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Figure 2. VSP-based calibration of thin-bed stratigraphy in 3-D seismic image space. The rigid welding of stratigraphic depth to VSP image time described on figure 1 is used here to interpret a 3-D data volume. In this example, the VSP image from figure 1b must be advanced (moved up) by 18 ms to optimally align with the 3-D seismic image. Because the stratigraphy penetrated by the VSP well is welded to the VSP image, the positions of the stratigraphic time windows in the 3-D image must also move up by 18 ms. The fact that VSP technology provides not just a time-versus-depth calibration function but also an independent image that can be time shifted to correlate with a surface-recorded image is the unique feature that makes VSP calibration of stratigraphy to 3-D seismic image time more reliable than check-shot-based stratigraphic calibration.