Oceans of noise - Whale and Dolphin Conservation Society

Energy source
(Air gun array)
28
Sea surface)
Hydrophone array
(receivers /detectors)
Figure 3.3. The basic components of a marine seismic reflection survey (the layers of the seabed
are shaded).
3.3.2. Marine seismic surveys
Marine seismic surveys are commonly either two-dimensional (2-D) or three-dimensional (3-D),
and the type of survey used not only affects the type of data collected but can also affect the
extent and duration of exposure to high sound levels.
Two-dimensional surveys employ a single airgun array and hydrophone streamer, and are
conducted along single lines, or pre-determined tracks within an open grid, to produce a vertical
slice or 2-D image through the earth's crust (McCauley 1994; Gulland and Walker 1998). The
source in this type of survey is often made as strong as possible for maximum penetration
(Gulland and Walker 1998). Two-dimensional seismic surveys provide little or no information
about the true position of reflecting points in the sub-surface (Gulland and Walker 1998) and are
typically used for speculative surveys covering large geographical areas (McCauley 1994).
Three-dimensional seismic surveys are characterised by the need to record a grid of data, with
each grid being close enough to allow processing along both grid axes (Gulland and Walker
1998), and where each grid point is the centre of a grid cell (usually 25 m by 25 m). The
appropriate processing grids can be used to produce a 3-D reconstruction of the traversed surface,
giving a much higher resolution than conventional 2-D surveys. By their nature, 3-D surveys
require accurate positioning and use multiple parallel hydrophone streamers often in conjunction
with multiple airgun arrays. Tracklines are often separated by 50 or 100 m (McCauley 1994).
Three-dimensional seismic surveys are typically used to define potential and / or existing
hydrocarbon deposits and fewer exploration wells are needed as a result. However, there are now
moves to use 3-D surveys for speculative exploration, with the extra cost being balanced by the
quality and quantity of data produced (McBarnet 1993 in McCauley 1994).
~12m