSDM but also as the exclusive tool to build the velocity model. In our tests, building a velocity model by means of a priori information unrelated with the recorded data or by means of the exclusive use of refraction tomography, PSDM produces depth images that have an imprint of the velocity models and not a result coming only from seismic reflection data.
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Groundwater Storage in a Karst Vadose Zone Evidenced Using Gravimetric and Surface-to-borehole ERT Monitoring Systems
Authors A. Watlet, O. Kaufmann, O. Francis and M. Van CampSummaryHydrogeophysical tools allow characterizing the heterogeneity of karsts and evidencing groundwater storage in their subsurface. A permanent gravimetric monitoring highlights seasonal variations in gravity that can be related to vadose zone effects after the discrimination of the saturated zone signature with direct measurements. A surface ERT monitoring improved with daily borehole-to-surface measurements through the vadose zone allows identifying preferential flow paths. Groundwater storage variation is then monitored by time-lapse inversion of the ERT datasets.
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Assessing Electrical Resistivity Tomography for Hydrofacies Detection Using a Sensitivity Dependent Probabilistic Method
Authors T. Hermans and J. IrvingSummaryAlluvial aquifers are generally composed of several facies with complex architectures and interconnections depending on the fluvial system. In this context, electrical resistivity tomography (ERT) may provide important information on the spatial distribution of hydrogeological parameters. However, ERT inversion introduces some bias in the resulting resistivity distribution due to regularization and resolution issues. In this study, we refine ERT inversions by incorporating prior information in order to improve the identification of facies through a probabilistic relationship derived from collocated measurements. We then analyze with synthetic cases the effect of spatially varying sensitivity on the probabilistic relationship. As expected, when sensitivity decreases, the distributions of resistivity for the different facies tend to be superimposed. A mean distribution thus overestimates the ability of surface ERT to discriminate hydrofacies in depth.
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Integration of Geoelectrical Imaging and Hydrochemistry Data in a Coastal Aquifer in India
Authors P.R. Pujari, R. Sanam, C. Padmakar and P. LabhasetwarSummaryGroundwater is a vital source of drinking water on earth. The quality of groundwater is under increasing stress due to various anthropogenic stresses. The impact of ash ponds has been a major concern and is a subject of research. The present study seeks to assess the impact of an ash pond on the near sub-surface through Resistivity and IP imaging survey. The Resistivity and IP imaging data has been acquired on profiles close to the ash pond and at some far off locations. Groundwater and surface water samples have been collected from the ash pond and also from the wells in and around the ash pond. The study indicates very low resistivity formations in the near sub surface at location close to the ash pond as compared to the profile approximately 1 km away from the pond. The water chemistry data from the monitoring wells indicates very high Total Dissolved Solids (8658 mg/L) and chloride (1402 mg/L).
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Ground Penetrating Radar Applications in Seismic Microzonation
Authors S. Santos-Assunçao, V. Perez-Gracia, R. González-Drigo, V. Salinas and O. CasellesSummaryThe work presents a methodology that combines a first GPR survey and a subsequent measurement of seismic ambient noise vibration. The GPR signal characteristics used in this analysis are two: the amplitude of the background noise in the A-scans, and the frequency content of the received signal. The background noise could be consequence of three main sources: clutter as consequence of external reflections, electronic noise and energy randomly scattered in the medium. The first source could be identify in the GPR B-scans and usually produce anomalies similar to those caused by reflections in the targets inside the medium. The second source produce a continuous noise in the A-scans characterized because the average value is approximately constant. The third source introduces noise in the A-scans but its amplitude depends on the randomly backscattered energy. As consequence, the amplitude of this noise could be used in order to identify sudden changes in the shallow geology, always depending on the grain size distribution. The frequency content depends strongly on the water content. The analysis of the spectrum, combined with the analysis of the B-scans and the backscattering noise in the A-scans could be used in order to locate active subterranean streams.
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Efficient Acquisition and Holistic Analysis of Rayleigh Waves
Authors G. Dal Moro, S.S.R. Moustafa and N. Al-ArifiSummarySurface-wave analysis is nowadays widely considered for a number of goals ranging from geotechnical investigations to seismic-hazard studies. The most common approach is surely represented by the analysis of active or passive multi-channel data used to depict the phase velocity spectrum from which dispersion curves are then depicted and inverted.
The presented methodology attempts to obtain a robust sub-surface model in spite of the light equipment (a single [at least] bi-axial geophone) and the straightforward field procedures.
The active method here presented can be considered as an improvement of the classical MFA (Multiple Filter Analysis) method and is based on the joint analysis of the vertical and radial group-velocity spectra, together with the Radial-to-Vertical Spectral Ratio (RVSR).
After introducing the fundamental points charactering its implementation, a case study is presented also comparing the obtained sub-surface model with the one obtained considering a standard approach based on multi-channel active (MASW — Multichannel Analysis of Surface Waves) and passive (ESAC — Extended Spatial Autocorrelation) data jointly considered with the Horizontal-to-Vertical Spectral Ratio (HVSR).
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Sensitivity Analysis of Rayleigh-wave Ellipticity with Application to Near Surface Characterization
By M. CercatoSummaryThe joint inversion of surface-wave measurements and Rayleigh-wave ellipticity has gained popularity in recent years for near-surface soil characterization. The common approach is to use low-frequency, single-station ellipticity data in conjunction with high-frequency dispersion measurements obtained employing small aperture arrays. A complete understanding of the diagnostic potential of ellipticity in such conditions can be assessed only with a complete sensitivity analysis.
To this end, a new analytical method is presented for computing the sensitivity of Rayleigh-wave ellipticity with respect to the structural parameters of a layered elastic halfspace. This method employs a layer stacking procedure based on the subdeterminant formulation of the surface-wave forward problem and is numerically stable at high frequencies.
The sensitivity of the ellipticity curve is then evaluated quantitatively with specific focus on near-surface examples and compared to the dispersion patterns and sensitivity of modal phase velocity.
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Approximate Direct Estimate of S-wave Velocity Model from Surface Wave Dispersion Curves
Authors L.V. Socco and C. CominaSummarySurface wave dispersion curves are inverted to obtain 1D local VS model. The inversion is based on a layered model and the result is non-unique. A linear relationship can be found between weighted average S-wave velocity at different depths and surface-wave phase velocity at different wavelengths. This relationship, if known for a category of models, can be used to directly estimate an approximated velocity profile from the dispersion curve without the need for inverting. We show on a synthetic case that the error between estimated and true velocity is of the same order of that obtained by inversion.
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Measurements of Surface Waves Phase Velocity with a Large Ring Laser Gyroscope and a Seismometer
Authors A Simonelli, J. Belfi, N. Beverini, G. Carelli, A. Di Virgilio, E. Maccioni, R. Santagata, G. De Luca and G. SaccorottiSummaryIn recent years the development of large ring laser gyroscopes gave birth to a new discipline: the rotational seismology. This field of study is rapidly expanding and many areas of geophysics are or can be involved, from geodesy to seismology to the monitoring of critical structures. New rotational sensors based on optical interferometry and Sagnac effect has been developed in the last two decades. These sensors are called ring laser gyroscopes (RLG) and are now able to cover the range of rotations of seismological interest i.e. from 10–2 to 10–9 rad/sec. A measure of rotations in seismology is of fundamental interest for several reasons:
Quantitative seismology requires a measure of all the six degrees of freedom that characterize a rigid body’s motion.
Standard seismological observations are contaminated by rotations that produce a bias in the estimate of ground translation.
Rotational observables provide important informations about the elastic properties of the subsoil. The extension of rotational observations to exploration seismology can improve resolution and accuracy of subsurface imaging.
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SH-wave Seismic Reflection at the Patigno Landslide and Integration with P-wave Reflection Data
Authors A. Tognarelli, E. Stucchi, A. Ribolini, E. Lauriti and L. MeiniSummaryIn this work we describe the acquisition and processing, up to the depth migrated image, of an SH-wave reflection seismic survey carried out on a complex deep seated landslide located in the Northern Apennines in Italy. We also show a comparison with a recently acquired P-wave seismic reflection profile that investigates the same landslide body. The P-wave survey was able to delineate the deep sliding discontinuity, but was unable to give a detailed description of the small reactivation slip surfaces delineating minor landslides at shallow depths, that are responsible of the major observed damages. Our experience shows that the combined use of both P-waves and SH-waves offers the possibility to obtain detailed insights of the whole landslide body from the deepest discontinuity up to the very shallow portion of the landslide, overcoming the limitations due to the low resolution of P-wave method for imaging shallow horizons and the low investigation depth of SH-wave method. The deeper knowledge of the landslide internal setting that can be gained by a joint application of both methodologies is of primary importance to plan adequate and effective defence strategies.
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Time-lapse Monitoring of the Slopes of a Heritage Earthwork by Means of Near-surface Seismic Techniques
Authors P. Bergamo, S. Donohue, D.A. Gunn, B. Dashwood, S. Uhlemann, J.E. Chambers and D. WardSummaryA significant portion of UK’s infrastructures earthworks was built more than 100 years ago, without modern construction standards: poor maintenance and the change of precipitations pattern experienced in the past decades are currently compromising their stability, leading to an increasing number of failures. To address the need for a reliable and time-efficient monitoring of earthworks at risk of failure we propose here the use of two established seismic techniques for the characterization of the near surface, MASW and P-wave refraction. We have regularly collected MASW and P-wave refraction data, from March 2014 to February 2015, along 4 reduced-scale seismic lines located on the flanks of a heritage railway embankment located in Broadway, SW of England. We have observed a definite temporal variability in terms of phase velocities of SW dispersion curves and of P-wave travel times. The accurate choice of ad-hoc inversion strategies has allowed to reconstruct reliable VP and VS models through which it is potentially possible to track the temporal variations of geo-mechanical properties of the embankment slopes. The variability over time of seismic data and seismic velocities seems to correlate well with rainfall data recorded in the days immediately preceding the date of acquisition.
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Structurally Constrained 4D ERT Monitoring to Image Hydrological Processes Leading to Landslide Reactivation
Authors S.S. Uhlemann, J.E. Chambers, S. Hagedorn, H. Maurer, P. B. Wilkinson, T.A. Dijkstra, B. Dashwood, A. Merritt and D.A. GunnSummaryFuture environmental and climate change will affect the frequency and magnitude of landslide occurrences. A major focus of research is therefore to gain an improved understanding of the processes contributing to unstable slopes and the associated triggering mechanisms. This study considers the use of a combination of one-off seismic and geoelectrical monitoring measurements (from a 34 month period leading to landslide reactivation) to gain detailed understanding of the hydrological conditions leading to landslide reactivation. The study site is the Hollin Hill landslide field observatory that comprises a suite of geophysical, geotechnical and environmental sensors, thus offering the opportunity to compare and inform interpretation of the different data streams. The 4D electrical resistivity tomography (ERT) inversion was structurally constrained employing results of a combined P- and S-wave seismic refraction tomography (SRT). The ERT results were temperature corrected and translated into values of gravimetric moisture content (GMC) using laboratory derived GMC-resistivity relationships. The results show seasonality effects for the first year of monitoring, followed by imaging of crack built up and deep moisture penetration leading to failure of the back scarp. Elevated moisture contents, as an effect of prolonged rainfall, were imaged throughout the landslide prior to its reactivation.
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Comparison of Geotechnical and Geophysical Prospecting Approaches in the Study of a Landslide in Northern Iran
Authors M. Kazemnia Kakhki, H. Rezaiy and A. SadidkhouySummaryOne of the most challenges in landslides is deciphering of slide surface. As we know, there are different landslide types which make us the first problem to create accurate observations to distinguish the type and the stage of landslide. The following step is to define where to focus our monitoring. Electrical resistivity measurement is one of the most common methods which can be applied in landslide studies, although it is not accurate in most cases. On the other hand, its lower expense makes it applicable. For this, 2D electrical resistivity observations and Geotechnical methods such as standard penetration tests were combined to distinguish the subsurface conditions on a big landslide surface in NargesChal, a village in north of Iran in Golestan province.
The results were controlled by geotechnical logs and the extracted slide surfaces were confirmed by direct subsurface information.
The depth of 11.5 to 12.5m in all profiles has confirmed as sliding surface in SPT and soil moisture variation diagrams which is somehow consistent to Geoelectric method.
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High-resolution Seismic Imaging of the Pian di Neve Glacier on the Adamello Massif (Italy)
Authors M. Giorgi, S. Picotti, R. Francese, F. Pettenati, A. Bondesan, C. Baroni and M.C. SalvatoreSummaryActive seismic investigations, generally employed to image the earth interior, are also widely applied in glaciology to map the thickness of glaciers and to study the basal sediment properties. In this work we present the results of a 1-km multichannel seismic survey carried out on the Pian di Neve, a high altitude glacier located in the Adamello massif (Italy), with the purpose to image the glacier bottom and obtain the ice thickness profile. The Pian di Neve occupies a 18-km2 plateau at an altitude of between 3100 and 3400 m, and together with the adjacent Mandrone glacier forms the largest glacier mass in the Italian Alps. From the maximum Little Ice Age extension to present, these two glaciers lost about 36% of their total area.
The seismic survey evidenced an ice thickness ranging from 170 m to 270 m and the presence of a relatively thick layer of sediments at the bottom. Comparison with previous investigations seems to indicate that, in the last 50 years, the ice thickness in the survey area has not significantly changed. This evidence is contrary to the general retreat trend and could be due to a sort of thermal inertia in this glacial accumulation area.
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Seasonal Monitoring of Snow Properties by WCR and up-GPR
Authors A. Godio, D. Franco, B. Chiaia, B. Frigo, L. Dublanc, M. Freppaz, M. Maggioni, E. Ceaglio and P. DellavedovaSummaryThe upward Ground Penetrating Radar (upGPR) permits to monitor in time lapse the snow properties using a single antenna or an array of several antennas, disposed on the soil and radiating upward (on the snow). Water Content Reflectometer (WCR) is a special TDR (Time Domain Reflectometer) system to monitor the electrical permittivity of soil and snow. We discuss the effectiveness of an integrated monitoring using an up-GPR with an antenna of 1500 MHz and WCR sensor on the snow and into the ground.
The data acquisition refers to the winter season 2014–15, in a test site of interest for monitoring and prediction of avalanche phenonema in the Italian alps, at elevation of 2.100 m a.s.l.
GPR data permit to monitor the different phases of snow accumulation and layering of the snowpack during the season, while the analisys of WCR data allow us to estimate with good accuracy density and water content of the snow. Particularly, the uppermost layers are characterized by density in the range between 250–300 kg/m3, while at deeper level, density values are around 400–450 kg/m3.
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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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Effect of Low Velocity Zones on Seismic Hazard Assessment - The Case of Taygetos Fault
Authors E. Mouzakiotis, V.K. Karastathis and P. KarmisSummaryShallow layers with low shear wave velocities may heavily influence the amplification of strong ground motion in a specific site. Such zones are often not taken into account in calculations of seismic hazard for a region, resulting in underestimated values of calculated ground acceleration. This issue is of great significance in areas that are adjacent to large seismic zones.
We assessed the effect of low velocity layers in the calculation of the amplification of ground motion, as well as in the calculation of the maximum values of acceleration. Additionally we evaluated the ability of several near surface geophysical techniques in detecting velocity reversals. We tested the MASW, MÁLW and refraction tomography techniques in conjunction with crosshole tomography. We found that only the MASW near surface methodology was able to reliably detect velocity reversals. We also proved that by not taking into account such low velocity layers in the calculation of seismic hazard, the results could be underestimated up to 25%.
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The Sensitivity Analysis as a Non-uniqueness Indicator in the Inversion of Rayleigh-wave Dispersion Curve
Authors M. Senkaya and H. KarsliSummaryIn the Rayleigh-wave dispersion curve (RDC) inversion, layer thicknesses are typically fixed during the inversion process and may be estimated using available a priori knowledge. But, the RDC inversion process frequently suffers from non-uniqueness due to the use of erroneous or conjectural layer thicknesses. In this work, sensitivity analysis of S-wave and layer thickness is used to reveal the reliability and accuracy of the RDC inversion results in the non-uniqueness situation. For this purpose, a synthetic data is inverted with three different initial models through the same inversion parameters and their sensitivity patterns are compared. The sensitivity analyses demonstrate that the sensitivity pattern of layer S-wave velocity and thickness may present useful information about the quality of estimated parameters. In this way, the possibility of getting trapped in non-uniqueness may be decreased.
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ERT Characterisation of La Calderilla Volcanic Caldera (Gran Canaria Island, Spain)
Authors M. Himi, A. Rodriguez-Gonzalez, J.C. Tapias, F. Pérez-Torrado and A. CasasSummaryMaar are a singular type of volcanic structure generated by explosive phreatomagmatic eruptions as a result of interaction between rising magma and groundwater. This kind of structures are characterised by circular craters, often filled with water and/or lacustrine sediments and surrounded by a ring of pyroclastic deposits. The whole of the volcano, including the pyroclastic ring surrounding the crater maar is called maar-diatreme.
Recently borehole was drilled at the bottom of the volcanic crater which penetrated about 8.7 m in its sedimentary sequence and the paleobotanical study has supplied the first evidence of paleoenvironmental evolution during the Holocene on the Gran Canaria Island.
The possibility of its sedimentary fill extends beyond of the Holocene is extremely attractive, since, for example, and there are few paleoenvironmental data regarding how much the last glaciation that affected the Canary Islands. In these circumstances, the knowledge of the total thickness of the lacustrine sediments is crucial to design a deeper borehole in the next future. Therefore, the subsurface characterisation provided by geophysics is essential for determining thickness and geometry of the sedimentary filling.
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Outpost Method of Computing Leaking and Normal Modes for Elastic Multilayered Half-space
Authors P. Znak and B. KashtanSummaryDispersion curves analysis of guided waves leaking modes provide P-wave velocity information in addition to the common techniques based on the Rayleigh modes dispersion curves inversion. Accounting for the leaking modes requires a fully automated and precise method for complex roots of dispersion equation computing. We have developed an extension of the so-called outpost method. That allows numerical computation of all the leaking and normal modes presented in the full wavefield in any elastic layered half-space. The method is based on the leaking modes property to come to the vicinity of complex plane real axis with increasing the frequency from zero to infinity. In principle, the approach consists of two steps. Firstly, we search for the moments of leaking modes crossing some line in the complex plane called outpost. We suggest a particular form of the outpost line. It crosses the branch cuts and is closed on the Riemann surface, accounting for all the possible leaking modes arising up in the wavefield. The second step is an individual continuation of every leaking mode as a curve parameterized by frequency starting from the outpost line. We give algorithms for leaking modes curves tracing through their peculiarities, which provides full method automation.
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Analysis of Parameters Sensitivity in Seismic Full Waveform Inversion Applied to Shallow Subsurface
Authors A.S. Spadini, L.D. Diogo, O.B. Bokhonok and R.P. PradoSummaryA Full waveform algorithm is applied to shallow seismic reflection data using vertical and radial components. Using an elastic formulation the desired parameters were P and S wave velocities, densities and the depths of layers. A restriction of flat layers with constant parameters was applied. To solve FWI we evaluated the sensitivity and behaviour of the objective function with respect to the model parameters. The results reveal a good sensitivity for the velocity parameters above the semi space and the stochastic algorithm should be improved for the estimation of the P wave velocity. This research is in the initial stage and more tests will be performed considering different offsets windows and about the mapping of the objective function.
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Approach to 3D Seismic Data Interpretation for Drilling Geohazard Assessment
Authors Y.E. Terekhina, M.Y. Tokarev, A.S. Pirogova, A.G. Roslyakov, N.V. Shalaeva and A.A. SemenovaSummaryIn the present there is an active offshore oil & gas shelf fields development. The standard methods applied for investigation of near surface geological conditions for site survey goals are high resolution seismic (HRS) for top hole drilling hazard assessment and ultrahigh resolution seismic (UHRS) mutually used with hydrographic surveying for sub-bottom geohazard assessment. On the other hand, application of 3D seismic technology for site survey investigations requires improvements both in data processing and interpretation. This paper focuses on the generalization of experience in the geohazards assessment in offshore fields based on the 3D seismic data. Involving of modern interpretation procedures standard for prospecting and exploration: inversion, wavelet analysis, pore pressure prediction based on spectral time analysis, Poisson ratio evaluation can significantly improve the accuracy and efficiency of 3D seismic data interpretation for site survey investigation. But still remain unsolved problems related to resolution of sub-bottom part, which caused by insufficient sources frequency range and absence of near offsets.
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Near-surface Voids in the Neapolitan Volcanic Tuff (Italy) Detected by Seismic Refraction Tomography
Authors S. Maraio and P.P.G. BrunoSummaryThe Neapolitan area (Italy) is affected by the presence of shallow man-made cavities in volcanic tuffs that were used in the past as building material. Therefore, the identification of these voids is a fundamental prerequisite for the minimization of subsurface collapse risks. In the past few years, several authors have shown that the geophysics prospecting methods can provide a most convenient solution to detecting underground voids. In this work we show how the seismic refraction tomography represents a powerful tool for the detection of near-surface tuff-cavities located in Casamarciano, Naples. The interpolation of the velocity models, allowed us to build a three-dimensional model of the tuff basement top and to show the lateral extend of the voids. The cavity location detected by seismic refraction tomography has been subsequently confirmed by four core-drilling made in the area.
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On the Stationarity of the Horizontal to Vertical Noise Spectral Ratio
Authors A. D’Alessandro, R. Martorana, P. Capizzi and D. LuzioSummaryThe Horizontal to Vertical Noise Spectral Ratio (HVNSR) method is nowadays widely used to estimate the resonance frequencies of geological structures. In the HVNSR method, seismic noise is considered as a stationary stochastic process. However, especially in industrialized/urbanized area, this is a very strict assumption seldom occurred. Several sources of noise can generate non stationary and anisotropic microtremor fields. To investigate the stationarity of microtremor, we have carried out several long-term measures of seismic noise with broad-band seismic sensors, in areas where the main source of anthropogenic noise is well known. The signals acquired have been analyzed both in frequency and in time domain. Our analysis have showed as the shape of the HVNSR curves can be strongly conditioned by the presence of near anthropogenic sources of noise. Useful information can be derived by the analysis of the data spectral density and by checking the azimuthal dependence of the HVNSR. The first one allows to easy identify different sources of seismic noise while the second one, in particular its time variability, is a useful instrument to evaluate the reliability of the data to estimate resonance frequencies of geological structures.
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First Attempt to Characterize Neoproterozoic Stromatolites at Chapada Diamantina (Brazil) by Geophysical Methods
SummarySince microbialites can be important reservoir rock, it is important to understand the paleoenvironmental conditions that favour the formation. Nevertheless, few studies have provided a detailed understanding of the strong lateral heterogeneity of these reservoirs and their complex development using near surface geophysical methods. The aim of this research is elucidate the internal architecture of a well exposed colonial stromatolite bioherms located at the Chapada Diamantina basin NE Brazil. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) sections were acquired in order to parameterize their geophysical signatures.
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Characterization GPR and FDTD Modeling of Asphaltic Sandstones of the Paraná Basin, Brazil - Preliminary Results
Authors L. Bertolla, J.L. Porsani and V.R. ElisSummaryThis paper shows the preliminary results of GPR profiles and numerical simulation by the finite difference method (FDTD) of asphaltic sandstones in the municipality of Anhembi, East Central study of the state of São Paulo, Brazil. Situated on the edge of the Paraná Basin, the study area has been the subject of great interest by Petrobrás (Brazilian Oil Exploration Company), that due to the existence of bitumen impregnated sandstones. With the results of the GPR profiles was observed shadow zones that could be simulated by FDTD modeling, although preliminary results suggest the presence of bacterial action, detailed studies are underway to test the hypothesis biodegradation.
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Introducing Particle Swarm Optimization to Invert Multimode Rayleigh Wave Dispersion Curve
Authors A. Zarean, R. Pourmirzaee and S. ChaychizadehSummaryRayleigh waves have been used increasingly as an attractive tool to obtain near-surface shear wave velocity profiles. Inversion of Rayleigh wave dispersion curves is demanding for most local-search methods due to its high nonlinearity and to its multimodality. Among many approaches which have been proposed for surface wave inversion so far, Intelligence Swarm algorithms effectively applied to solve it, and attempt to avoid trapping in local minima. In this paper, we proposed a new approach (Particle Swarm Optimization PSO) to overcome above disadvantages. To evaluate calculation efficiency and stability of PSO to inversion of surface wave data, we first inverted a broadband (1–30 Hz) synthetic dispersion curves from a synthetic model. After that we added first high mode to improve non- uniqueness of models. Results from all synthetic data demonstrate that PSO can be used for quantitative analysis of Rayleigh wave dispersion curves.
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Basic Role of the Electrical Tomography in the Exploration of Building Stones
Authors F. Šumanovac, S. Kolar and J. OreškovicSummaryIn the framework of exploration of building stones electrical tomography is an essential method. It is particularly effective in three types of geological models: boundary between carbonates and clastic rocks (1), boundary between rather compact igneous rocks and clastites (2) and fault and fracture zones in hard rocks (3). These are the most common models in surveying of building stones and great potentials of electrical tomography are presented on examples of two quarry explorations.
Model 1 and model 3 are shown on Vojnovec quarry where electrical tomography enabled precise definition of boundaries between carbonates and clastic rocks, as well as determination of margins of fractured zone within carbonates. Model 2 is presented by Hruškovec quarry where diabase bodies are in contact with clastites. Since high resistivities similar to the resistivity of diabase, also may be caused by dry sandstones magnetic method has been applied to distinguish between rocks of similar resistivity but different magnetic properties. In one case magnetic method was helpful in determination of the edge of diabase bodies, whereas in the second case the edge could not be determined due to changes in magnetic properties of diabases. Electrical tomography, in both cases, enabled determination of reliable two-dimensional model.
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A Comparison of DCIP Inversion Software
Authors M. Wennermark, P.I. Olsson, S. Johansson and K. HellmanSummaryElectrical resistivity tomography (ERT) is a commonly used geophysical method, which can conveniently be combined with measuring time-domain induced polarization. The combined measurement can be termed DCIP, and can be employed in a wide range of situations such as pre-investigations for construction, or contaminant monitoring to name a few. An integral part of a DCIP survey is the software used for processing and interpretation. There exists many such software packages. We compare the ubiquitous, commercial software, Res2DInv and two other common alternatives; the semi-academic AarhusInv and the academic, open source, BERT/GIMLi. The comparison is done by simulating measurements of resistivity and IP with a multiple gradient protocol over a model that represents a waste pond setting. The results show that there are quite a few similarities between the different softwares, but also some notable differences that one should be aware of during interpretation. This work is the first step towards an exhaustive comparison.
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Impact of Time-domain IP Pulse Length on Measured Data and Inverted Models
Authors P.-I. Olsson, G. Fiandaca, T. Dahlin and E. AukenSummaryThe duration of time domain (TD) induced polarization (IP) current injections has significant impact on the acquired IP data as well as on the inversion models, if the standard evaluation procedure is followed. However, it is still possible to retrieve similar inversion models if the waveform of the injected current and the IP response waveform are included in the inversion. The on-time also generally affects the signal-to-noise ratio (SNR) where an increased on-time gives higher SNR for the IP data.
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Identification of Possible Weakness Zones in Limestone Bedrock by Resistivity and Induced Polarization
Authors S. Johansson, C. Sparrenbom and T. DahlinSummaryIn connection to an environmental investigation of a former industrial site, the cretaceous sandy limestone bedrock in the surrounding area were mapped with resistivity and time domain induced polarization (DCIP). The aim of the DCIP survey was to delineate geological units and possible preferential pathways of DNAPL contaminated groundwater. Although IP anomalies corresponding to fractured crystalline bedrock have been observed previously and are commonly interpreted as results of clay weathering, there are not many studies of the IP behavior of limestone in previous research. In contrast to silicates, the weathering of calcite minerals in pure limestone results only in dissolution, without any weathering products in form of minerals. In this study, varying resistivity values and large IP anomalies were found in the sandy limestone bedrock, which are probable indications of weakness zones. The observed strong IP anomalies may be caused by calcite dissolution and precipitation processes or by clay mineral precipitation from chemical weathering of the silica grains contained in the sandy limestone. Another possibility may be sporadic presence of glauconitic sand in the limestone. More research is needed to confirm the sources of the observed anomalies.
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Diffraction Imaging for Edge Detection
Authors F. Alonaizi and R. PevznerSummaryIn this work a robust imaging algorithm based on detecting edge diffraction has been developed. The algorithm is based on measuring the coherency of the data using the semblance function computed along the traveltime curve of the diffracted wave. Based on the fact that waveforms on two sides of edge diffraction have opposite polarity, the proposed method exploit the phase-reversal phenomenon of edge diffractions in the computation process and use it as a criterion to separate edge diffractions from specular reflections. Utilizing the phase-reversal property of edge diffraction allows the detection of small features in the subsurface, and image hidden diffracted waves that could be masked by strong reflections. Two examples are provided to illustrate the validity of the method, the first example is a physical model of an edge diffractor, and the second example is real GPR data acquired over a lake.
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Imaging and Tomographic Interpretations of Seismic Refraction Data for Near Surface Basement Assessment in Somaliland
Authors R. De Franco, G. Caielli, A. Morrone and B. PetrucciSummaryWe present a comparison between the results of the processing of seismic profiles acquired in Somaliland, near the town of Boroma, in the frame of an UNESCO project for groundwater research. The main objective of the work was to support the siting of drilling water wells. The aim of geophysical survey was the identification of the geophysical parameters of the basement and overlying limestone unit.
The data processing was performed with a classical approach based on the tomographic inversion of P first arrivals. Whereas the picking of first arrival times is sometimes difficult, we present the application of the multi refractor imaging technique (MRI) based on the stack of refraction convolution sections of the refracted signals.
The results show that MRI is a suitable technique to processing and interpret the refracted data and it allows to obtain an accurate image of refracting interfaces. Further it complements and integrates the interpretation of the results obtained with tomographic approach also in presence of complex structures.
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Detection of Underground Hollow Objects Using Seismoacoustic Noise (On the Example of Buried Pipes Physical Model)
Authors Yu.I. Kolesnikov and K.V. FedinSummaryThe possibility of underground hollow objects detection using passive seismoacoustic measurements is demonstrated on the example of buried pipes physical model. The proposed method is based on the extraction of standing waves that can be generated by seismoacoustic noise in the space between the day surface and the hollow object. The experimental results showed that this method make it possible to detect a hollow objects at depths up to 5–8 their smallest horizontal sizes.
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Fully Non-linear Self-organizing Inversion of Cross-borehole Tomographic Data
By H. PaascheSummaryGeophysical model reconstruction by data inversion is usually ill-posed and suffers ambiguity due to limited number and accuracy of the available observations. Here, we formulate a new concept for an almost completely data-driven fully-nonlinear inversion to achieve an ensemble of different tomographic models fitting the underlying data set equally well. Since we parameterize the model reconstruction area using regular grids with grid cell dimensions not exceeding the spatial resolution of the tomographic data set we can find models of any reasonable complexity. To avoid the finding of models characterized by numerous anomalies below the spatial resolution limit of the data set we concurrently address a data misfit objective and a model structure objective formulated in fuzzy domain. The final ensemble of models addresses the data objective equally well but the model structure objective only to different degrees and allows for the finding of model ensembles illustrating the tomographic reconstruction ambiguity reasonably.
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Staggered Grids as a Tool to Improve the Model Resolution in Seismic Tomography for Elliptical Anisotropic Media
Authors E. Cardarelli and A. CerretoSummaryIn this study we present the use of staggered grid as a tool to improve the resolution of inverse model in seismic tomography in the case of elliptical anisotropic media. We analyze two synthetic models, both showing elliptical anisotropic characteristics, with different complexity: the first consists of a body with rectangular shape having inside an anomaly with higher velocity and the second one is characterized by morphological variation to simulate a watershed. This second case is studied for both poor and high ray coverage configuration. Successively, a previous field survey is analyzed introducing staggered grids and the results are compared with the old ones, solved using a constant cells’ size model. Staggered grid give enhanced results in terms of the velocity model and can be used as a tool to improve the knowledge of the studied domain by determining cells with variable shape and size, useful to infer a good initial model for a further inversion. This conclusion applies also to isotropic media.
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