Apport de la sismique marine très haute résolution à l’étude de la rade abri de Brest (France)

Dans le cadre de la modernisation du port militaire de Brest, deux reconnaissances geophysiques realisees avec le boomer Seistec (en 2004 et en 2006) ont permis de reconnaitre la couverture sedimentaire et la profondeur du socle. De precedentes investigations avaient etabli l’existence de decalages verticaux des couches geologiques grâce a des sondages. La vision "en continu" fournie par les profils sismiques THR permet d’apprehender de facon differente ces decalages, et de les associer a des structures ondulees remarquables, relatives a une phase de remplissage de la Baie. Cette vision globale des fonds permet de comprendre l’evolution geomorphologique de la rade dans son ensemble et de depasser la vision "ponctuelle". Ainsi, l’influence de l’Aulne, et non de la Penfeld comme le laissait supposer la disposition actuelle, s’avere preponderante. Translated version: The study of the military Harbour of Brest (France): contribution from a Very High Resolution seismic tool Within the framework of the modernisation of the military harbour of Brest, two seismic surveys have been conducted with the Seistec boomer (in 2004 and 2006) to recognize the sedimentary cover and depth of the rock basement. Previous investigations, based on drill holes, have shown the occurrence of vertical offset between geological strata. The "continuous" view acquired with the VHR seismic profiles allows a new approach of these vertical offsets and thus a new interpretation: they could be special undulating structures relative to the phase of sedimentary infilling of the bay. Such a global vision of the sea bottom floor allows to better understand the geomorphologic evolution of the harbour and to go beyond the "punctual" view. Whereas the Penfeld River seemed to be the actual active river, the influence of the Aulne River appears to have been preponderant in the sedimentary dynamics of the bay. Keywords: Brest harbor; Boomer seismic; Sediments; Faults; "Continuous versus punctual view".