Strategies for visco-acoustic waveform inversion in the Laplace-Fourier domain, with application to the Nankai subduction zone

Waveform inversion is a non-linear and ill-posed inverse problem, with the objective of utilizing the full information content of recorded seismic waveforms. A Laplace-Fourier domain implementation allows a natural `multiscale\u27 approach that mitigates the non-linearity and ill-posedness by inverting low-frequency, early arrival data in the initial stages of inversion. High-frequency components, and late arrivals are incorporated at a later stage. This allows the development of robust inversion strategies capable of handling large wide-angle crustal surveys, leading to reliable, high-resolution velocity and attenuation models of crustal structures. I apply waveform inversion to extract a P-wave velocity model of the active megasplay fault system in the seismogenic Nankai subduction zone offshore Japan, using controlled-source Ocean Bottom Seismograph data. The resulting velocity model includes detailed thrust structures, and low velocity zones not previously identified. The connection of large low-velocity zones in the inner and outer wedge suggests a significant distribution of overpressured regions in the vicinity of the megasplay fault, with the potential to strongly influence coseismic rupture propagation. I identify six-fold key strategies for successful waveform inversion; i) the availability of low-frequency and long offset data, ii) a highly accurate starting model, iii) a hierarchical approach in which phase spectra are inverted first, and amplitude information is only incorporated in the final stages, iv) a Laplace-Fourier approach, v) careful preconditioning of the gradient, vi) strategies for source estimation. Chequerboard tests and point-scatter tests demonstrate the resolution and the limitations of the acoustic implementation. I also compare four misfit functionals for optimization, and demonstrate that velocity information may be reliably extracted from phase alone, and that amplitude information is secondary in updating the velocity model. Finally I develop inversion strategies for retrieving both velocity and attenuation models. Cross-talk between these two classes of parameter estimates arises from the lack of parameter scaling in the gradient of the objective function, and primarily affects the attenuation model. I show the cross-talk can be suppressed by the combination of an appropriate attenuation damping parameter, and by the use of smoothing constraints. Initial velocity-only inversions also help in reducing the effects of cross-talk in subsequent velocity-attenuation inversion

2D PET backprojection acceleration through a 2D predictive cache

Reduction of image reconstruction time is a key point for the development and spreading of PET scans. Thus this article presentes a hardware/software architecture which aims at accelerating the 2D reconstruction on a SoPC (System on Programmable Chip) plateform, the new generation of reconfigurable chip. Issue posed by the latency of memory accesses has been solved thanks to the 2D Aptative and Predictive cache (2D-AP cache).Le développement et la diffusion des équipements TEP passent par la réduction des temps de calcul de la reconstruction des images acquises. Aussi cet article présente une solution mixte logicielle/matérielle pour l'accélération de la reconstruction 2D sur une plateforme SOPC (System on Programmable Chip), la nouvelle génération de circuits reconfigurables. Le verrou technologique posé par la latence des accès mémoire est levé grâce au cache 2D Adaptatif et Prédictif (cache 2D-AP)

Application d'un cache 2D prédictif à l'accélération de la rétroprojection TEP 2D

Le développement et la diffusion deséquipements TEP passent par la réduction des temps de calcul de la reconstruction des images acquises. Aussi cet article présente une solution mixte logicielle/matérielle pour l'accélération de la reconstruction 2D sur une plateforme SOPC (System on Programmable Chip), la nouvelle génération de circuits reconfigurables. Le verrou technologique posé par la latence des accès mémoire est levé grâce au cache 2D Adaptatif et Prédictif (cache 2D-AP). Abstract : Reduction of image reconstruction time is a key point for the development and spreading of PET scans. Thus this article presentes a hardware/software architecture which aims at accelerating the 2D reconstruction on a SoPC (System on Programmable Chip) plateform, the new generation of reconfigurable chip. Issue posed by the latency of memory accesses has been solved thanks to the 2D Aptative and Predictive cache (2D-AP cache)