3.2 Airgun Arrays for Non-Experts
An expert is someone who knows some of the worst mistakes that can be made in his subject, and how to avoid them. Werner Heisenberg (1901–1976)
3.2.1 Source Directivity
Figure 3.12 shows a seismic vessel with two airgun arrays towed 355m behind (measured from the navigation reference point). An example of an airgun array configuration with 28 active guns in three strings is shown in plan view in Figure 3.13a. Te individual gun volumes in this example range from 20 in3 250 in3
(0.3 l) to (4.1 l). Te total
volume is 3,090 in3 (50.7 l). Te array contains a number of ‘cluster guns’, where two guns sit so close together that their air bubbles coalesce after the guns have fired. Cluster guns produce sound more efficiently
than a single large gun with the same volume as the cluster. Te source array dimension is 15m (inline) x 20m (cross-
line). Inline and cross-line refer to the direction the ship sails. Te source signature from the array at 5m depth is displayed
in Figure 3.13b. Observe that the bubble oscillations are strongly damped. Te primary-to-bubble ratio is PBR=35.6. Te amplitude spectrum is shown in Figure 3.13c. Te notches at frequencies 0, 150, 300 and 450 Hz are caused by the source ghost. Note that the primary-to-bubble ratio is frequency dependent. In seismic surveying, airgun arrays are designed to direct
a large proportion of the sound energy downwards. Despite this downward-focusing effect, relatively strong sound pulses will propagate in all directions. Te radiation from an array will depend on the angle from the vertical, so that the radiated source signature is directional. Tis effect is called directivity. Each array has its own specific radiation pattern. Tis
pattern, which will be different for different frequencies, varies relatively slowly from low to high frequencies. Te radiation pattern will also be different for different array tow depths. For the gun array in Figure 3.13a, source directivity can
be modelled. Figure 3.14 shows the radiation pattern for frequencies 0–150 Hz, in in-line (top) and cross-line (bottom) directions. Vertical direction is 0 degrees. At the edge of each circle 90 degrees corresponds to horizontally propagating energy. Observe that the radiation pattern is concentrated
100
Figure 3.12: Seismic vessel with two seismic sources (two airgun arrays) and 12 hydrophone cables equipped for a 3D investigation.
downwards, and that the source pulse gets attenuated for angles that differ from the vertical. Te amplitude levels emitted horizontally tend to be 18–29 dB lower than the vertical. Note that the sound produced by the array is not distributed evenly across the frequency spectrum. Te amplitude is largest in the 20–100 Hz interval but some
energy will be present up to 500–1,000 Hz. Te high-frequency components are weak when compared to the low-frequency components, but strong when compared to ambient noise levels.
3.2.2 The Effect of the Water Layer and Seafloor
Te seismic signal from the airgun array will be affected by the physical properties of the water layer and the seafloor. Te sound travelling with small to moderate angles to the
vertical axis will reflect at and refract into the water bottom. Te reflection strength is given by the reflection coefficient for the interface between the water layer and the layered bottom. For small angles, the reflection coefficient is small, typically 0.2, so that most (~80%) of the sound enters the subsurface. However, sound hitting the seafloor at angles larger than a
critical angle to the vertical, determined by the ratio of sound velocity in water and sea bottom, will be reflected back into the water layer (Figure 3.15). Te water layer, bounded above by the sea surface and below by the water bottom, then forms
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