Seismic refraction lines fired to ocean bottom seismometers (OBS) and to the ocean sub-bottom seismometer (OSS) at Site 581 provide some of the highest quality refraction data yet obtained in the deep ocean. A particularly useful subset of these data had a Soviet air gun as a source and an OBS as a receiver. Excellent data with a shot spacing of less than 250 m are well recorded out to a range of 45 km. These data are analyzed with WKBJ, r-p inversion, and reflectivity modeling which are used to obtain seismic models. The resulting model for the Northwest Pacific crust agrees with that obtained by Anosov et al. (1982) for the same region. It differs from "standard" ocean crust models in that it contains a low-velocity zone for both P and S waves near the top of Layer 3. Strong shear conversion at the base of the sediments is easily identified on the different components, yielding an accurate sediment shear velocity of 210 m/s. The value of using a very large air gun as a refraction source and geophones as sensors is obvious from the results obtained. 106 ?igure 1. Map showing locations of the ocean sub-bottom seismometer (OSS IV) experiment at DSDP Sit 581. F. K. DUENNEBIER ET AL.

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