Page 114 - Acquisition and Processing of Marine Seismic Data

2.4 SPECIFIC ACQUISITION TECHNIQUES 105

FIG. 2.59 Comparison of (A) conventional streamer and (B) ocean bottom cable P wave seismic sections. After Hall, S.A.,
MacBeth, C., Barkved, O.I., Wild, P., 2005. Crossmatching with interpreted warping of 3D streamer and 3D ocean-bottom-cable data at
Valhall for time-lapse assessment. Geophys. Prospect. 53, 283–297.

characteristics due to the more consistent source and is attached onto an iron frame or anchor
and receiver positioning (Eriksrud, 2014). (Fig. 2.60). It is recovered with an acoustic release
system: On completion of the survey, the device
2.4.1.2 Ocean Bottom Seismometer is separated from the anchor by an acoustic
Oceanbottomseismometers(OBS)aredevices release and ascends to the surface by sending a
that contain a 3C seismometer and a hydrophone coded acoustic signal emitted by the ship’s trans-
to record long offset seismic or seismological ducer to the transponder of the OBS. The instru-
data. They are not permanent devices located ment also has a radio beacon to be detected by
on the sea floor, and are typically designed to the ship’s radar, as well as a flash light and a flag
work down to 6000 m water depths for a maxi- for an easy detection and retrieval onboard.
mum operational period of approximately OBS systems are commonly used for seismo-
3months.TheOBSsystemisbuoyantinthewater logical research by academia and oceanographic

FIG. 2.60 (A) Geomar, (B) Sercel, and (C) USGS ocean bottom seismometers.