below 300 hertz. Platform Henry, also rated noisy, similarly was rated 

 noisy in the two lower bands. It was engaged in drilling and production, 

 and generated its own prime power with a gas turbine, so it might be 

 expected to be noisy. A rise in the broad band spectrum in the frequency 

 region above 2000 Hz was observed for some platforms, including man-made 

 Rincon Island. It may be hypothesized that this is due to breaking waves, 

 or surf-like noise which was prominent at some rigs, including 

 Rincon Island. Because of the high swells, there was a great deal of 

 water splashing and run-off from various structural members of the platforms, 

 and from auxiliary boat landing aids, such as rubber tires, etc. This is 

 visible in the photograph of Platform Houchin (Fig. 10). It may be noted 

 that the broad-band spectra for the various platforms in general were 

 shaped approximately like the sea-state spectrum curves, which are related 

 to breaking waves at the sea surface. The variability in shape and level 

 of the continuous spectrum portion for the various platforms might be ex- 

 pected, based on specific structural differences in those areas washed by 

 the passing swells. 



2. Variation of Underwater Sound Level With Time 



A continuous chart record of the sound pressure level of the noise at 

 Platform Hondo was made for five days starting at approximately 1000 hours 

 on Monday, 19 January 1981. The termination of the data was inadvertently 

 caused by a gradual loss of marking density by the recording pen. Although 

 the trace began to fade at 1500 hours on Thursday, 22 January, it remained 

 slightly visible through 24 January. 



F-J 



