358 OCEANOGRAPHIC INVESTIGATIONS 



used, and problems of unknown effects due to nearby metal objects were avoided. Subsequently, 

 any desired direction could be measured merely by stretching a string from the rose in the 

 unknown direction and reading the bearing from the rose. By means of this technique the mag- 

 netic heading of Sealab II on the bottom was found to be about 78 degrees. 



The surveyor's tape is a standard Lufkin tape refill #0506ME mounted in a plastic reel 

 designed for simple and foolproof underwater operation. Use of the compass rose and tape 

 together can provide range and bearing data, and two roses (separated by a known amount) or 

 two tapes (from known positions) can provide triangulation data. The disadvantages of this 

 technique are that (a) the divers are separated (and often out of sight of each other), but are 

 connected by the tape or line so that hand signals may be used, and (b) the tape or line must be 

 pulled straight and is susceptible to hanging on objects near its middle, causing possible un- 

 known errors. Also, this procedure is cumbersome if ranges in excess of several hundred 

 feet are involved. 



Another device which proved to be useful for this work was the NAVMINDEFLAB Divers 

 Observation Board (1), which consists of a writing surface combined with built-in compass, 

 depth gage, inclinometer, bubble level, and ruler and which has receptacles for pencil, 1.8-m 

 (6-ft) folding rule, stem thermometer, and 7.6-m (25-ft) circling line. 



Ambient Noise Conditions 



Due to the presence of generators and other loud sound sources, it was realized from the 

 outset that the true oceanographic ambient noise level could not be measured near Sealab II. 

 However, for future design consideration, and for certain underwater audio experiments, there 

 was the need to know acoustic levels, both outside and inside Sealab II. Records of ambient 

 noise level were made by use of a NAVMINDEFLAB Sound Measuring Set, which provides the 

 capability to record, on both analog strip chart and magnetic tape, sound levels in three filter 

 bands to an accuracy of ±1 dba.* The hydrophone, positioned at the observer's station of the 

 human-factors program acoustic range, was about 40 ft from the after port side of Sealab II. 

 Recordings were made of (a) ambient noise outside Sealab II, (b) ambient noise inside Sealab II 

 both with and without the Arawak pumps operating, (c) helium speech, and (d) calibration level 

 signals from the sound -measuring set. The tape recordings will be further analyzed in narrower 

 filter bands to obtain the spectral distribution of energy. Acoustic levels observed in the water 

 were quite high, namely, of the order of 40 dba (with occasional peaks going to 55 dba) in the 

 frequency range 400 cps to 30 kc. Analysis of sound levels inside Sealab II will be undertaken 

 after obtaining calibration data on the microphone provided with the tape recorder. 



Surface Wave Measuring System 



A problem of potential importance to future Sealab type experiments for which there is 

 no staging vessel at the surface is that of determining the surface-wave condition. Wave ef- 

 fects directly observable at the bottom (e.g., pressure variations) yield information only about 

 waves of length greater than roughly twice the water depth; hence a method for obtaining in- 

 formation about the shorter, and usually more energetic, wind-generated waves is needed. A 

 very simple, yet effective, approach toward solution of this problem was tried during Sealab 

 II. The technique consists of floating at the surface a small buoy which effectively follows the 

 surface motion; i.e., it rides up and down with the wind waves. This float is attached to a 

 small, light, nylon-covered wire whose length is adjusted so that a small weight (approximately 

 one pound) attached to the lower end is positioned about 4 or 5 ft above the bottom. Divers 

 then observe and measure the up-and-down motion of this weight to obtain surface-wave height 

 data. It proved to be very easy to position the origin of a meter stick at the lowest point of a 

 wave cycle, i.e., at the trough, and then to observe the height on the meter stick of the highest 

 point of motion, i.e., at the crest. In this manner it was feasible to use the standard method 

 of measuring 30 waves and averaging the highest 10 to obtain the significant wave height. To 

 make a measurement of significant wave height requires only about 5 to 10 minutes. It is 

 realized that there is some error in this technique, since such a float-mass system does not 

 exactly follow the surface waves; however, as long as the float remains above the surface, 

 this error can never exceed the height of the float (about 15 in. in the trials described here), 



^'Reference: 1 dyne/cm^ 



