ALBERT

Description: Wide-angle seismic data acquired by use of air-guns and Ocean Bottom Seismometers (OBS) contain strong direct water arrivals and multiples, generally considered as noise and thus not included in the modelling. However, a recent study showed that standard ray-tracing modelling of the water multiples recorded off the Bear Island, North Atlantic, provided a reliable estimate of the velocity distribution in the water layer. Along the profile, the water velocity is found to change from about 1450 to approximately 1490 m/s. In the uppermost 400 m the velocities are in the range of 1455-1475 m/s, corresponding to the oceanic thermocline. In the deep ocean there is a velocity decrease with depth, and a minimum velocity of about 1450 m/s is reached at about 1.5 km depth. Below that, the velocity increases to about 1495 m/s at approximately 2.5 km depth. Here, we demonstrate that including the amplitudes in the modelling provide valuable information about the VP contrast at the seafloor, as well as the VP/VS ratio and attenuation (QP) of the uppermost sediments. The VP contrast at the seafloor is estimated at about 250 m/s, within a precision of approximately ±30 m/s. The VP/VS ratio in the uppermost sedimentary layer is modeled in the range 2.25-2.50, and the QP factor is estimated at 1000 for the water and 30-50 and 40-50 for the uppermost sediments. The values obtained for the sediments suggest a lithology dominated by silty clays, with porosity below average.