Doing tomography differently: building the imaging tools of tomorrow
Geophysics
Final Report Abstract
The CLEARVIEW project is part of an international joint effort that has been underway for ten years, and is still ongoing. The goal is to develop effective numerical methods and corresponding software tools for the complete inversion of waveforms, the most promising way to improve the resolution of tomographic images. By construction, this approach is very costly in terms of computation because it requires a large number of approximate numerical solutions of the wave equation in a 3-D medium. This holds for both the amount of forward problems and the amount of frequencies to cover in an inversion. Methods. To overcome the difficulties associated with the nonlinearity and ill-posedness of waveform inversion, we have developed a new optimization method under constraint that improves the descent direction computed from the gradient of the cost function while remaining less expensive than Newton’s approach, which requires inverting the full Hessian. We have also developed new algorithms that allow us to take into account a priori information on the models by means of regularization terms. Finally, we have investigated the effect of attenuation in the inversion problem, which is important in explaining the amplitude of waveforms. All the inversion methods developed for imaging open up important perspectives, particularly for imaging the Earth, at all scales, from the global Earth to the scale of the reservoir (e.g. storage of CO2 , geothermal energy), or medical imaging. Major results. The new methods developed in this project will allow us to image the interior of the Earth with unprecedented spatial resolution. Our first studies have focused on subduction zones: Andes, Cascades, and Japan, and in particular on regions where mega-earthquakes (M > 8) occur, with the hope of better understanding how these major destructive events develop. Another application, on a more local and superficial scale, concerns the imaging of systems that produce natural hydrogen, for example by interaction between water and mantle rocks. Summary. The CLEARVIEW project has enabled the development of a new generation of hybrid seismic wavefield modeling codes on a regional scale. This is an important breakthrough because it paves the way for full waveform inversion imaging. The development of new inversion methods, converging more quickly to more robust solutions, also represent a significant improvement for waveform inversion problems.
Publications
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Efficient simulation of challenging PDE problems on CPU and GPU clusters. PhD Thesis, University of Stuttgart
Malte Schirwon
