Page 90 - Acquisition and Processing of Marine Seismic Data

2.3 3D MARINE SEISMIC ACQUISITION 81

dimensions from the array center, and in gen- acquisition techniques, such as slant streamer
eral, horizontally radiating amplitude levels or fan mode acquisition, as well as multiazimuth
are approximately 20 dB lower than the verti- surveys, are also explained.
cally radiating amplitudes. The simulated Conventional 3D acquisition has become
amplitudes vary with the frequency of the emit- standard for hydrocarbon exploration before
ted signal and tow depth, and there is less ampli- an offshore drilling decision has been made. In
tude difference between the horizontally and the very first applications of 3D acquisition, only
vertically radiating signals at low frequencies. one single streamer and two identical source
The difference arises from a number of factors: arrays (Fig. 2.38A), or two parallel streamers
the array dimensions and horizontal design of with one source array (Fig. 2.38B), were used,
the guns, irregular distribution of gun volumes which doubled the number of acquisition lines
within the array, and other oceanographic con- for each sail line at the beginning of 1980s. Then
ditions such as variations in water temperature the introduction of twin source and flip-flop
and salinity both in the vertical and horizontal acquisition concepts increased the number of
planes. acquisition lines to four in the case of two-
streamer acquisition (Fig. 2.38C). Since then,
the number of towed streamers and their overall
2.3 3D MARINE SEISMIC lengths have increased consistently. Today,
ACQUISITION modern seismic vessels tow 8–16 parallel
streamers, each 6–10 km long with a 100 m
In the last two decades, 3D seismic explora- streamer separation shooting with dual-source
tion has become a standard tool for the hydro- design to achieve 16–32 acquisition lines per sail
carbon industry following the introduction of line. Typically, array separation is half of the
sophisticated streamer steering and positioning streamer separation, which enables a crossline
systems, and it has replaced 2D surveys, which bin size that equals to one-quarter of the
are commonly still used for academic research streamer separation. Total crossline coverage is
purposes and site surveys for engineering pur- directly proportional to the number of streamers
poses, in addition to the reconnaissance surveys in the spread. For instance, the total lateral dis-
prior to a detailed 3D survey today. Although tance of an 8-streamer spread with 100 m sepa-
the equipment, data acquisition and data pro- ration is 700 m, and the total crossline
cessing in 3D exploration are much more costly coverage achieved is 350 m. Today, up to 960
than in 2D seismic surveys, the information and recording channels per streamer are used to
data quality provided are not comparable to record seismic data in 3D surveys. Unlike 2D
those of 2D acquisition. Since 3D acquisition acquisitions, which provide one single shot
covers the entire survey area with no gaps gather for each shot, 3D seismic provides several
between the subsurface bins, the results and shot gathers recorded from each individual
interpretations are much more correct and accu- streamer for every shot. Fig. 2.39 shows an
rate than those deduced from 2D seismic data. In example shot gather group from a 3D dataset
particular, the development of real-time and off- recorded with eight streamers and 480 channels
line QC and QA applications during and after per streamer.
3D surveys has provided more reliable and Marine seismic contractors develop complete
accurate 3D seismic datasets. In this section, acquisition systems to provide solutions to the
the components of 3D surveys, especially the issues of the conventional 3D acquisition and
steering and positioning devices dedicated to to improve seismic data quality, especially in
3D acquisition, are introduced. Some specific challenging areas such as the regions of salt