# Cross-line fold of a towed-streamer survey

On page 29 of the book 3D Seismic Survey Design by Gijs O. Vermeer, Vermeer says that the equation for a maximum fold in a 3D survey is the following:

$M = M_i \times M_c$

where $M_i$ is the inline fold and $M_c$ is the crossline fold. For a towed streamer, the inline fold is the same as for a 2D geometry which can be expressed as:

$M_{i} = \frac{N_{chan}\Delta r}{2\Delta s}$.

For the crossline fold he states that:

"The midpoint range in the crossline direction of one boat pass equals the number of streamers times the streamer interval divided by two ... normally, the next boat pass is shifted in the crossline direction with this distance, ensuring adjacent midpoint ranges of neighboring boat passes, i.e., no overlap in midpoint coverage between boat passes; hence, crossline fold $M_c = 1$. Therefore, in marine streamer acquisition, usually $M = M_i$ "

I feel that I understand why the inline fold is the same for the case of a 2D acquisition geometry, but I am having difficulty understanding the concept of the crossline fold. Specifically from the previous quote, could someone help me to understand what Vermeer means when he says midpoint range and coverage?

Any help will be greatly appreciated.

## 1 Answer

By midpoint range he refers to the size (length in the crossline direction) of the area in the subsurface that is illuminated by one pass of the boat. In a simple case of flat and parallel layers, that range is half of the size of the "spread" or receiver patch. BTW, this is true both in inline or crossline directions.

By coverage he means how the subsurface is illuminated by the combination of many passes of the patch over the area of a 3D survey. In common practice, passes are overlapped in the surface by half the crossline size of the patch so that the areas of subsurface illumination are contiguous with no overlap.