LONG-RANGE REFRACTION EXPERIMENTS IN DEEP OCEAN

Long-range refraction experiments as a potentially powerful tool to elucidate the structure of the oceanic lithosphere are reviewed. Although the necessity for long-range experiments has been well appreciated for some time, difficulties of undertaking such experiments, especially the lack of access to reliable and low-noise OBS (ocean-bottom seismograph), have prevented full implementation of these experiments. However, several long-range experiments have been performed by utilizing land seismographs, which led to various interesting findings: the existence of a high-velocity layer (Vp of 8.4-8.7 km/s) beneath the Moho discontinuity, and offsets of travel times that suggest the existence of a low-velocity layer beneath the lithosphere. Nevertheless, the use of land stations inherently limits the location of the experiments, as well as the resolution of their analyses, since the profile inevitably includes an ocean-continent (or island) transition structure that is probably complex. After examining ambient noise spectra at ocean bottoms and a reconnaissance survey by a low-noise OBS system, four long-range experiments using OBSs have been performed to date by Japanese scientists. Although not all were successful, some interesting results were obtained. The profiles lay over the western Pacific basin where the oldest oceanic lithosphere is postulated. The existence of an 8.6-km/s layer in the lower part of the lithosphere, and a rather thin low-velocity layer that is bottomed by a 8.6-km/s layer, have been found from a N-S profile east of the Japan trench (1974 profile). However, a profile which is rougly perpendicular to the 1974 profile suggests considerably different velocities; the difference probably comes from an anisotropy. From a study of the profile east of the Mariana trench, which is about 2000 km from the 1974 profile, a variability in lithospheric structures is indicated. Groups of scientists in Europe and the U.S.A. have initiated long-range programs which also make use of their own OBSs; they have been making good progress in elucidating the fine structure of the oceanic upper mantle. Some problems which current long-range experiments are encountering are also reviewed. © 1979.