Page 129 - Acquisition and Processing of Marine Seismic Data

120 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.73 Several hydrophones are connected in parallel to form hydrophone arrays for each recording channel, which
results in each hydrophone receiving the reflected energy with a time delay. When summing up the outputs of the hydro-
phones, this causes a specific filtering termed the directivity effect, the influence of which decreases with decreasing
incidence angle.

an incidence angle of 45 degrees is 7 dB, whereas For several reasons, long offsets are required to
it is 14 dB for a 100-Hz signal. obtain better quality data, which include:
In seismic streamers, group interval and
• Increasing the number of channels also
number of hydrophones forming the group is
increases the nominal fold of the data.
fixed. Therefore, the only way to reduce the
• Long offsets are crucial for amplitude versus
amplitude decrease is to lower the incidence
offset (AVO) analyses for hydrocarbon
angle of the incoming signal. For a constant tar-
exploration.
get depth, the incidence angle depends on the
• Some of the multiple reflection elimination
source-receiver distance, that is, the offset. Small
techniques require long offset data.
offset corresponds to small incidence angle,
• Seismic velocitiesof deeperlayerscan bemore
which means that less amplitude attenuation
precisely resolved if long offsets are available.
occurs for high-frequency components. The
effect of minimum offset distance may be signif- • Longer streamers provide higher inline
icant in shallow water surveys. In relatively coverage and slightly better azimuth
distribution.
deep waters, such as two times the streamer
length, high-frequency amplitudes are not sig-
nificantly decreased since the incidence angles 2.5.1.5 Sail Line Azimuth
are already small. Sail line azimuths should be chosen in parallel
The distance between the last (or the farthest) to the structural dip of the subsurface if there is
channel and the source center is termed the max- vintage seismic data for the planning of the
imum offset. For a constant group interval, acquisition. This also ensures a more reliable
increasing the number of recording channels migration and prevents recording side-sweeps.
increases the maximum offset distance of the If the purpose of the survey is to map the faults,
data. As a general rule, maximum offset distance then the sail lines should be perpendicular to the
should approximately equal to the target depth. fault strikes.