Page 118 - Acquisition and Processing of Marine Seismic Data

2.4 SPECIFIC ACQUISITION TECHNIQUES 109

and require careful analyses, since they may eas- receiver types are involved in the acquisition.
ily be masked by differences in the acquisition Logistics, operational and geographical difficul-
properties, parameters and geometry of both ties as well as near surface conditions often
datasets, as well as the noise level embedded affect the quality of the acquired data, because
in the data. Therefore, the reliability of 4D data the working environments involve different sur-
strongly depends on the accuracy and repeat- ficial conditions, such as land, swamps, reefs,
ability of the sensor and shot positions between beaches, marsh zones with fresh or salt water
two successive surveys. intrusions, etc.
4D seismic surveys can provide valuable Although different types of seismic sources
information for the management of the reser- (dynamite, Vibroseis or air guns) and receivers
voir, and monitoring of the dynamic structure (geophones or hydrophones) can be used
of the reservoir may lead to higher production (Fig. 2.64), it is preferred to utilize one type of
and a better estimate of the reserves while it source and receiver for the entire survey to pre-
minimizes the further infill drilling costs. 4D vent the phase changes in the seismic wavelet
seismic can also be used in monitoring the injec- from one type of source to another and to obtain
tion fluids, such as water, gas, steam, CO 2 , etc., a stationary source signature for the whole data-
into the existing reservoir during the develop- set. Dynamite detonated in shallow holes both
ment stage, and can reduce costs by optimizing on land and shallow marine areas can be used
well injection programs (Lumley, 2001). as a suitable seismic source when the safety
issues are not concerned.
A hydrophone cable is often used to receive
2.4.3 Transition Zone Acquisition
the signals. However, placing the hydrophone
Transition zones are the most challenging cables in correct position without dynamic depth
regions of very shallow marine, lake and river levelers (birds) is very difficult and the cable is
environments where the acquisition of seismic typically anchored to the sea floor. OBC or
data can be quite complex and problematic. OBN acquisition can often be suitable choice
Since these are the regions where the conven- for transition zone acquisition up to 30 m water
tional land or marine acquisition techniques depths. In addition, dedicated receiver cables
are not applicable, generally mixed source and and receivers compatible to the existing land

FIG. 2.64 Schematic illustration of different types of sources (top) and receivers (bottom) used for transition zone surveys.
Vertical scale is arbitrary.