PYRenees Observational Portable Experiment – PYROPE

Studies of the Earth structure and dynamics are currently experiencing a strong and rapid development in various countries. In North America, the 'Bigfoot' component of the USArray program will cover United States with a transportable array of about 400 broadband instruments and a spacing of 70 km. Such a project will allow seismologists to image deep Earth structures with unprecedented resolution. These images will undoubtedly profoundly modify our views of mantle dynamics and plate tectonics. Similar ideas have been promoted for studying the European continent. Among them, the ongoing TOPO-IBERIA project will cover the entire Iberian Peninsula with a spacing of about 70 km. The third stage of this deployment, which is planned for the end of 2010, will cover the northern part of the Iberian Peninsula. The present project is intended to be the first big-scale contribution to a 'France-Array' initiative and proposes to seize the unique opportunity that will be offered at the end of 2010 to deploy a dense broadband array across the Pyrenees in coordination with Spain. This experiment will allow the Earth Sciences community to give a new look at the Pyrenees, which is one of the very few easily accessible natural laboratory where the fundamental concepts of plate tectonics can be directly confronted to geological and geophysical observations. Indeed, the Bay of Biscay-Pyrenees is one of the best areas to study the evolution of extreme crustal thinning, mantle exhumation and continental breakup resulting in the formation of an oceanic basin, and its subsequent inversion to form a mountain range. The project will also initiate the development of a large seismological database that will be accessible to the whole scientific community at the end of the experiment. This database will not only distribute seismograms, but also different seismological observables such as travel times, amplitudes, or splitting intensity, that can be extracted from seismological data. These products of seismological data analysis will first be exploited to construct high resolution tomographic models of the Pyrenean lithosphere, using classical imaging approaches. However, to fully exploit the wealth of information offered by dense arrays of broadband stations such as the one we plan to deploy, we will also develop new high resolution imaging methods, based on the utilization of finite-frequency kernels and full waveform inversion. While the calculations of 3-D kernels using the adjoint method is now possible at both regional and global scales, their utilization on large datasets is still prohibitively expensive from a computational point of view. It will thus be necessary to develop fast and practical algorithms, able to handle a large number of data in a reasonable computation time. The new tools that will be developed will also be made available to the whole community. Our project will thus embrace the whole imaging process, from data acquisition to inversion. For this reason, we have gathered a large number of scientists, with a broad range of expertise fields, for this ambitious project. The complete global coverage of France (and Europe) remains the long term objective of this experiment. An integrated European Research Structure EPOS (European Plate Observatory System) is on the way within the ESFRI Roadmap (European Strategy Forum on Research Infrastructure). The major French contribution to this European infrastructure is the development of a seismological and geodetic antenna in France (EPOS-RESIF) that will be funded as a TGE (Très Grand Equipement) by the French Ministry of Research. EPOS-RESIF will be composed of a multi-scale permanent antenna installed in Metropolitan France, which will be supplemented by a pool of about 250 mobile seismological stations. Part of this flexible component will be dedicated to complement the large scale and broadband component of the permanent antenna in order to cover France with a homogeneous and dense broadband network, in three or four consecutive stages. This 'France-Array' initiative will strongly improve the capacities of the permanent antenna, especially for the imaging of the deep structures beneath France.