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71st EAGE Conference and Exhibition - Workshops and Fieldtrips
- Conference date: 08 Jun 2009 - 11 Jun 2009
- Location: Amsterdam, Netherlands
- ISBN: 978-94-6282-103-3
- Published: 08 June 2009
21 - 40 of 112 results
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Imaging scattered seismic surface waves with nonuniform receiver arrays
Authors A. Kaslilar and X. CampmanSurface waves are widely used for determining properties of the subsurface on different scales. In most applications, the dispersion of surface waves is analyzed to yield depthdependent velocity profiles. However, the heterogeneity of the shallow subsurface complicates such an analysis because of the scattering it causes. Here, we use an inverse scattering method to determine the location and properties of the heterogeneity. First, we illustrate this method, using a regular receiver array. A synthetic example shows that the scatterer is imaged at the correct location with good resolution. In practice, we may want to take advantage of existing non-uniform arrays, such as those deployed for global seismic studies. In our next example, we generate scattered data at irregularly spaced locations. By simultaneously inverting the scattered wave field excited by different sources, we observe that the image is well focused on the scatterer. Further work is on the way to better understand the influence of non-uniform (and sparse) sampling on the image quality and what are the sampling requirements.
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Multimodal Inversion of Surface Wave Data at Sites with Shallow Bedrock
Authors C. Cesare, F. Sebastiano and M. MargheritaIn this paper results of a characterization campaign, conducted by means of surface wave tests, are presented. Tests are conducted on sites with shallow bedrock and the presence of stiff seismic interfaces. The sites are the location of some stations of the Italian Accelerometer Network in the Liguria Region (Italy) and the experimental tests are part of a the development of the Italian strong motion database. Due to the peculiar shear wave velocity profile of the sites, a multimodal inversion is required because the effective dispersion curve is strongly affected by the presence of higher modes of propagation. Three examples are presented, in order to show that a multimodal inversion is of paramount importance in order to increase investigation depth, avoid errors in the retrieved soil profiles due to mode misidentification and improve the resolution on shallow layers, particularly in situations in which the effect of higher modes could be relevant (i.e. velocity inversions or strong seismic contrasts). The used inversion algorithm is a stochastic algorithm, which demonstrates its ability of properly taking into account higher modes of propagation allowing for a correct definition of seismic velocities and interfaces depth in different stratigraphic conditions. Combined refraction interpretation performed on the same seismic dataset provides an useful comparison to assess the reliability of the results.
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Love and Rayleigh waves dispersion analysis from microtremor measurements at Bevagna (Italy)
Authors K. Tokeshi, S. Foti, S. Parolai, M. Picozzi, R. Puglia, M. Massa and E. D’AlemaIn the last decades, researchers focused their attention on using the Rayleigh waves dispersion characteristics to estimate the shear wave velocity profile of a site, since their dispersion characteristics can be extracted also from only one component recordings (namely the vertical ones) of active and passive seismic tests. In particular, inverse analysis of empirical surfacewaves dispersion curves from microtremor measurements (passive seismic test) is a very attractive tool since it allows to keep the cost of investigation relatively low and to avoid the use of active sources that might be prohibitive in urban areas. However, until now the full potential of seismic noise array methods was not fully exploited, and, in particular, the possibility of better constraining the subsoil structure by extracting also the Love wave part from microtremors recordings has not been fully investigated.
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Global surface wave tomography
By J. TrampertI will present an overview of the state-of-art surface wave analysis which consists of three steps: (i) The measurement of the average dispersion between source and receiver (or between two receivers on the same great circle path) based on a non-linear waveform inversion. (2) The construction of anisotropic phase velocity maps which summarises the measurements on the sphere. (3) The local depth inversion of the phase velocity maps for elastic parameters. At each step, uncertainties are evaluated and all parameters are expressed in terms of probability density functions. Examples will show a global crustal model, transition structure and probabilities of anisotropy in the upper and lower mantle.
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Time-lapse monitoring of surface waves
Authors P. Hatchell, P. Wills and C. DidragaObservations of surface and near-surface waves show that shallow velocities change significantly in response to reservoir compaction that often takes place thousands of meters deeper. The compacting reservoir induces vertical and horizontal displacements and strains at the seafloor that lead to anisotropic changes in the shallow p- and s-wave velocities. Analyzing permanent OBC data from the Valhall field we observe large changes in the arrival times and phase velocities of Scholte and critically refracted compressional waves between several surveys acquired over a 5 year interval. The changes in the shallow velocities are compared with seafloor strains predicted by geomechanical models and a reasonable agreement is found. Shallow waves present us with an excellent laboratory for calibrating the stress and strain dependence of our rock physics models and are useful for constraining geomechanical models and monitoring the compaction of deep reservoirs.
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SASW measurements in civil engineering applications: using wavelengths to tailor profiling depths from 5 cm to 500 cm
By K. H. StokoeField seismic testing is an active and growing area in Civil Engineering, especially in the subdisciplines of geotechnical, transportation, and structural engineering. Traditionally, intrusive seismic methods have been used. Over the past 15 years, nonintrusive surface-wave methods have undergone significant developments. One surface-wave method, the spectral analysis of surface waves (SASW), is briefly discussed. Development of the SASW method began in the mid-1980’s out of needs in geotechnical earthquake engineering to determine shear wave velocity (Vs) profiles in hard-to-sample soils (e.g. liquefiable gravelly soils) and in pavement engineering to evaluate stiffness profiles without boreholes. Because the method is nonintrusive, applications within Civil Engineering have mushroomed. A brief background of the SASW method is presented. Case histories illustrating the range in applications and profiling depths are discussed. Some examples are: (1) detection of the onset of fatigue cracking in the asphaltic surface layer of a pavement using 2- to 20-cm long wavelengths, (2) evaluation of the improved zone in a liquefiable sand due to dynamic compaction using 0.3- to 30-m long wavelengths, and (3) determination of Vs profiles at Yucca Mountain, Nevada using 1- to 1000-m long wavelengths.
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Attenuating scattered surface wave: two faces of interferometry
Authors D. F. Halliday, A. Curtis and K. FinlaySurface wave interferometry is the process of estimating inter-receiver surface waves by cross-correlation, cross-convolution or deconvolution of wavefields recorded at each receiver. We study seismic interferometry of scattered surface waves in attenuating media. One of the starting assumptions for interferometry by correlation is that the medium of interest is lossless, however the near surface of the Earth is often strongly attenuating. On the other hand interferometry by convolution makes no assumptions about medium losses. By investigating both correlation-type and convolution-type interferometry we identify how to successfully approach the problem of estimating inter-receiver scattered waves, and illustrate our findings using real and synthetic examples. Such estimates have applications in the attenuation of scattered ground roll and in near surface imaging and characterization.
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Verification of Rayleigh-wave dispersion characteristics using synthetic microtremors
Authors K. Tokeshi and L. V. SoccoAn attempt to verify the characteristics of experimental Rayleigh-wave dispersion curve from passive microtremor measurements is carried out through synthetic microtremors. As case study, array microtremor measurements were performed at an Alpine site. The average experimental vertical Rayleigh dispersion curves obtained by the f-k spectral method was used to estimate subsoil models by random search (Monte Carlo inversion). Also, the predominant frequency of experimental H/V spectral ratio was calculated to improve the inversion process. Several researchers have reported that the fundamental frequency of theoretical H/V spectral ratio or the predominant frequency of experimental H/V spectral ratio would represent an approximate threshold for the lowest frequency of experimental vertical Rayleigh dispersion curve. However, the highest velocity of the experimental Rayleigh dispersion curve occurred at a frequency higher than the value of the experimental H/V predominant frequency. In consequence, this experimental high velocity would not correspond to the fundamental Rayleigh dispersion curve. For verifying purposes, synthetic microtremors using a similar array to the one used at the site, were calculated for one of the subsoil models estimated through Monte Carlo inversion. The synthetic vertical Rayleigh dispersion curve was in good agreement with the experimental one, verifying the above assumptions.
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On the feasibility of surface waves approximation to interpret ambient vibrations wave field
Authors D. Albarello and E. LunedeiRandom vibrations monitoring and analysis represents a cheap and effective tool for the seismic characterization of the subsoil. A standard approach to the physical interpretation of ambient vibrations measurements is based on the hypothesis that surface waves dominates the relevant wave field. To evaluate the reliability of this hypothesis, a general physical model for ambient vibrations generated by anthropic activities has been. In this model, the complete wave field generated by a random distribution of independent point loads at the surface of a weakly dissipative layered Earth has been considered. Horizontal to Vertical Spectral Ratios computed from the complete wave field solution have been compared with those deduced in the surface waves hypothesis. This comparison suggests that this approximation supplies reliable only results in the frequency range above the resonance frequency of the local subsoil. At the resonance frequency, a reasonable agreement is obtained when a source-free area exists around the receiver with a radius of the order of few tens of meters.
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The role of intermodal cross terms in seismic interferometry
Authors W. P. Kimman and J. TrampertThe exact interferometric equation gives the relation between the retrieved Green’s function and signals arriving from an enclosed surface integral of sources (monopole and dipole sources). This relation is exact, even for complicated media. If only monopole sources are assumed as needs to be done for all practical cases, retrieval can be far from perfect in heterogeneous media. We show that this is due to intermodal cross terms that will appear even if the source distribution is perfect. Especially when sources are present at the free surface only, cross terms have the potential to overwhelm higher modes completely. Luckily one-bit correlation overcomes this problem by the amplifying higher modes.
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Marine surface wave investigation in a harbour in southern Sweden
Authors R. Wisén and M. SvenssonA marine surface wave seismic investigation was performed in order to characterize the sediments and bedrock in Ystad harbor, southern Sweden. The dispersive nature of Sholte waves was analyzed and fundamental mode dispersion curves extracted. Using inverse modelling a few layer shear wave velocity, Vs, profile was successfully retrieved to a depth of 5-10 m below sea bottom. The seismic results were confirmed with result from geotechnical probing. The measurements were made with a 12-channel neutral buoyancy streamer with 2 m hydrophone separation. A small number of led plates were attached to the streamer in order to make it sink and for each measurement the ends of the streamer were fixed on the bottom using small concrete blocks. The streamer was pulled up on the vessel before moving to the next measurement position. As energy source a small amount of explosives was detonated on the sea bottom with the shot positioned at 15-30 m distance from the nearest hydrophone and inline with the streamer. The production rate of this approach was 30 measurements in one field day, which should be compared to the 1 or 2 geotechnical soundings that can be achieved at the same time.
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Dispersion accuracy using Slant Stack transformation
More LessLast decencies show the increasing interest in using Multichannel Spectral Analysis of Surface Waves (MASW) for ground structure recovery. Using MASW the dispersion characteristics of surface wave field is often determined using so called p−τ transformation known as Slant Stack (SL). It is commonly assumed that the acquisition geometry can affect the dispersion results and several general rules in current MASW practice are observed when this geometry is designed. They suggest for example to observe the minimum offset in order to avoid the interferences of surface waves with the body waves, the maximum offset in order to reach the required depth and maximum geophone spacing in order to avoid spatial aliasing. In the same time there are few reports on the influence of the array geometry on the resolution (i.e. accuracy) who deal with the array involved uncertainty of a dispersion curve. This paper aims a more detailed assessment of above problem: The accuracy of the SL procedure is studied as the function of the acquisition system like receiver spacing, receiver number, aperture length, sampling rate and signal frequency. It is shown how the not adequate array geometry design can corrupt the dispersion result and lead to erroneous interpretation. The easy to use criteria are formulated.
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Developments in seismic interpolation and regularization and their applications to acquisition and processing
By R. L. AbmaBecause of the cost, seismic data are seldom acquired with a sampling that is optimal for seismic processing. To produce data with optimal sampling, interpolating or regularizing the seismic data may be used, not only to improve the resolution of the results, but to reduce the cost of acquiring seismic surveys. Multi‐dimensional interpolation methods that allow seismic data to be de‐aliased can provide significant improvements in quality and ease‐of‐use over older interpolation methods.
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Accurate data reconstruction through simultaneous application of statistical and physics-based constraints to multiple geophysical datasets.
Authors A. Baumstein and R. NeelamaniSpatial aliasing is the key problem that affects seismic data interpolation techniques. Although existing methods can interpolate data "beyond aliasing" under certain assumptions, their performance degrades as shot and receiver sampling become coarser. Most existing data interpolation techniques overcome aliasing by making assumptions regarding either the statistical properties of the data or the physics that explains the data.
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Four methods of data regularization
More LessWhen seismic data regularization is formulated as an inverse problem, it requires mathematical regularization, a method for imposing constraints on the reconstructed data. Mathematical regularization can take four different forms: a differential operator (such as a prediction-error filter or a plane-wave destructor), an integral operator (such as a recursive inverse of prediction-error filtering or a plane-wave constructor), a sparseness constraint in a special domain (such as Fourier or seislet), or a shaping operator. Similar results can be achieved with different methods but at a different computational cost. Using both onedimensional toy examples and seismic field data applications, we compare and illustrate properties of the four methods of data regularization.
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Beating Nyquist by randomized sampling
Authors G. Tang, R. Shahidi and F. J. HerrmannSeismic exploration is rooted in the paradigm of regular Nyquist sampling. Recent developments in the area of “compressive sensing” have shown that regular-Nyquist sampling may not be most favorable for signals that are compressible, e.g., by curvelets. In that case, appropriate randomization of the acquisition; such as by jittered1 sampling, creates favorable recovery conditions from data collected at a limited number of randomly placed sources and receivers. By virtue of the randomization, and the subsequent nonlinear recovery by transform-domain sparsity promotion, significant improvements can be made in the frequency content and quality of regularly sampled data volumes obtained from highly undersampled randomized measurements. In this new paradigm, the effective sampling rates are no longer dictated by Nyquist but by transformdomain sparsity. Consequently, twice as compressible signals can be recovered from only halve the randomized sample points. This removes a major impediment of acquisition where costs are proportional to sample rates and survey areas. During this talk, we will discuss multidimensional and continuous2 (opposed to gridded) extensions of randomized jitter sampling. These will include investigations of Poisson disk sampling3 and Farthest Point sampling4 and we will demonstrate that these randomized schemes perform better than the wellestablished regular sampling protocol.
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Operator-oriented CRS interpolation
Authors G. Hoecht, P. Ricarte, S. Bergler and E. LandaIn common-reflection-surface (CRS) imaging the reflection arrival time field is parameterized by a kinematic description of the recorded wavefield. Using the CRS approach locally in the unmigrated prestack data domain opens a potential for trace regularization and interpolation. The presented CRS interpolation operates in the original prestack data domain and accounts for irregular geometries. A distinction to existing methods is the use of local operators of the second order which are less sensitive to aliasing but imply a considerable increase of the computational expense.
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Improving old tricks: FX interpolation beyond alias for irregularly sampled data, for data with large gaps and for data with spatially variant dips
Authors M. Naghizadeh and M. SacchiFor almost two decades, FX prediction filtering (Spitz, 1991) and FK interpolation (Gülünay, 2003) have provided a robust route to interpolate aliased seismic data. These are probably the only two interpolation methods that were built on sound and solid arguments for the interpolation of aliased events. They both have precise assumptions that one must honor to optimally interpolate seismic data. In essence, the data must be composed of a finite superposition of events with linear moveout. The latter is often validated by windowing the data. In addition, both methods require regularly sampled data and a good SNR in the low frequency portion of the spectrum where the vital non‐aliased Fourier coefficients reside.
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Seismic data reconstruction with application to SRME for walkaway VSP data
Authors E. Otnes and K. HokstadInterferometry is used to reconstruct surface data from Walkaway VSP data. Together with a common receiver gather of a WVSP experiment we formulate a data-driven surface demultple scheme for WVSP data. The theory will be presented together with real data examples from the Nordkapp basin in the Barents Sea, an area well known for its complex geological structures.
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Green's theorem framework for data reconstruction
By A. RamirezThere is a tremendous and pressing need to improve our ability to effectively extrapolate, interpolate and regularize seimic data. That drives the interest in methods of data reconstruction in general.
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