58 Well seismic surveying and acoustic logging the value of the permeability thus found is significant compared to the well over a distance equal to the dominant half-length of the incident compression wave (Huang and Hunter, 1981). An example of tube wave analysis in a reservoir zone is presented by J.L. Mari (1989). Figure 2.6 Creation of tube waves by compressional waves crossing a permeable zone, and schematic representation of these waves on the VSP record (from Hardage, 1985). Figure 2.7 shows a VSP with a high level of tube waves, labeled TW1 to TW6. We can identify the direct downgoing wave (first arrival), a set of upgoing reflected arrivals intersecting the direct arrival and some downgoing and upgoing tube waves. The surface waves generated by the source create a field of tube waves (TW3) that is reflected at the well bottom (TW4), and at the top of a porous and permeable zone located at a depth of 440 m (TW5). TW5 is reflected again at the surface on the fluid-air contact (TW6). The downgoing P-wave that enters the permeable zone at 440 m creates a tube wave (TW1) that is reflected at the well bottom (TW2). Secondary tube waves with a low apparent velocity can be noted, due to the tool itself. Stoneley waves can also be used to obtain information on the shear wave velocity of the formation, and to detect fracture zones and karsts. An example of the use of tube waves to detect karstic levels of a near-surface carbonate reservoir is presented in Chapter 5. In this case, it is preferable to use a hydrophone as a seismic sensor.
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