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  • 1
    Electronic Resource
    Electronic Resource
    Quantitative imaging of complex structures from dense wide-aperture seismic data by multiscale traveltime and waveform inversions: a case study (2004)
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Science Ltd
    Geophysical prospecting 52 (2004), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An integrated multiscale seismic imaging flow is applied to dense onshore wide-aperture seismic data recorded in a complex geological setting (thrust belt).An initial P-wave velocity macromodel is first developed by first-arrival traveltime tomography. This model is used as an initial guess for subsequent full-waveform tomography, which leads to greatly improved spatial resolution of the P-wave velocity model. However, the application of full-waveform tomography to the high-frequency part of the source bandwidth is difficult, due to the non-linearity of this kind of method. Moreover, it is computationally expensive at high frequencies since a finite-difference method is used to model the wave propagation. Hence, full-waveform tomography was complemented by asymptotic prestack depth migration to process the full-source bandwidth and develop a sharp image of the short wavelengths. The final traveltime tomography model and two smoothed versions of the final full-waveform tomography model were used as a macromodel for the prestack depth migration.In this study, wide-aperture multifold seismic data are used. After specific preprocessing of the data, 16 frequency components ranging from 5.4 Hz to 20 Hz were inverted in cascade by the full-waveform tomography algorithm. The full-waveform tomography successfully imaged SW-dipping structures previously identified as high-resistivity bodies. The relevance of the full-waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling (VSP) log available on the profile. The prestack depth-migrated images, inferred from the traveltime, and the smoothed full-waveform tomography macromodels are shown to be, on the whole, consistent with the final full-waveform tomography model. A more detailed analysis, based on common-image gather computations, and local comparison with the VSP log revealed that the most accurate migrated sections are those obtained from the full-waveform tomography macromodels. A resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wide-aperture components of the data, allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged.The processing flow that we applied to dense wide-aperture seismic data is shown to provide a promising approach, complementary to more classical seismic reflection data processing, to quantitative imaging of complex geological structures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.2004.00452.x
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    A 2-D Velocity Model of the Vrancea Region In Romania: Prediction of Teleseismic Waveforms (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In previous studies, anomalies in arrival time and amplitudes of depth phases pP and sP recorded at teleseismic distances and produced by large intermediate-depth events of the Vrancea region were pointed out (Perrot et al. 1994). the modelling of major recent events has shown that the presence of a dipping interface and thick low-velocity sediment layer in the upper crust above the hypocentral area can explain such anomalies. Simulating broad-band records of the 1990 May 31 earthquake, it is shown that the 2-D crustal velocity model derived by Perrot et al. (1994) can also be used to explain observed waveforms in an extended azimuthal range from 105° to 243° and is valid for earthquakes having different hypocentral locations in the deep Vrancea seismic zone. Using records in the azimuthal range where a classical spherical model produces a good fit to the observed waveforms, the source is best modelled by two point sources at 90 and 93 km depth releasing the same amount of seismic moment, and separated by 1.5 s in time. Synthetics calculated for this source time history and the 2-D velocity model above the source give a much improved fit of the observed waveforms over what could be achieved with a 1-D spherical model. the results suggest that teleseismic waveforms could be reasonably well predicted for any event in the region with the 2-D velocity model, which accounts for the main structural features in the upper crust of the Vrancea region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb00016.x
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  • 3
    Electronic Resource
    Electronic Resource
    Source investigation of a small event using empirical Green's functions and simulated annealing (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We propose a two-step inversion of three-component seismograms that (1) recovers the far-field source time function at each station and (2) estimates the distribution of co-seismic slip on the fault plane for small earthquakes (magnitude 3 to 4). The empirical Green's function (EGF) method consists of finding a small earthquake located near the one we wish to study and then performing a deconvolution to remove the path, site, and instrumental effects from the main-event signal.The deconvolution between the two earthquakes is an unstable procedure: we have therefore developed a simulated annealing technique to recover a stable and positive source time function (STF) in the time domain at each station with an estimation of uncertainties. Given a good azimuthal coverage, we can obtain information on the directivity effect as well as on the rupture process. We propose an inversion method by simulated annealing using the STF to recover the distribution of slip on the fault plane with a constant rupture-velocity model. This method permits estimation of physical quantities on the fault plane, as well as possible identification of the real fault plane.We apply this two-step procedure for an event of magnitude 3 recorded in the Gulf of Corinth in August 1991. A nearby event of magnitude 2 provides us with empirical Green's functions for each station. We estimate an active fault area of 0.02 to 0.15 km2 and deduce a stress-drop value of 1 to 30 bar and an average slip of 0.1 to 1.6 cm. The selected fault of the main event is in good agreement with the existence of a detachment surface inferred from the tectonics of this half-graben.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06022.x
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  • 4
    Electronic Resource
    Electronic Resource
    Seismicity, normal faulting, and the geomorphological development of the Gulf of Corinth (Greece): the Corinth earthquakes of February and March 1981
    Amsterdam : Elsevier
    Earth and Planetary Science Letters 57 (1982), S. 377-397 
    Details
    ISSN: 0012-821X
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0012-821X(82)90158-3
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  • 5
    Electronic Resource
    Electronic Resource
    Azimuthal distortion of the seismic focal sphere: application to earthquakes in subduction
    Amsterdam : Elsevier
    Physics of the Earth and Planetary Interiors 84 (1994), S. 247-270 
    Details
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0031-9201(94)90045-0
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  • 6
    Electronic Resource
    Electronic Resource
    1-D P-velocity Models of Mt. Vesuvius Volcano from the Inversion of TomoVes96 First Arrival Time Data (2000)
    Springer
    Pure and applied geophysics 157 (2000), S. 1643-1661 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —We applied a revised version of the 1-D τ–p inversion method to first P-arrival times from the active seismic experiment performed at Mt. Vesuvius (southern Italy) in 1996 (TomoVes96 Project). The main objective of this work is to obtain 1-D velocity models of Mt. Somma-Vesuvius volcano complex and surrounding area. Moreover we show that combining the 1-D information we provide a reliable 2-D initial model for perturbative tomographic inversions. Seismic and geological surveys suggest the presence of a refractor associated with the contrast between carbonate basement and volcanic/alluvial sediments; synthetic simulations, using a realistic topography and carbonate top morphology, allowed us to study the effect of topography on the retrieved velocity models and to check that the 1-D τ–p method can also approximately retrieve the refractor depth and velocity contrast. We analysed data from 14 on-land shots recorded at stations deployed along the in-profile direction. We grouped the obtained models in three subsets according to the geology of the sampling area: Models for carbonate outcrop area, models for the Campanian Plain surrounding the volcano edifice and models for Mt. Somma-Vesuvius volcano complex. The found 1-D P-velocity models show important vertical and lateral variations. Very low velocities (1.5–2.5 km/s) are observed in the upper 200–500 m thick shallow layer. At greater depths (3 km is the maximum investigated depth) P velocities increase to values in the range of 4–6 km/s which are related to the presence of the carbonatic basement. Finally we interpolated the 1-D models to demonstrate an example of misfit for a 2-D interpolated model whose residuals are confined in a narrow band around zero.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00001054
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  • 7
    Electronic Resource
    Electronic Resource
    Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997 (2000)
    Springer
    Journal of seismology 4 (2000), S. 377-386 
    Details
    ISSN: 1573-157X
    Keywords: Seismicity ; Central Italy ; Umbria-Marche ; Aftershock sequence ; seismic crisis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1026568419411
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  • 8
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    Chapter 13 Modelling Seismic Wave Propagation for Geophysical Imaging (2021)
    InTechOpen
    Details
    Publication Date: 2024-04-04
    Description: The Earth is an heterogeneous complex media from the mineral composition scale (10−6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (Günther et al., 2006; Rücker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields.
    Keywords: seismic wave ; geophysical imaging ; seismic wave ; geophysical imaging ; Boundary value problem ; Finite element method ; Free surface ; Frequency domain ; Seismology ; Time domain ; Velocity ; thema EDItEUR::P Mathematics and Science::PD Science: general issues
    Language: English
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    E-BOOK
  • 9
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    Source investigation of a small event using empirical Green's functions and simulated annealing (1996)
    Oxford University Press
    Details
    Publication Date: 1996-06-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 10
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    Combining asymptotic linearized inversion and full waveform inversion (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-16
    Description: A method for combining the asymptotic operator designed by Beylkin (Born migration operator) for the solution of linearized inverse problems with full waveform inversion is presented. This operator is used to modify the standard L 2 norm that measures the distance between synthetic and observed data. The modified misfit function measures the discrepancy of the synthetic and observed data after they have been migrated using the Beylkin operator. The gradient of this new misfit function is equal to the cross-correlation of the single scattering data with migrated/demigrated residuals. The modified misfit function possesses a Hessian operator that tends asymptotically towards the identity operator. The trade-offs between discrete parameters are thus reduced in this inversion scheme. Results on 2-D synthetic case studies demonstrate the fast convergence of this inversion method in a migration regime. From an accurate estimation of the initial velocity, three and five iterations only are required to generate high-resolution P -wave velocity estimation models on the Marmousi 2 and synthetic Valhall case studies.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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