• The underwater hearing of large whales is optimized. Because the ocean is a noisy 

 place, an acoustic system will be limited by noise before it is limited by sensitivity; 

 therefore, a detection threshold of dB will be required for a signal to be heard 50 

 percent of the time. 



• The hearing bandwidth is 1/3 octave. 



• The receiver is omnidirectional. 



In deep water (greater than 100 fathoms), a good approximation for transmission loss 

 is given by spherical spreading (20 log r). The estimation detection range can be approximated 

 by: 



Range (m) = 



10 



SL(peak)-(Ns+101ogBW) 



20 



Where: SL(peak) = Peak source level (dB re 1 n?a at 1 m) 



BW 



= Background noise level (dB re 1 ixPa) 



= Critical bandwidth at the frequency of the signal. 



Attenuation is also a factor in range determinations. The attenuation coefficient (a) 

 is frequency dependent, and at frequencies below 1 kHz is approximately 0.05 dB/kyd. In 

 the following calculations attenuation was considered insignificant and was ignored. 



In shallow water, transmission loss is sensitive to many variables, particularly the sea 

 surface, the water medium and the bottom. Thus, in the absence of specific knowledge of 

 the variables, especially the sound velocity and density structure of the bottom, transmission 

 loss in shallow water is only approximately predictable (see ref 27). Therefore, for shallow 

 water, the formula above at best approximates a "minimal detectable range" in the absence 

 of further information. 



The values in table 1 show that noise generated from oil and gas drilUng activities may 

 be detected at considerable distances from the driUing sites. Favorable propagation character- 

 istics could extend these ranges further. 



FREQUENCY 



(kHz) 



SOURCE 

 LEVEL 



(dB) 



BACKGROUND 

 NOISE 



(dB) 



BANDWIDTH 



(Hz) 



RANGE 



AREA OF A 



CIRCLE WITH 



A RADIUS = 



TO RANGE 



(SQ NMI) 



Kilometers 



NMI 



0.02 

 0.10 

 1.00 



160 

 150 

 180 



60 

 50 

 50 



8 

 15 

 25 



38 

 17.4 

 174 



21.0 



9.3 



94.0 



1.3x10^ 



2.7 X 10^ 



2.8 x 10"* 



Table 1. Estimated minimum distances from which noise from oil and gas 

 drilling activities might be detected by marine mammals. 



H-l[ 



