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

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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−1 at t = 0 to 0.77 s−1 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.