III. ASSUMPTIONS FOR APPLICATION OF THE MODEL 



A. Source 



The sound source is assumed to be a localized (point) source 

 radiating into the water uniformly in all directions (omnidirectional 

 source). The acoustic strength of each platform is characterized by "source 

 level." This is a well established concept which describes the source by a 

 sound pressure level in decibels relative to the underwater sound reference 

 zero of one micropascal, and at a reference distance of one yard. The sound 

 pressure level is actually measured at distances of ten to several hundred 

 yards, and the source level at one yard is calculated by applying an 

 appropriate level vs. distance rule, such as that of inverse square (6 dB per 

 distance doubled). This concept of source level at a standard distance serves 

 as a convenient means of comparing the acoustic source strength of various 

 noise sources such as oil platforms, work boats, etc.; furthermore, it 

 provides a standard input to sound propagation calculations of the sound 

 pressure level at various distances from the source. 



At this early point in the study, very little i's known of the actual 

 mechanism of radiation of sound from offshore oil platforms. Various 

 mechanisms may contribute to the coupling of vibratory energy into the water. 

 Possible pathways are illustrated in Figure 1. A major potential noise source 

 is the prime energy source, S, usually a diesel or gas engine or turbine. Its 



G-12 



