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- Volume 9, Issue 6, 2011
Near Surface Geophysics - Volume 9, Issue 6, 2011
Volume 9, Issue 6, 2011
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Surface waves: use them then lose them. Surface‐wave analysis, inversion and attenuation in land reflection seismic surveying
Authors Claudio Strobbia, Laake Andreas, Peter Vermeer and Anna GlushchenkoABSTRACTWhile in other domains of applied geophysics the surface‐wave is considered a source of information for near‐surface characterization, in the seismic industry the so‐called ground roll has been traditionally regarded only as coherent noise to be filtered out as soon as possible. This difference of perspective is mainly due to the limitations of conventional land acquisition. The Rayleigh waves, which constitute a large part of the recorded energy, can be acquired properly, analysed and inverted to characterize the near‐surface with a surprisingly high resolution, even in large 3D surveys, with point receiver acquisition. Surface waves can play a new role: they contribute to a better near‐surface characterization for the perturbation correction and can be used for velocity modelling and geological modelling. Their proper identification enables alternative filtering strategies. Surface waves are not coherent noise but a signal that can be lifted from the seismic record and exploited in a variety of well‐established geophysical solutions. In this paper we discuss a workflow for the analysis, inversion and attenuation of surface waves with 3D land data, showing examples from a land 3D survey in Egypt.
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Geotechnical characterization of a river dyke by surface waves
Authors Lutz Karl, Thomas Fechner, Mattias Schevenels, Stijn François and Geert DegrandeABSTRACTThe need for effective and reliable methods to survey and monitor the structure of earth‐fill dams recently became pressing in light of the increasing number of flood events in central Europe. Among geophysical techniques, dam imaging using electrical resistivity methods is applied in most cases. Occasionally, ground‐penetrating radar is applied in the framework of the search for subsurface facilities. Seismic methods are rarely used.
This paper focuses on the multichannel analysis of the surface waves (MASW) method to determine dynamic soil properties and aims to extend its application field to dyke and dam structures. The standard processing procedure of the MASW assumes a flat free surface of infinite extension. The flat surfaces of a dyke, in contrast, are in the order of 1–10 times smaller than the wavelengths in the soil; disturbing side reflections will occur. Even though MASW has already been applied on a few dyke sites, the effect of such an obvious breach of preconditions needs to be studied before the method can be recommended.
In this paper the influences of the dyke’s topography on the test results are studied by means of a numerical analysis. Typical cross‐sections are modelled using 2.5D finite and boundary elements. The results of models taking the topography into account are compared with models neglecting the topography. The differences are evaluated on the level of the dispersion curves and for one cross‐section on the level of the S‐wave velocity. They were found to be insignificant for dykes with a width‐to‐height ratio larger than four.
A testing campaign was conducted providing the chance to collect experience in the practical use of the MASW method on dykes. Test results obtained at two test sites are selected and compared to the results of borehole logs and cone penetration tests. A remarkable relation between the S‐wave velocity and the consistency of the clay sealing was found at one site; a distinct positive correlation to the measured cone tip resistances was achieved on the other test site. Valuable information on the composition of the dyke body and base could be obtained but the resolution of the method to identify small areas of inhomogeneity should not be overestimated.
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Interpretation of microtremor 2D array data using Rayleigh and Love waves: the case study of Bevagna (central Italy)
Authors R. Puglia, K. Tokeshi, M. Picozzi, E. D’Alema, S. Parolai and S. FotiABSTRACTIn the last decades, geophysicists and seismologists have focused their attention on the inversion of empirical surface‐waves’ dispersion curves from microtremor measurements for estimating the S‐wave velocity structure at a site. This procedure allows a fast and convenient investigation without strong active sources, which are difficult to deploy especially in urban areas.
In this study we report on a 2D seismic noise array experiment carried out at Bevagna (central Italy) near the station BVG of the Italian Accelerometric Network (RAN). The site was investigated within the DPC‐INGV S4 Project (2007‐2009). The Rayleigh‐ and Love‐waves dispersion characteristics were estimated using different methods. The inversion of the dispersion curves was then performed independently, obtaining two estimations for the S‐wave velocity profiles. The results of cross‐hole logging near the seismic station are used for a comparison.
The shear‐wave velocity profiles estimated by microtremor analyses range up to 150 m depth. The two independent procedures provide consistent shear‐wave velocity profiles for the shallow part of the model (20–30 m in depth) in agreement with the results of the cross‐hole logging. Some problems arise between 30–40 m in depth in the profile estimated by surface waves. In this range cross‐hole logging evidences an inversion of the S‐wave velocity. Although the cross‐hole logging stops at 40 m of depth, we are confident about the results provided by the Rayleigh‐wave analysis below 40–50 m. This case study suggests that greater efforts should be devoted to exploit the potential of a coupled analysis of Rayleigh and Love waves from microtremor array measurements.
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Structure of an ambient vibration wavefield in the frequency range of engineering interest ([0.5, 20] Hz): insights from numerical modelling
Authors Dario Albarello and Enrico LunedeiABSTRACTThe expected structure of an ambient vibration wavefield at the top of a shallow soft layer overlying a rigid bedrock is explored by applying a full wavefield physical model, under the hypothesis that ambient vibrations are the effect of a uniform distribution of random independent point‐like harmonic sources at the surface of a flat, weakly dissipative Earth. The comparison of the results provided by this model with those deduced on the assumption that surface waves dominate the wavefield allows evaluation of the respective roles of body and surface waves (Love and Rayleigh) in their fundamental and higher modes. This analysis reveals that the structure of the ambient vibration wavefield strongly depends on the subsoil structure (P‐ and S‐wave velocity profiles and thickness of the uppermost soft sedimentary layer) and on the distribution of ambient vibration sources around the receiver. This dependence also changes along with the frequency range of interest. In this regard, three frequency domains are identified, each showing a different sensitivity to the relevant parameters: below the fundamental resonance frequency for S‐waves , above the frequency , where is the resonance frequency for P‐waves and in‐between. A consequence that emerges is that a number of possible combinations of body and surface waves are possible, which could account for the heterogeneous results obtained from experimental studies. These findings also indicate constraints on the use of simplified models based on the assumption that surface waves dominate the ambient vibration wavefield, as is currently the case in most engineering applications.
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The meaning of surface wave dispersion curves in weakly laterally varying structures
Authors Daniele Boiero and Laura Valentina SoccoABSTRACTThe analysis of surface wave dispersion is efficiently applied to estimate 1D subsurface velocity profiles. The same approach is even applied at sites that present weak lateral variations under the assumption that the estimated dispersion curve is representative of the average properties beneath the receiver spread. We verify this assumption by discussing the meaning of the dispersion curve in weakly laterally varying structures using the path‐average approximation (PAVA). Using PAVA we compute synthetic data for different lateral variations and we extract dispersion curves using the wavefield transform. If the phase slowness is linearly varying along the propagation path and the wavenumber resolution of the measuring array does not allow for separating the different wavenumbers of the propagating surface waves, the estimated dispersion curve provides the average slowness. On the contrary, if the phase slowness is not linearly varying or if the wavenumber resolution of the measuring array is enough to discriminate the wavenumbers, the retrieved dispersion curve does not represent any specific velocity of the subsurface. To mitigate this problem windowing can be successfully adopted to make the retrieved dispersion curve representative of the local property of a subsoil column that coincides with the window maximum.
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Comparative application of dispersion curve inversion strategies. Case study of noise arrays in the Euroseistest site, Greece
Authors Héloïse Cadet and Alexandros SavvaidisABSTRACTCurrently, the use of ambient noise arrays has become fairly routine for site characterization applications. Conventionally, the first step in the analysis of ambient noise arrays is the computation of the dispersion curve, defined as the Rayleigh wave phase velocity dependence on frequency. The second more complex step is the inversion of the dispersion curve to obtain a shear‐wave velocity profile. In many engineering applications, where only the time‐averaged shear‐wave velocity, termed Vs30, is needed, a relationship between the Rayleigh phase wave velocity at a given wavelength, VR(λ), and Vs30 is commonly developed for the specific area of study. We compare the results from two recently proposed inversion strategies, the first one is based on the misfit criteria, the second on the Akaike criteria, which we apply to experimental data acquired from the Euroseistest site in Greece. We also show that the two inversion strategies have their limits: for the first strategy, using misfit criteria and constraints, the restriction of describing the dispersion curve within the frequency band defined from the fundamental frequency to ten times this frequency is sometimes difficult to fulfil. On the other hand, the second strategy, using the Akaike criteria, is time‐consuming and requires large data storage. Finally, we conclude that the VR(λ=37m)‐Vs30 relationship is promising but should be confirmed with more data analysis.
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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)