Sediment velocities from sonobuoys: Bay of Bengal, Bering Sea, Japan Sea, and North Pacific

Abstract

Measurements of upper unlithified sediment layer interval velocities using the sonobuoy technique were made on several expeditions in the Bay of Bengal and adjacent areas, the Bering Sea, the Japan Sea, and the North Pacific. The results are presented in figures, regression equations, and tabulated station data by geographical areas. In each area the sediment surface velocity was based on measurements in cores that were corrected to in situ values. These sediment surface velocities ranged from 1454 m/s in the Tatar Strait to 1585 m/s in the Aleutian trench. An average linear velocity gradient was computed from 0 to 0.5 s (one-way travel time) for each of 13 areas in which the sediments were mostly turbidites. The values at each 0.1-s interval were then averaged. These averaged gradients decreased from about 1.32 s -• at t = 0 to 0.77 s -• at t = 0.5 s. Such average values of velocity gradients can be used with measured or predicted sediment surface velocities and one-way travel times (measured from reflection records) to compute predicted true sediment layer thicknesses for areas in which no interval velocity data are available. One of the most important uses of interval velocity information is in computations of true thicknesses of sediment layers. When no interval velocity measurements are available, there are usually three alternatives in computing sediment thicknesses: (1) use equations and curves from an area of similar stratigraphy and sediment type, (2) use a predicted linear velocity gradient and sediment surface velocity (as was done above), or (3) assume an interval velocity. Of these three alternatives the last is most commonly used and is the least accurate. There is a tendency in marine geophysical literature to assume interval velocity values for upper unlithified sediment layers that are usually too high (e.g., 2000 or 2150 m/s). In 15 areas covered by Houtz and his colleagues and this report the average interval velocities for layers in which one-way travel times are 0.1 and 0.2 s are 1619 and 1719 m/s. This report presents the results of measurements of sediment layer interval velocities using the sonobuoy technique on several expeditions during the past 6 years. Measurements were made in the Bay of Bengal during two Scripps Institution expeditions: the Circe expedition in 1968 and the Antipode expedition in 1970. In the Bering Sea and Gulf of Alaska, measurements were made in 1970 during the joint Naval Undersea Center-U.S. Geological Survey expedition. Measurements in the northeast Pacific and the Japan Sea were made on several Naval Undersea Center expeditions. The above measurements are grouped and reported geographically.It is possible to approximate velocity changes with depth in the sea floor by corrections to surface velocity for variations with depth caused by overburden pressure, reduction of porosity, temperature increases with depth, and a number of other complex and interrelated factors [Hamilton, 1959[Hamilton, , 1964[Hamilton, , 1965. However, at best these approximations are tenuo...

Sediment Velocities and Deep Structure from Wide-Angle Reflection Data around Leg 116 Sites

“…Sediment Velocity Structure The sediment velocities obtained here are in reasonable agreement with the limited amount of sonobuoy data published from this area (Naini and Leyden, 1973;Hamilton et al, 1974;Curray et al, 1982;Stein and Weissel, in press). Although generally consistent with a gradually consolidating turbiditic lithology to basement, it is likely that prior to the large-scale development of the Bengal Fan (pre-mid Miocene-the '0' sediments of Curry and Moore, 1971) a greater proportion of the sediment was pelagic chalks and clays.…”

“…Houtz et al (1970) using results from several hundred sonobuoy stations in the North Pacific have found average velocity gradients of 0.6-0.8/s. Hamilton et al (1974) present a regression equation based on variable angle reflections from sonobuoys in the Bay of Bengal, Bering Sea, Japan Sea, and North Pacific which predicts a decrease in average linear velocity gradient from 1.3/s at the surface to 0.8/s at a depth of 1000 mbsf. Bachman et al (1983) give a near-surface velocity gradient for the southern Bengal Fan of 1.18/s.…”

Tectonics, structure, and sedimentary framework of the Peru-Chile Trench

“…Where possible, maximum sediment thicknesses in the trench were measured to the nearest one tenth of a second of two-way travel time from seismic reflection profiles. Travel times were converted to thicknesses in metres using sonobuoy refraction velocity data from the Aleutian Trench (Hamilton and others, 1974), because no sediment velocity data are available for the Peru-Chile Trench. Because both trenches contain rapidly deposited, land-derived turbidites, the sediment interval velocities should be roughly comparable.…”

Crustal structures of the Peru continental margin and adjacent Nazca plate, 9°S latitude

“…Prince and Kulm (1975) reported the trench fill in this area to be composed of turbidites. Velocities of 1.7 to 1.9 km/sec from Hamilton and others (1974) were used to model the turbidite fill. Due to several misfires of explosives at short distances, it was not possible to analyze the sonobuoy 9 data to the west for the velocity and structure above the 5.55 km/sec layer.…”