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A determination of the sound velocities in the sediments near each of the sites investigated on Leg 20 of the Deep Sea Drilling Project (Heezen et al., 1972) was attempted by recording an oblique seismic reflection profile using an expendable SSQ-41 sonobuoy and an air gun sound source (Figure 1). The equipment and techniques employed were practically the same as those described by Le Pichon et al. (1968), except that an electrically fired, instead of a free-firing, airgun was used. The latter was triggered every 10 sec with a solenoid valve activated by a switch on the inboard recorder. Simultaneously, a normal incidence profile displayed the configuration of the subbottom layers along the shooting line (Figures 2 to 8). Because of the maneuvering that was often necessary before the precise drilling location was finally established, it was not practicable to shoot the profiles on the approach to each site. The normal procedure was to deploy a sonobuoy on completion of drilling as soon as the ship attained a constant speed of about 5 knots on a steady course to the next site. Because time was at a high premium, at only one site (194) was preliminary positioning of the Glomar Challenger carried out to allow the sonobuoy to be dropped directly over the acoustic positioning beacon. The distances between buoys and their respective drilling sites are listed in Table 1. With the exception of the line on Ita Maitai Guyot, it was necessary to use the sonobuoy hydrophone at its maximum depth of 100 meters to minimize the considerable ship-generated acoustic interference. Refracted arrivals were not seen on any of the profiles despite the fact that several were shot out to ranges at which refractions from the basement would have been expected. Their absence is probably largely a result of the poor response of the receiving system and recorder filter below 30 Hz.

Determination of Sedimentary Velocities Using Expendable Sonobuoys at DSDP Leg 20 Drilling Sites, Northwest Pacific