Effect of crosscurrents

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	12
Pages	469 - 484
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

Problem 12.4

Assuming that 96 hydrophone group centers in Figure 12.3a are 50 m apart and that the ship speed is 6 knots, calculate the crosscurrent at two locations. Locations are plotted for every 25th source. The active streamer length is 5000 m.

Solution

The effects of streamer feathering were discussed in problem 7.10.

Toward the left the angle that the streamer makes with the ship’s traverse is about $38^{\circ }$ decreasing to about $20^{\circ }$ to the right. The tangents of these angles equal the ratio of the crosscurrent velocity to the ship’s velocity. Hence if the ship’s speed is 6 knots, the crosscurrents are, respectively, 4.7 and 2.2 knots.

Relying on feathering to obtain offsets perpendicular to the ship’s track produces unequal distributions of offsets and azimuths. The areal coverage that feathering produces is capricious as currents change. The strip of subsurface coverage shown in Figure 12.3a varies from about 1800 m at the left side of the figure to about 700 m at the right.

Figure 12.3a.  Sideways drift of streamers because of crosscurrent.

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Table of Contents (book)

Also in this chapter

• Spatial sampling restrictions
• Bin size in marine work
• Effect of crosscurrents
• Number of seismic sources
• Circle shooting
• Ocean-bottom cable surveys
• Vibroseis land survey
• Loop layout for a 3D survey
• Fault interpretation using time slices
• Acquisition direction for marine 3D surveys

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