abstract

Four fully or partly reversed seismic refraction measurements were made during the winter of 1950 in the North Atlantic Basin by the whaleboat method. Velocities of 2.61 km/sec. and 3.85 km/sec. were found offshore from Martinique at a depth of about 1,000 fathoms. Velocities of 6.05 km/sec. in 3,160 fathoms and 7.07 km/sec. in 3,300 fathoms at lat. 22° 55′ N, long. 64°23′ W and lat. 28°28′ N, long. 56°28′ W respectively were found in the deep area south and southeast of Bermuda. Assuming that the sediment velocity lay between 1.0 and 1.2 times that of sound in water, this gave sedimentary thicknesses between 0.76 and 0.93 km. and 0.34 and 0.42 km. respectively. At a point 150 miles east of Bermuda (lat 32°20′ N, long. 62°02′ W) two distinct velocities were found: 4.12 km/sec. (2.36 km. thick) and 5.97 km/sec. The thickness of unconsolidated sediment would be of the order of 200 m. The velocities of 6.05 km/sec. and 5.93 km/sec. could correspond either to granitic or to low-velocity basic rocks. 7.07 km/sec. is definitely in the ultrabasic range.

Owing to limitation on profile length of the whaleboat method, the thickness of the high-velocity rocks of the latter three was not determined. Recent data obtained in the same general area show that they are underlain by rocks with velocity of about 8 km/sec. and are of the order of 5 km. thick.

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