poor. The technique looks promising. A means and variance program was developed 

 and promises to reveal sub-bottom impedance layers from the acoustic data. How- 

 ever, the results of this particular test indicated that the shallowest major reflector 

 was approximately 100 feet down, or considerably greater than the core samples 

 obtained. Further tests will be made using varying pulse lengths for a given fre- 

 quency, and the presentation of the structures will be compared. This technique is 

 in the formative stage and is undergoing modification. 



A similar approach will be made on the next bottom study cruise. A 12-kcps 

 capacitance discharge device will be towed near the bottom. When a 0. 5-msec pulse 

 length is used, the layer thickness resolution should be about one foot, with penetra- 

 tion of from 50 to 100 feet. This device is similar to the pinger unit used in most 

 underwater camera systems; but it will not be battery powered, it will serve as a 

 receiver as well as transmitter, and the repetition rate will be varied and controlled 

 from the surface ship, as deemed necessary. Later, if the evaluation of this tech- 

 nique proves satisfactory, this device will be modified to operate in the 1 to 4 kcps 

 frequency range. Whether acoustic impedance layers several inches thick or less are 

 significant contributors to the overall energy loss is a subject currently under studj . 

 It appears that the controlling feature is not the thickness of the layer but the magni- 

 tude of the rate of change in impedance. 



As more is known about the geologic-acoustic relationships and about the be- 

 havioral reactions of each property in the soil mass, we shall embark upon a program 

 to measure t^ie absolute values of a number of the parameters in situ. This will 

 include the use of: (1) gamma radiation devices which are available to determine bulk 

 density in a sediment mass; (2) vane shear devices, fall cone penetrometers,, etc., 

 which are available for in-situ measurement of shear strength; and (3) the acoustic 

 probe technique which has been utilized at NEL and will be attempted this year at USL 

 using lower frequencies and greater depth penetration. Hopefully, coring pipes or 

 other elongated tubes will be instrumented for in-situ measurements of sound velocity. 



This instrumented probe technique is well developed in the petroleum and well- 

 logging industries. With the advances in marine drilling, this realization may be 

 more immediate than we are at present prepared for. 



LABORATORY DISCUSSION 



The laboratory analyses of the core samples have been performed at the Univer- 

 sity of Rhode Island's Soil Mechanics Laboratory, in the Department of Civil Engi- 

 neering. The following engineering properties were determined at one-foot intervals 

 along the core, and at smaller intervals where required: specific gravity, water 

 content (based upon 100% saturation), atterberg limits, void ratio, bulk wet density, 

 organic content, carbonate content, median grain size, sorting coefficient, gross 

 determination of grain shape, and sound velocity. 



380 



