52 Well seismic surveying and acoustic logging However, the lateral range of investigation of a seismic walkaway is limited (to between several hundred meters up to a kilometer) compared to that of a classic seismic reflection profile. This underlines the local nature of a reservoir study using a well seismic method. VSP, OVSP and seismic walkaway implementations provide an image of the geological formations below the well. During drilling, the drill bit itself can be used as a well source, thereby permitting the imaging of formations that have not yet been reached while the well is being drilled (prediction ahead of the bit). The use of a well source enables reverse well seismic surveys (source at the bottom and receiver on the surface) to be carried out, along with well-to-well seismic surveys. This aspect has been developed in Chapter 1, which focuses on the measurement of formation shear velocities. Well-to-well seismic surveying can provide images of formations between wells in the form of seismic reflection sections, which give acoustic impedance contrasts (Figure 2.3 - left) or velocity models obtained by inversion of first arrival times (tomography by transmission, Figure 2.3 - right). Figure 2.3 Examples of well-to-well seismic surveying. Left: example of well-to-well seismic measurement – reflection of S-waves between a vertical well and a deviated well (from Becquey et al., 1992). Right: Example of transmission tomography in civil engineering (F. Lantier, consultant) The various implementations of well seismic methods can be grouped under the general term of well seismic profiling.
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