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Propagation des ondes à l’échelle du laboratoire - Benoit DE CACQUERAY

22 novembre 2012 ( dernière mise à jour : 23 janvier 2013 )

Sujet de thèse : Elastic waves identification and extraction within Agar-agar gel

Encadrants  : Philippe Roux

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The problems associated to the presence of surface waves are the ground truth since the beginning of exploration geophysics. For deep explorations, surface waves are very energetical and represent up to 70 % of the recorded energy. They mostly hide a large part of the information coming from the earth subsurface through body waves. Then, filtering and/or modelling surface waves to cancell their impact is a constant subject of work. This supposes ever more complex simulations and tests, in particular due to the growing number of channels. Multi-component technologies and bigger design of acquisition lead to simultaneous record of several tens of thousands channels. In this condition, a simple test represents months of work for many persons. On the other hand, simulation does not take into account all the complexity of the on-site conditions. To investigate some situations, experimentation in lab environment is often an attractive alternative.

In a gel with about 6% of Agar-agar, velocities are about 1500 m/s for P waves and around 9-10m/s for S and Rayleigh waves. The speed ratio between P and Swaves make the first one nearly invisible below 1000 Hz since their wavelength is much larger than the propagation medium. Working in gels below 1000 Hz means then dealing with shear and rayleigh waves. with a velocity ratio of about 95 %. This mix is a good approximation of the geophysical environment where the surface and body waves velocities are similar. In this context, surface waves studies at laboratory scale is a exible way to evaluate new acquisition designs and processing.

This thesis shows how gel-based phantom can be successfully used to study wave mixing in the context of geophysics prospection. Small-scale experiments provide the record of thousands of traces which enables wave identification and extraction with the goal to adapt recent array processing algorithms to geophysicallike designs. Body wave extraction, passive acquisition, 4D effect and other subjects are studied.

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