Page 136 - Acquisition and Processing of Marine Seismic Data

2.5 DATA ACQUISITION PARAMETERS 127

(Table 2.5). If not, it may require a decrease in the disks by the seismic recorder. Record length is
vessel speed to supply more time for compres- a function of depth and seismic velocity of the
sors to fill in the guns between two successive deepest horizon and is directly proportional to
shots, which ultimately affects the overall time the target depth. It should be selected long
schedule of the survey. enough to record the reflections from the dee-
In 3D acquisition of flip-flop shooting, sepa- pest horizon of interest and to collapse the dif-
ration between the two air gun arrays affects fractions from the deepest reflector for a
the crossline bin size and hence spatial resolu- reliable migration.
tion of the seismic data. In general, the lateral As a general convention, the record length, t r ,
distance between two arrays is half of the should satisfy
streamer separation to obtain a regular CMP line
t r t d +2 t FL +1:5 s (2.4)
distribution.
where t d is two-way time to the deepest horizon
2.5.1.11 Tow Distance and Vessel Speed of interest at maximum offset, and t FL is the time
Tow distance states the distance between the length of the longest filter to be applied (e.g.,
stern and towed equipment, such as gun arrays 400 ms for frequency filtering). An additional
and streamers. It is not related to the resolution 1.5 s is for NMO/DMO correction and
or frequency content of the data, but is directly migration.
associated with the operational noise level of In practice, record length is also related to the
the data. If smaller, operational noise level source strength or maximum penetration depth
increases. Distant tow requires additional of the signal. If it is selected too long, then it
umbilicals and lead-in cables with appropriate causes recording of an unnecessarily large vol-
length, as well as extra data telemetry units for ume of useless data beyond the penetration
long-distance data transmission. depth of the source signal, which may signifi-
Speed of the seismic vessels is expressed as cantly increase the processing time and hence
nautical miles per hour, or knots: 1 knot equals the overall cost of the survey.
1 mile/h and is 1853 m/h. Vessel speed during
the survey affects the total survey program, 2.5.2.2 Sampling Rate
and is closely related to the record length and The amplitudes of reflected energy are per-
shot interval (Table 2.5). In addition, higher ceived by the hydrophones and are digitized
tow speeds induce higher noise on the recorded at regular time intervals before recording to
data and decrease the overall S/N ratio. In 2D the tapes or disks. This time interval is constant
and 3D seismic acquisition, vessel speed is kept for all traces of all shots along the seismic line
constant between 4.0 and 4.5 knots, even though and is termed the sampling rate. Based on the
it can be increased during the line changes sampling theory given in Section 4.10, sampling
depending on the strength limitations of the rate directly affects the maximum frequency
towed equipment. value that can be recorded, termed the Nyquist
frequency, f N , which is
2.5.2 Parameters for Data Recording 1
f N ¼ (2.5)
2.5.2.1 Record Length 2Δt
Record length is the time span that the reflec- where Δt is the sampling rate. As the sampling
tion amplitudes arriving at the receivers imme- rate increases, the Nyquist frequency decreases.
diately after a time break signal are perceived by Even if the source produces high-frequency
the hydrophones and recorded to the tapes or components, these cannot be recorded. For