Page 47 - Acquisition and Processing of Marine Seismic Data

38 2. MARINE SEISMIC DATA ACQUISITION

Seismic surveys use elastic waves generatedby the data-processing steps and required proces-
artificial explosions or vibrations on land, and sing parameters. In general, parameters and out-
commonlyhigh-pressureairdischargesinmarine puts of some specific data-processing steps
surveys. Some of the generated waves are strongly depend on the data acquisition parame-
reflected back from the interfaces separating the ters. Therefore, a processor should be aware of
different stratigraphic units in the earth, and we the basic concepts of acquisition to design the
receive and record the amplitudes as well most suitable processing flow with optimum
as arrival times of this reflected energy on the processing parameters. This section includes
earth or ocean surface using specific sensors, basic principles of marine seismic data acquisi-
called geophones for land acquisition and hydro- tion as well as the equipment used in 2D and
phones for marine surveys. All types of marine 3D surveys in addition to some specific types of
seismicsurveysconsistofoneortwosourcearrays surveys, like P-cable and 4C acquisitions, in
and a specific configuration of several hydro- order to provide a complete source from the
phones assembled in a receiver cable, termed a acquisition to the end of data processing.
streamer. The configuration varies based on the Offshore seismic surveys are conducted both
purpose of the survey: it may be one single cable for academic purposes, to understand the struc-
(for 2D acquisition), several parallel cables (for ture and dynamics of the continental margins
3D and 4D acquisition), or receivers lying onto and deep basins, and for exploration, produc-
the seafloor (for 4C acquisition), etc. tion and development of offshore oil and gas
Coherent and random events observed in the fields. Seismic acquisition is a very specific area
seismic data determine the characteristic specifi- of expertise that requires professional field
cations of the data recorded. Most of the coherent teams to conduct the surveys and collect the data
noise components are generated by the acquisi- in required quality limits without any complica-
tion components; for instance, operational noise tions. Although it may vary according to the
is produced by the survey vessel, or bird noise requirements of the specific project, Fig. 2.1
arises from the movements of the wings of depth shows a general offshore seismic survey crew
levelers mounted at regular intervals along the list and structure. The HSE (Health, Safety and
streamer (Chapter 3). Data processors should Environment) team ensures that all the opera-
be familiar with the nature and origin of these tions onboard are conducted in accordance with
specific noise types originating from the nature the safety regulations based on the implementa-
of the seismic acquisition, to suitably determine tions determined and declared in the project’s

FIG. 2.1 A general crew structure for an offshore seismic survey.