John Northrop

Virgin islands bathymetric survey

Abstract A bathymetric survey in the vicinity of the Virgin Islands showed that Anegada and Jungfern Passages, which connect the Atlantic Ocean with the Caribbean Sea between the Virgin Islands Platform and St. Croix Island, are the deepes charted passages between the two seas. The 1,072-fathom sill depth of Jungfern Passage is the limiting factor in the exchange of deep water between the Atlantic Ocean and the Carribean Sea. Furthermore, it was found that the Virgin Islands Basin, which lies between Anegada and Jungfern Passages, has a flat floor 2,400 fathoms deep. It is bounded on the north and south by sea scarps having apparent slopes of 9 to 43 degrees. The eastern end of the basin is divided into two arms which embrace a 420-fathom sea knoll. Both these arms terminate at sills which separate them from Anegada Passage and St. Croix Basin. The western end of the basin is connected with a smaller basin, 2,200 fathoms deep, which is bordered by Jungfern Passage on the south and by Grappler Bank on the west.

Bathymetry of the Gibbs Hill area, Bermuda

Abstract Precision depth recordings were made South of Gibbs Hill, Bermuda, along a number of closely spaced sounding tracks. The accuracy of transit navigation and the sounding coverage were considered sufficient to justify correcting the soundings for both sound velocity and slope. The corrected soundings show that a steep sided submarine spur extends south-south-east from the shelf break off Gibbs Hill. The spur has a topographic relief of 300 fm and is about 3 miles long and 1 2 mile wide. At the 1000 fm curve, the feature broadens into a platform that extends another 12 miles seaward to the 2000 fm curve. True slopes along the steep portion of the spur are as high as 50° and the bottom echoes appear to be from a number of highlights causing “crescent” on the record. Where a definite break in slope occurs, the apparent bottom shown on the bathygram is that of a diffraction pattern from an edge and all points on this crescent migrate to a point, which is the true position of the break in slope.

SEISMIC REFRACTION PROFILES ON THE CONTINENTAL SHELF SOUTH OF BELLPORT, LONG ISLAND, NEW YORK

Seven reversed seismic refraction profiles were made extending from the Heliport Coast Guard Station, off Long Island, New York, to 45 miles offshore. The profiles show a thin layer of mud above the sedimentary wedge that covers the basement. The mud layer was found to have a compressional velocity of 5030–5260 ft/sec, the unconsolidated sediments, 5700–6040 ft/sec, the semiconsolidated sediments, 6370–6770 ft/sec, and the consolidated sediments, 7470–10,700 ft/sec. Consolidated sediment was found only in the profile farthest offshore. Basement velocities ranged from 16,300 to 18,300 ft/sec. The thickness of the sedimentary column in a section extending 45 miles offshore was found to increase from 1927 feet to 7598 feet, the greatest increase coinciding with the appearance of the consolidated layer. Comparison of the seismic profiles with well logs obtained in the area indicates that the unconsolidated sedimentary layer can be correlated with the Upper Cretaceous Magothy Formation of Long Island and the semiconsolidated sedimentary layer can be correlated with the Upper Cretaceous Raritan Formation which overlies the gneisses and schists that constitute the basement beneath Long Island. Strong evidence for a fault was found 8 miles offshore and again 19 miles offshore.

Comment [on “Forensic seismology and the sinking of the Kursk”]

The January 23,2001 issue of Eos published a diagnosis of the explosions and sinking associated with the Oscar-class Russian submarine Kursk in August of last year, based on a study of data from seismic stations in the region. The article prompted the comment and reply below.