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Seismic experiments Towed-streamer acquisition Another illustration of towed streamer acquisition A marine 18-airgun array configuration A towed-streamer acquisition with three boats Snapshots of wave propagation Examples of events in towed-streamer data Simulated seismic data Snapshots of wave propagation Simulated seismic data The reciprocity theorem A shot diagram The reciprocity theorem Pluto 1.5 dataset Pluto 1.5 dataset 2D/3D seismics Rough sea Swell noise Swell noise Swell noise Swell noise Swell noise Interference noise Interference noise Interference noise Interference noise Over/under streamers Monopole and dipole radiations Principle of 4C-OBS acquisition Original SUMIC sensor Z-system Tommeliten Alpha Tommeliten Alpha (Cont'd) Alba channel Alba channel (Cont'd) Bright-spot BSR/gas hydrates Principle of CO2 sequestration Land survey Linear and nonlinear sweeps Vibroseis sweep correlation Vertical cables and surface receiver arrays Synthetic common-shot gather Synthetic common-shot gather Airwave, a surface wave, and reflections Groundroll Tommeliten field An illustration of zero-offset VSP experiments VSP experiment Walk-above experiment Salt-proximity experiment Shear-wave experiment Marine seismic vibrator Marine-seismic vibrator developed by the Norwegian Geotechnical Institute Marine-seismic vibrator developed by the Norwegian Geotechnical Institute (Cont'd) Marine-seismic vibrator developed by the Norwegian Geotechnical Institute (Cont'd) 3D walkaway VSP survey over Ekofisk Imaging results of the 3D-WVSP data Hydrophone recording in a zero-offset VSP experiment run offshore Norway in 2001 An illustration of vertical cable acquisition Example of primaries, receiver ghosts, and free-surface multiples in VC data Downgoing and upgoing waves A real VC data Primaries and receiver ghosts VC sampling Surface data and VC data Angular coverage A narrow azimuth survey A multiazimuth survey A multiazimuth survey A multiazimuth survey A multiazimuth survey A multiazimuth survey A multiazimuth survey A rose of diagram of azimuth distribution A comparison between a narrow azimuth navigated data and multiazimuth navigated data

Images in: Chapter 7

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Seismic experiments

Images - Chapter 7 Seismic experiments are generally divided into land, transition zone, and marine.

The concept of homogeneity, heterogeneity, acousticity, and elasticity controls the type of waves that we can generate and the physical quantities that we can record. Therefore their implications for seismic acquisitions are profound. For instance, the division of seismic experiments into land and marine is primarily related to the fact that (1) in marine cases the acquisition is conducted in water, which can be treated as acoustic and homogeneous, whereas (2) in land cases, the acquisition is conducted in heterogeneous elastic media.

Actually, the differences between the various acquisition scenarios presented here come down to the difference between generating and recording waves in a homogeneous fluid and in a heterogeneous elastic medium (solid). The fluid is generally considered homogeneous, with a relatively flat air-water interface. It supports only P-waves, and only pressure variations are recorded, although particle velocity can be deduced from pressure measurements, as we will see later.

The solid is generally considered a heterogeneous elastic medium with a nonflat air-solid interface at the earth'surface or a nonflat water-solid interface at the sea floor. It supports P- and S-waves, and the three components of particle velocity can be recorded.

Towed-streamer acquisition

Images - Chapter 7 An illustration of towed-streamer acquisition. The vessel tows an array of airguns and streamers of hydrophones behind the boat while traveling at a roughly constant speed (it takes about 15 seconds for a typical seismic boat to move 50 m).

Again this figure illustrates a towed-streamer acquisition, in which a ship is towing a set of cables containing receivers to record signals generated by seismic sources as the vessel maneuvers across potential petroleum reservoirs. These cables of receivers, generally called streamers, are towed at a depth of between 5 to 10 m below the sea surface.
A typical streamer today is 5,000 m to 10,000 m long. It carries several hundred sensors, known as hydrophones, which record pressure changes. In conventional acquisition, each seismic receiver is composed of 12 to 24 hydrophones which are summed before or after recording, depending on the processing objectives (we will discuss some of these processing objectives in Chapter 8). The spacing between receivers (i.e., the center of a group of hydrophones) is generally 12.5 m.

Typical acquisition vessels today can tow 12 to 16 streamers spaced 50 to 100 m apart. One of the major challenges with towed streamers is maintaining constant streamer spacing.\footnote{Present seismic data processing requires that the data be uniformly sampled in space. That is why the distance between streamers must be constant.} Currents, tides, and other forces can cause streamers to feather, or drift laterally, from programmed positions, and in extreme cases they get tangled. Tangled streamers have to be reeled back to the vessels and untangled manually, resulting in nonproductive time for a detailed discussion of these technological challenges and current solutions).

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

A marine 18-airgun array configuration

Images - Chapter 7 (a) An example of a marine 18-airgun array configuration without streamers. Arrays are sometime fired in alternating ways to allow recharge of the other arrays and to improve the acquisition time. (b) A conventional receiver array with 24 hydrophones. Notice the 24-hydrophone array is formed with overlapping groups of 12 sensors. Notice also the 12-sensor groups are nonuniformly spaced.

A towed-streamer acquisition with three boats

Images - Chapter 7 A schematic diagram of a towed-streamer acquisition with three boats.

Note that in some special cases, two or more boats can be used to collect towed-streamer data. For instance, we here describes a towed-streamer experiment in which three boats are used to collect long-offset (as long as 20 km) data and in a split-spread configuration;
i.e., receivers can be on either side of the shot point. So in some special cases, towed-streamer data can be split-spread. Long offsets are needed to increase the angular coverage for deep targets like sub-basalt rock formations, as discussed in Chapter 1.

Snapshots of wave propagation

Images - Chapter 7 Snapshots of wave propagation in a model made of two homogeneous layers and a half-space. The properties of this model are given in the previous figure. Notice that the impedance constrasts are particularly large in this example, because we wanted see internal multiples as well as primaries, free-surface multiples and ghosts.

Examples of events in towed-streamer data

Images - Chapter 7 Examples of events in towed-streamer data. These events can be grouped into direct waves, primaries, receiver ghosts, source ghosts, free-surface multiples, and internal multiples.

Simulated seismic data

Images - Chapter 7 Simulated seismic data. The physical quantity displayed here is the pressure. Offset is the distance between the shot position and a receiver position along the x-axis. The depth of the source is 225 m, and that of the receivers is 275 m.

Snapshots of wave propagation

Images - Chapter 7 Snapshots of wave propagation in a model made of two homogeneous layers and a half-space. The properties of this model are given in Figure 7-5. Source and receivers are located very near to the sea surface (zs=5 m, zr=10 m) compared to the previous figures (zs=225 m, zr=275 m).

Simulated seismic data

Images - Chapter 7 Simulated seismic data. The physical quantity displayed here is the pressure. Offset is the distance between the shot position and a receiver position along the x-axis. The depth of the source is 5 m, and that of the receivers is 10 m.

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