Seismic surveys
Seismic survey
techniques are used to build up an image of the rock formations
beneath the seabed during the first part of the exploration phase.
Later in the life of a potential oil field further seismic surveys
are undertaken to gain more detailed information about the geology
around proposed well sites.
In general
terms, seismic surveying equipment works on the same principle as
radar or sonar devices. Pulses of low-
frequency sound are released at regular intervals, which pass
through the water and the seabed to be reflected by the geological
layers beneath the surface of the earth (Figure 1
- Diagram illustrates pulses of sun reflected from the seabed to the
surface). The reflected
echoes are recorded and analysed, and the results are used to create
a computer-generated image of the geology far below the bottom of
the sea.
Survey
engineers use an array of underwater airguns towed behind the survey
ship to release a 'pop' of compressed air. These brief pulses occur
approximately every ten
seconds. The echoes coming back from the subsurface rock formations
are measured by a towed array of cables fitted with hydrophone sound
detectors while the survey vessel sails along a pre-determined line
several kilometres long.
Each line
sailed yields data which is used to produce a two-dimensional image
of the earth in cross-section. These cross-sections can indicate the
geology of rock layers down to 20,000 ft below the bottom of the
ocean. If enough closely-spaced seismic survey lines are sailed,
analysts can use the data to create a three-dimensional image of the
subsurface rock by layering the two-dimensional cross-section images
side by side
(Figure 2
- shows the positioning of the hydrophone streamers behind the
seismic vessel which receive the reflected signal).
The future of Seismic Imaging Technology
Reservoir imaging
technologies are progressing rapidly with major advances being made
in all of the imaging tools, e.g. seismic, gravity and
electromagnetics (EM). However, in addition, it is recognised
that combining the strengths of these approaches by jointly
acquiring and processing the various signals gives an added and
powerful insight, not just into the structure of the reservoir but
also the presence, or absence, of hydrocarbons. This is the
core of current subsurface activity in ITF - for further information
contact Duncan Anderson (d.anderson@oil-itf.com).
Useful links:
British Geological Survey
Geological Society of London
Petroleum Exploration Society of
Great Britain
First Break
CDA (Common Data Access Limited)
PILOT (formerly OGITF- Oil
and Gas Industry Task Force)
BERR (formerly DTI)
Industry Technology
Facilitator (ITF)
Operations Index
