61 research outputs found
A study on the variability of Kappa (Îș) in a borehole: Implications of the computation process
Knowledge of the acceleration spectral shape is crucial to various applications in engineering seismology. Spectral amplitude decays rapidly at high frequencies. Anderson and Hough (1984) introduced the empirical factor Îș to model this attenuation. This is the first time Îș is studied in a vertical array consisting of more than two stations. We use 180 earthquakes recorded at a downhole array with five stations in soils and rock to investigate the effect of soil conditions on Îș. Given that Îș computation processes vary across literature when following the classic AndersonâHough method, we investigate its variability with the different assumptions that can be made when applying the method. The estimates of Îș0 range between 0.017 and 0.031 s at the surface and between 0.004 and 0.024 s at rock. This variability due to the assumptions made is larger than the error of each estimate and larger than the average difference in values between sediment and rock. For this data set, part of it can be attributed to the type of distance used. Given this variability, Îș0 values across literature may not always be comparable; this may bias the results of applications using Îș0 as an input parameter, such as groundâmotion prediction equations. We suggest ways to render the process more homogeneous. We also find that Îș at rock level is not well approximated by surface records from which we deconvolved the geotechnical transfer function. Finally, we compute Îș on the vertical component and find a dependence of the verticalâtoâhorizontal Îș ratio on site conditions
Free and smooth boundaries in 2-D finite-difference schemes for transient elastic waves
A method is proposed for accurately describing arbitrary-shaped free
boundaries in single-grid finite-difference schemes for elastodynamics, in a
time-domain velocity-stress framework. The basic idea is as follows: fictitious
values of the solution are built in vacuum, and injected into the numerical
integration scheme near boundaries. The most original feature of this method is
the way in which these fictitious values are calculated. They are based on
boundary conditions and compatibility conditions satisfied by the successive
spatial derivatives of the solution, up to a given order that depends on the
spatial accuracy of the integration scheme adopted. Since the work is mostly
done during the preprocessing step, the extra computational cost is negligible.
Stress-free conditions can be designed at any arbitrary order without any
numerical instability, as numerically checked. Using 10 grid nodes per minimal
S-wavelength with a propagation distance of 50 wavelengths yields highly
accurate results. With 5 grid nodes per minimal S-wavelength, the solution is
less accurate but still acceptable. A subcell resolution of the boundary inside
the Cartesian meshing is obtained, and the spurious diffractions induced by
staircase descriptions of boundaries are avoided. Contrary to what occurs with
the vacuum method, the quality of the numerical solution obtained with this
method is almost independent of the angle between the free boundary and the
Cartesian meshing.Comment: accepted and to be published in Geophys. J. In
Elastic full waveform inversion in the frequency domain
International audienc
Potential of muon flux density and electrical resistivity imageries for detecting and characterizing discontinuities in a clay medium at the Tournemire URL
International audienc
In situ seismic measurements in claystone at Tournemire (France)
International audienceCompressional and shear wave seismic measurements were performed in an old railway tunnel and in galleries excavated in a 250-m-thick Toarcian claystone formation in the Tournemire experimental station (France). Three component (3C) geophones and three orthogonal orientations of the vibroseismic force source were used. Additionally, vertical seismic profiling (VSP) measurements were recorded with a 3C borehole geophone, a hydrophone and a microphone in a 159 m deep borehole (ID180) in the tunnel. The seismic data show that Toarcian claystone has strong transverse isotropy (TI) with a vertical symmetry axis. The qP, SH and qSV wave propagation velocities in horizontal directions-the plane of isotropy of the TI medium-are measured as 3550, 1850 and 1290 m s-1, respectively. The zero-offset VSP reveals that only one shear wave propagates in the vertical (depth) direction and the P- and S-wave velocities are 3100 and 1375 m s-1, respectively. Four elastic moduli of the TI medium are determined from the seismic velocities and from the bulk density of 2.53 g cm-3 c11 = 31.9 GPa, c33 = 24.3 GPa, c44 = 4.5 GPa and c66 = 8.7 GPa. A walkaway VSP with the borehole geophone at 50 m depth in borehole ID180 and shot points in the galleries leads to oblique seismic ray paths which allow us to determine the fifth elastic modulus of the TI medium to c13 = 16 GPa. The tube wave recorded by a hydrophone in the water filled lower part of the borehole propagates with 1350 m s-1, which confirms the estimate of the elastic constant c66. The analysis of body wave and surface wave data from a seismic experiment in Galerie Est shows reflections from several fracture zones in the gallery floor. The thickness of the excavation damaged zone (EDZ) in the floor of Galerie Est is estimated to 0.7 m. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society
An Unknown active fault revealed by microseismicity in the South-East of France
Geophysical Research Letters, v. 30, n. 15, p. 1782, 2003. http://dx.doi.org/10.1029/2003GL017171International audienc
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