Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
Studies of gray whale (Malme et al. 1983) and bowhead whale (Ljungblad et
al. 1985) response to noise from air guns have shown that much higher effective
peak pressure levels are tolerated before a 50% avoidance probability is
reached when compared with the results from constant level playback studies.
This is believed to be the result of mammalian hearing characteristics, as
discussed earlier. The 50% avoidance probability has been found to occur for
an average peak .pressure level of 170 dB for gray whales and 160 dB for
bowheads. These responses are for transient signals having a spectrum peak at
about 100 Hz and a duration typically less than about 50 msec.
The 50% avoidance criterion for air guns has been determined as an
average of the overall peak pressure levels for the pressure waveform. The
air gun array and vibroseis array signals presented in the industrial noise
data base are given in terms of peak level in a 1/3 octave band to be consistent
with the other data sets. As a result, it is necessary to specify the
overall peak pressure level of air gun signals in terms of the peak 1/3 octave
band pressure spectrum to determine an equivalent criterion level for the zone
of influence estimate. Measurements of the air gun array operation on the
seismic survey vessel WESTERN POLARIS (Miles et al. 1987) showed that the
ratio of the average peak pulse pressure to the pressure obtained from a power
sum of the peak levels in the dominant 1/3 octave bands was 12 dB. Therefore,
when using peak 1/3 octave spectra instead of the pulse pressure waveform, an
effective received level of 158 dB is used as the gray whale 50% avoidance
cl-iterion for air gun array signals.
Moving sources may have a zone of influence which extends beyond the
limits determined by the range at which the received level drops to the 50%
probability of avoidance criterion. The behavioral response model incorporates
a reference response time of 2 hours as the integrazion period in
determining Leq. The total energy of all sounds received within a two hour
period is considered as potentially influencing a behavioral response. As a
result of this concept, moving sources can be considered to leave a trail or
"footprint" which remains along the path of the source for a period of two
hours. The effective zone of influence becomes elongated and has an area of:
where RZ is the range at which the sound level is equal to 50% probability of
avoidance criterion level (km)
S is the speed of advance of the source (km/hr).
On the other hand, as a result of the 2-hr averaging used in determining
Leq, the estimated radius of influence around a moving source may be reduced
from that expected if the same sound were present for the entire 2-hr period.
In effect, the zone of influence would be determined by the range at which Leq
equals the criterion level rather than by the range at which the maximum
received level (Lr) equals the criterion. Since the concepts of acoustic
response time and equivalent level estimation as applied to marine mammal
hearing and behavior need further study, for the present, maximum received
level values are used to estimate zone of influence radii for both fixed and .