reverberation levels which did not prevent the 

 detection of the fish. The best possible shape 

 for the operation of the sonar would be a tank 

 similar to the USNEL. Transdec transducer 

 test facility but this tank was not available. 

 Therefore, the next possibility was a tank 

 which would scatter the sound and prevent 

 direct reflection from the walls into the hydro- 

 phone and sonar. The only shape that was 

 available and could possibly perform this 

 function was that of the circular holding tanks 

 at Sea World, an oceanarium located on 

 Mission Bay in San Diego. 



Evaluation at Sea World 



Permission was granted by M. Shedd, the Sea 

 World Director, for an evaluation test to be 

 made in one of the circular concrete holding 

 tanks, 40 feet (12.2 m.) in diameter and 10 feet 

 (3.0 m.) deep. We checked the holding tank 

 for reverberation at all angles of incidence 

 and depth of the transducer-projector array. 

 The reverberation level was found to be 

 exceedingly high and of such a character as to 

 mask small targets conopletely. Using the 

 tank for the evaluation of signal returns from 

 fish was therefore impossible. This problem, 

 even though learned early in the program, 

 raised considerable difficulties because now 

 it forced operation to an open-water environ- 

 ment. The problem of containing a fish at some 

 point in the sonar beam in open-water tests 

 is more difficult. 



Mission Bay Yacht Club Pier 



For open salt-water experiments a site was 

 selected in Mission Bay, a shallow bay north 

 of San Diego. A series of tests was made at 

 the Mission Bay Yacht Club pier to evaluate 

 the signal return from various materials that 

 could be used for containment of fish specimens 

 in open-water tests. Mission Bay was selected 

 because during a normal week day the boat 

 activity within a radius of 1 mile (1.6 km.) is 

 very low. Tests could be suspended during the 

 infrequent and brief passage of motor launches. 

 The tests were made to determine the reflec- 

 tion capability or target strength of several 

 materials. The materials tested included: 

 4-inch-mesh (10 cm.) nylon net; 1-3/8-inch- 

 mesh (3.5 cm.) nylon net; and sheets of 

 polyvinyl, 0.002-inch (0.05 mm.) and 0.007- 

 inch (0.18 mm.) thick. In all tests, the net 

 material gave signal reflections high enough 

 to mask small targets viewed through the 

 net. The sheets of polyvinyl material passed 

 the test with only small reflected signals. 

 Most of the reflections were due to clinging 

 air bubbles, which give extremely strong 

 sonar returns. A 4-foot (1.2 m.) diameter 

 cylindrical, polyvinyl bag 15 feet (4,6 m.) 

 long and suitable for operation in the water 

 adjacent to the floating pier at the Mission 



Bay Yacht Club was constructed. This bag 

 was perforated with several thousand 3/8 -inch 

 (9.5 mm.) diameter holes to allow oxygenated 

 water to enter the bag during tests. A boom 

 was constructed to hold the polyvinyl bag at 

 the necessary distance from the pier. Figure 5 

 shows the top view of the test site and indicates 

 the location and distances in the test on several 

 fish specimens. The problem of obtaining test 

 specimens was very difficult because the 

 waters off and near San Diego had a red tide 

 that repelled fish. Therefore, the polyvinyl 

 enclosure was evaluated in a series of tests 

 with fish that were not necessarily those of 

 greatest interest in this program. The fish 

 that were used did give sufficient information 

 for the equipment and data-processing tech- 

 niques to be further developed. 



During the first of these tests aSCUBAdiver 

 observed the position of two fish--kelp bass 

 ( Paralabrax clathratus )- -within the enclosure. 

 The procedure was as follows: a specimen 

 was placed in the enclosure; the enclosure was 

 moved into the sonar beam by rotation of the 

 boom at its pivot point; the diver entered the 

 water, observed the fish's position, attitude, 

 and behavior; the diver left the water. A data 

 run was taken; the diver reentered the water 

 and observed the location of the fish; it was 

 assumed that the behavior was reasonably 

 consistent during the two observations. The 

 actual location of the fish within the enclosure 

 during a test was never determined owing to 

 the poor visibility under the red tide condition. 

 The fish tended to go directly to the bottom of 

 the bag; when they did they were no longer in 

 the major beam of the sonar. In subsequent 

 tests a 6-foot-diameter (1,8 m.) bag was 

 constructed with a "false" bottom heat-sealed 

 into the walls of the polyvinyl cylinder. This 

 arrangement linnited the range of the fish in the 

 bag to about 8 feet (2.4 m.) and ensured re- 

 tention of the fish in the major lobe of the 

 sonar beam. 



The results of the fish tests with the diver 

 indicated a problem in reverberation, not with 

 the walls of the enclosure as had been deter- 

 mined in the previous tests, but between the 

 surface and bottom reflections in the shallow 

 water, which was 15 feet (4.6 m.) deep at 

 high tide. This problem manifests itself in the 

 output of the equipment as multiple returns 

 from a single target. The tape output for 

 example showed two strong returns from one 

 SCUBA diver. Rather than nnove the operation 

 to deep water, we attempted to determine the 

 signature of fish within the enclosure in the 

 presence of the surface and bottom reflections. 

 Unfortunately, the returns were confused to 

 such a degree that suitable presentation of the 

 motion of the fish was impossible. The tape 

 recordings carried indications of fish motion, 

 but certainty as to the behavior of the specimen 

 in the sonar was not possible. Further tests of 

 the CTFM sonar were abandoned and work was 



