Abstract

A marine seismic system for recording near-vertical incidence to wide-angle reflected waves and refracted waves with penetration from the ocean bottom to the upper mantle (deep seismic sounding or DSS) has been developed. Signals from six individual hydrophones suspended at 45 m depth from a 600 m cable trailed behind the receiving ship are recorded in digital form. Using charges ranging from 2.3 to 280 kg, two reversed DSS profiles were recorded in the region of Explorer ridge during 1974. A preliminary interpretation of the profiles based on first-arrival information in the range 4 to 80 km has been made.The reversed profile run across the ridge showed no anomalous effects as the ridge was crossed; the profile on Juan de Fuca plate, paralleling the ridge, exhibited traveltime branch offsets and delays. These have been interpreted as due to faulting with a vertical component of offset of about 4 km. The reversed upper mantle velocities are 7.85 and'7.30 km/s indirections perpendicular and parallel to the ridge. Anisotropy is proposed to explain these different velocities and gives a 7% anisotropic effect. The data require that 'layer 2' comprise at least two layers with velocities of 4.13 km/s and 5.25 km/s and individual depth extents ranging from 1 to 2 km. Compared with crustal sections from other ridge areas, the interpretation gives a thick 'layer 3' (up to 6 km) near the ridge crest. The sub-bottom thickness of the oceanic crust varies between 7 and 9 km, except in the faulted region, where the 7.30 km/s material is present less than 3 km from the bottom.

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