61 2. Well seismic surveying Figure 2.9 Wave ray paths in well-to-well seismic surveying. For well-to-well seismic surveying (Figure 2.9), the observed wave field is complex. In general, the following waves are observed: • Direct wave. • Waves reflected and/or refracted on markers located above or below the depth of the receiver, with and without conversion phenomena. • Conversion waves created by tube waves generated by the source. These conversion phenomena (tube waves-volume waves) occur at layer boundaries associated with strong acoustic impedance contrasts and at the bottom of the well. When two or more wells are available, a tomographic survey can be performed to finely describe the area between the two wells. Several implementations are possible: • Transmission tomography. • Reflection tomography. • Diffraction tomography. Figure 2.3 -right is an example of transmission tomography in civil engineering, conducted in vertical wells ahead of a tunnel digger used for laying out a large diameter sewer pipe. It shows a significant alteration of the rocks in the project zone.
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