Page 67 - Acquisition and Processing of Marine Seismic Data

58 2. MARINE SEISMIC DATA ACQUISITION

Each bird transmits its depth information initialization of the streamers and digitizing
obtained from its depth sensor to the control modules, and analyze some specific information
unit. The wing angles of the birds are modified from the streamer cables, such as temperature
during the acquisition to maintain the depth of and pressure information on the digitizers or
the streamer generally within a depth range of the tension on the streamers if a tension sensor
1 m. This process is done automatically: A tar- is available. Some important data acquisition
get depth to each bird unit is assigned, and con- parameters such as trace summing, record
trol software keeps the birds at the desired depth length and sampling rate are set in the recorder.
range during the survey by modifying the wing Modern seismic recorders are of low instru-
angles automatically. Depths and wing angles of ment noise, harmonic distortion and power
each bird are read by the software normally at line interference. They are capable of recording
each shot position and graphically displayed tens of thousands of channels from several
on the screen, fed into the navigation system streamers as well as automated analyzing and
and logged into the disk files for further offline QC methods (Lansley, 2006), and also recording
QC processes after the survey. and managing the auxiliary channels. They are
Somebirdsareequippedwithafluxgatemagne- expected to have a dynamic range as high as
tometer, termed compass birds, which measures possible. The dynamic range is defined as the
and transmits the azimuth angle of the birds in ratio of largest signal amplitude that can be
addition to the depth information (Fig. 2.15A). If recorded to the smallest amplitude above
the compass birds are mounted on the streamer the system noise (Section 2.5.2.4). Seismic
at regular intervals, then it becomes possible to recorders may commonly have various fre-
observe the feathering angle of the streamer in quency filters. The most common filter types
areas of strong surficial currents and streamer are specific low-pass antialiasing and 50- or
bending at each line change (Fig. 2.15E). Further- 60-Hz notch filters to prevent recording of the
more, a specific inflatable device can also be aliased signal and power line interference,
mounted onto the birds, termed streamer recovery respectively.
device or retriever, which recovers the streamer Starting with a trigger signal from the inte-
sections in case of an accidental failure during the grated navigation system, the seismic recorder
acquisition (Fig. 2.15B). The retrievers are placed immediately initiates data logging from all
above the cable leveling devices and are auto- available active and auxiliary channels into the
matically activated to inflate when the streamer disk and/or magnetic tape units for a predeter-
sinks to a depth of 48 m. mined time span, that is, the recording length. It
also opens the navigation port and receives the
shot and channel positions from navigation sys-
2.1.6 Seismic Recorder
tem to record into the seismic data headers.
The seismic recorder is the system that Modern recorders perform demultiplexing and
receives all the seismic amplitudes at each time DC offset removal on the seismic data before
sample from all available streamers at each shot, constructing the raw shot gathers. The raw seis-
and stores the raw seismic data into its internal mic data is recorded in a specific format, as
hard disk drivers or external storage devices. In binary coded data files. A number of sophisti-
analog seismic systems, the recording unit also cated seismic data formats are defined by the
has an analog-to-digital (A/D) conversion unit. Society of Exploration Geophysicists (SEG).
In modern digital systems, there is no need to The most common format types are known as
have an A/D converter since the incoming data SegD for raw data and SegY for data exchange
is already digitized by modules mounted to the between the different processing and interpreta-
streamer(s). Seismic recorders control the tion platforms (Section 5.2).