Recording of 4-C seismic data

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


Marine 4-C data are recorded by using ocean-bottom cables with receiver units, each containing one hydrophone to detect the pressure wavefield and three geophones to detect particle motions in a Cartesian system. The receivers used in marine multicomponent surveys are usually of gimballed type; as such, the vertical geophone is guaranteed to measure the vertical component of the particle motion. The two horizontal components measure the particle motions in two orthogonal directions, and they are intended to be oriented in such positions that one of them is aligned in the direction of the receiver cable. Shown in Figure 11.6-4 is a diagram of an ideal three-component geophone layout that would be possible to achieve in land surveys. The orientations of the three components coincide with a right-handed Cartesian coordinate system. This means that the vertical z-component is positive downward, while the inline x-component is defined to have positive direction when the crossline y-component is clockwise with respect to the x-component.

The geophone orientation of the layout shown in Figure 11.6-4 is not achievable in an ocean-bottom survey. Although the vertical geophone is indeed oriented in the vertical direction and it measures the particle motion as positive downward, the two horizontal geophones are not guaranteed to be in the inline and crossline directions. Instead, these two geophones position themselves at various different, still orthogonal, directions. As a result, the horizontal geophones associated with a common-shot record measure particle motions in arbitrary directions, rather than the desired common inline and crossline directions (Figure 11.6-5). This arbitrary horizontal geophone orientation is primarily caused by the seabed conditions such as currents, unconsolidated sediments, and the roughness of the seabed surface.

The sensor systems used in OBC surveys are of two types — the node systems and cable systems. A node is an individual unit that houses a single hydrophone and three geophones in the Cartesian orientation. The nodes are pressed into the seabed sediments by a remotely operated vehicle. Most nodal systems are derivatives of the SUMIC system pioneered by Statoil. The more widely used cable systems deploy different designs for housing the hydrophones and geophones. The cable usually is dragged a certain distance and then draped down to the seabed along the desired traverse.

The recording geometry for a 4-C OBC survey is an adaptation of a typical land 3-D survey geometry (3-D survey design and acquisition). As illustrated in Figure 11.6-5, two or more cables are laid down on the seabed parallel to each other, and data are recorded using a conventional seismic vessel with source locations aligned in the direction perpendicular to the receiver lines.

Shown in Figure 11.6-6 is a composite common-shot gather from a 4-C OBC survey made up of four records. The individual components are the hydrophone record, and the inline, crossline and vertical geophone records. This survey was conducted using two receiver cables laid in parallel, 300-m apart on the seabed. While the total cable length is 6000 m, maximum offset for some shots was up to 9000 m. Each cable carried 240 receiver units at 25-m interval; hence, each of the four records shown in Figure 11.6-6 contains 240 traces. The same common-shot gather is displayed in Figure 11.6-7 with AGC applied in order to better examine the signal and noise character in each record. Note that the hydrophone and vertical geophone records exhibit events with similar moveout since they both contain P-waves. The records associated with the two horizontal components exhibit events with much larger moveout since these records contain S-waves which travel with a slower velocity than P-waves, almost twice as slow in many rock types. Therefore, when identifying the same event on both a P-wave record and an S-wave record, keep in mind that the event time in the latter can be twice the event time in the former. This means that if you plan a 5-s record length for conventional P-wave data, you would need to record the 4-C data using a 10-s duration. The slower S-wave velocities would also result in a much larger moveout on the S-wave record compared to the moveout on the P-wave record. This means that spatial aliasing would be more serious when applying multichannel processing applications, such as f − k filtering or migration, to S-wave data than P-wave data.

Figure 11.6-8 shows a common-receiver gather which was created by sorting the common-shot gather data as in Figure 11.6-6. The same receiver gather with AGC applied is shown in Figure 11.6-9. The horizontal geophone records from both the common-shot gather (Figure 11.6-7) and the common-receiver gather (Figure 11.6-9) exhibit events with relatively more complex moveout than those observed on the hydrophone or vertical geophone records. You should not expect a one-to-one correspondence between the events on the two sets of records. Because layer boundaries give rise to differences in P-wave and S-wave impedance contrasts, there may be some events that appear on both records with different strengths, or some events may be present in one record set and absent in the other.

Figure 11.6-10 shows a close-up portion of the composite shot gather shown in Figure 11.6-7 and the spectra of the individual components. Note the polarity reversal from one side of the cable to the other on the inline record as a consequence of the right-handed recording convention (Figure 11.6-5). The frequency content of the hydrophone data appears to be broader than the vertical geophone component; this is because of the imperfect coupling of the geophones with the seabed sediments. This difference in bandwidth because of the coupling issue is observed also in the common-receiver gather shown in Figure 11.6-11.

Acquisition of 4-C OBC data is different from conventional streamer recording in respect of the receivers. In fact, it is like land acquisition at the seabed. When the seabed has irregular geometry, it gives rise to both long- and short-wavelength statics. Therefore, in processing 4-C data, receiver statics need to be calculated and applied to both PP and PS data.

References

  1. 1.0 1.1 Gaiser, 1999b, Gaiser, J. E., 1999b, Applications of vector coordinate systems of 3-D converted-wave data: The Leading Edge, 1290–1300.

See also

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Recording of 4-C seismic data
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