APPORT DE LA SISMIQUE TRES HAUTE RESOLUTION EN DOMAINE PORTUAIRE : IDENTIFICATION DES STRUCTURES GEOLOGIQUES ET TECTONIQUES SOUS- MARINES. Very high resolution seismic : a tool for Identification of geological and tectonic sub marine structures in harbours.

In the frame of a local modernisation of the military harbour planning in Brest, a geophysical survey has been conducted to determine the thickness of the sediment cover and the depth of bed rock. A focus point was the existence of 2 escarpments, interpreted as faults in previous documents. This was the main target of this survey : determine wether these escarpments have a tectonic origin, or whereas they have a sedimentological explanation. Thus, 20 profils were acquired with the Boomer Seistec in the Brest Harbour (July 04). Due to its specificities, small dimensions and acoustic characteristics (1 to 10 kHz), the boomer Seistec penetration could reach a 80 meters depth of penetration with a vertical resolution of 25 cm, depending on the sediment type. So, the profiles allowed us to observe the bed rock and to determine that the escarpments are from a sedimentological origin and not from a tectonic origin. This assumption is based on the sub bottom reflectors which are continous and not affected by the potential « faults ». Mots Clefs: port de Brest, Sismique THR, amenagement, tectonique, sedimentologie