Analyse des intercorrélations du champ d'onde sismique ambiant - Application à la tomographie de l'Ouest de la France

摘要

The continuous excitation of the Earth’s surface due to oceanic wave-seafloor coupling, is widely used for both analyzing the seismic ambient wavefield properties and for imaging the deep structures. A new statistical approach based on the phase redundancy of cross-correlations is developed to provide quantitative information on the ubiquitous seismic signal. Two dense arrays of broadband seismometers have been temporarily deployed over the southern and western France, and over the northern Spain, providing a good opportunity to reveal the crustal and uppermost mantle features beneath this region. A Monte-Carlo Markov chain inversion algorithm is used to translate, for each station pair, the frequency dependent energy diagram of the ambient noise cross-correlation, into probability density distribution of the shear wave velocity as a function of depth. The 1D velocity profiles are then combined together using an anisotropic regionalization scheme, to build a 3D velocity model with an unprecedented lateral resolution of 75 km. The isotropic velocity maps correlate well with the geology up to 10 km depth. While negative velocity anomalies are associated with the sedimentary basins, the crystalline Variscan basement displays positive anomalies. At 20 km depth, the Bay of Biscay exhibits positive velocity anomalies, whose eastern boundaries can beinterpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale.