Russell W. Raitt

SEISMIC-REFRACTION STUDIES OF THE PACIFIC OCEAN BASIN

On the Mid-Pacific and Capricorn expeditions seismic-refraction observations were made at 42 stations scattered widely within an area of the Central Equatorial Pacific Ocean extending from latitudes 22° S. to 28° N. and longitudes 162° E. to 112° W. At 29 of these stations velocities of about 8 km/sec, or more were reached at the depth of greatest penetration of the refracted waves. The mean of these velocities was 8.24 km/sec. The crustal thickness, defined as the depth below the sea floor at which the 8 km/sec, velocity is reached, ranges from 4.8 km to 13.0 km. The distribution of thicknesses is bimodal with seven anomalous stations giving values between 10 and 13 km. The group of 22 stations believed to be typical of the deep Pacific Basin has an average thickness of 6.31 km with a standard deviation of 1.01km.

Topography and structure of the Peru-Chile trench☆☆☆

During the Scripps Institutions IGY Expedition Downwind of 1957–1958 the research vessels Horizon and Spencer F. Baird spent 41 ship-days conducting geological-geophysical studies in two portions of the Peru-Chile Trench, concentrating their efforts off northern Chile and central Peru. Data from 5400 miles of echo-sounding traverses and eight seismic-refraction stations (three off Peru, five off Chile) are here reported. The peru-Chile Trench, lying off southern Ecuador to central Chile, is interrupted off southern Peru by the northeast-trending Nasca Ridge. North of Nasca Ridge the trench reaches a maximum depth of nearly 6500 m. Characteristically, the trench floor is flat; this flatness is attributed principally to land-derived sediment, introduced by the intermittenly-flowing rivers of the coastal zone, passing down the trench flank. South of Nasca Ridge, off the Atacama Desert, relatively little sediment reaches the narrow trench bottom. Here the maximum depth is slightly more than 8000 m. South of 27° 30′S the trench shoals in a series of nearly flat-floored basins. River-transported solid phases here form a major portion of the trench-floor sediment. Seismic refraction studies of two sections across the trench off Chile and Peru are very similar in that both show that crustal thickening begins westward of the trench and reaches a thickness of about 11 km beneath the trench axis. The Mohorovicic Discontinuity, which is found at a depth of about 17 km below sea level at the trench axis, plunges steeply to the east with a slope not inconsistent with the depths of about 65 km beneath the Andes, estimated from the observations of Tuve and Tatel.

Reverberation in the Sea

The apparatus used to measure ocean reverberation at 24 kc consists essentially of a sound projector which sends a signal of adjustable duration into the water; a hydrophone which translates the backward scattered sound into electrical voltage; an amplifier which increases this voltage; a cathode‐ray oscillograph which converts voltage fluctuations into spot movements; and finally a camera which records the movements on a moving film. Simple reverberation theory indicates (i) that reverberation level increases 10 db with a tenfold increase of pulse length; (ii) that volume reverberation level decreases 20 db for a tenfold increase in range; and (iii) that surface reverberation level decreases 30 db for a tenfold increase in range. At certain times and under certain conditions, presumably when ocean conditions are those postulated by theory, observed reverberation levels agree with theoretical values. Such agreement, however, is relatively uncommon. Under most conditions, deep scattering layers cause volum...

Propagation of Explosive Sound beneath the Sea Floor

Sound waves propagated through the sea floor from explosions near the seas surface have yielded much knowledge of the velocity structure of the oceanic crust. The interpretation of the observations is customarily based on the relation between the wave travel time and the distance between shot and receiver. The records of received sound are usually used merely for identification and measurement of the time of the first arrivals of significant waves. It is also of interest to consider the problem in terms of the intensity and spectrum of the entire train of waves propagated through the sea floor. The sea observations have the advantage of a uniformity and elastic simplicity of the medium at the source and receiver not possessed by similar observations on land. (This paper represents results of research carried out by the University of California under contract with the U. S. Navy Department.)