59 2. Well seismic surveying Figure 2.7 Example of VSP with tube waves (ENGIE – IFPEN). When the source is offset, we can observe conversion phenomena. To properly understand the wave propagation it is necessary to record data with multi-component receivers. Figure 2.8 shows an offset VSP recorded with a two-component well geophone, vertical component Z and horizontal component H. For both components the first arrival is the direct P-wave. We can observe a downgoing S-wave with a low apparent velocity that is more visible on the horizontal component. Upgoing waves are visible on both components, the apparent velocity of S-waves being lower than that of the P-waves.
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