Rapid response to the earthquake emergency of May 2012 in the Po Plain, northern Italy

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On the linearity of cross-correlation delay times in finite-frequency tomography,

International audienceWe explore the validity of the linear relation between cross-correlation delay times and velocity model perturbations that is required for linearized finite-frequency tomography. We estimate delay times from a large number of 'ground truth' seismograms computed with the spectral element method in 3-D models. We find that the observed cross-correlation delays remain sufficiently linear, depending on frequency, for sharp velocity contrasts of up to 10 per cent in a checkerboard model. This significantly extends the domain of linearity beyond that of inversions based on direct waveform differences. A small deviation from linearity can be attributed to the Wielandt effect (i.e. the asymmetry in the effect of positive and negative anomalies on the traveltime). Smoother Gaussian covariance models can have velocity variations twice as large and cross-correlation delay times still remain sufficiently linear for tomographic interpretations

Comparison of ray- and adjoint-based sensitivity kernels for body-wave seismic tomography

International audienceWe compare finite-frequency sensitivity kernels computed from ray-theoretical wavefields ("banana-doughnut" kernels) and from full waveform computations (often called "adjoint" kernels) in order to evaluate resolution, accuracy and computational cost. We focus here on body-wave seismic tomography at regional and local scales. Our results show that: (1) for homogeneous reference media, ray-based and adjoint kernels agree except for the expected differences in the regions close to the source and the receiver, where near-field contributions are neglected in the ray-based kernels; (2) for a smooth 3D background velocity model, the differences in predicted delay times for the two methods are generally well below 10 % of the delay for P waves, though as much as 20 % for S-waves, suggesting that extra care should be taken when performing S-wave tomography with ray-based "banana doughnut" kernels

Parametric Study on the Interpretation of Wave Velocity Obtained by Seismic Interferometry in Beam‐Like Buildings

International audienceAbstract In this article, we propose an interpretation of the propagation velocities of the pulse wave obtained in vertical structures through seismic interferometry by deconvolution. The novelty of this article is to propose a parametric study applied to canonical finite‐element models of fixed‐base buildings from pure‐shear to pure‐bending beam‐like buildings, adjusted to equivalent Timoshenko beam‐like structures. For given input seismic motions, the time histories of the horizontal displacement at each floor are obtained and used to estimate the propagation velocity of the pulse wave by deconvolution. A frequency–wavenumber technique is used to highlight the dispersive characteristics of the pulse wave. The obtained velocity is compared with the theoretical dispersion curve of the Timoshenko beam‐like structure and interpreted according to the nature of the structure. We propose a corrective coefficient to link the first resonance frequency of the building to the velocity obtained by deconvolution, according to the shear‐to‐bending ratio. Finally, we compare specific Timoshenko beam models with a number of previously published studies on the experimental interpretation of velocity in real‐case buildings for which soil–structure interaction conditions are different from the fixed‐base conditions of the Timoshenko beam‐like structure

Cross-borehole tomography with correlation delay times,

International audienceWe evaluated a comprehensive numerical experiment of finite-frequency tomography with ray-based ("banana-doughnut") kernels that tested all aspects of this method, starting from the generation of seismograms in a 3D model, the window selection, and the crosscorrelation with seismograms predicted for a background model, to the final regularized inversion. In particular, we tested if the quasilinearity of crosscorrelation delays allowed us to forego multiple (linearized) iterations in the case of strong reverberations characterizing multiple scattering and the gain in resolution that can be obtained by observing body-wave dispersion. Contrary to onset times, traveltimes observed by crosscorrelation allowed us to exploit energy arriving later in the time window centered in the P-wave or any other indentifiable ray arrival, either scattered from, or diffracted around, lateral heterogeneities. We tested using seismograms calculated by the spectral element method in a cross-borehole experiment conducted in a 3D checkerboard cube. The use of multiple frequency bands allowed us to estimate body-wave dispersion caused by diffraction effects. The large velocity contrast (10%) and the regularity of the checkerboard pattern caused severe reverberations that arrived late in the crosscorrelation windows. Nevertheless, the model resulting from the inversion with a data fit with reduced χ2red=1 resulted in an excellent correspondence with the input model and allowed for a complete validation of the linearizations that lay at the basis of the theory. The use of multiple frequencies led to a significant increase in resolution. Moreover, we evaluated a case in which the sign of the anomalies in the checkerboard was systematically reversed in the ray-theoretical solution, a clear demonstration of the reality of the "doughnut-hole" effect. The experiment validated finite-frequency theory and disqualified ray-theoretical inversions of crosscorrelation delay times. Read More: http://library.seg.org/doi/abs/10.1190/geo2013-0059.

An efficient algorithm for sampling the shear-modulus reduction curve in the context of wave propagation using the elastoplastic Iwan model

International audienceSUMMARY The elastoplastic Iwan model has been used since the end of the 1970s to simulate nonlinear soil behaviour in seismic wave propagation. In this work, we present an automatic algorithm to efficiently sample the shear-modulus reduction curve in function of shear deformation, which constitutes the exclusive ingredient of the elastoplastic model. This model requires the data from the shear- modulus reduction as a function of shear deformation, which are readily available in the literature and from specific laboratory tests. The method involves a discretization and interpolation of these data to be used. The quality of the solution depends on the number of interpolated points. However, a larger number of them produce an increase of the computational time. To overcome this, we present an automatic algorithm to efficiently sample the shear-modulus reduction curve. We numerically prove that the chosen discretization of the curve has a strong impact on the calculation load, in addition to the well-known dependence on the input motion amplitude level. Two tests of nonlinear wave propagation in 1-D and 3-D media show the clear gain in computation time when using the proposed automatic sampling algorithm

Identification of two vibration regimes of underwater fibre optic cables by Distributed Acoustic Sensing

International audienceSummary Distributed Acoustic Sensing (DAS) enables data acquisition for underwater Earth Science with unprecedented spatial resolution. Submarine fibre optic cables traverse sea bottom features that can lead to suspended or decoupled cable portions, and are exposed to the ocean dynamics and to high rates of marine erosion or sediment deposition, which may induce temporal variations of the cable’s mechanical coupling to the ocean floor. Although these spatio-temporal fluctuations of the mechanical coupling affect the quality of the data recorded by DAS, and determine whether a cable section is useful or not for geophysical purposes, the detection of unsuitable cable portions has not been investigated in detail. Here, we report on DAS observations of two distinct vibration regimes of seafloor fibre optic cables: a high-frequency (>2 Hz) regime we associate to cable segments pinned between seafloor features, and a low-frequency (<1 Hz) regime we associate to suspended cable sections. While the low-frequency oscillations are driven by deep ocean currents, the high-frequency oscillations are triggered by the passage of earthquake seismic waves. Using Proper Orthogonal Decomposition, we demonstrate that high-frequency oscillations excite normal modes comparable to those of a finite 1D wave propagation structure. We further identify trapped waves propagating along cable portions featuring high-frequency oscillations. Their wave speed is consistent with that of longitudinal waves propagating across the steel armouring of the cable. The DAS data on cable sections featuring such cable waves are dominated by highly monochromatic noise. Our results suggest that the spatio-temporal evolution of the mechanical coupling between fibre optic cables exposed to the ocean dynamics and the seafloor can be monitored through the combined analysis of the two vibration regimes presented here, which provides a DAS-based method to identify underwater cable sections unsuitable for the analysis of seismic waves