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Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 14-18, 2014
- Location: Athens, Greece
- Published: 08 September 2014
1 - 100 of 185 results
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Enhancement of SRT And ERT Interpretations Using Time-lapse Measurements and Cross-plot Analysis
More LessSummaryAccording to the 2013 ASCE report card for America’s infrastructure, dams and levees in the U.S. are in a poor to fair condition and exhibit signs of deterioration with strong risk of failure.
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Nuclear Magnetic Resonance (NMR) Logging -Lessons Learned at the USGS Cape Cod Toxic Substances Hydrology Research Site, Massachusetts
Authors C.D. Johnson, J. Sorenson, D.R. LeBlanc and J.W. LaneSummaryOver the last 30 years, advancements have been made in developing borehole geophysical tools and methods for determining aquifer hydraulic properties that control solute transport. More recent advances in technology allow for nuclear magnetic resonance (NMR) logging of small-diameter boreholes typical of environmental studies. Advantages of the NMR logging method are that (1) NMR does not use active nuclear sources to determine water content, and (2) NMR provides depth-dependent estimates of pore-size distribution that allow for the distinction between bound- and mobile-water fractions, which is not possible with active-source logs.
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Automatic Generation of Groundwater Model Hydrostratigraphy from AEM Resistivity and Boreholes
Authors P.A. Marker, N. Foged, A.V. Christiansen, E. Auken and P. Bauer-GottweinSummaryRegional hydrological models are important tools in water resources management. Model prediction uncertainty is primarily due to structural (geological) non-uniqueness which makes sampling of the structural model space necessary to estimate prediction uncertainties. Geological structures and heterogeneity, which spatially scarce borehole lithology data may overlook, are well resolved in AEM surveys. This study presents a semi-automatic sequential hydrogeophysical inversion method for the integration of AEM and borehole data into regional groundwater models in sedimentary areas, where sand/ clay distribution govern groundwater flow. The coupling between hydrological and geophysical parameters is managed using a translator function with spatially variable parameters followed by a 3D zonation. The translator function translates geophysical resistivities into clay fractions and is calibrated with observed lithological data. Principal components are computed for the translated clay fractions and geophysical resistivities. Zonation is carried out by k-means clustering on the principal components. The hydraulic parameters of the zones are determined in a hydrological model calibration using head and discharge observations. The method was applied to field data collected at a Danish field site. Our results show that a competitive hydrological model can be constructed from the AEM dataset using the automatic procedure outlined above.
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Inverting for Lithology Using Resistivity Models and Boreholes
Authors A.V. Christiansen, N. Foged and E. AukenSUMMARYWe present an automatic method for parameterization of a 3D model of the subsurface, integrating lithological information from boreholes with resistivity models through an inverse optimization, with the objective of further detailing for geological models or as direct input to groundwater models. The parameter of interest is the clay fraction, expressed as the relative length of clay-units in a depth interval. The clay fraction is obtained from lithological logs and the clay fraction from the resistivity is obtained by establishing a simple petrophysical relationship, a translator function, between resistivity and the clay fraction. Through inversion we use the lithological data and the resistivity data to determine the optimum spatially distributed translator function. Applying the translator function we get a 3D clay fraction model, which holds information from the resistivity dataset and the borehole dataset in one variable. We apply the concept to the Norsminde survey in Denmark integrating approximately 700 boreholes and more than 100,000 resistivity models from an airborne survey in the parameterization of the 3D model covering 156 km2. The final 3D model differentiates between clay materials and different high resistive materials from information held in resistivity model and borehole observations respectively
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Assessing Soil Wetness with Airborne Radiometric Data
By D. BeamishSUMMARYA valid interpretation model for UK radiometric data requires a joint assessment of both soil and bedrock variations. Although the geological bedrock (the parent material) provides a specific radiogenic level with associated radiochemical attributes, attenuation of the signal level is controlled by soil wetness in conjunction with the density and porosity of the soil cover. Peat soils, in particular, produce readily identifiable attenuation zones. Other soil types are also predicted to attenuate radiometric signal levels but at lower wetness sensitivities. The amount of water stored in the soil is of fundamental importance to agriculture
Studies of soil attenuation zones have now been undertaken using a number of modern UK radiometric data sets. Case studies are presented which highlight the procedures and results obtained. A variety of implied increases in wetness, at the scale of the airborne measurements, have been identified across a range of soil types, including those categorized as freely draining. The level of attenuation (and hence wetness scale) remains uncalibrated since ground information on wetness is not generally available at the appropriate scal
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Salt Water Intrusion Imaging at Tybaki (Greece) Using Geophysical Methods
SUMMARYA geophysical survey was conducted at Tybaki, Crete, Greece in order to support the geological and hydrogeological studies of the existing phenomenon of seawater intrusion. This survey employed seismic refraction, transient electromagnetic (TEM) and geoelectrical methods. The later, consisted of Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT). The objective of this survey was to image the saltwater intrusion at this area and map possible favorable sea water pathways. We present the combined interpretation of vintage geophysical data obtained under the research project MEDIS and the ClearWater project data. At Tybaki, the geophysical survey provided images of the saltwater intrusion which is present at the north west of the basin.
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Geophysics for the Determination of Hydrological Parameters of Karst Systems in Yucatan, Mexico
Authors S. Gruber, R. Supper, A. Schiller, D. Ottowitz, J.H. Kim, A. Maqueda Estrada, M. Hendrick and P. RenardSUMMARYIn the karstic system of Yucatan, airborne geophysics as well as ground geophysics and groundwater levelling, which were performed between 2006 and 2013, led to the generation of a general groundwater model of the area between the town of Tulum and the biosphere reserve of Sian Ka’an. However, for calibration of the model, significant parameters, e.g. the geometry of the cave system, estimates on flow velocity, as well as knowledge on the hydrological properties of the karst matrix are still missing, a necessity to derive reliable modelling results. Therefore, within a small part of the conduit system, the subsurface structure was determined in detail, applying surface geoelectrical measurements of different configurations, hole-to-surface measurements, borehole logging, drilling and tracer tests. Additionally, a low cost method based on laser scanning was developed and tested to derive a 3D model of the cave shape, which was used for calibration purpose. Results coming from different methods are compared and interpreted to derive advanced knowledge about hydrological parameters of the karst system of Yucatan.
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Complex Electrical Properties of Sand-clay Mixtures and Alluvial Sediment Samples
Authors S. Inzoli, M. Giudici, M. Mele and E. RondoliniSUMMARYThe interpretation uncertainty of DC-resistivity surveys is reduced applying Spectral Induced Polarization methods. The aim of this work is to compare the complex electric resistivity behaviour, in the frequency range from 10 mHz to 100 kHz, of saturated sand-clay mixtures prepared in laboratory and alluvial muddy sands or sandy mud samples extracted from Po plain quarries (Italy). The comparison is made on the basis of the coarse-to-fine granulometric fraction ratio, defined with a cut-off diameter of 0.063 mm. Electrical analyses are conducted within a cylindrical sample-holder equipped with impolarizable potential electrodes and results are discussed in terms of magnitude and phase spectra. Large differences in the phase spectra are observed. They are interpreted in terms of mineralogical composition, especially of the fine-grained fraction, and organic matter content. In fact, the effect of water electrical conductivity on these samples is already showed by a series of systematic tests and the textural assemblage is controlled. To evaluate the contribution of sediments’ geochemical properties, data fitting through the Cole-Cole model is proposed with satisfactorily results. Since phase spectrum is quite sensitive to the fitting parameters, also a multi-objective approach that consider separately the fit of the magnitude and phase spectra has been tested.
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Fast Mapping and Analysis of Anthropogenic Noise Sources Affecting MRS Experiments
Authors J.J. Larsen, E. Dalgaard, P. Christiansen and E. AukenSummaryThe applicability of the magnetic resonance sounding (MRS) technique in urbanized areas is limited by the inherently weak signal from the subsurface water which is often deeply buried in noise from anthropogenic sources. The noise can have spatially variations on even sub-100 m scales and the signal to noise ratio of an MRS measurement can therefore be optimized by a careful choice of the exact location. In this paper we present an instrument for fast mapping and analysis of the noise conditions at a site prior to MRS experiments. The main components of the instrument are two induction receiver coils connected to amplifiers and a digital oscilloscope controlled by a PC. Analysis of the gathered data reveals the spatial distribution of the different noise components, in particular powerline harmonics and spikes. A comprehensive noise map of a test site is presented and analyzed.
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Geoelectrical Monitoring of Complex Hydrological Processes in a Riparian Wetland
Authors S.S. Uhlemann, J.P.R. Sorensen, J.E. Chambers, P.B. Wilkinson and D.C. GooddySummaryWetlands or groundwater dependent ecosystems in general provide vital habitats for diverse aquatic and terrestrial flora and fauna. Such systems usually show a highly complex hydrological regime and are very sensitive to environmental changes. Thus non-invasive methods have to be used to investigate those processes. This paper presents the application of 2D geoelectrical monitoring to such environments which is, due to its high sensitivity to changes in moisture content and pore water resistivity, aiding in improving our hydrological understanding of these systems. After correcting the resistivity data for the seasonal temperature variations, our results highlight the need to divide the alluvium into two hydrological layers showing different characteristics. While the uppermost layer shows significant responses from biogeochemical cycling, with decreasing resistivities during spring and summer, the lower part of the alluvium shows increasing resistivities due to upwelling of more resistive pore waters from the underlying gravels. These processes, and thereby also the resistivity changes, were proven by sensors at different locations and depths, showing the same results, but with higher accuracy and sampling rate.
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GPR Tomography Feasibility Study in the Process of Air-filled Voids Search in Engineering Structures
Authors M.S. Sudakova, A.Y. Kalashnikov and E.B. TerentievaSUMMARYNon-destructive testing of buildings and structures appears acutely on the agenda. The load-bearing capacity of structures apart from the concrete strength depends on the defects of the internal structure. These defects can be caused by voids of various size and origin. The GPR is known as one of the nondestructive methods of internal structure study. In some cases the conventional GPR sounding doesn’t allow solving the problem of voids’ detection. The GPR tomography can afford a better solution, eliminating the disadvantages of the sounding and offering a quantitative result: the electromagnetic waves’ velocity and amplitude values. The present paper focuses on the analysis of GPR tomography potential aimed at the search of voids of various dimensions by the example of isometric concrete structure (a construction or a bridge pillar). Mathematic modeling and in situ measurements on a pillar with a known structure, containing a void inside were carried out in order to attain the claimed objective. The paper demonstrates that the voids within the range of two wavelengths (20sm) can be successfully delineated, regardless of their position, on the basis of the following indicators: ray bending at the boundary of the void and velocity increase by more than 30%.
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Combined Analysis of Ambient Vibration HVSR and Active MASW Method
Authors F.M. Gouveia, I.F. Lopes and R.C. GomesSummaryThe well-known non-uniqueness inversion problems of dispersion curves as well as the reduced resolution at higher depths could be improved by performing a combined analysis of data sets of different types. In this study, Nakamura’s technique or HVSR technique was applied to several ambient vibration recordings performed at the surface of alluvium deposits, in order to estimate their fundamental frequency. The shear wave velocity profiles of the soil of the studied sites were previously determined using the active MASW method. The theoretical HVSR associated to each MASW profile was determined using the program ModelHVSR and compared to the experimental HVSR curves. This program was also used for the inversion of the experimental H/V curve, in order to improve the resolution of the deepest part of soil profile and identify the depth of the bedrock. Furthermore, the HVSR deduced from surface waves approximation was also computed for each profile to evaluate the reliability of the hypothesis that surface waves dominates the wavefield.
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Different Processing and Inversion Methods for Resolving Vs Profiles in Engineering Geophysics Using Surface Wave Data
Authors A. Savvaidis, G. Kritikakis, I. Papadopoulos, N. Papadopoulos, U. Dikmen, M.D. Mangriotis, P. Soupios and A. VafidisSummaryRecent advances in Seismic Codes increased the necessity in Engineering Geophysics to use surface wave analysis as a tool.
The shear wave velocity profile had to be resolved to calculate the site amplification in case of an earthquake event, necessary also for seismic hazard studies. In our approach we present the results from Surface Wave data acquired in different site in Crete Island (Greece) at strong motion sites. Passive and Active data at linear and circular geometries have been analyzed and the corresponding processing and inversion results are compared. Although one would comment that there are discrepancies on Dispersion Curve results the resulted ground profiles up to a shallow depth show considerable agreement. However, this is site dependent and in order to increase the accuracy and reliability of results one should be careful with the assignment of different modes in surface wave analysis and also combine the results provided.
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Integration of Magnetics and GPR Methods to Map Buried Tubes of a Pipeline
Authors M.A. Barsottelli-Botelho, A.R. A.S. Meneses and L.A. MesquitaSummaryIt is tested the magnetic method efficiency to detect and map buried tubes in a pipeline and also one second procedure which integrates the magnetic method with the electromagnetic method of Ground Penetrating Radar (GPR). Both geophysical methods acquire data on saturated and not saturated soils, the research uses areas where the pipeline crosses one river. The data acquisition with both geophysical methods uses profiles perpendicular to the pipelines situated at two areas, one at São Paulo and other at Bahia states, Brazil. The definition of pipe position on magnetic data map uses forward modelling in order to establish patterns of signatures of magnetic anomaly and its derivatives. The modelling illustrates approximately the response of pipes in terms of magnetic dipoles associated to cylindrical and spherical bodies. The GPR is very effective to locate the pipes and it is also able to inform the depth of the buried pipes but at saturated soils, as the ones close to the rivers, the GPR usually decreases its ability to detect the tubes and the use magnetic methods became more important. The combination of GPR with the magnetic method increase the success of mapping buried tubes of pipelines.
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Tunnel Pre-investigation - Towards a Resistivity to Rock-quality Transform
Authors S. Bazin, A.A. Pfaffhuber, H. Anschütz, G. Grøneng and C. ChristensenSummaryThe aim of our study is to assess whether resistivity may be transformable into an engineering parameter such as the Q-value and can substitute seismic investigation when not practicable. ERT has the advantage of being efficient, reliable, silent and non-destructive, therefore suitable in urban areas.
We have integrated Q-values derived from core drillings and geological logging during tunnelling and resistivity derived from surface ERT on two tunnel projects in Norway. In both cases, it is qualitatively evident that lower resistivity points towards weaker rock. Indeed, preliminary results can partially show the expected correlation where resistivity data co-align with boreholes or the tunnel.
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Geophysical Methods for the Assessment of the Ejection of Sand Due to the Liquefaction in the Mirabello Area
Authors L. Baradello, F. Accaino, S. Pepi, C. Vaccaro and D. NietoSummaryThe principal effects of the earthquake (ML 5.9) happened at May 20th, 2012 in Emilia Romagna (Northern region of Italy) were the ruptures and the ejection of sand due to the liquefaction of sand layers present in the subsurface. This study has pointed out the ability of the geophysical high resolution techniques to map the shallow subsurface extent of both fractures and, under certain conditions, the liquefied sand bodies too. In fact while Ground Penetrating Radar investigations detected the fractures in the upper part, seismic reflection profiling gets the possibility to trace back these fractures at greater depths. The results achieved in a test site, a paleo-river bed of the Reno river in Mirabello, permit geologists to obtain proper characterization of the subsurface of the damaged buildings and engineers to aid in formulating appropriate site-specific solutions.
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Detection of Water Caused Channels and Voids in Loos Soils
Authors E. Toros, Z. Pronay, P. Tildy and B. NeduczaSummary2010 was a wet year in Hungary. The heavy rains caused serious damages, especially in loose soils. Two case studies are introduced, how geophysical investigations are supplementing the geotechnical studies to detect water damages. In the first case the influence of successive rainwater on loess area was investigated, secondly the geological structures susceptible to river jetting were looked for alongside the River Danube. The research tool primarily was GPR to look for near surface water caused channels and sinkholes and carefully planned 3D resistivity measurement was used to find sandy aquiferous structures on water jet area.
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Quarry Blasts Assessment and Their Environmental Impacts on the Nearby Oil Pipelines
By A. MohamedSummaryAbstract
Ground vibrations induced by blasting in the cement quarries are one of the fundamental problems in the quarrying industry and may cause severe damage to the nearby utilities and pipelines. This research paper presents the results of ground vibration measurements induced by blasting at the National Cement Company (NCC) southeast of Helwan city, Egypt. The aim of this study is to investigate the influence of the quarry blasts on the oil pipelines of SUMED Company. The seismic refraction for compressional waves and the shear wave velocity obtained from the Multi channel Analysis of Surface Waves (MASW) are used to evaluate the closest site of the two pipelines to the quarry blasts. The results demonstrate that, the closest site of the two pipelines is of class B, according to the International Building Code (IBC) and the safe distance to avoid any environmental effects is 650 m, following the deduced Peak Particle Velocity (PPV) and Scaled Distance (SD) relationship (PPV = 440.64 × SD-1.225) in mm/s and the Air over Pressure (Air Blast) formula (Air Blast = 152.87 × SD-0.071) in dB. In the light of the prediction analysis.
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Detection of Underground Cavities and Weak Zones from Multi Channel Analysis of Surface Waves
By F.A.I. HamdiSummaryThe Multichannel analysis of ground roll dispersion data was shown to be a very effective and adaptable non-destructive tool for the detection of adverse strata and cavities within the underground of the sites for engineering projects. The two examples, which are selected, cover some of the applications of the technique in small-scale surveys to define ground conditions, which may affect civil engineering construction works.
The technique should be allied with the results of calibration boreholes drilled at selected locations along executed seismic profiles. Using this information hazardous ground conditions can be accurately described delineated and where necessary, successfully treated.
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Non-linear Ground Motion Simulation at the Sites of Atalanti Gulf Area for the Case of Real and Hypothetical Earthquakes
Authors T.I Novikova and V.K. KarastathisSummaryWe perform numerical simulation of the ground motion and liquefaction at several sites located at the coastal area of Atalanti Gulf (Greece). To incorporate region-specific geologic effects in the ground motion and liquefaction hazard estimates, subsurface information, particularly Vs and dynamic properties, are compiled and evaluated with the aid of geophysical methodologies and borehole testing respectively.
The possible non-linear soil behavior at the investigated sites is examined for three selected earthquakes. In two cases, real records, obtained from the NOA database, are used as input motion. In third case the input motion is computed using a stochastic simulation technique, for the scenario earthquake of the 1894 Atalanti main shock with M=7.0.
The results of the modelling show that in the case of an earthquake with magnitude Mw=5.4 all sites amplify the input signal to some degree. In case of Mw=6.7 earthquake almost all investigated sites will exhibit liquefaction initiation. An exception is two sites: one, where strong amplification of input motion by the shallow soft layers is expected; another one, located very close to the eastern segment of Atalanti fault, where obtained surface motion is not significantly changed through the structure. The Atalanti earthquake produces both effects liquefaction and amplification.
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Analysis of Ground Motion Parameters from the 2011 Earthquake in Lorca (SE Iberian Peninsula) Based on Seismic Data
Authors J. Pérez-Cuevas, P. Martínez-Pagán, M. Navarro and A. García-JerezSummaryIn the present work we study how the ground motion parameters had a relevant influence on the buildings structural damages due to the 2011 earthquake in Lorca city. We quantified the severity of subsurface motion through the estimation of peak ground acceleration (PGA), peak ground velocity (PGV), and Arias Intensity (AI) at bedrock level. Those values were obtained by means of S-wave velocity data from Multichannel Analysis Surface Waves (MASW) method which was conducted along the streets of Lorca city. Remarkably, PGA and PGV maps present high values in those urban areas which reported severe building damages. Besides, the highest PGA, PGV and AI values are located in the easternmost area of the city. In fact, in that area was reported significant building damages by the 2011 earthquake. These ground motion maps add some insights into both the interaction soil-building and the type of structural damages associated to buildings during the occurrence of an earthquake.
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Combined Geophysical Survey for the Detection of an Archaeological Theatre in Amphiareion of Oropos, Attica, Greece
Authors G. Apostolopoulos, G. Amolochitis, S. Stamataki, A. Giannopoulos and P. KofakisSummaryThe Amphiareion of Oropos, a sanctuary dedicated in the late 5th century BCE to the hero Amphiaraos, among the ancient monuments, a theatre not whole been revealed by excavation is to be detected by geophysical investigation. Electromagnetic (EM) measurements with conductivity meter in a certain field design but most important by a suggested processing and interpretation have outlined the area of theatre seats that have been covered by loose sediments. In addition other probable features been detected by EM indicate the position of subsequent ERT profiles. The later show in detail all features of the underground proving what EM has indicated along with other very useful information for the archaeologists in order to design their further research.
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Investigating Pre-Columbian Ceremonial Features at El Caño Archaeological Site, Panama, through Geophysical Surveys
Authors A. Mojica, M. Lezcano, J. González, J. Thiesson, L. Pastor, C. Camerlynck, N. Florsch, A. Tabbagh and R. VanhoeserlandeSummaryBetween 2005 and 2006, a series of magnetic and electrical surveys were conducted over the site El Caño, one of the most important pre-Columbian ceremonial site of the Isthmus of Panama (Central America). This site, of 3.4 ha in area presents a set of alignment of columns of carved basalts and tuff and mounds, with a first organized occupation between 100 and 400 BC. Today, these surveys have been extended to electromagnetic prospecting with the aim of identify with accurately the spatiotemporal organization of the site, funeral archaeological features of the mounds and surrounding. Integration and interpretation of the geophysical data allowed us to identify a set of circular magnetic anomalies in the base of mounds due to the strong runoff during the rainy season and topography, but in the base the runoff is weak (by flatness) and infiltration is higher; these results explain the difference between the calculated resistivity values obtained inside and outside of these mounds through electrical resistivity tomographies. Another larger circular magnetic anomaly was identified in the flatness southeast zone of the mounds; this result allowed the discovery of one of the most important pre-Columbian funerary complex in Panama.
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Three-dimensional Ground-penetrating Radar and Magnetic-gradient Surveying of the Roman Castle Irgenhausen (Switzerland)
Authors C. Schmelzbach, H. Horstmeyer, K. Scanlan, M. Kälin, S. Brändli and P. NagySummaryCombined three-dimensional (3-D) ground-penetrating radar (GPR) and magnetic-gradient (vertical gradient of the total magnetic field) surveying were used to investigate the remains of an older Roman villa complex buried under a surface-exposed younger Roman castle. Magnetic-gradient measurements allowed for efficient mapping of a large area around the castle in short time, whereas the 3-D GPR provided detailed information at depth of a selected area. The 3-D GPR data volume revealed several linear and rectangular features within the topmost around 2 m below the surface interpreted as walls and floors of houses belonging to the villa complex. Some of the identified walls could be correlated with the remains of a house with floor heating excavated and re-buried in the early 20th century. Furthermore, the joint interpretation of the 3-D GPR and magnetic-gradient data allowed identifying an extensive and rectangular, highly reflective feature in the GPR data collocated with a prominent magnetic anomaly as the fire place/oven used to drive the floor heating.
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Joint Analysis of Self Potential and Electrical Resistivity Tomography Data for Studying Alexandrovsky Settlement
Authors V.A. Shevnin, A.A. Bobachev, S.V. Ivanova and K.I. BaranchukSummarySelf potential (SP) method was used to study archaeological Alexandrovsky settlement, filtration anomaly and diffusion-absorption (DA) anomaly. Electrical resistivity tomography (ERT) data allowed estimating layered structure, lithology of layers and helped to prove DA origin of SP anomaly. Quantitative interpretation of filtration anomaly gave derelict spring yield estimation that could be used for water supply of the settlement in old days.
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ERT and VLF Measurements Contributing to the Extended Revelation of the Ancient Town of Trapezous (Peloponnesus, Greece)
Authors J.D. Alexopoulos, S. Dilalos, A. Tsatsaris and S. MavroulisSummaryA geophysical survey was carried out in the archaeological site of Kyparissia (Megalopolis, Greece) applying the Very Low Frequency technique, as preliminary combined with highly detailed Electrical Resistivity Tomography. Settlements of the ancient city of Trapezous (limestone slabs 0.5m height × 0.5m width) have already been uncovered from the local Ephorate of Prehistorical and Classical Antiquities, revealing a regularly planned town of the classical period. The geophysical research was performed at the non-excavated area of the plain, vicinal to the already exposed remaining, where, according to the expected geometry, the buried walls and drainage channel should be continued to. Fifteen parallel sections with distance 5m and spacing interval 1m were carried out. The processing with the application of Fraser and Karous-Hjelt filters led to the construction of respective maps, indicating some resistive lineation which could be a result of the buried settlements. Moreover, two detailed ERT sections were carried out with spacing 0.20 and 0.25m and total length 200m. The processing using the robust inversion, which is indicated for such environments, adumbrates quite clearly a sharp resistive target that could be the limestone slabs of the ancient walls or drainage channels, confirming the expected lineated geometry of the town.
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Detecting Shallow Medieval Features in the Church of St. George, Slovakia
Authors J. Panisova, J. Haličková, P. Brunčák, R. Pašteka, V. Pohánka, J. Papčo and P. MiloSummaryIn the framework of OPIS-Operational Programme Information Society co-funded by the European Regional Development Fund (ERDF) the most important monuments of Slovak cultural heritage are recorded using modern geodetic techniques. More than two hundreds objects, e.g. historical bridges, cemeteries, sacral buildings, folk architecture reservations, rock-cut dwellings, have been documented so far. The Church of St. George was recorded using laser scanning combined with very precise geodetic measurements. We present here the results of microgravity and GPR surveys carried out in the nave of this church in order to delineate the position of burial vaults beneath the floor. In microgravity data processing we focused on the calculation of the building correction. For this purpose a detailed polyhedral model of the church was created. Integrated interpretation of geophysical results has confirmed the presence of two medieval crypts predicted from historical archives. Moreover, the remains of the oldest Romanesque construction stage of the church were successfully detected by both methods.
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How to Characterize a Recharge Area by ERT Time Lapse and Cluster Analysis - Application to Lascaux Cave
Authors S. Xu, C. Sirieix, J. Riss and P. MalaurentSummaryThe decorated cave in Dordogne needs more attention while being studied and conserved. Geophysical methods enable in a non-invasive way to detect the underground environment of the decorated cave. Associated with the analysis of the effective rainfall (groundwater recharge) in this site and the flow in the cave, the Electrical Resistivity Tomography monitoring using pole-dipole and gradient arrays has identified a zone which probably stores water in relation with the cave. There is a high electrical contrast between the surrounding limestone and the probably recharge zone. Then, with a multivariate analysis through the resistivity values allows us to characterize the model blocks showing a specific behavior over time, especially the blocks the lowest electrical resistivity. The main result of this study is that these blocks are assumed to be probably a part of the recharge area; systematic electrical survey after raining event could be used to predict the beginning of the underground flow in order to give advice for preserving the Lascaux Cave.
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Improving Near Surface Characterization by Combining Reprocessed Vintage Seismic and Geophysical Passive Datasets
Authors B. Benjumea, A. Macau, A. Gabàs, M. Esquerda and S. FiguerasSummaryThis study reveals the importance of reprocessing vintage seismic data, originally focused on deep targets, in order to retrieve near-surface velocity model and structure. This information can be verified and complemented using cost-effective geophysical methods (passive seismic and audio-magnetotelluric -AMT data technique). This methodology has been applied to the Empordà Neogene basin (NE of Spain) where oil reflection datasets are available. 41 H/V stations were deployed along one seismic reflection profile to detect seismic impedance contrast between Quaternary/Neogene sediments and basement (Palaeozoic and Mesozoic). In order to calculate this contrast depth, shear-wave velocity profile has been obtained using seismic noise array technique. The final model obtained from the different seismic datasets show a basin shape with a gentle dip at the SW end of profile and an abrupt dip at the NE side interpreted as the Roses Fault. Bedrock depth reaches 660 m at the center of the profile according to H/V results. Finally, an AMT survey was undertaken at 10 sites to report a detailed 2D geoelectrical image across the Roses fault. The electrical resistivity model allows to characterize both structure and fluid properties associated with fractures network within the fault.
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Introducing Particle Swarm Optimization (PSO) to Invert Refraction Seismic Data
Authors R. Poormirzaee, R. Hamidzadeh Moghadam and A. ZareanSummarySeismic refraction method is a powerful geophysical technique in near surface study. In order to achieve reliable results, processing of refraction seismic data in particular inversion stage should be done accurately. In this study refraction travel times inversion considered by Particle Swarm Optimization (PSO) algorithm. In this year PSO algorithm, as a metaheuristic optimization method, is used in many fields of studies. Application of this method in different geophysical data inversion shows that this algorithm is powerful, fast and easy. We developed PSO code in Matlab for inversion of seismic refraction data. For efficiency evaluation of developed code, different synthetic models are inverted. Finally PSO inversion code investigated in a case study at the part of Tabriz city in north-West of Iran for hazard assessment. Then field dataset inverted by PSO code. Obtained model was comprised by the geological information of study area. The results emphasize the reliability of the PSO code to invert refraction seismic data with an acceptable misfit and convergence speed.
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Vibroseismic Acquisition with Long Landstreamer on Disko, Greenland
Authors E. Nørmark, P. Gulbrandsen, H. Sabra and O.R. ClausenSummaryA seismic landstreamer with a length of 500 m, has been applied for a reflection seismic survey on the island of Disko in the western part of Greenland. As source signal a vibrator has been used. The surface consisted of snow and ice and in some places of the exposed surface from a dry riverbed. Sledge mounted geophones and gimbaled geophones were used for the survey. In total 192 geophones channels were applied. Both types of geophones worked well but the gimbaled geophones performed slightly better in the present terrain. In total 10 km seismic profiles have been acquired. Conventional seismic processing has been applied. FK-filtering has been applied in order to suppress surface waves. The present survey has demonstrated that it is possible to use the landstreamer technique with a relatively long streamer in a terrain with snow and ice and exposed surfaces.
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The IGGE UAV Aero Magnetic and Radiometric Survey System
More LessSummaryThis paper expounds the advances in the development of a novel prototype UAV aero magnetic and radiometric integrated survey system developed by IGGE. The magnetometer consists of a CS-VL cesium vapour sensor and an AARC-510 real-time compensator and data logger. The gamma-ray spectrometer has 5 NaI(TI) crystal detectors. All the instruments were set up onboard a middle size UAV which can take off and land autonomously and fly for 10 hours with a 145kg payload at 180 km/h. A field trail survey of 3000 line-km was carried out at a exploration altitude of 120m AGL in 5 days to test the exploration performance of the system. Four flights were accomplished during the night. The survey data quality are competitive over traditional airborne survey system and meets the CGS survey specifications. However, some more optimization research will be required to improve the reliability and safety, simplify the operation, lower the costs, and make it a commercially usable system.
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Comparison of Different Stabilizing Functional in 3D Regularized Inversion of Direct Current Resistivity Data
Authors N.Y. Gundogdu and M.E. CandansayarSummaryIn this study, a new three dimensional (3D) direct current resistivity (DCR) data inversion algorithm is developed. In the developed algorithm, Tikhonov parametric functional is used for regularized inversion. Mostly, smoothing stabilizer is preferred in 3D inversion algorithms of DCR data. In the developed algorithm, different stabilizers are used and their effects on the inverse solution are investigated. It is showed that, inversion with "minimum support" and "first-order minimum entropy" stabilizers gives results with sharper boundaries than inversion with smoothing stabilizer. The algorithm is tested with synthetic data and also field data which is collected in archeological area.
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Classification of Unexploded Ordnance Using Geophysics – A Practical Reality
Authors D. Mortimer and N. ValleauSummaryDuring an unexploded ordnance (UXO) clean-up project, most of the items excavated are not UXO, but are harmless scrap metal. If these items can be determined to be non-hazardous without digging, the cost of a site clean-up can be cut by a substantial amount.
Reliable classification of suspected UXO targets using geophysical survey data is now possible. This paper provides a summary of new advanced electromagnetic sensors and related software technologies to consistently classify suspected UXO and buried munitions vs. non-hazardous buried metallic objects. The technology and the tools have matured to the point where classification can be (and are) routinely applied to UXO clean-up projects in a transparent and reproducible approach. This methodology is now being accepted and required by US government program managers and regulators in clean-up projects that span a range of site conditions.
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Geoelectric Reference Technique, Efficient Tool to Eliminate External Noise in SIP Data
By T. RadicSummaryExternal noise disturbs measurements carried out with the method of Spectral Induced Polarization (SIP), and lessens its data quality. In densely populated areas the disturbance signals mostly originate from high voltage power lines and the electrical devices supplied by them. However, disturbances (tellurics, lightnings) also appear in remote areas. At frequencies up to 250 kHz, disturbances through VLF and LW transmitters are also be taken into account. Stacking techniques are very time-consuming. We have implemented and tested the novel Geoelectrical Reference Technique, which with reasonable additional hardware costs effectively reduces most noise types without loss of time. External noise is characterized through a high spatial coherence. The signals of disturbance source can be observed in a large area. The Geoelectrical Reference Technique uses this fact by recording the noise at one or more remote locations. In doing so, the technique predicts and eliminates the coherent noise components contained in the SIP records. In order to use the Geoelectric Reference Technique in a wide frequency band, we developed a completely new hardware (SIP-Fuchs-4). Field measurements show that the Geoelectric Reference Technique improves the data quality at medium frequencies by a factor of up to 40.
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Time-domain Induced Polarization Profile with Multiple-current Transmission
Authors Y. Yamashita, F. Lebert, J.C. Gourry, B. Bourgeois and B. TexierSummaryWe propose the method of calculating time-domain chargeability on multi current transmission using Code-Division Multiple-Access technique. The multi current transmission technique is proposed for highly efficient resistivity surveys aiming at lowering costs in large number of measurements or improving time resolution in monitoring applications for example. In the purpose of expanding multi current transmission technique to time-domain IP survey, we developed the method for calculating time-domain chargeability using measured potential data obtained by multi-transmission resistivity measurement. We conducted a simple numerical experiment and a field experiment of multi-transmission measurement for confirming our method. We showed it is possible to obtain time-domain chargeability from multi-transmission data. Currently, it is needed to be improved for adapting practical situation such like instability of current injection.
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Non-conventional 3D Electric Resistivity Tomography Arrays
Authors D. Vargas, G. Cifuentes, J.E. Hernandez and R.E. ChavezSummarySpecific conditions of the studied area, where natural or anthropogenic obstacles do not allow carrying out conventional Electric Resistivity Tomography (ERT-3D) arrays, employing parallel transects. It is necessary to construct new ERT designs able to be adapted to any particular zone under investigation. ‘L’-and ‘Corner’-arrays were designed to surround the target to acquire reliable geophysical information from the subsurface. Combinations of these arrays employing more conventional settings (Wenner-Schlumberger, Dipole-Dipole, and so on), are applied to design alternative electrode geometries. The circle, the triangle and the horseshoe arrays are investigated to study the subsoil of different types of scenarios. The efficiency of such alternative arrays was verified on the examples considered. Such studied areas are: a pre-Hispanic circular pyramid, a seawater intrusion and a small urban neighborhood. Results are quite encouraging, where coverage at depth was adequately obtained, and inverted results computed were very interesting.
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Mapping and Migrating Reflected Surface Waves
Authors C.W. Hyslop and R.R. StewartSummaryIdentifying sharp lateral change in the near-surface is needed to accurately locate anomalies. We directly account for sharp lateral change by imaging surface wave reflections. We first show synthetic and field results from a processing flow that produces a space-depth reflectivity map across a seismic survey line. Then we use wave-equation migration to image surface-wave reflections adjacent to the survey line. Our results indicate that reflectivity images from both imaging methods can be used as an attribute in highlighting the location of anomalies and faulting. Using wave-equation migration with an imaging condition based on deconvolution resolves sharp discontinuities at high resolution while preserving polarity of the reflection. Frequency dependent surface-wave reflectivity is converted to depth resulting in 2D and 3D images.
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Three-Dimensional Advanced Fluxgate Magnetometers for Near Surface Geoscience Measurements
More LessSummaryThree new types of fluxgate magnetometers are presented in this paper, able to monitor the vector of the ambient field, all of them being based on the principle of rotational excitation field. The first type employs Yttrium- Iron Garnet (YIG) single crystal film, magnetized with rotational field on its plane, where the 2nd, 4th and 6th harmonics offer the three components of the ambient field with sensitivity better than 1 pT at 0.2 Hz, its size being 25 cubic centimeter. The second type is based on permalloy film, where the rotational excitation field on its plane offers change of magnetoresistance with sensitivity better than 10 pT at 1 Hz, uncertainty of 1 ppm and size ∼ 8 centimeter. The third type, is based on amorphous film, where the rotational field mode offer sensitivity better than 100 pT at 1 Hz, uncertainty of 10 ppm and size ∼ 10 cubic millimeter.
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On the Very Low Frequency (VLF) Measurements Using an Unmanned Aerial System (UAS)
Authors B. Tezkan, R. Eröss and J.B. StollSummaryWe performed very low frequency measurements by employing an unmanned aerial system as an innovative geophysical measuring platform. An appropriate suspension system had to be developed to carry out VLF measurements using the super high frequency coil triple (SHFT) sensor from Metronix and the ADU-07 logger. The survey was realized on a test site in Neuchatel/Switzerland on several profiles crossing a buried pipeline and a power cable. The locations of the buried anomalies were successfully detected by the UAS-VLF data and compared with conventional ground VLF measurements. Finally, the UAS-VLF data were interpreted by 2D conductivity models.
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A Multidisciplinary Approach for the Determination of Subsurface Structure of Leonessa Plain (Central Apennines, Italy)
Authors K. Skrame, M. Di Filippo and M. Di NezzaSummaryIn this study, detailed gravity survey and microtremor observations were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits of the Leonessa plain.
The study involved a test area of 50 km2 occupied by 300 gravity stations and by 30 sites of 3-component single-site observations. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. The microtremor data were analyzed by means of the horizontal-to-vertical spectral ratio (H/V). A realistic density of the unconsolidated Quaternary deposits (1.75–2,00 g/cm3) determined on core samples and a density of 2.60 g/cm3 for the basement limestone were used to constrain the 2D gravimetric models.
The gravity and microtremor data were compared with a collection of existing well logs and geophysical data. The 2D models match quite well with the information determined from the previous works. The data obtained with these different methodological approaches allowed us to reconstruct the geometry of the depression and to evaluate the thickness of the Quaternary sedimentary infilling.
Finally, on the basis of the obtained morphology, we were able to reproduce and to define the 3D model and the Quaternary evolution of the Leonessa plain.
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Edge Detection of Magnetic Sources Related with the Philippi Granitoid in N. Greece
Authors A. Stampolidis, G.N. Tsokas, P.I. Tsourlos and G. VargemezisSummaryMagnetization differences in rocks are related with differences in the content of magnetic minerals within rock units. Assuming that magnetic properties (susceptibility and/or remanence magnetization) are discernible between the various rock units, magnetic field anomalies could provide valuable information for the texture of lithological units and even be correlated with distinct lithologies. Delineating edges of magnetized structures is a common application of magnetic data to geological interpretation. We evaluate a number of edge mapping techniques in order to study the magnetic anomaly caused by Philippi granitoid and to estimate its lateral extension. Edge mapping methods can definitely improve the interpretation of magnetic data. They can delineate the locations of the edges of the subsurface magnetic sources. Applied on the magnetic data of the Philippi granitoid edge mappers succeed to map the edges of the granitoid. The PSG-HGM mapper is a robust method that can detect contacts even in the presence of noise. TDXAS filter was able to detect signal from shallow and deeper contacts.
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Keynote Speech - Near-Surface Geophysics and Australia’s Challenge
By K. SutoSummaryThe definition of “near-surface” in geophysics has not reached a consensus. There are two classes of geophysicists who work on the “near-surface” problems: one interested in physical properties of near surface material and the other trying to understand near surface to improve deeper image. The former is typically for engineering and environmental applications and the latter is for resource exploration.
Engineering geophysics is slowly penetrating to geotechnical community in Australia but many engineers are not aware of its effectiveness and geophysical techniques are not utilised to their full potential. Geophysics is only a minor part of geotechnical engineering curriculum. In the field of environmental geophysics, airborne electromagnetic surveys are used to address Australia’s salinity problem.
Declining discovery rate of minerals in Australia prompted a new initiative to collaborate the governments, research organisations, academia and industry. Geophysics is expected to be a major contributor in mapping out the thickness and properties of the cover material in the area as large as Europe. We are facing a big challenge in providing accurate information over a large area in a timely manner.
While the challenges are within Australia, the issues addressed here are universal, and the result achieved may be useful globally.
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Quantitative Depth to Bedrock Extraction from AEM Data
Authors H. Anschütz, C. Christensen and A.A. PfaffhuberSummaryA new road segment is being planned northeast of Norway’s capital city, Oslo. In this context, knowledge of sediment thickness is vital, as is information about occurrence and extent of highly sensitive marine clay (so-called quick clay).
Airborne EM measurements were conducted to provide information of depth to bedrock/sediment thickness between drilling sites and guide the further drilling program. AEM data indicate a variable bedrock depth with a general trend towards shallower bedrock in the northeastern part of the investigation area. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings.
Various methods for estimating depth to bedrock were compared: (1) Using a constant resistivity threshold either survey-wide or for each profile line; or (2) by using an appropriate resistivity value as a function of position. A contouring algorithm was developed to for the second method to incorporate both borehole and AEM data and hence account for apparent variations in resistivity at the sediment-bedrock boundary. Though both exhibit the same general trends, predictions for some locations differed significantly. Based on the AEM results recommendations for further drillings were given, thus reducing the overall costs of the project
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Assessing Climate Effects on Railway Earthworks Using MASW
Authors S. Donohue, D.A. Gunn, P. Bergamo, E. Hughes, B. Dashwood, S. Uhlemann, J.E. Chambers and D. WardSummaryMany parts of the UK’s rail network were constructed in the mid-19th century long before the advent of modern construction standards. Historic levels of low investment, poor maintenance strategies and the deleterious effects of climate change have resulted in critical elements of the rail network being at significant risk of failure. The majority of failures which have occurred over recent years have been triggered by extreme weather events.
Advance assessment and remediation of earthworks is, however, significantly less costly than dealing with failures reactively. It is therefore crucial that appropriate approaches for assessment of the stability of earthworks are developed, so that repair work can be better targeted and failures avoided wherever possible. This extended abstract briefly discusses some preliminary results from an ongoing geophysical research project being carried out in order to study the impact of climate or seasonal weather variations on the stability of a century old railway embankment on the Gloucestershire Warwickshire steam railway line in Southern England.
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Do We Get the Right Position?
Authors J. Endom, C. Holst and M. StrickerSummaryLiterature for applied geophysics present rules of good practice for field work and descriptions on the different methods. Parameters like depth, frequency, resolution and further conditions are discussed in detail. The fundamental requirement that geophysical data have to be linked to a position seems to be self evident in times of GNSS and laser total stations.
Whereas surveying is an engineering discipline, the today positioning equipment implies that everyone can produce exact positioning data. Many geophysical experts perform their positioning with quite rudimentary surveying knowledge and blind faith into GPS & Co.
In civil engineering applications we observe rectangular shaped inspection areas as well as long linear structures like railway tracks or roads. Surveying a soccer field sized site requires different positioning technologies than surveying along highways. Standard positioning methods do not longer apply to this problems.
Focussing on GPR in civil engineering a survey speed of 80 km/h is required today. With velocities of more than 22 m/sec latency of surveying systems gets quite relevant.
Different available technologies of surveying are discussed in this presentation. Manufacturers as well as users are addressed to work on a unified methodology that could be implemented as a standard in future.
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Void Detection at Chania Airport by Combining ERT and Borehole Data
SummaryA detailed geophysical survey was carried out at Chania airport located at the north-west part of Crete, Greece in order to detect cavities in carbonate rocks. This survey combines electrical tomography and borehole data from Chania airport where new buildings are planned to be constructed. The site mostly consists of recrystallized carbonate rocks belonging to Trypali unit. Electrical resistivity tomography applied in dense grids at Chania airport provided images of the subsurface which conducted a borehole program for the detection of karstic forms in the carbonates. At the area under investigation according to resistivity imaging as well as borehole and surface geological data, the formations are mostly characterized as highly fractured. The size of the existing voids ranges from 0.5m to 6m.
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The Use of GPR in the Rehabilitation of Built Heritage
Authors N. Barraca, M. Almeida, H. Varum and M.J. Senos MatiasSummaryRehabilitation of city centres and of buildings of architectural interest is an increasing activity in Civil Engineering works. A thorough, economic and careful project demands à priori information about the original construction materials and techniques, changes to the original structure, infrastructures (water, electricity, sewage, etc) defects and foundations. Direct physical intervention on the buildings is not advisable and, therefore, non intrusive and non destructive techniques must be used.
Herein it is proposed to give an account, using selected data, from a larger on going GPR survey in the study and characterization of an Art Deco building (from early XXth century). Data were collected with shielded antennas and are discussed both in 2D and 3D and, finally, the benefits of the survey are appraised confirming the potential of GPR in the investigation to support conservation interventions.
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Soil Classification and Looseness Evaluation by Rock Physics Based Cross-plot Analysis for Unsaturated Soils
Authors C. Konishi, K. Hayashi, T. Tanaka and T. AbeSummaryRock physics-based methodology for cross-plot analysis in the interpretation of the integrated geophysical survey is proposed to overcome an issue of the current cross-plot analysis. By using the rock physics models, we can avoid ambiguity or subjectivity associated with the determination of threshold values for discriminating the data in the cross-plot, which is the critical part to identify the vulnerable location in the survey area. In addition to the vulnerability section, the method provides more detailed soil type classification and looseness evaluation profiles by using theoretical cross-plots individually. We applied the methodology to actual field data obtained at the river levee where the levee body is supposed to be replaced with new one. After the survey, several points of the levee body are exposed so that we can observe actual soil types and conditions and verify our interpretation results. As a result, the estimated soil type and looseness are fairly consistent with actual soil type and conditions. Therefore, it is considered that the methodology is applicable and beneficial for the interpretation of the integrated geophysical survey.
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Mapping Overpressure Zones Using Acoustic and Seismic Techniques
Authors J.N. Makris, L. Marechal and J.E. PapouliaSummaryA combination of acoustic and seismic methods was applied in order to define hazard on the sea floor and overpressure zones for planning drilling activities in the Caspain Sea. The following methods were used:
- Multibeam Echosounder Swath mapping (MES), in order to produce accurate bathymetry and map seafloor irregularities, faults and obstructions that could influence drilling operations.
- Side-Scan Sonar mapping (SSS), needed to identify obstacles on the seafloor with a very high resolution and particularly to locate positions of abandoned wells.
- Sub-Bottom Profiling and multichannel high-resolution seismic mapping (MCS) with the intention to resolve the stratigraphy of the area, identify faults, map high pressure zones and provide the link between the very shallow structures and those dominating at depth.
The obtained results proved the efficiency of the methods proposed in satisfying engineering requirements.
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Geophysics in the Context of Disaster Mitigation -Results from the Landslide at Pechgraben, Austria
Authors R. Supper, D. Ottowitz, J.H. Kim, B. Jochum, S. Gruber and S. PfeilerSummaryIn early June 2013 a large landslide was triggered at the area of Pechgraben, situated in a mountainous area of Upper Austria. Only several hours after the onset of the first movements, a geophysical investigation program was started to support the planning of the disaster response actions as well as the design of a follow up early warning system. The geophysical methods included geoelectric profiling, geoelectrical monitoring, borehole logging, GPS, inclinometric, piezometric, soil humidity and optical monitoring as well as airborne geophysics. Within this paper results of different methods are given and evaluated on their role within disaster mitigation.
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Time-lapse VSP Monitoring of Shallow Gas Injection at the Maguelone Experimental Site (Languedoc, France)
Authors H. Perroud, O. Jaafar, P. Pezard and J. LofiSummaryThe Maguelone experimental site has been the subject of surface and downhole geophysical monitoring of gas injection experiments in a shallow subsurface reservoir. In particular, vertical seismic profiles (VSP) were conducted in two holes located within meters of the gas injection hole, before, during and after a series of injections. The times of first-arrival were obtained by picking and interpreted in terms of seismic velocity models. Before injection, the lateral variability of seismic velocities at the site is underlined. The comparison of seismic velocities between experiments for a single hole brings out the impact of gas injection on the elastic properties of the porous clastic sediments at the injection site.
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Statistical Analysis of Three-years Time-lapse Electrical resistivity Imaging for the Study of the Petacciato Landslide
Authors G. Calamita, R. Luongo, A. Perrone, S. Piscitelli, T.A. Stabile and V. LapennaSummaryIn the last ten years, time-lapse electrical resistivity tomography technique has been applied to monitor the temporal and spatial changes of resistivity values in different environmental and natural processes like saline intrusion, aquifer recharge, water content change, etc. The growing use of this technique is mainly due to the significant technological improvements occurred in recent years. Indeed, the introduction of new static multi-channel systems has allowed the reduction of acquisition time, the remote control of system and the monitoring of temporal variation of resistivity also in areas where the accessibility can be complicated.
This paper reports the results of a statistical analysis of time-lapse ERT data acquired in a landslide area located in Molise region (Italy). A prototype geoelectrical system was installed in the area with the aim to indirectly estimate the water content change in the subsoil by continuously measuring the resistivity. The analysis was performed on apparent resistivity values without applying any inversion software. In this way no artefacts or errors due to the inversion process were introduced.
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Evaluation of Time Lapse Acoustic Monitoring of Immiscible Fluid Flows in Near Surface by Attenuation Examination Method
Authors B. Hassan, S.D. Butt and C.A. HurichSummaryVelocity information alone, in acoustic monitoring of fluid fronts and interfaces when non-aqueous spills occur in the near surface unconsolidated sediments or water table/aquifers, and of those associated with consequent remediation, may not describe such immiscible flows and displacement fronts adequately. Attenuation characteristics examination, for its greater sensitivity to subsurface fluids, can offer better resolution in describing such subsurface processes. Characterization of controlled immiscible fluid displacement process signifying examination of attenuation characteristics, by comparing spectral curves, so, is demonstrated as extended results of an experimental study. In the experiment oil is displaced with brine, flowed through a saturated pack of ceramic beads against gravity under controlled constant head flow conditions. P-wave data were acquired employing pulse transmission with 1MHz sensors. Pertinent concise review of well studied wave attenuation mechanisms and associated anelastic and viscoelastic models is provided for connective lucidity of presented results and interpretation. Application of the method following a benchmarked time-lapse type workflow for data analyses, results presentation and plausible inferring/interpretation is effectively evaluated. Cited method unambiguously resolved, oil from brine and the interfacial mixing and evolution. Subtle insight instilling and interpretation enhancing frequency dependent combined effects related to fluid density, viscosity and flow rate were observed, too.
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Optimized Arrays for Surface-to-tunnel ERT Measurements
Authors K. Simyrdanis, P. Tsourlos, N. Papadopoulos and P. SoupiosSummaryIn this work optimum protocols using the criterion of Jacobian matrix were tested to increase the resolvable power of surface-to-tunnel electrical resistivity tomography measurements. The problem was approached through synthetic numerical modeling, experimental data collected in a controlled tank environment that was constructed for this purpose and data that were retrieved from a real case scenario in Crete, where an aqueduct tunnel exists in the archaeological site of Eleftherna. The resistivity inversion images were reconstructed through a 2-D inversion algorithm that was modified to account for the special surface-to-tunnel ERT measurements. The optimized data sets contained fewer measurements with respect to the original data sets (∼30%) but the optimized inversion images show comparable resolvable capabilities with the original inversion models.
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Geoelectrical and Geotechnical Investigations at Tenth of Ramadan City, Egypt - A Structure-Based (SB) Model Application
Authors M. Attwa and A. El ShinawiSummaryGeoelectrical measurements were used in association with soil engineering tests in environmental engineering investigations at Tenth of Ramadan city, east Nile Delta, Egypt. The genetic algorithm (GA) approach with structure-based (SB) model is applied towards an optimal interpretation of DC resistivity measurements. The inverted models together with borehole data and soil classification exhibit two soil types, gravelly sand and clayey sand with free lime. The survey demonstrates that the combined use of the GA with SB model provides valuable information to constraint the number of interfaces. The reliable resistivity inversion results are considered as a key role in studying the geotechnical parameters such as Atterberg limits unconfined compressive strength and swelling tests. The clay sediments are described by medium plasticity and sensitivity for swelling. Further, the free lime with water makes gill action with decreasing the compressive strength. Therefore, such soil, in turn, may cause soil water appearance on the ground surface and geotechnical problems during or after the process of any construction. In this respect, the integration of the geoelectrical survey with soil engineering properties is efficient and less time consuming for characterizing the engineering-geologic zone.
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Hydrogeophysical Tools for Investigating Groundwater Storage in the Subsurface of a Karst System
Authors A. Watlet, K. Van Noten, T. Lecocq, J. Chambers, P. Meldrum, O. Francis, M. Van Camp and O. KaufmannSummaryThrough the use of three different geophysical techniques, we highlight structures and groundwater storage changes in the subsurface of a karst system, in the so-called epikarst layer. An Electrical Resistivity Tomography (ERT) has been installed onsite informing on conductivity variations directly related to changes in groundwater content within the unsaturated zone. Both relative and absolute gravimetric measurements monitor the charges and the discharges of the groundwater storage within the epikarst. Analysis of Horizontal to Vertical spectral ratio of seismic ambient noise allows investigating the structure of the epikarst and validating the robustness of ERT data.
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Geophysical and Geochemical Study of Geropotamos Aquifer in the North-central Coast of Crete, Greece
Authors D. Kalisperi, P. Soupios, S. Kershaw, M. Kouli, P. Barsukov, F. Vallianatos and N. Lydakis-SimantirisSummaryThe geological setting of Geropotamos aquifer on the north-central coast of Crete, Greece, is considered complex, while the local tectonic regime of the study area is characterized by two sets of faults orientated NW-SE and NE-SW. Investigation of the aquifer using the Transient ElectroMagnetic method (TEM) has resulted initially in 1D models of geoelectric structures and a final 3D geoelectrical model was constructed, depicting the zones of salination of groundwater in the aquifer. Groundwater samples were analysed and the most important chemical parameters were determined to provide an independent dataset for comparison with the TEM results, while Groundwater Quality Maps were produced. TEM and geochemical data correspond and provide verification of the TEM approach. As a result, saline intrusion is likely to occur along fractures in a fault zone through bedrock, and this work emphasises the critical role of fracture pathways in salination problems of coastal aquifers.
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Magnetic Resonance Sounding Signals Retrieval Based on Empirical Mode Decomposition and a Statistical Method
Authors M. Fallah-safari, R. Ghanati and M.K. HafiziSummaryThe technique of magnetic resonance sounding (MRS) is a non-invasive hydro-geophysical tool for estimating the distribution of water content and the pore size distribution with depth in the subsurface. It is well-know that the quality of MRS measurements is strongly affected by even very low noise. In the present study, in order to enhance the performance in the noisy environments, a two-step noise cancellation approach based on the empirical mode decomposition and a statistical method is proposed. In the first stage, a significant part of the signal noise is eliminated using the decomposition of the signal by the empirical mode decomposition algorithm and based on the detection and removal of the noisy IMFs. Then the de-noised signal is reconstructed through the no-noise information. In the second stage, the signal obtained from the initial section enters an optimization process to cancel the remnant noise, and consequently, estimate the signal parameters. The strategy is tested on a synthetic MRS signal contaminated with Gaussian noise, spiky events and harmonic noise. By applying successively the proposed steps, we can remove the noise from the signal to a high extent and the performance indexes, particularly signal to noise ratio, will increase significantly.
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3D Hydrogeological Model Building Using Airborne Electromagnetic Data
Authors G.A. Oldenborger, C.E. Logan, M.J. Hinton, V. Sapia, A.J.M. Pugin, D.R. Sharpe, A.I. Calderhead and H.A.J. RussellSummaryWe develop a 3D geological modelling procedure supported by the combination of helicopter time-domain electromagnetic data, seismic reflection data, and water well records for the Spiritwood buried valley aquifer system in Manitoba, Canada. Our procedure is an innovative hybrid of knowledge-driven and data-driven schemes that provides a clear protocol for incorporating different types of geophysical data into a 3D stratigraphic model framework. The limited spatial density of water well bedrock observations precludes detection of the buried valley bedrock topography and renders the water well records alone inadequate for accurate hydrogeological model building. The expert interpretation of the geophysical data allows for leveraging of a spatially extensive dataset with rich information content that would be otherwise difficult to utilize for lithostratigraphic classification.
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Contribution of Geophysics to Geochemical Study
Authors S. Barhoum, R. Guérin, D. Valdès, L. Bodet, P. Gombert and Q. VitaleSummaryA study of spatial heterogeneity of groundwater geochemistry is conducted within an abandoned underground quarry of chalk.
An associated geophysical study (electromagnetic mapping at low induction number, and electrical resistivity tomography) is carried out on the surface.
It allows to determine the structural geometry of superficial formations, in order to understand the link between the geochemical spatial variability of the lakes and the properties of the environmental medium.
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Water in Karst Hydrosystems Unsaturated Zone; MRS Evidences within an Integrated Hydrogeophysical Approach
Authors K. Chalikakis, S.D. Carrière, N. Mazzilli, C. Danquigny, A. Legchenko and C. EmblanchSummaryKarst unsaturated zone (UZ) is recognized to play a key role in karst hydrosystems recharge and contaminant attenuation processes. Its characteristics are identified as an important factor for karst groundwater vulnerability assessment and resource management. A large scale hydrogeophysical experiment was undertaken over the last 4 years combining several surface-based geophysical methods and measuring techniques in a unique experimental site within the Fontaine de Vaucluse (FDV) hydrosystem in southern France. The aim was to apprehend the UZ structure and functioning of typical urgonian karstified mediterranean limestone. Geological structure is being well recognized by electrical, electromagnetic and seismic measurements. Magnetic Resonance Sounding (MRS) results clearly identify water presence and evidence seasonal variations within the karst UZ. Due to these results a conceptual hydrogeological model is proposed. On May–June 2014 a drilling campaign is being programmed and a cross-validation of the MRS results is also planned.
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Geo-electrical Characterization and Monitoring of a Waste Landfill for Its Future Exploitation as a Bioreactor
Authors A. Arato, E. Agostini and A. GodioSummaryThis work deals with the geoeletrical characterization of a municipal solid waste landfill, and with the time-lapse monitoring of an infiltration test within the waste mass. The main objective is to understand the hydraulic behaviour of the wastes under a controlled water supply from the surface, in order to delineate the paths through which the water flows.
In fact, the landfill will be exploited as a bioreactor, in order to speed up the biodegradation of the organic fraction of the waste. Then, ensuring the adequate moisture content, through re-circulation of water or leachate, is essential to enhance the bacterial activity, and therefore the biogas production.
Geoelectrical characterization helped to locally understand the water infiltration processes, supplying useful knowledge for future plans of leachate re-circulation system.
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Sensitivity Analysis of a Light Fixed-wing Airborne TDEM System for the Characterization of Karstic Environments
Authors G.-A. Sab, C. Schamper, F. Rejiba and A. TabbaghSummaryDetailed mapping of karstic aquifers is of prime importance for water resources management. To achieve these surveys objectives, airborne measurements are the only methods which allow both large and dense coverage. Electromagnetic induction methods such as time-domain electromagnetic (TDEM) help to map conductors like clayey accumulations surrounding karstic conduits. Fixed-wing TDEM systems using light aircrafts do not exist on the market, despite being able to offer higher cost effectiveness and performance close to that reached with helicopter-borne EM systems in hydrogeological surveys. A sensitivity analysis of key acquisition parameters is undertaken to evaluate the capabilities of this kind of system for mapping karstic environments. Despite a flight altitude of 50m, higher than the average altitude of 30m reached with helicopter-borne systems, the results are encouraging and indicate a good capacity for investigation from 10m to about 150m depth.
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Geoelectrical Studies by VES/TDEM Joint Inversion in the Parana Sedimentary Basin, Brazil
Authors J.L. Porsani, D.N. Leite, C.A. Bortolozo, M.A. Couto Jr. and J.D.R. CampanãSummaryIn this work the authors shows results of VES/TDEM joint inversion for Urupês region, located in Paraná basin, São Paulo State, Brazil. The objectives were to map the sedimentary and crystalline aquifers. The sedimentary aquifer is characterized by Adamantina Formation (Bauru aquifer) and the crystalline aquifer is characterized by fractures zones in the basalt layers of the Serra Geral Formation. The joint inversion results are promising and they permit to map the tick of the Adamantina Formation, as well as locate a fracture zones inside of the basalt layer of the Serra Geral Formation.
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Application of Cross-hole Seismic Tomography in Characterization of Heterogeneous Aquifers
Authors S.I Ehosioke and T. FechnerSummaryDetailed characterization of subsurface geological heterogeneities which control fluid flow and transport is still an unresolved challenge in the geoscience community. While surface geophysical methods have helped in obtaining large scale parameter distributions, the decrease of resolution with depth limits their application for delineating heterogeneities at high resolution. This limitation has encouraged the use of cross-hole geophysical techniques such as cross-hole seismic tomography. Cross-hole seismic tomography which involves measuring travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocities between the wells offers an approach for characterizing geological heterogeneities at a high resolution. Its application range from near surface aquifer studies, geothermal, to deep oil and gas reservoirs (Sainkov et al 2005; Tselentis et al 2011). Becht et al., (2007), evaluated the use of cross-hole seismic tomography for high resolution aquifer characterization. In this study, we present a case application of cross-hole seismic tomography for a high resolution characterization of the Prakla-Seismos field test site. Detailed velocity distributions are correlated with available borehole geological data to better delineate subsurface heterogeneity which provide useful information for an improved groundwater flow and contaminant transport modeling in the area.
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Effectiveness of the Reciprocity Principle for Dipole-dipole Electrical Resistivity Survey in Field
More LessSummaryThe reciprocity principle of source and receiver array holds in dipole-dipole electrical resistivity survey. To examine the aspects of source voltage and current and of the potential difference of reception electrodes for heterogeneous medium, a vertically contacted half space model involving high and low-resistivity medium is defined, and the numerical simulation is carried out in the case of dipole-dipole electrode array configuration. And to examine the effectiveness of the electrode optimization based on the reciprocity principle in actual field situations, the numerical experiments for the model involving high and low resistivity with random noise are performed and analyzed.
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Effective Array of Electrical Survey for Detection of Piping Zone of Embankment
Authors S.H. Oh, H. Lee, W.Y. KIM, E.S. IM and H.J. CHUNGSummaryPiping through embankment or foundation of concrete dam has been issued as a main factor of various menaces for safe maintenance of the water facility structure. In this study, a new approach to find the location of the piping point by geoelectrical method is proposed.
Our approach is to install the current line source in the water of upstream and downstream, and then let the current flow from upstream to downstream area. The potential will be measured at downstream to find any electrical anomalies due to piping condition.
Numerical study made it possible to adjust various conditions to configure optimal configuration for successful locating of the piping point. And lab test was successfully performed and the result implied a new way of electrical application for safety assessment of water facility structure for the piping problem.
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How Calculate DOI Index to Assess Inverted ERT model?
Authors S. D. Carrière, K. Chalikakis, C. Danquigny and L. Torrès-RondonSummaryAssessing Electrical Resistivity Tomography (ERT) reliability is critical for interpretation step and decisions making. Scientists try to provide solutions with several quality indicators: Depth of Investigation (DOI) index, sensitivity, uncertainty and resolution matrix. Each indicator, presenting strengths and weaknesses, has to be cross-validated with others. Most of these indicators are already included in the commercialized inversion softwares.
This paper deals with DOI index calculation. Two calculation methods appear/exist in the Literature: the one uses inverted resistivity and the other uses logarithm of inverted resistivity. Both solutions are compared in this paper with several synthetic models and field data set. In general, DOI index calculated using inverted resistivity point out systematically a less reliability in resistive zone of the model. DOI index map calculated using logarithm of inverted resistivity appears more robust in various configurations. This calculation method highlights quite accurately inversion artifacts.
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Near Surface Ore Exploration Using TAU-transformation of Time-domain IP Data
Authors E. Turai, M. Dobroka, A. Kiss, G. Petho and M.K. BaraczaSummaryThe so-called critical elements can be connected to polimetallic ores therefore geophysical measurements were carried out in the area to explore and describe ore bodies in volcanic geological environment.
Geoelectric, especially the Induced Polarization (IP) method yields good results. For the interpretation of IP curves the TAU-transformation was developed, which provide the spectral information of polarization with the spectrum of time-constants. With introducing the Weighted Amplitude Value (WAV) it was possible to give quantitative characterization of the ore. With the application of these processing methods, two different type of mineralization could be distinguished: a stock-vein type ore which is mainly affected by redox processes in the western part and primary porphyritic mineralization on the northern part of the exploration area. The results proved the applicability of TAU-transformation and WAV in case of ore exploration. Based on the WAV, corrected apparent specific conductivity metallic and redox polarization maps, three points were marked out for drilling.
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Geophysical Exploration of a Complex Metavolcanic Environment
Authors G. Petho, E. Turai, N.P. Szabo, N. Nemeth, M.K. Baracza, D. Bulla and K. KormosSummaryThe Triassic stratovolcanic complex of the East Bükk Mts. in North Hungary was the target of our geophysical exploration because of its previously known REE and critical element occurrence. There is no direct geophysical survey to locate these elements, for this reason complex geophysical survey including magnetic, very low frequency (VLF) electromagnetic, spectral radiometry, multielectrode resistivity and multielectrode induced polarization measurements were made along profiles to gain more knowledge about the possible host rocks of REE and critical elements being relatively close (5–30m) to the surface. In the course of interpretation the core description and well logs of the boreholes drilled earlier were also taken into account. It was experienced that the ore zones were overlying mainly high resistivity and low chargeability metarhyolites. The combination of the magnetic and VLF measurements were used to locate a covered boundary separating limestone and metarhyolite. Among the applied geophysical methods the simultaneous interpretation of the multielectrode resistiviy and induced polarization measurements provided the best geological image.
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Developing a Near Surface Electrical Resistivity Model of Great Britain
Authors J.C. White, D.C. Entwisle, J.P. Busby, R. Lawley and I.L. CookeSummaryA model of the near surface electrical resistivity of Great Britain is presented. The first geological unit beneath the base of soil layer is attributed with electrical resistivity.
Geotechnical data are used to derive modeled resistivity distributions for each of the lithostratigraphic units described in the near surface geological model of Great Britain. Resistivities are calculated using an effective medium methodology and these values are tested against electrical resistivity sounding data and apparent resistivity estimates from the airborne electromagnetic surveys. The central moments of the statistical distributions are coupled to the geological map and the resulting resistivity distribution shows a general increase in resistivity from SE England to NW Scotland. This pattern reflects the age of the bedrock geology of Great Britain.
The methodology described in this paper can be applied to other regions where suitable geotechnical information and geological mapping is available.
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Application of Geoelectrical Techniques in the Investigation of a Coastal Sand Dune Field
Authors J.D. Alexopoulos, S. Dilalos, S.E. Poulos, G. Ghionis and S. MavroulisSummaryA geophysical research was carried out to investigate the lithostarigraphic substratum characteristics of the dune field of the central Kyparissiakos Gulf (W. Peloponnese, Greece), which is characterised by the presence of four dune lines. For this purpose, the geophysical techniques of Electrical Resistivity Tomography (ERT) and Vertical Electrical Soundings (VES) were applied, along with detailed morphological mapping and the collection of geological and geomorphological information. The processing of the geophysical data that were collected from two ERT soundings, perpendicular to the general direction of the dune lines, and seven geoelectrical soundings revealed four geoelectrical layers with the two surficial resistive layers adumbrating the extent and thickness of the four sand dune ridges and the associated slags. The third layer, corresponding to the underlying geoelectrical formation of 50–75 Ohm.m, is interpreted as a layer of transgressive Holocene deposits saturated with fresh water, while the deepest geoelectrically identified layer seems to correspond to the “Neda” formation, which consists of marls, sandy marls and conglomerates. It is concluded that the combined application of the aforementioned techniques has the potential to provide valuable data for the investigation of complex coastal depositional environments.
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Describing the Pressure Dependence of Lamé Coefficients on Coal Samples
Authors J. Somogyi Molnar, A. Kiss and M. DobrokaSummaryUnderstanding the relationship between pressure and rock physical parameters, such as acoustic velocity, elastic moduli, porosity is essential for exploring and exploiting of natural reserves. Therefore a petrophysical model for describing the pressure dependence of P and S wave velocities was developed. The advance of the model is that it provides the required physical explanation of the pressure dependence with its three-parameter exponential function. On the basis of the model the pressure dependent Lamé coefficients were deduced. To prove the applicability of our method laboratory data of coal specimen was inverted to estimate the model parameters. Since the model valid for P and S wave velocity has a common parameter a joint inversion technique was applied. The velocity data were measured under varying confining pressure with pulse transmission technique on dry samples. A very good fitting between measured and calculated data was found thus inversion results strengthened the applicability of the petrophysical model.
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High Resolution 3D Seismic Reflection Investigation in Urban Environment for Assessment of a Sinkhole Feature
Authors C. Hartmann and U. SwobodaSummaryIt is well known that full spatial information about the underground geology generally can be retrieved by 3D seismic investigations. The advantage of recording multidirectional data information allows imaging of complex underground features, such as fault systems, sinkholes or dome structures.
The downside of a 3D seismic investigation is the operational requirement of laterally free access to the investigation site at the surface to lay out the seismic recording equipment and deploy the seismic source. This is typically no problem, for example, for large scale operations in desert-like areas for the oil and gas industries. In urban areas, though, access is limited due to building structures or transport infrastructure. Moreover, other constraints such like the public interests require significant considerations during all stages of a seismic operation.
This work summarizes the possibilities and limitations of carrying out a detailed 3D seismic reflection investigation in urban environment by presenting a case study investigation program which has been carried out across a small community in Germany.
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Correlation between S-wave Velocities and Building Damage Distribution in Bolu (Turkey) During the 1999 Düzce Earthquake
By A.T. BasokurSummaryThe ‘Düzce Earthquake’ of magnitude Mw= 7.2 took place on 12th November 1999 in the northeast of Turkey. Besides the effect of earthquake parameters such as epicentre distance, magnitude, duration, source mechanism and frequency content of earthquake waves, this large earthquake indicated the importance of the local soil conditions on building damage especially for incorrectly designed and improperly constructed structures.
In order to examine the connection between damage and the local subsoil condition, the two-dimensional inversion of shear-wave refraction arrival times performed on 149 profiles having a total length of 10370 meters. A strong correlation was found between the local shear-wave velocity and building damage distributions that also comparable with the geology of the studied area. The consistency between seismic velocities and building damages shows the importance of seismic refraction survey to estimate the local site effects that extracted from the transmission properties of shear waves.
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Applying Time-frequency Analysis in Borehole Seismic Travel-time Picking
More LessSummaryThe data reductions of existing borehole seismic methods are currently based on travel-time analysis in which first-arrival times of the seismogram are manually picked. However, manual picking is not efficient and depends heavily on the data quality and analyst’s experience. Besides the difficulty of picking first arrivals in the time domain, no information is obtained regarding the effective frequency of the analysis. Data reduction method based on the time-frequency domain was proposed in the study. The aim of the study was to propose a semi-automatic procedure for more objective first-arrival time picking. Evaluation using field data shows that the semi-automatic time-frequency spectrum analysis works well in PS-Log testing. The picked first arrival times from the proposed method agree well with the manually-picked results. The procedure is stable and yields reasonable results. But it should be noted that the method inherit the tradeoff between frequency and time resolution from the time-frequency spectrum analysis. The parameters used in the time-frequency analysis should be optimized for different source types.
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Proving the Applicability of the Petrophysical Model Describing Acoustic Hysteresis of P and S Wave Velocities
Authors J. Somogyi Molnar, A. Kiss, H. Szegedi and M. DobrokaSummaryDue to the increasing demand for hydrocarbons and the depletion of the known hydrocarbon fields there is a growing claim to predict rock physical parameters more accurate at non-conventional conditions also. It is well known that acoustic velocity in rocks strongly depends on pressure which influences the mechanical, transport and elastic properties of rocks as well as wave propagation under pressure is very nonlinear and the quasistatic elastic properties of rocks are hysteretic. Characterization of hysteretic behavior is important for mechanical understanding of reservoirs during depletion. Therefore a quantitative model - which provides the physical explanation - of the mechanism of pressure dependence is required. In this paper a petrophysical model is presented which provides the connection between the propagation velocity of acoustic waves (both P and S) and rock pressure both in case of pressurization and depressurization phases as well as explains the mechanism of acoustic hysteresis. The developed model is based on the idea that the pores in rocks close under loading and reopen during unloading. The model was applied with success to acoustic P and S wave velocity data sets.
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Development of a Prototype 3-Component Borehole Electromagnetic Receiver
More LessSummaryA proto-type borehole electromagnetic receiver has been developed and compared the response to the commercial audio-frequency magnetotelluric receiver BF6. The proto-type showed comparable sensitivity and decay of magnetic field to BF6. The purpose of the receiver development is for wide application of electromagnetic tomography techniques, that is readily developed the interpretation software in last two decades. The barrier of the EM tomography is that appropriate hard-ware or measurement system cannot be found in the market. Once developed, the system can be applied to monitoring the fluid migration in EOR and CO2 sequestration and various engineering purpose. In the presentation, we expect to show more advanced version of system and test results as well.
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Three-dimensional Modeling of GREATEM Data
Authors T. Mogi, S. Abdallah and E. FomenkoSummaryThe GREATEM (GRounded Electrical source Airborne Transient ElectroMagnetics) survey uses a grounded electrical dipole source of 2∼3 km length as a transmitter and a three-component magneto-meter in the bird towed from a helicopter as a detector. The GREATEM system is considered to be an airborne version of the Long Offset Transient ElectroMagnetics (LOTEM) system ( Strack, 1992 ). We developed a numerical forward modelling scheme using a staggered-grid finite-difference method (SFD, Fomenko and Mogi, 2002 ) adding a finite-length electrical dipole source routine. To construct a three dimensional (3D) resistivity model for GREATEM data, we have to overcome to compute a model composed of huge number of grid at wide frequency ranges because GREATEM has obtained resistivity data in time domain at a large number of sites.
In order to verify the validity of the code, we compare the results with those obtained by the 2D numerical code of the LOTEM case developed by Mitsuhata (2000) for a 2D quarter space model and a trapezoidal hill model. Finally, we simulate GREATEM responses for 3D modeling with a conductor placed at coast-land boundary and a topography model.
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Simple and Efficient Approaches to Integrate Data into Geophysical Inversion Problems - Examples from Se
Authors M.K. Karaoulis, P.I. Tsourlos, M.P.E. Kleine and P.P. KruiverSummaryData integration in geophysics provides additional information to elucidate subsurface structure and reduce non-uniqueness of inverted models.
In this work we demonstrate two alternative and easy methods to integrate data. A) the use of image-guided inversion, where structural information is taken directly from a high-resolution geophysical image (e.g. ground penetrating radar or seismic reflection) or from a geological cross-section. This structural information is introduced into the inverse problem through a weighted smoothing matrix, where it correlates and favors formations related to a specific structural feature and not just uniformly across the entire model. B) Definition of spatially weighted distribution of the Lagrange parameter, where the inversion solution favors model that incorporates naturally a-priori information from different data types. It is important to notice that in both scenarios, no petrophysical expressions are necessary and the modification of existing codes is limited to the stabilazation term.
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3D Airborne Electromagnetic Inversion in the Areas with High Conductivity Contrasts and Rugged Topography
Authors M.S. Zhdanov and L. CoxSummaryThis paper introduces an approach to 3D modeling and inversion of the airborne electromagnetics (AEM) that is suited to arbitrarily complex earth models with very high conductivity contrasts and rugged topography, yet is fast enough to consider large surveys. We use a hybrid FE-IE method, which directly avoids errors associated with numerical differentiation and interpolation of the electric vector potentials at the edges of the elements containing the receiver. This approach is stable and accurate and for conductivity contrasts in excess of 10 :1, as is typically required for practical AEM interpretation. We incorporate the moving sensitivity domain method into this modeling framework to increase the modeling speed for an entire survey by several orders of magnitude. A case study for the 3D inversion of 90 line km of DIGHEM data from the Reid-Mahaffy test site is presented to demonstrate the efficacy of our method.
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Combined Inversion of Seismic Waveforms and Traveltimes for Acoustic Waves
Authors E. Manukyan, H. Maurer, A. Nuber and R.G. PrattSummarySeismic full waveform inversion (FWI) is a powerful method for obtaining high resolution subsurface images, but the objective function of waveform data typically contains many local minima that may lead to inappropriate solutions. This can be avoided by choosing an initial model that is close to the true model, and by incorporating only low frequencies at an early stage of an inversion run. Here, we demonstrate that the local minima problem can also be mitigated by simultaneously inverting waveforms and first arrival travel times. Using a simple two parameter example we show, how the addition of travel time data can remove a local minimum present in the waveform misfit function. Inversions of synthetic 2D acoustic data show that our novel combined travel time and waveform inversion approach leads to superior results compared with a pure FWI inversion.
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Inversion to Recover Electrode position Displacements on 3D Electrical Resistivity Tomography Monitoring Grids
Authors P.B. Wilkinson, S.S. Uhlemann, J.E. Chambers and P.I. MeldrumSummaryElectrodes installed on active landslides and vulnerable earthworks to monitor changes in resistivity associated with moisture dynamics can be subject to movement. This affects the geoelectrical data and leads to errors in the resulting Electrical Resistivity Tomography (ERT) images. This paper demonstrates the selection of appropriate ERT measurements to provide sensitivity to electrode displacements in both directions on a plane. A combination of linear and equatorial dipole-dipole measurements are considered, which permit use on rectangular grids of any aspect ratio. A Gauss-Newton inversion scheme is developed that allows for the incorporation of constraints based on the magnitude and direction of movement. The effects of the constraints are demonstrated using synthetic data designed to simulate a planned laboratory experiment. In the presence of realistic levels of data noise and uncertainty in the preferred directions of movement, the inversion method is able to recover the displacements of the electrodes with a root-mean-square misfit of less than 1% of the electrode spacing.
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Hybrid Approach for Travel Time Seismic Tomography in Elliptical Anisotropic Media
Authors E. Cardarelli and A. CerretoSummaryHybrid approach for seismic travel time tomography is proposed in the case of elliptical anisotropic media. A sequential scheme is presented that combines simulating annealing with linear least square inversion. Simulated annealing was implemented to obtain a velocity model that can be used as initial guess for linear least square inversion. Linear Traveltime Interpolation raytracing approximation is used to trace rays and calculate traveltimes. The procedure is tested both for a synthetic model and a real case. The real case comes from a previous study, that was solved by linear least square inversion. In both synthetic and real cases comparison is performed between the linear least square inversion results and hybrid approach; in the field study we compare the old results with those inferred by sequential approach.
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2D Cole Cole Inversion of Time Domain Induced Polarization Data - Model Studies and Field Measurements
By B. TezkanSummaryA 2D inversion algorithm for time domain induced polarization data has been developed. The IP transients are inverted into 2D Cole Cole models including resistivity, chargeability, relaxation time, and frequency constants. In order to speed up the inversion procedure, an approximate forward algorithm has been used which is highly accurate and its validity was tested with exact solutions up to 3D. In the 2D inversion procedure, a 3D DC forward algorithm has been used as forward operator. The method requires a conventional 2D DC inversion and a homogenous half space Cole-Cole inversion to derive the Cole Cole parameters of each cell. Synthetic model studies and a field example will be demonstrated.
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1D Joint Inversion of Rayleigh Fundamental Mode and Electric Sounding Curves - Synthetic Tests
Authors M. Senkaya and H. KarslıSummaryLayer thickness is important model parameter as far as S-wave velocity that contributes to Rayleigh phase velocity. But, S-wave velocity is only inverted parameters in the traditional Rayleigh-wave inversion process. Thus, using erroneous thickness information in inversion process will cause unreliable or non-unique S-wave velocity profile. However, layer thickness can be incorporated into process by the joint inversion approach to increase accuracy of sub-surface model. This study presents 1D joint inversion of Rayleigh fundamental mode curve with electric sounding data, to use layer thickness as variable in the inversion process, contrary from separate inversion of Rayleigh dispersion curve. The proposed method was tested on noise-free and Gaussian noisy synthetic data. Results demonstrate that reliable vertical shear velocity profile can be obtained by the joint inversion approach, even there is no a priori.
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Combined Onshore and Offshore Seismic Investigations at the Geothermal Field of Aedipsos-Yaltra, Central Greece
Authors V.K. Karastathis and E. MouzakiotisSummaryA clear picture of the fault that facilitates the hydrothermal flux in one the most important geothermal fields of the Greek mainland, in the North Euboean Gulf, has been acquired through the implementation of a combined onshore and offshore seismic survey.
The results of the seismic investigations accurately map the structural characteristics of the fault providing us with precise estimations of its dip and throw.
An efficient exploitation of the field can be achieved by selecting the proper drilling sites according with the results of the seismic surveys.
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Surface-wave Dispersion Inversion versus SH-wave Refraction Tomography in Saturated and Poorly Dispersive Quick Clays
Authors S. Pasquet, G. Sauvin, M.R. Andriamboavonjy, L. Bodet, I. Lecomte and R. GuérinSummaryA seismic survey involving two distinct acquisition setups, with vertical and horizontal component geophones, has been carried out along the same line on a site presenting a simple vertical structure (peat, quick clays and bedrock) and no strong lateral variations. SH-wave refraction tomography and Rayleigh-wave dispersion inversion provided the same shear-wave velocity gradient in the quick clays. SH-wave tomography failed to correctly depict the peat layer and to reach the bedrock. A collection of Rayleigh-wave dispersion images were extracted along the line using windowing and stacking techniques. A thorough analysis of these images made it possible to give a complete description of the site velocity structure, illustrating the complementarity of both methods.
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Real-time or Full-precision CRS Imaging Using a Cloud Computing Portal - Multi-offset GPR and Shear-wave Reflection Data
Authors Z. Heilmann, H.P. Müller, G. Satta and G.P. DeiddaSummaryThe presented cloud-computing portal allows automated imaging of near-surface structures either in full-precision using a global simultaneous search for the best fitting spatial CRS stacking operator or in realtime using a pragmatic sequence of line searches. The simultaneous search even though computationally very expensive becomes necessary when the pragmatic search fails due to strongly varying velocities, low CMP fold and high noise levels. In our first case study we apply real-time imaging by reprocessing multi-offset Ground Penetrating-Radar data with a good signal/noise ratio. Generating stacked and prestack time migrated sections, consistent with conventional results, required less than 10 minutes. In our second case study we analyze a challenging SH-wave seismic reflection data, recorded in an urban environment, where a high level of ambient noise and landfills below the sealed surface down to 2 m depth resulting from recent reconstruction work hampered seismic data quality severely. Here, we successfully applied our newly developed full-precision CRS imaging approach in addition to conventional CMP stack aided by VSP. In this urban environment, the relevance of applying both, high resolution SH-wave seismic and VSP surveys was demonstrated. Two recently drilled wells confirm the aquiclude to be found as predicted by the seismic forecast.
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Distorting Effects of the Near-surface Layer on Seismic Imaging - Physical Model Investigations
Authors Y.K. Mo, S.A. Greenhalgh, J.O.A. Robertsson and H. KaramanSummaryThe 2D ultrasonic model facility at ETH Zurich was used to investigate how a variable near-surface weathered layer can adversely affect the imaging of reflection data. The simulated geological structures are constructed from 2 mm thick metal and plastic sheets, cut and bonded together into various shapes. The experiments entailed the use of a piezoelectric source driven by a pulse amplifier at ultrasonic frequencies to generate Lamb waves in the plate, which are detected by piezoelectric receivers and recorded digitally on a National Instruments recording system, under SignalExpress software control. High quality reflection seismic data were collected. However, the pre-stack migrated image for the deeper reflectors deteriorates as the near-surface becomes more complex and as block anomalies are inserted within the layers. Crosshole and VSP seismic data were also acquired, and first arrival travel-time tomography used to reliably reconstruct the velocity distribution. The 2D physical scale model system provides a valuable complement to numerical modeling to study various aspects of wave propagation and subsurface imaging.
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Long-term Geoelectrical Monitoring to Support Nuclear Decommissioning at the Sellafield Site, UK
Authors O. Kuras, P.B. Wilkinson, P.I. Meldrum, L.S. Oxby, S.S. Uhlemann, J.E. Chambers, A. Binley, J. Graham, G. Dewey and N. AthertonSummaryThe reduction of risk and hazard associated with the UK’s civil nuclear legacy is a strategic priority for the UK’s Nuclear Decommissioning Authority (NDA). A full-scale field trial using 4D ERT has been completed successfully at the Magnox Swarf Storage Silos at the Sellafield Site, which has monitored a controlled injection experiment. This paper summarises the achievements of the trial and lays out future plans for geoelectrical monitoring at Sellafield. The potential of ERT as a long-term monitoring tool is explored, and electrode longevity estimates are made based on the results of the MSSS trial.
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Field Application of Resistivity and Spectral Time Domain IP to Investigate Geoelectrical Signatures of Free-phase PCE
Authors S. Johansson, G. Fiandaca and T. DahlinSummarySoils beneath former dry cleaning sites are often contaminated with the chlorinated hydrocarbons PCE and its degradation products, which are highly toxic and cancerogenic. In this study, resistivity and time domain induced polarization (DCIP) were measured at a former dry cleaning site in order to investigate if the source contamination of PCE could be detected. 80 meter long 2D-profiles were measured across the site with the ABEM Terrameter LS instrument. The DC and full IP decay data were inverted using the 2-D DC/IP inversion algorithm developed by Fiandaca et al. (2013) , giving access to the spectral information contained in the IP decays. The inversion parameters DC resistivity (ρ), chargeability (m0), the relaxation time (τ) and the frequency factor (C) were compared to reference data from boreholes. The reference data consisted of geological classification and concentration of free-phase chlorinated hydrocarbons, obtained from soil samples. A high resistive anomaly was observed at a location along the profile where soil sampling have confirmed high concentrations of free-phase chlorinated hydrocarbons. High IP-effects, in terms of the spectral parameters m0 and C, was seen next to this area. It is suggested that the IP-effects arise in a zone next to the free-phase source plume.
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Complex Conductivity Measurements of Silver Nanoparticles in Unsaturated Porous Media
Authors E A Atekwana, G.Z. Abdel Aal and D.D. WerkemaSummaryThe emergence of engineered nano-materials (ENMs) in the global marketplace and their accidental introduction into the subsurface pose a potential risk to the environment and public health. There is a need for the development of techniques to detect their presence and transport in the subsurface. Complex conductivity measurements have been shown to be sensitive to the presence of metals. Laboratory experiments were conducted to investigate the complex conductivity response of silver (Ag) nanoparticles in a sand column under different nanoparticle concentrations (0, 2, 4, 6, 8 and 10 mg/g) and moisture content (5%, 10%, 15%, 20% and 30%). Complex conductivity measurements were obtained between 0.1–1000 Hz. The results showed that at different water saturation the magnitude of the imaginary conductivity component increased by one order with increasing concentration of Ag nanoparticles with minimal changes in the real conductivity component. On the other hand, at different concentrations of Ag nanoparticles the magnitude of the imaginary conductivity and real conductivity components increased by one and a half order and one order of magnitude, respectively. Our results demonstrate that complex conductivity measurements are sensitive to the presence of nanoparticles in unsaturated porous media which potentially could be used in guiding the detection and remediation processes of such contaminants within the vadose zone.
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Distribution of 137Cs in Forest Components of Selected Areas in the Czech Republic
Authors S. Takahashi, I. Suchara, K. Okamoto, K. Umegaki and R. FujiyoshiSummaryMigration process of 137Cs in soil has been investigated by measuring its activity concentration with depth. However, surface portion of soil (organic components) have various properties with different thickness. It is necessary to classify the distribution of 137Cs into each forest component.
Therefore, we investigated forest components (litter, fermentation, humus, Ah horizon, upper and lower part of mineral soil) in several forest areas of the Czech Republic, and tried to clarify the present situation about 30 years after the Chernobyl accident.
The highest value of 137Cs activity concentration was obtained to be 1.60 kBq/kg in fermentation.
Cesium-137 is therefore still present in organic layer. We’ve got previous results on distribution of 137Cs in humus horizon at the same forest area. Apparent half-life of 137Cs in humus horizon was estimated to be 20.2 year. This value gave initial deposition of 137Cs in humus layer to be 3.9 kBq/kg, comparable with that (4.3 kBq/kg) derived from the apparent burial rate of 137Cs. This is a new finding on initial deposition of 137Cs in humus layer with different ways.
In order to predict future situation in forest components affected by nuclear accidents, long term monitoring is found to be important.
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3D Geophysical Model of the Vajont Rockslide from Seismic and Geoelectric Surveys
Authors G. Boehm, R. Francese and M. GiorgiSummaryThe Vajont rockslide represents the most catastrophic massive failure of a mountain slope ever reported in historical times. The reconstruction of the geometry and of some key physical properties represents a major step forward in the understanding of the slide mechanism and of the collapse dynamics.
The landslide body and the surroundings were then targeted with a large 3D geophysical campaign aimed to define the seismic and the electrical properties of the various geological units comprising its body.
Data analysis and interpretation was constrained by a series of additional seismic and electrical measurements collected on the outcropping rocks below the village of Casso.
The larger values of P- and S-wave velocities as well as terrain resistivity appear to be located in the eastern block while in the western one these parameters are much lower values. These variations in the rock mass coherency are somewhat related to the larger stress field acting on the western block before and during the failure. Furthermore the slide displacement of the eastern block is lower than the western one due a small amount of rotation during the collapse.
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Delineation of Clay Layers in a Landslide Area in Norway Using Frequency Domain Helicopter-borne EM Survey
Authors V.C. Baranwal, E. Dalsegg, H. Dretvik, J.S. Rønning, J.F. Tønnesen and I.L. SolbergSummaryThere was a quick-clay landslide in Byneset on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrocks at several places. Prehistorically, the sea-level was ∼160 m higher than the present sea-level. The sediment in the area mainly consists of marine clay. Subsequent leaching by fresh groundwater alters the chemical composition of the pore water and “quick clay” may develop. Quick clay completely liquefies when remolded under stress and results in large landslides.
Resistivity survey is a powerful tool to investigate such clay layers but it is time consuming. Airborne EM survey can be a faster way to investigate large areas. Seismic refraction survey is useful to delineate depth of bedrock. Therefore frequency domain helicopter EM (FHEM), resistivity and refraction seismic surveys were performed in the area last year. There is a good agreement of the results from all the geophysical surveys. Resistivity and FHEM data show various clay layer boundaries well in case of thin marine clay deposits. However, FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits due to low skin depth in a conductive environment.
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Improved Time-lapse ERT Monitoring of Dense Non-aqueous phase Liquids (DNPLs) with Surface-to-horizontal Borehole Arrays
Authors C. Power, J. Gerhard, P. Tsourlos, P. Soupios, K. Simyrdanis and M. KaraoulisSummaryElectrical resistivity tomography (ERT) has long been recognized to exhibit the potential to significantly improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs).
However, the technique has not become a common tool for mapping such contaminants due to the complexity of the DNAPL target coupled with the inherent limitations of the commonly used surface and cross-hole ERT configurations. The objective of this work is to evaluate the potential of surface-to-horizontal borehole (S2HB) ERT for mapping the spatio-temporal evolution of DNAPL mass during remediation. A coupled DNAPL-ERT model was employed to provide an initial, theoretical evaluation of S2HB ERT, relative to surface ERT, for monitoring the remediation of a realistic, field-scale DNAPL source zone. A laboratory experiment was then performed to demonstrate the S2HB ERT approach for a real system involving a changing NAPL distribution over time. Four-dimensional ERT inversion was employed for both numerical and experimental data to generate time-lapse resistivity images of the subsurface regions experiencing NAPL changes. Results demonstrate significantly improved resistivity imaging with S2HB ERT and its potential as a non-invasive DNAPL site tool for mapping DNAPL mass changes during remediation.
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Mapping of Olive Oil Mills’ Wastes (OOMW) through ERT - A Case Study from Alikianos Site in Eastern Crete (Greece)
Authors N.G. Papadopoulos, P. Soupios, J.H. Kim, K. Simyrdanis, S. Kirkou and P. TsourlosSummaryThe production of olive oil in Greece and in Mediterranean is of major economic importance and goes hand in hand with a large food-processing industry. Thus it constitutes an important source of employment, especially in regions where it still remains today one of the major financial activities. However, these activities produce large waste amounts which are disposed in the environment without control. What is needed, apart from the adoption of specific policies regarding waste management, is the development and application of automated systems that will allow the rapid identification of environment degradation incidents and thus assist the decision making and the determination of specific measures and actions to be taken in order to protect and improve degraded areas. To this direction Electrical Resistivity Tomography was tested in an Olive Oil Mill Waste (OOMW) site in western Crete trying to explore its resolvable capabilities to image for such pollutants. The results indicate that OOMW can be mapped due to their conductive signature in relation to their hosting material rendering ERT a significant tool in the direction of providing a specific environmental tool in order to define a strategic framework for addressing this problem.
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Simultaneous 4D Cross-borehole ERT Inversion to Guide Leak Monitoring in Nuclear Decommissioning
Authors L. Oxby, O. Kuras, P.B. Wilkinson and P.I. MeldrumSummaryNuclear decommissioning efforts at the Sellafield site, UK, are increasingly supported by near-surface geophysical monitoring. Electrical Resistivity Tomography (ERT) is being applied to help manage the increased risk of leakage from the Magnox Swarf Storage Silos (MSSS) during planned waste retrievals.
A full-scale field trial of ERT has been undertaken in the immediate vicinity of the MSSS site. Six borehole electrode arrays were installed to monitor a subsurface volume equal in size to a proposed imaging cell under the MSSS structure. An automated geoelectrical imaging system was used to observe dynamic resistivity changes within the ground caused by simulated leaks.
Controlled injections into the vadose zone of environmentally benign conductive simulants were designed to test a number of realistic leak scenarios. The complex geology and low conductivity contrasts provided a significant challenge for ERT to detect and characterise the simulated leaks.
We show that full, simultaneous 4D ERT inversion successfully resolved the pathways adopted by the tracer fluid; these were found to be compatible with historic contamination detected in cores retrieved from the boreholes. The trial has demonstrated the value of ERT for nuclear decommissioning by enhancing our conceptualisation of likely behaviour of potential future leakage at MSSS.
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