Survey design for vertical cable seismic acquisition
Carlos Rodriguez-Suarez, Robert R. Stewart
The vertical cable recording geometry is showing considerable promise as a seismic acquisition technique. It is especially suitable in marine areas where any of the following are present: 1) obstacles (e.g. platforms or buoys) for a vessel towing one or more streamers, 2) deep to very deep water, and 3) a complex subsurface geology. Vertical cables also overcome the problems of pipelines on the sea bottom, which may prohibit the use of ocean bottom cables (OBC). On land, use of vertical cables may be useful anywhere one would use a VSP survey.
This paper discusses the design of the surveys that can best image the target geology in the most economic way. Thus, we are interested in questions of source and receiver distribution and the resultant target coverage.
Analysis of coverage (fold) for regular grids of cells (bins) in vertical-cable acquisition was done for a straightforward, 2.5-D synthetic case. The model describes a passive continental margin environment in a Mesozoic basin. Acquisition geometries with a different numbers of vertical cables, hydrophones per cable, numbers of shot points and water depths are analyzed. Three-dimensional ray tracing was used to obtain the reflection point from a target layer. Converted (P-to-S) waves, offset and azimuth distribution and a land case are also analyzed.
The results show that good coverage can be obtained using only a single cable, a reasonable number of hydrophones per cable, and a modest shot point spacing. Azimuth and offset distributions are poor for a single bin with a single-cable geometry, but improve as more cables are used. Deeper water acquisition shows better fold homogeneity than shallow water when several cables are used on an optimized configuration. For converted-wave imaging, longer offsets are necessary, but higher fold is obtained close to cable positions.