FINE-STRUCTURE OF SCHISTES LUSTRES BASAL CONTACT IN HAUTE-MAURIENNE BASED ON BOREHOLE SEISMIC

In the framework of the Lyon/Turin High Speed Railway Link project, dr illing and seismic campaigns are currently performed to determine prec isely the geometry and nature of the subsurface in Kaute-Maurienne (Sa voie, France). La Norma, east of Modane, is one of the test sites. A p reliminary analysis of a high resolution surface seismic profile close io the La Norma borehole permitted us to identify 3 reflectors: the t op and the base of an anhydrite layer at depths of 420 and 480 metres respectively, as well as the base of the schistes lustres at a depth o f 803 metres. However, many seismic arrivals are also visible within t he first 400 milliseconds on the vertical seismic profile (VSP) and on two oblique seismic profiles (OSP) recordings. Detailed analysis of t hese seismic arrivals not only enabled us to refine the geometry of th e 3 reflectors identified by HR seismics, but it also revealed new ref lectors such as a graphitic/non-graphitic schistes lustres interface a nd the base of the deepest anhydrite unit. These observations are cons istent with a kink band structure reworking a former thrust slice. Fur thermore, analysis of the downgoing seismic arrivals allowed us to geo metrically constrain the dimension of a cavity (extending 110 m N-S) w hich was met at a depth of 444 metres in the borehole. This study thus illustrates the usefulness of borehole seismic data for a detailed in terpretation of complex geological structures.