The pen traced the overall sound pressure level in a wide frequency 

 band. The band was determined by the "C" weighting on the sound level 

 meter which has its -3 decibel roll-off points at 30 and 8000 Hz. This 

 indicates that the two spectral lines at 4 and 28 hertz (Fig. 4, Hondo) 

 were out of the pass band, so that the overall level was principally made 

 up of the components clustered between 70 and 280 Hz, and those at 3.4 and 

 4.3 KHz, plus the broad band noise component which is estimated to have an 

 overall level comparable to, or perhaps exceeding that of the spectrum 

 lines above 30 Hz. 



In general, the overall level was quite stable, except when a work boat, 

 or personnel boat was in the near vicinity. It was quite apparent that the 

 noise of these boats was dominant, particularly when their propellers were 

 cavitating, as when maneuvering and when at cruising speed or above. Figure 

 11 shows a work boat alongside Platform Hope during drilling operations. Note 

 that Hope and Heidi (in background) each have two large cylindrical legs. 



Data presented will show the variability of the overall sound level at 

 the hydrophone location below the west edge of the platform at a depth of 

 30 feet, and will relate the level to local activity. For reference purposes 

 a value of 100 decibels is assigned to the minimum overall sound pressure 

 level which was observed at midnight (2400 hours) on Wednesday, 21 January, 

 at which time the wind and sea were probably relatively calm. At this time, 

 and during other relatively quiet periods the level varied over about a 

 four decibel range, rising repeatedly to maxima of about 104 to 105 dB, 

 and falling to minima of about 100 to 101 dB. On one occasion, at approxi- 

 mately 0720 hours on Friday, 23 January the level rose to 114 dB, with 



F-9 



