Survey Design For Vertical-Cable Seismic Acquisition

Abstract

The vertical cable recording geometry has shown considerable promise as a seismic acquisition technique. It is<br>especially suitable for marine areas where any of the following are present: 1) obstacles (e.g. platforms or buoys) for a<br>vessel towing streamers, 2) deep to very deep water, 3) a complex subsurface geology, 4) pipelines on the sea bottom,<br>and 5) a hard (e.g., carbonate or basalt) sea floor. The last two situations may prohibit the use of ocean bottom cables<br>(OBC). On land, it can be seen as a 3D-VSP with one or more wells (cables).<br>If we are to acquire vertical cable data, we need to be able to design the surveys to best image the target geology in the<br>most economic way. Thus, we are interested in questions of source and receiver distribution and the resultant target<br>coverage.<br>Analysis of fold for regular grids of bins in vertical-cable acquisition was done for a straightforward, still realistic, 2.5-D<br>synthetic case. It describes a passive continental margin environment in a Mesozoic basin. Acquisition geometries with a<br>different numbers of vertical cables, hydrophones per cable, numbers of shot points and water depths were analyzed.<br>Three-dimensional ray tracing was used to obtain the reflection point from a target layer. Converted (P- to S-) waves, and<br>a land case were also analyzed.<br>The results show that good coverage can be obtained using only a single cable, a reasonable number of hydrophones<br>per cable and a fair shot point spacing. Deeper water acquisition shows better fold homogeneity than shallow water when<br>several cables are used on an optimized configuration.