51 2. Well seismic surveying The lateral range of investigation of the VSP can be improved by offsetting the source with respect to the well. This technique is called Offset Vertical Seismic Profiling (OVSP). The image obtained after processing is thus a single-fold seismic section. A Seismic Walkaway is a series of offset VSPs, with the surface source situated at several locations corresponding to successively increasing offsets with respect to the borehole. The image obtained after processing is a section with a low degree of multiple fold coverage. In this type of setup, the number of positions of the well geophone is generally limited. Figure 2.2 illustrates the implementation of a seismic walkaway and shows an example of the type of imagery obtained in the oil industry. The horizontal axis represents the distance of the mirror point with respect to the well. Figure 2.2 Example of imaging with seismic walkaway (document: ENGIE - IFPEN).
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