OCEANCXJRAPHIC INVESTIGATIONS 353 



scalar irradiance at depth. Data obtained from this instrument are being analyzed for corre- 

 lation with water- visibility measurements taken by divers. 



Tides— Changes in the water level above Sealab were monitored by (a) a Hytech Model 

 4000 water-level monitor and (b) a Braincon temperature-Depth recorder, Type 148. The 

 Hytech monitor utilizes a precision Mechmetal bellows to alter the tension in a vibrating be- 

 ryllium copper wire to measure changes in hydrostatic pressure. The output of this instru- 

 ment was recorded on a Rustrak recorder located in Sealab. This instrument was in operation 

 during Team 2's occupancy. In the Braincon depth recorder the movement of a helical Bourdon 

 tube is contact printed on 70- mm photographic film. This instrument was attached to Sealab; 

 the transducer of the Hytech unit was pyositioned some 15 m (50 ft) from Sealab. The location 

 of both of these instruments is shown by Fig. 130. The Braincon unit was in operation from 

 Sept. 6 to Oct. 8. The records from both of these units, after corrections for changes in baro- 

 metric pressure, will be correlated with records from a Coast and Geodetic Survey tide gage 

 located at the end of Scripps pier. The film record from the Braincon unit is presently being 

 processed. The maximum tide range recorded by the Hytech unit was approximately 2.3 m 

 7-1/2 ft), which was in good agreement with the 2.4 m (7.8 ft) maximum predicted tide range 

 for this period. 



Specific Problem Areas 



Of the 15 specific research tasks planned, useful results were obtained from eight. In the 

 interest of completeness, however, aU of the planned tasks will be discussed giving either pre- 

 liminary results or reasons why a task was not completed or successful. Also included in this 

 discussion are several problem areas which came to light during the writer's occupancy of 

 Sealab. 



Horizontal Visibility measurements by Swimmers — Although considerable progress has 

 been made in recent years concerning the visibility of objects by swimmers, there is a paucity 

 of adequately controlled experimental data with which to verify existing prediction theories. 

 Sealab H offered the opportunity to obtain needed data at depths sufficient to insure uniformity 

 of the radiance distribution, for a time interval of sufficient duration to obtain a wide range of 

 light levels and transparency, and in water of excellent spatial uniformity. In order to take 

 advantage of this opportunity, the S.I.O. water- clarity meter (4) was used to monitor the optical 

 characteristics of the near-bottom seawater, while swimmer measurements were made at a 

 specially constructed horizontal visibility range. This range, located about 21 m (70 ft) from 

 Sealab (Fig. 130), consisted of four visual targets arranged as shown by Fig. 133. The targets 

 consisted of a white square, a black circular disc, a white cross, and a yellow triangle. The 

 three angular targets were of equal area (906 sq cm), 140 sq in), while the black disk was 

 somewhat smaller (707 sq cm) (109 sq in.). The apparent reflectances (R^,) of the submerged 

 targets as measured by the S. I. O. visibility laboratory were: white square and white cross 

 0.875; black disk 0.506. The origin of a 15-m (50 ft) measuring tape was attached to the sup- 

 port stake of the black disk. The procedure for obtaining visibility measurements was as fol- 

 lows. Two swimmers, starting at the end of the measuring tape, swimming horizontally at or 

 slightly below the target level slowly and carefully approached the targets until some target 

 was first detected. The swimmers' position along the tape was then recorded as the range of 

 detection. The approach then continued in this manner until the detection range of each target 

 was recorded. In like manner the range at which the shape of each target could be discerned 

 was recorded. Since the measurii^ tape was not in the center of the range, both swimmers 

 swam to the right of the tape in order to minimize error due to the angular spread of the tar- 

 get array. Resulting from each visibility run were two sets (detection and identification) of 

 four ranges for each swimmer. A total of 20 visibility runs was made. Extreme detection 

 ranges varied from a minimum of 2.4 m (8 ft) to a maximum of 9.1 m (30 ft). Extreme iden- 

 tification ranges varied from 1.5 m (5 ft) to 8.5 m (28 ft). A preliminary inspection of the data 

 has shown that the black disk even though smaller in size, was always the first target to be 

 detected and identified. A detailed analysis of these data is being conducted, and an attempt 

 will be made to use these results, in conjunction with the water-clarity-meter data, to verify 

 the Duntley-Preisendorfer underwater visibility range prediction theory. 



