Oceans of noise - Whale and Dolphin Conservation Society

The source used during 3-D surveys is often less strong than those used in conventional 2-D
surveys because subsequent processing is able to handle the resulting difference in data quality
(Gulland and Walker 1998). However, the staggered firing of multiple arrays, to reduce signal
interference, increases the period of noise exposure (McCauley 1994). Additionally, the need to
grid concentrates seismic activity in a small area for a prolonged period, subjecting resident fauna
to high levels of sound for protracted periods and may consequently have greater long-term
effects (McCauley 1994).
3.3.3. The seismic source (airguns and airgun arrays)
Most seismic surveys now use airguns as their noise source; these are pneumatic devices that
produce an acoustic signal by rapidly releasing a volume of compressed air into the water
column.
In general, single air-guns produce broadband source levels between 215 and 230 dB re 1 µPa-m,
with highest energies falling in the range 10 - 300 Hz (McCauley 1994, Greene et al. 1995). The
waveform of this signal resembles a damped sinusoid, and depends on the energy contained in the
compressed air prior to discharge (Turnpenny and Nedwell 1994). Although most energy is
produced at lower frequencies, considerable energy above ambient noise levels may be produced
at frequencies up to 22kHz (Gordon and Moscrop 1996).
Broadband peak to peak source levels of 230 – 255 dB re 1 µPa-m in a downward direction are
reported for air-gun arrays, with peak frequencies covering the range 10-100 Hz (McCauley
1994, Greene et al. 1995). Recently, however Goold and Fish (1998) reported levels as high as 90
dB re 1 µPa 2 / Hz at a frequency of 20kHz and at a range of 1km from the seismic source.
Although the direction of greatest sound intensity is downwards, a considerable amount of energy
is radiated in directions away from the beam axis (McCauley 1994). The far-field signature may
be detected many kilometres from the source (for example, 50-75 km in water 25-50 m deep in
Greene and Richardson (1988). Seismic activity off Nova Scotia is prominent in the acoustic
background off the Bahamas and along the Mid-Atlantic Ridge, several thousands of kilometres
away (Weilgart pers. comm.). These propagation characteristics imply that the sound levels
received by an animal in close proximity to the source will depend on its depth and position
relative to the array's axis. Those animals perpendicular to the array's axis will experience a given
sound pressure level at greater range than those in the line axis of the array. Similarly, at medium
depths (several hundred metres) and assuming free field propagation, animals deeper in the water
column and directly below the array will receive a higher intensity sound than those animals
closer to the surface but at the same range from the array.
There is now a world-wide trend towards increasing oil exploration in deeper waters, and this has
brought with it an increased potential for conflict between those species of cetacean thought to be
most vulnerable to seismic pulses; for example the low frequency specialists, such as mysticete
whales, and deep divers, like the sperm and beaked whales.
3.4 Industrial noise associated with oil and gas exploration and production
3.4.1 Noise sources
Noise is generated during all phases of oil and gas production, noise sources may be continuous
or impulsive and can be described as being transient or permanent (Table 3.4). Activities
generating noise are many and varied, ranging from seismic surveys (exploration), through pile
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