Geoscience Reference
In-Depth Information
Fig. 2.1
Relationship between target area and acquisition area.
Modern powerful computers enable data processing to begin in the field or on the
boat shortly after the acquisition has started, leading to rapid delivery of products
even for the largest surveys. This is important not just for the financial implications
of improved turnaround time, but also because decisions on data quality, such as the
effects of bad weather and increased swell noise, can be made by examination of real
data quality. Only the most time-consuming processes such as pre-stack 3-D migration
need dedicated processing centres with large powerful computers.
2.1
Marine 3-D data acquisition
In general, 3-D marine data acquisition is simpler and faster than land acquisition since
in all but the most heavily developed offshore areas there are few obstacles, leading
to routine and rapid data gathering. In standard marine acquisition, a purpose-built
boat
(fig. 2.2)
is used to tow one or more energy sources and one or more cables
containing (pressure sensitive) receivers to record the reflections from the underlying
rocks. At present, the source is nearly always an array of air guns tuned to give an
energy pulse of short duration with directivity characteristics that concentrate the energy
vertically downwards. In the past, other sources such as water and steam guns were
used, and these may be encountered on older 3-D surveys. Evans (
1997
) gives a good
description of the workings of an air gun; briefly, the expansion and collapse of the
air bubble in the water acts as an acoustic source that sends sound waves through
the water and into the rock layers below the seabed. At changes in the rock acoustic
