Geoscience Reference
In-Depth Information
Fig. 2.3
Basics of marine acquisition. The boat travels through the water and every few metres
fires the source which emits a sound wave into the water. This travels through the water and into the
rock layers. At changes in the acoustic properties of the rock, as generally occur wherever the
lithology changes, part of the sound wave is reflected back. The reflection travels up to the surface
where it is captured by receivers within a long cable towed behind the boat. The receivers transmit
the recorded signal back to the boat where it is stored on tape and may be partly processed.
A
B
Fig. 2.4
Basic 3-D acquisition. After shooting a line, the boat turns with a relatively large radius
before shooting a line in the opposite direction. The boat then turns again and shoots a line adjacent
to the original line. This is repeated several times until finally the line AB is shot. The full survey
may contain several repetitions of this basic design.
eight subsurface lines for each pass of the boat. The guns fire alternately; each gun
generates as many subsurface lines as there are cables being towed. The gun separation
is such that the lines recorded by one gun are interleaved with the lines shot by the
second gun. Usually the separation between lines is between 25 and 37.5 m. It is diffi-
cult to reduce this further with a single pass of the boat without an unacceptable risk of
cables becoming entangled, but occasionally surveys are shot with 12.5 m line spacing
