The abstract of the Indian Ocean report follows (Hamilton et al, 1977, p. 3003). 



New measurements of interval compressional wave velocities were made in the first 

 sediment layer, using the sonobuoy technique. These measurements were made during two 

 expeditions in the Bay of Bengal, in the Andaman Sea and over the Nicobar Fan and Sunda 

 Trench. Sediment interval velocities from these areas were added to those previously re- 

 ported, and revised diagrams and regression equations of instantaneous and mean velocity 

 versus one-way travel time are furnished for four areas of the Bengal Fan and for the Anda- 

 man Basin, Nicobar Fan, and Sunda Trench. The velocity gradients directly below the sea 

 floor were used to separate the Bengal Fan into four geoacoustic provinces. In the north 

 and west, the velocity gradients are 0.86 s~l and 1.28 s~^, respectively; whereas in the cen- 

 tral part of the fan, the gradient is 1 .87 s~^. These variations indicate lesser increases of 

 velocity with depth in the sea floor in the north and west. They are due probably to more 

 rapid deposition, less consoHdation and less Hthification near the riverine source areas of 

 the sediments. The near-surface velocity gradients in the other areas are: the Andaman 

 Basin, 1.53 s~l ; the Nicobar Fan, 1.63 s"^ ; and the Sunda Trench, 1.41 s~K In a second 

 part of the paper, the linear velocity gradients (from the sediment surface to a given travel 

 time) in 17 areas of the Indian Ocean, Pacific area, Atlantic Ocean, and Gulf of Mexico 

 were averaged at each 0.1 s from to 0.5 s of one-way travel time. These averaged gradients 

 ranged from 1 .32 s~^ at t = to 0.76 s~^ at t = 0.5 s. The regression equation for the veloc- 

 ity gradient, a, in s~^ as a function of one-way travel time, t, in seconds is: a = 1.316 - 

 1 .1 17t (for use from t = to 0.5 s). These average velocity gradients can be used with sedi- 

 ment surface velocities and one-way travel times (measured from reflection records) to com- 

 pute sediment layer thicknesses in areas of turbidites lacking interval velocity measurements 

 in the first sediment layer. 



Three of the figures in Hamilton et al (1977) are reproduced here. Figure 4 shows 

 the sonobuoy stations in the Northeast Indian Ocean; Figure 5 illustrates the type and scat- 

 ter of the data; Figure 6, the averaged velocity gradients in these and other areas. 



Current Studies 



At present (September 1977), sound velocity gradients in the principal types of 

 marine sediments are being studied. These types include sands, terrigenous sediments (di- 

 rectly from land sources), calcareous ooze, and siHceous ooze. The emphasis in these studies 

 will be on averaged values which can aid in predictions. 



ATTENUATION OF COMPRESSIONAL (SOUND) WAVES IN MARINE SEDIMENTS 

 AND ROCKS 



Introduction 



Some years ago it became apparent that sound propagated through the sea floor at 

 certain frequencies and at certain grazing angles. In such cases quantitative knowledge of 

 sound attenuation in marine sediments became a required property in understanding sound 



14 



