MODELING SHEAR-WAVES IN OBS DATA FROM THE VORING BASIN (NORTHERN NORWAY) BY 2-D RAY-TRACING

Three component recordings from an array of five ocean bottom seismogr aphs in the northwestern part of the Voring basin have been used to ob tain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves a re identified by means of travel-time differences compared to the comp ressional (P) waves, and by analyzing their particle motions. The mode l has been obtained by kinematic (travel-time) ray-tracing modelling o f the OBS horizontal components. The shear-wave modelling indicates th at mode conversions occur at several high velocity interfaces (sills) in the 4-10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set. An average V-p/V-s rati o of 2.1 is inferred for the layers above the uppermost sill, indicati ve of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in the V-p/V-s ratio (1.7) below the first sill may in part be attributed to well consolidated sediments, and to a cha nge in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5-10 km depth V-p/V-s ra tios of 1.85 indicate a lower sand/shale ratio consistent with the exp ected lithologies. The average V-p/V-s ratio inferred for the crust is 1.75, which is consistent with values obtained north of Voring, in th e Lofoten area. An eastward thinning of the crystalline basement is su pported by the shear-wave modelling.