Circle shooting

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Problem 12.5

What are the advantages and disadvantages of marine circle recording as opposed to acquiring data along parallel lines?

Background

Circle shooting is sometimes used to acquire 3-D data over salt domes and other features. One type involves the ship encircling the prospect in circles of ever-increasing radius [Figure 12.5a(i)], and another type involves circles of the same radius where the centers of the circles progress along a straight line (Figure 12.5a(ii)).

Solution

Where ship traverses are parallel, the acquisition assembly has to turn around to acquire the next traverse and the ship has to turn very slowly when towing a lot of equipment to keep the towed equipment under control. Typically, it takes two or more hours to turn around, that is, the cost of turning is about the same as that of acquiring about 15 km of data. With circle shooting, no time is lost turning around.

With the method of figure 12.5a(i), all azimuths from the center are acquired in the same way so that acquisition differences should be minimized and comparisons of data at different azimuths should be freer from acquisition bias. Multiplicity should be nearly constant because end-of-line tapers are almost eliminated. However, processing almost always requires a rectangular grid so a polar acquisition pattern has to be converted to rectangular bins for processing. This type of acquisition is usually not suitable for structures that are long in one direction. Difficulties with locating acquisition points are not a problem with today’s navigation systems, although they were earlier.

Figure 12.5a.  Two types of marine circle shooting.

With the method of Figure 12.5b(ii), multiplicity varies considerably, and much of the time that would be lost in turning the ship is lost in oversampling and undersampling different portions of the area. This technique is rarely used.

For structures with some radial symmetry, such as salt domes and their associated faulting, the technique shown in Figure 12.5a(i) works well.

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