DETERMINATION OF THE SUPERFICIAL VELOCITY-FIELD FROM FIRST BREAKS INVERSION IN THE CADOMIAN DOMAIN OF THE ARMORICAN BELT (FRANCE)

First arrival times of the deep seismic profile Armor AR2; recorded in 1995 in the northern part of the Armorican Belt, are inverted in orde r to obtain the superficial velocity field. Initially, first breaks fr om the unit shots are taken as the observed traveltimes data set. Then , they are compared to ones computed by ray-tracing using an initial v elocity model. An iterative algorithm, based on damped least-square me thod, is used to invert the data, until the velocity model converges t o an acceptable solution. This is later discussed in terms of resoluti on and uncertainties before being compared with geological observation s. The results allow us to explain the upper seismic velocity layer as a shallow weathered zone probably corresponding to a Tertiary palaeos urface. The weak velocity zone, observed at depth in the central part of the profile, underlines the importance of deformation and fracturat ion on weathering development linked to the North Armorican Shear Zone . ((C) Academie des sciences / Elsevier, Paris.).