50 Well seismic surveying and acoustic logging subsurface. After processing, the VSP provides a seismic trace without multiples, which is directly comparable to a surface seismic section recorded in the vicinity of the well. With the added constraints of log data (sonic and density), this trace represents an acoustic impedance log in the well and below the bottom of the well. The well may be an open hole, a cased hole (steel and/or PVC) or a cemented cased hole. For the latter, a cementation control (acoustic logging) is recommended. Figure 2.1 shows an unprocessed VSP recording of a steel-cased vertical well. In this representation, the horizontal axis shows the various depths of the well geophone, and the vertical axis represents listening time. In this example the receiver depth varies between 25 and 90 m, and the surface source is slightly offset (5 m) with respect to the wellhead. The distance between two successive geophone positions in the well is 5 m. The sampling rate in time is 0.25 ms for a listening duration of 250 ms. In the Figure, the listening time has been limited to 100 ms. Figure 2.1 Example of VSP recorded in a vertical well at depths of between 25 to 90 m.
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