Page 115 - Acquisition and Processing of Marine Seismic Data

106 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.61 An example (A) vertical and (B) horizontal component OBS data from south China Sea. Different arrivals from
crustal structures are also indicated. After Zhao, M., Qiu, X., Xia, S., Xu, H., Wang, P., Wang, T.K., et al., 2010. Seismic structure in
the northeastern South China Sea: S-wave velocity and V p /V s ratios derived from three-component OBS data. Tectonophysics 480, 183–
197.

research groups to obtain information about the data. In the case of using a source vessel at the
upper crust using earthquake waves (Fig. 2.61). surface, the only reference to match the recorded
Recorded data is generally interpreted along data with shooting events is the time. Each OBS
with the ray tracing modeling results to obtain has an accurate internal clock calibrated by GPS
a reliable model of the upper crust and Moho. time to minimize the timing drifts between the
They are also used to record microearthquakes surface equipment and the recorded data within
and monitor the active faults offshore. In such the OBS.
large-scale observations, several OBS devices The OBS devices for seismological research
are deployed several kilometers apart within and seismic exploration have different instru-
the research zone. For hydrocarbon-related mentations. Generally, much heavier and larger
exploration, however, much more dense OBS sensors are used for seismological OBS systems
spacing, sometimes as close as 100 m, is used because the earthquake signals have a relatively
and a source vessel at the sea surface generates lower frequency band of 0.01–30 Hz. These
the seismic signal. For reservoir-related surveys, types of OBS devices also have larger battery
OBS data is typically used for AVO analyses, packages to supply power for much longer oper-
since it may provide very long offset seismic ating time periods. Short period OBSs can record