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3DSymSam, The Netherlands
Corresponding author: gijs@3dsymsam.nl
Last minute note: At the SEG Annual Meeting in Salt Lake City, I noticed that Input/Output found a very ingenious solution to the vector fidelity problem of bottom cables. In its new design there is a much reduced coupling between sensor unit and cable.
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The special section in the August 2002 issue of TLE was of great interest to me—subjects that are near to my heart, sufficiently near to be called "relentless" in the Introduction by Cambois et al. I have written some comments to four papers in that section. The comments to two papers (Cambois et al., and Cordsen and Galbraith) are lumped together to avoid overlap. I hope these ideas and replies by the authors will generate further discussion about this important subject.
Comments on "Wide-azimuth acquisition: True 3D at last!" by Cambois et al. and "Narrow-versus wide-azimuth land 3D seismic surveys" by Cordsen and Galbraith. The first thing one has to decide upon when designing a 3D seismic survey is the type of geometry to be used. This is an important choice, because the properties of different geometries are different. There are three main geometries available to choose from: parallel geometry, orthogonal geometry, and areal geometry. From a geophysical point of view, orthogonal geometry and areal geometry should preferably be as wide as long—i.e., have aspect ratio equal to one, for the simple reason that the properties of these geometries are essentially the same in the x (inline) and in the y (crossline) direction. (For orthogonal geometry, C-wave acquisition constitutes an exception. Due to the asymmetric nature of the raypaths, the maximum crossline offset should preferably be larger than the maximum inline offset.) Often, practical, nongeophysical reasons force the designer to select an actual geometry with aspect ratio different than 1.
On the other hand, again from a geophysical point of view, parallel geometry should be selected as narrow as possible. Wide parallel acquisition geometries used in streamer acquisition tend to produce illumination gaps when shooting downdip. The wider the configuration the larger the gaps. Apart from reducing the width of
Sunbury-on-Thames, England
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