Buffington, Hamilton, and Moore 



MEASUREMENT OF SOUND VELOCITY AND ATTENUATION 



Knowledge of sound velocity and attenuation in sea-floor sedi- 

 ments is now considered to be a basic requirement for experimental 

 and theoretical studies in bottom-bounce, underwater acoustics. At 

 the Navy Electronics Laboratory, since 1954, measurements have 

 been made in the laboratory, and in situ by scuba divers, and from 

 the bathyscaphe TRIESTE. The present program in the DEEPSTAR 

 is a continuation of previous work. These in situ measurements are 

 necessary to validate laboratory studies on sediments sampled from 

 surface vessels, and to determine the corrections that must be made 

 on laboratory measurements to obtain in situ values. In addition, it 

 is necessary to measure sound velocity and attenuation in natural, 

 undisturbed sediments in which the original sediment structural 

 strength is present. 



Successful measurements of sound velocity and attenuation were 

 made during five dives in the present DEEPSTAR program. These 

 stations, made from the submersible, were supplemented by 12 sta- 

 tions made from a diving boat (using the same equipment) with scuba 

 divers to emplace the equipment and take sediment samiples. 



The equipment used for the measurements was a second model 

 of the probes used during the TRIESTE program. The new probes 

 allow measurements at 14, 7, and 3. 5 kc/s. The equipment con- 

 sists of three probes, 2f inches in diameter, fastened to a rigid 

 frame in such a manner that when the frame is on the sediment sur- 

 face, the probes are inserted to a variable, preset depth in the sedi- 

 ment; this depth varied from 6 inches to about 2 feet during the 

 DEEPSTAR tests. The sediments were cored by plastic tubes 

 attached to each end of the probe frame (Fig. 5). 



Three barium titanate transducers, designed by NEL's Trans- 

 ducer Division, were used as sound source and receivers. Velocity 

 was determined by measuring travel time over a 1- meter path 

 length between receivers 1 and 2. Attenuation was measured relative 

 to that in the bottom water (assumed to be zero for 1 -meter). Path 

 length between receivers was calibrated by using a water velocimeter. 



The unique feature of these probes is the ability to measure 

 sound velocity and attenuation at three frequencies (14, 7, and 3. 5 

 kc/s) without removing the probes from the sediment. 



The large diameter of the probes (2f inches) was necessary to 

 obtain measurements at lower frequencies. Insertion of the probes 

 into the sediment to about 2 feet is necessary to obtain measurements 

 at 3. 5 kc/ s, and about 1 foot for measurements at 7 kc/ s. In a sedi- 

 ment containing appreciable amounts of sand, the DEEPSTAR could 

 not insert the probes a sufficient depth to obtain measurements at 

 any frequency, and even in high porosity clayey silt the probes could 

 not be inserted a sufficient distance to measure at 3. 5 kc/ s. However, 



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