pressure corrections are made (laboratory to in situ) to 

 a sediment sound speed, using Wilson's tables for the speed 

 of sound in sea water. 6 ' 7 An independent verification of 

 the method was obtained when a laboratory measurement 

 on a gravity core sample from the Mohole Site (table 8), 

 when so corrected, was only 3 m/sec from that computed 

 from reflection data in the same area. 



These studies with the TRIESTE called attention to 

 the possibilities of applying temperature corrections to 

 vertical sound-velocity profiles within the sediment body. 

 Within the past decade there have been enough studies of 

 heat flow, sediment conductivities, and thermal gradients 

 to allow predictions of this effect in many areas of the 

 oceans. A paper by Gerard et al. gives measurements for 

 the Atlantic, and work by Maxwell, Revelle, Bullard, and 

 Von Herzen, and others has been summarized recently by 

 Bullard. 36 ' 37 Except over ridges, rises, and a few other 

 restricted areas, a representative value for the thermal 

 gradient in the upper sediments is 0.05°C/m. In the 

 Atlantic this gradient is linear in at least the upper 13 

 meters of sediment. 36 At the Mohole (Guadalupe) Site, a 

 linear gradient of 0.137°C/m was measured through the 

 whole of the sediment section. 38 These gradients are 

 appreciable and are important in studies of the vertical 

 sound-velocity gradient in oceanic sediments, especially 

 when they are as high as that measured at the Mohole Site. 

 In addition to temperature corrections, hydrostatic pressure 

 corrections within the pore water of the sediments can and 

 should be applied. 



At the Mohole Site, the temperature in the sediment 

 at the water-sediment interface is 1.6°C; the thermal 

 gradient is0.137°C/m. At a depth of 100 meters in the 

 sediments the temperature is a surprising 15.3°C; the hydro- 

 static pressure has increased in the pore water of the sedi- 

 ment by 10.5 kg/cm 2 . The temperature and pressure cor- 

 rections (both positive) from Wilson's tables for the speed 

 of sound in sea water indicate an increase in sound velocity 

 (due solely to these two factors) of 53 m/sec from the sedi- 

 ment surface to a depth of 100 meters within the sediment. 



30 



