Page 111 - Acquisition and Processing of Marine Seismic Data

102 2. MARINE SEISMIC DATA ACQUISITION

Ocean bottom cables and nodes are not used • Acquisition of data with virtually unlimited
for exploration purposes, but are the methods to far offsets as well as very close to zero-offset
monitor the existing reservoirs during their pro- can be achieved, which is not possible in
duction period in order to understand ways to conventional surface seismic acquisition
enhance the hydrocarbon recovery (Fig. 2.56). except for TopSeis and FreeSeis techniques
Ocean bottom seismometers, however, are typi- (Section 2.3.8).
cally used to understand the crustal structure of • A better inversion of seismic data can be
the earth using earthquake waves, mostly uti- obtained by using ocean bottom data with a
lized by academia. Today, there are OBS appli- more complete far offsets, low frequency
cations to obtain long offset seismic data for amplitudes, as well as S wave information.
reservoir mapping, especially by amplitude ver- • Bandwidth of the seismic data is improved
sus offset (AVO) analyses. In each case, the sen- because the receiver ghost is eliminated,
sors are located on the sea floor and a separate which provides much higher resolution
source vessel is used to generate seismic signal. seismic data.
The separation of the sensors and the seismic • It is possible to separate up- and downgoing
source provides data acquisition in different azi- wave fields by ocean bottom recording,
muths with different shooting geometries. which provides a better attenuation of
There are several advantages of ocean bottom multiples.
recording:
The main shortcomings of the ocean bottom
• Ocean bottom seismic acquisition provides S recording techniques are their relatively higher
wave data, which allows us to obtain cost with respect to the towed streamer acquisi-
important supplementary information about tion and more difficult processing of the data.
the reservoir, such as the V P /V S ratio or Since elastic wave field recording of S waves
attenuation characteristics of the in a marine environment requires the sensors
reservoir, etc. to be located on the seabed, coupling of the
• It becomes possible to record full-azimuth receivers to the sea floor is important, especially
seismic data in different offsets since the in OBC and OBN surveys. Inconsistent coupling
source and receivers are separated, which can cause poor vector fidelity (equivalent com-
provides better target illumination due to a ponent response to the same ground motion)
wider diversity of ray paths, especially for and can degrade the data quality of horizontal
complex geological environments such as salt components.
intrusions and long offset data for deeper S waves produced by a mode conversion of
reservoirs. incoming P waves are termed C-waves or PS
• 4D repeatability is increased since the source waves. PS wave data used in conjunction with
and receiver repositioning can be done more the conventional P wave data from towed
accurately. streamers provides important additional inter-
• Design flexibility makes it possible to acquire pretive information about the reservoir proper-
data where the towed streamers fail due to ties. Both types of data can be used to
the different obstructions at the sea surface, determine rock properties such as V P /V S or
such as production platforms. Poisson’s ratio of the subsurface, which is sensi-
• S/N ratio is improved because the sensors are tive to the changes of some specific rock charac-
located in a quieter environment, away from teristics, such as lithology, porosity or reservoir
the surface noise. Downtimes because of bad fluid type, since V S is more sensitive to pore con-
weather conditions are reduced. tent than V P and can provide a correlation with