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- Volume 12, Issue 6, 2014
Near Surface Geophysics - Volume 12, Issue 6, 2014
Volume 12, Issue 6, 2014
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Seismic ringing effect in the shallow subsurface
Authors Shimon Rubin, Vladimir Shtivelman, Shmariahu Keydar and Aner LevWe refer to the seismic ringing effect as a dynamic response of a localized shallow heterogeneity in the ground to an incident wave field originated either from a seismic source operating at the surface or from a background noise. On the seismograms the seismic ringing wave field usually appears as a long wave train with a pronounced resonant character concentrated above the localized subsurface heterogeneity.
Based on the theoretical and numeric analysis of the frequency and amplitude characteristics of the ringing signals, we suggest different mechanisms that can explain the underlying features of the effect. Quantitatively, the results of our study show an excellent agreement, with an error less than 5% between analytical, numerical models and field experiments for the invoked normal modes inside the localized subsurface heterogeneity, and at least 85% agreement between the analytical model for the flexural modes and field experiments.
Potentially, the ringing effect may be exploited in shallow subsurface surveys, and structural mapping of subsurface terrain.
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Travel time inversion from ground level to gallery: protocol for the characterization of P‐wave seismic signature in a fractured‐porous Urgonian platform at hectometric scale
Authors Emeline Maufroy, Stéphane Gaffet, Stéphane Operto, Yves Guglielmi and Daniel BoyerA tomographic ‐wave velocity model is inferred from a ground level‐to‐gallery vertical 500 m × 800 m seismic experiment conducted at the inter‐Disciplinary Underground Science and Technology Laboratory (LSBB, France). No initial knowledge of the velocity structure of the surrounding fractured‐porous carbonates was previously available. Ninety‐four shots at the surface were recorded by a line of 189 seismometers on the steep slope of the topographic surface and by a line of 150 geophones in an 800 m‐long, 250‐500 m‐depth gallery. The ‐wave velocities inferred from first‐arrival travel time inversion display a relatively large set of values ranging from 4000 to 6000 m/s. Such variations correlate well with the 5 to 20% porosity variations between the main geological units that consist of two sedimentary facies affected by a complex cemented fault zone.
Taking advantage of the known geology of the site, this study explores the influence of the acquisition geometry impacted by the topography and of the near‐surface weathered zone onto the shallow tomography resolution ability. Considering the mesoscopic scale of the targeted medium, reliable imaging of hectometric geological bodies with 10% contrasts in porosities can be achieved only with the simultaneous association of (i) a high density of sources and receivers in the monitoring array geometry, and (ii) the equal consideration of surface‐to‐gallery and surface‐to‐surface first‐arrival travel times, as an essential constraint to correctly image the underlying structures.
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High resolution seismic imaging at the planned tunnel entrance to the Forsmark repository for spent nuclear fuel, central Sweden
The Swedish Nuclear Waste Management Company (SKB) plans to build a repository for storage of high‐level radioactive spent nuclear fuel at the Forsmark site in central Sweden at a depth of about 470 m. The planned repository will cover an area of about 3.6 km2 at this depth. Prior to beginning excavation and tunneling, some detailed geophysical surveys are being performed at the planned site. One of these was a refraction seismic survey to determine depth to bedrock in the vicinity of the planned access ramp. Two lines, each about 300 m long and spaced about 35 m apart, were acquired in August 2011. Since the bedrock topography is known to be highly variable, a close receiver (2 m) and source (6 m) spacing was required to map it. This close spacing allowed the data also to be treated as reflection seismic data and some adjustments to the acquisition procedure were made in the field with this in mind to aid in the later processing. The main adjustment was that seismic data were recorded on all geophone stations simultaneously. That is, as shots were fired along one line, data were recorded along both that line and the other one. Likewise, when shots were fired along the other line, data were recorded along that line and the first line. This adjustment allowed semi‐3D coverage between the lines. Results from first break traveltime tomography along the lines indicate a depth to bedrock that is greater than that found from geotechnical observations along the lines. This discrepancy is attributed to the uppermost bedrock being highly fractured and having a velocity significantly below that expected from the intact bedrock deeper down. Reflection seismic processing of the data shows a reflection at about 20 ms (about 60 m). The reflection is interpreted to have a gentle northwesterly dip component to it. Comparison with core data in the area suggests that the reflection is from a thin (a few metres thick) fracture zone, although none of the boreholes actually penetrate the reflector where it is mapped by the seismic data. This fracture zone may be part of a larger fracture zone mapped by core drilling further to the east. The newly mapped reflector may be crossed by the ramp when excavation begins. Further seismic surveying towards the west is required to verify if this will be the case.
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Estimating location of scatterers using seismic interferometry of scattered rayleigh waves
Authors A. Kaslilar, U. Harmankaya, K. van Wijk, K. Wapenaar and D. DraganovFrom non‐destructive testing to medical imaging and seismology, estimating the location of scatterers is of high importance. The location estimation can be achieved using a method inspired by seismic interferometry. This method correlates only the isolated scattered fields from a scatterer, and inverts for the travel times to estimate the scatterer’s location. The correlation eliminates the influence of the path between a source and a scatterer. We illustrate the potential of the method using data from a scaled laboratory model, representing geophysical field problems. We use ultrasonic data recorded on an aluminum block containing many scatterers at the surface represented by vertical drill holes. To estimate the horizontal coordinates of a scatterer, we use the scattered Rayleigh‐wave fields recorded along two lines due to one source. We address the problem of selecting scattered fields along the two lines that pertain to the same scatterer using simple geometrical considerations, but also during the inversion. We show that the inversion does not converge when scattered fields coming from different scatterers have been chosen for the correlation.
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Reliability of MASW profiling in near‐surface applications
Authors Hannes Steinel, Jörg Hausmann, Ulrike Werban and Peter DietrichPseudo‐2D surface wave profiling (MASW) has become a powerful state‐of‐the‐art tool for the characterization of near‐surface features in recent years. The rapid gathering and interpretation of surface wave data, i.e. Rayleigh waves, that this method offers is applicable to many investigations and environments. However, most studies only perform unidirectional active surface wave profiling, i.e. without data evaluation by measuring the same profile in the opposite direction. Uncertainties arising from using the MASW method are connected to the one‐dimensional inversion of a two‐dimensional subsurface. Moreover, the occurence of lateral inhomogeneities and dipping layers distort the flat‐layered 1D assumption used in surface wave inversion. In this study, we present data from two different MASW surveys. For both locations, we reveal shot‐geometrical effects and prove the necessity of bidirectional profiling, especially since these check shots can be implemented with only little extra effort. The results of this investigation show that, in general, more attention should be paid to data evaluation.
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Shear‐wave velocity based seismic microzonation of Lorca city (SE Spain) from MASW analysis
Many populated areas located in moderate seismic risk regions have been hit by earthquakes of moderate magnitude which, surprisingly, have caused very serious damage to buildings and led to the loss of human life. The city of Lorca (SE Spain) is a clear example. On 11 May 2011, two main shocks occurred in the vicinity of Lorca city with a maximum magnitude of 5.1 Mw causing some casualties and serious widespread damage in the city and its surroundings.
Most of the damage was concentrated in certain districts in the city, while other parts remained intact. Actually, in Lorca, investigators detected both a clear example of seismic site effect and a seismic wave amplification occurrence due to the type of geological materials on which the city is located. For this reason, several studies are being carried out in Lorca city to assess the actual contribution of the soil conditions to the seismic amplification phenomena.
In these studies, the shear wave velocity plays a key role as a parameter for evaluating the dynamic behaviour of the shallowest geological materials. Consequently, site characterization applied to calculating seismic hazard is usually based on the near surface shear‐wave velocity distribution. This study looked at the average shear‐wave velocity for the uppermost 30 m of ground, which is referred to as Vs30. The Vs30 values obtained from multichannel analysis of surface waves (MASW) were used to create a new soil classification map of Lorca city. Thus, the derived Vs30 map was transformed into the NEHRP and Eurocode 8 (EC8) standards. In Lorca city, the softness and the thickness of shallow geological formations have been observed as two important factors that affected the level of ground shaking and the degree of damage. The results show that there is a significant correlation between the Vs30 values and the damage distribution within the city.
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A new stable downward continuation of airborne magnetic data based on Wavelet deconvolution
Authors Maysam Abedi, Ali Gholami and Gholam‐Hossain NorouziThis paper describes an efficient wavelet‐based deconvolution method for the downward continuation of airborne potential (magnetic and gravity) field data. We formulate the downward continuation process as a linear ill‐posed deconvolution problem. To obtain a reasonable downward continued field data, it is stabilized in a wavelet domain by minimizing the L1‐norm of the coefficients subject to the constraint that is the agreement of the upward response with the original observed data up to a white Gaussian noise level. The resulting convex constrained problem is then solved very fast and efficiently via the split Bregman iterations method. The generalized cross‐validation (GCV) criterion as a plotted curve versus the iteration count with new formulation is used to determine the optimum number of Bregman iterations without prior knowledge about noise properties. A synthetic magnetic field data embedded in a Gaussian noise is constructed from isolated multi‐source anomalies, whose results show that the airborne magnetic data can be continued stably downward by the proposed automatic sparse deconvolution method in a few iterations. When compared with conventional methods such as Tikhonov and Edge‐preserving with the proposed method, similar results were obtained. To test the performance on real data, we chose to use the airborne magnetic data of the central Iranian volcanic‐sedimentary belt. The enhanced downward continued data to a depth level of 200 m beneath the ground surface indicates adequate matching of high potential zones with previous working mines and copper deposits. Obtained results from both synthetic and real data confirm that the performance of the proposed technique is as good as the classical deconvolution ones.
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On the value of combining surface and cross‐borehole ERT measurements to study artificial tile drainage processes
Authors R. Clément, S. Moreau, H. Henine, A. Guérin, C. Chaumont and J. TournebizeMany articles have shown the usefulness of electrical resistivity tomography (ERT) where characterizing spatial electrical resistivity variations over time associated with water content variations. In a case study of artificial drainage processes in comparison to isolated measurements of the temporal variation in water content, ERT could be used to obtain additional qualitative spatial information (2D or 3D). Indeed, there have been no articles relating the advantages of using ERT to study tile drainage processes. The aim of this article is to explore the effectiveness of the ERT method for delineating soil moisture spatial patterns, following their temporal variations in the context of artificial drainage. First, in the numerical description, taking into account the current knowledge on artificial drainage processes, the authors evaluate the combination of surface and cross‐borehole ERT measurements and different array types. In a field application, the surface and cross‐borehole combination helps to obtain better information on the soil structure and to optimize electrical resistivity monitoring during tile drainage processes. In the second part of the article, the comparison between the ERT data sets and ancillary data from TDR sensors provides a useful critical evaluation of ERT for studying water transfer in waterlogged soil influenced by artificial drainage. The field results showed that ERT contributes additional 2D information on resistivity as related to the water content, which complements single TDR measurements.
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Analysis of fluvial, lacustrine and anthropogenic landforms by means of ground‐penetrating radar (GPR): field experiment
More LessA field experiment aiming to study the influence of measurement conditions on depths of penetration and resolution of GPR surveys was conducted in the middle Obra valley (western Poland). The tests were carried out along five profile lines in three sites representing fluvial, lacustrine and anthropogenic landforms. The GPR surveys were conducted at various hydrogeological conditions: high and low groundwater levels, and in winter conditions when the ground was under snow/ice cover and frozen. The best measurement conditions for imaging the internal structure of fluvial landforms were found during a period of low groundwater levels. At high groundwater levels the bottom parts of radar profiles were obscured by reverberations originating from the differences in the water content between unsaturated and saturated parts of the studied sections. Some sedimentary structures within lacustrine landforms were better seen at high levels when all the deposits were saturated. In winter conditions, high quality radar profiles were obtained only when measurements were conducted on an elevated, anthropogenic landform, unaffected by dispersion and attenuation of the radar signal in the other landforms due to a water layer situated between the ice cover and land surface. It was also shown that it is possible to image sedimentary structures of landforms built of deposits with significant admixtures of silts, although the depths of penetration are reduced in such cases.
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A new reconfigurable stepped frequency GPR system, possibilities and issues; applications to two different Cultural Heritage Resources
Authors Raffaele Persico, Marcello Ciminale and Loredana MateraSome results achieved from a reconfigurable stepped frequency GPR system will be presented. The aim is to show the possibilities of a reconfigurable architecture for a GPR system within the noninvasive prospecting of the Cultural Heritage. In particular, two sites (indoor and outdoor, respectively) have been chosen, in order to test the system in different situations that pose different problems. The same sites have been prospected also with a commercial pulsed system, in order i) to work out a comparative heuristic analysis and ii) to plan possible future advancements improving the reconfigurable system.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)