Directivity of marine arrays

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Problem

Figure 8.9a shows the directivity effect of group length typical of end-on marine shooting. How will the curves change (a) with arrival time, (b) as offset increases, and (c) for greater stacking velocity? Is it better to tow the recording system in the updip or the downdip direction?

Solution

Aside from the effects of the number and spacing of the elements, angles of approach, and wavelength, the array response is affected by NMO and dip moveout. These add in the downdip direction and subtract in the updip direction.

Figure 8.9a.  Array response to apparent dip. Array length 50 m, offset 500 m, time 1.0 s.
  1. As increases, the NMO decreases and hence has less effect, so the curves in Figure 8.9a become more symmetrical.
  2. As the offset increases, the NMO increases and the curves become more asymmetrical.
  3. For greater stacking velocity, NMO decreases and the curves become more symmetrical.

Because NMO and dip moveout effects add in the downdip direction, it is better to shoot in the updip direction.

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