In this paper results are presented from a seismic refraction experiment (CANOBE) carried out in southern Norway. Ten explosions, fired at sea, were recorded on land by shifting 13 recording instruments along a profile with an average station spacing of 5 km. The main line runs in a northeasterly direction from the south coast at Lista along the western margin of the Oslo Graben into the NORSAR array, with a total length of 5 15 km. A separate leg runs across the Graben just north of Oslo, for the first time allowing a direct comparison of seismic records in the Graben with those in the adjacent Precambrian shield area. The laterally varying crustal structure along the profile is examined by modelling of travel times and amplitudes of P-wave arrivals. The Moho, which is the major discontinuity in the lowermost crust, appears to 'sink' beneath the coast from 27 km on the seaward side to 34 km onshore. The two-dimensional modelling procedure adopted proves invaluable in explaining the characteristic amplitude pattern observed in this area. Beyond the coastal area our picture of the crust is that of a relatively homogeneous one, as expected for a shield area. There are no indications of significant discontinuities in the crust along the first 300 km of the profile, although from the large Pg amplitudes within this distance range we infer a strong velocity gradient in the lower crust. Two structural models are proposed for the Oslo Graben where the Moho appears to be elevated to between 25 and 29 km.

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BERGE, A.M., DRIVENES, G., KANESTR@M, R. and BESKOW, B. 1986, Acoustic Modeling of the Seafloor, Geophysical Prospecting 34, 11-29.Data from routine seismic surveys contain considerable information about the geoacoustic properties of the seafloor. Waves are reflected at a wide range of angles of incidence from near-vertical reflections (higher multiples) to supercritical reflections (primary and lower multiples). The reflection coefficient is approximately constant for small angles of incidence ( < 10") but varies greatly for larger angles of incidence. Near-vertical reflections are used to determine the seafloor density. The P-velocity in the seafloor is determined in advance from the critical distance using the amplitude variation of the primary as well as the multiples. The Vp/% ratio is determined by modeling the amplitude variation with the angle of incidence. The primary reflection from the seafloor and the first three multiples are included in the modeling.Seismic data obtained with both conventional and superlong airgun arrays have been modeled. Data collected from the Barents Sea show that even if the P-velocity is the same at different sites, the Vp/K ratio, density and Poisson's ratio vary significantly. The most extreme example shows that for a P-velocity of 2.80 km/s the V,/V, ratio varies between 1.9 and 6.0. The corresponding densities vary from 2.36 g/cm3 to 1.80 g/cm3 and the Poisson's ratio varies from 0.3 1 to 0.49.The acoustic modeling offers a method of assessing the mean geotechnical or mechanical properties of larger volumes of marine sediments in terms of incompressibility, shear modulus and Poisson's ratio.