for four principal tonal components, including what appears to be the 

 fundamental frequency at 12 Hz. 2 The two production platforms differed in 

 several respects, as did their radiated noise spectra, so source levels of 

 both are being presented in Tables 2 and 3. 



2. Minimum Detectable Signal (MPS = NL + CR) . Minimum detectable 



signal level is obtained at each of the signal frequencies in the SL data for 



three ambient noise conditions (NL) and one or two critical ratio (CR) 



assumptions, depending on whether the frequency is above or below 450 Hz where 



the CR = 20 dB. Above 450 Hz the CR is that of the solid line in Figure 4. 

 Below 450 Hz a "best estimate" is taken from the solid line, and a 

 "conservative estimate" is taken as 20 dB. 



2ln describing a spectrum containing many discrete lines, each representing 

 a tonal component of the spectrum, the term fundamental frequency is often 

 used. These spectra, often called "line spectra" frequently contain a series 

 of components, .ach at a frequency (f^) ^^ich equals an integer (n ^ 1, 2, 

 3, 4, 5, etc.) multiplied by a number which is called the fundamental 



frequency (f^). ^hus fn = nfo. When n = 1, fn = fo - fundamental 

 frequency. If n - 3, f^ , 3 f^_ j^^^ ^^ ^.j,^ ^^^^^ harmonic, etc. For 

 the example of the Alaska drilling (SSD-1 ) the spectrum shown in Figure 7 

 shows line frequency components at 12, 72, 100, and 250 hertz. These 

 correspond to n = 1, n = 6, n = 15, and n = 21 respectively. Therefore, the 

 fundamental is at a frequency of 12 hertz and the other components are the 

 6th, 15th, and 21st harmonics. 



G-29 



