24 



survey. The size of the fine-grained deposit, its thickness and area, would help determine 

 how much of the original material remained on site and was buried under the sand. 



The 24 kHz acoustic survey at CSDS on August 10 was conducted to try to provide 

 answers to this question. The subbottom profiler system gathered both surface and 

 subsurface sediment density values. Changes in subsurface densities (from sand to mud) 

 should become apparent as the acoustic signal passes from surface sand to the underlying 

 mud. On the surface, density values should decrease as the signal passes over the center of 

 the fine-grained deposit. The results of the surface density analysis did show a predominance 

 of values greater than 1.6 g-cc 1 concentrated north of 41° 12.75' N. These values do 

 correspond to the location of coarser grained material and shell lag identified in the 

 REMOTS® photographs. An exact correlation between the REMOTS® photographs and the 

 measured surface density values is not clear. As the survey passed over the fine-grained 

 material at REMOTS® locations G8 and G9, the surface density values ranged from 1.5 to 

 1.8 g-cc 1 , higher than expected for the fine-grained material that was documented in the 

 REMOTS® photographs. 



Subsurface density values were collected by the 24 kHz acoustic survey at 15 cm 

 intervals below the seafloor. If the fine-grained material had been detected by this system 

 (i.e., the mud was located at 15 or 30 cm below the surface), there would have been a 

 corresponding drop in density values. The density results around the mound (at 15 cm below 

 the seafloor) all exceed 2.0 g-cc" 1 . In the REMOTS® photographs, the top of the fine-grained 

 deposit was detected within 8 cm from the seafloor. Only if the deposit exceeded 7 cm thick 

 would it have been discernible by the subbottom system. Based on depth differences before 

 and after the deposit of the North Cove material, the mound formed was 50 cm high at its 

 apex. This is reason to believe that there should have been fine-grained material detected in 

 the 24 kHz acoustic survey. 



The lack of success with the 24 kHz acoustic survey may have been due in part to the 

 resolution of the system, but attention to and adjustment to varying system parameters in 

 future surveys may result in more successful distinction between subbottom layers. 



Monitoring Cruise at the Cornfield Shoals Disposal Site, August 1992 



