Page 30 - Acquisition and Processing of Marine Seismic Data

1.2 MARINE ACOUSTIC METHODS 21

Single-channel acquisition is relatively simple as raw seismic data, and therefore, it is not pos-
and more economical in terms of equipment sible to obtain subbottom stratigraphy during
deployed and the data acquisition methodology, the acquisition because multichannel seismic
as compared to multichannel data acquisition. data requires various additional data-
Processing of single-channel data is also less com- processing steps to obtain the subsurface struc-
plex. However, it does not allow us to obtain the ture (Fig. 1.13B).
subsurface velocity distribution in 1D or 2D since After NMO correction and stacking
we only have the information of arrival times of (Chapter 10), multichannel seismic data also
thereflectionsshownbyt(x)inFig.1.13A.Because becomes zero offset since NMO correction
the arrival time of a specific reflection is a function removes the offsets between the source and
of reflector depth and the propagation velocity, receivers in CDP gathers. The effect of stacking
and because we cannot know the depth of the on the data quality is spectacular and the seismic
reflectors originating the recorded reflections, image is of a much higher S/N ratio. As an
we cannot obtain the seismic velocities of the example, Fig. 1.14 compares two zero-offset sec-
subsurface sediments by single-channel seismics. tions along the same 2D survey line: One is
Applications of single-channel acquisition are single-channel seismic data (Fig. 1.14A), and
limited to the engineering surveys before the set- the other is 48-fold stacked multichannel seismic
tlements of offshore geo-engineering structures. data (Fig. 1.14B). The data quality of the stack
Today, modern marine seismic data is acquired section is superior with a better trace-by-trace
with some offset between source and several consistency and higher S/N ratio.
recording channels located within a single (2D Before the introduction of 3D seismics, the
acquisition) or multi (3D acquisition) receiver data were collected along straight lines to record
cables (streamers). The term offset is the distance 2D data. In 2D acquisition, a number of closely
between source location and each recording spaced parallel (as well as crossing) lines are
channel. In multichannel acquisition, offset dis- necessary to reliably map the survey area. In
tances of each recording channel are different, any case, the gaps between the 2D survey lines
and in general, it is regularly increasing as we are interpolated during mapping. Today, multi-
move away from the source location along the channel marine seismic data is collected by sev-
streamer. Utilization of multichannel acquisi- eral receiver cables (multistreamers) lying
tion with different offsets provides several parallel to each other to obtain 3D seismic data
advantages, such as: without any gaps in the survey area. A 2D sur-
vey provides subsurface information with data
• It is possible to obtain subsurface velocity
coverage limited to survey line locations,
model from multichannel seismic data after
whereas it is possible to explore each specific
velocity analysis.
point independently from the survey line loca-
• Stacking process suppresses most of the
tions in 3D seismic data, which provides a better
random and significant amount of
image of the subsurface that is very close to the
coherent noise.
real world (Fig. 1.15). Advantages and disad-
• Multiples can be suppressed using different
vantages of 2D and 3D conventional seismic sur-
approaches using prestack seismic data.
veys are summarized in Table 1.7.
• Amplitude variations with increasing offsets
The output of a 2D survey consists of sparsely
may indicate subsurface hydrocarbon
located seismic lines, whereas the output of a
accumulations.
3D survey is a seismic data volume, which
In multichannel seismic acquisition, specific covers both inline and crossline directions of
trace groups, called shot gathers, are obtained the survey area (Fig. 1.16A). We can extract 2D