--> Figure 2. Assumed low-velocity gas chimney extending through the thick shale interval above the Ekofisk Formation. Towed-cable seismic data have significant time sags when their two-way raypaths have to pass through the low-velocity chimney. In contrast, offset-VSP data have minor time delays, because only short segments of each VSP travel path have to traverse the low-velocity zone.

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Figure 2. Assumed low-velocity gas chimney extending through the thick shale interval above the Ekofisk Formation. Towed-cable seismic data have significant time sags when their two-way raypaths have to pass through the low-velocity chimney. In contrast, offset-VSP data have minor time delays, because only short segments of each VSP travel path have to traverse the low-velocity zone.