62 Well seismic surveying and acoustic logging Reflection tomography can also be used to image the zone between wells by processing data such as those obtained in an offset VSP (Figure 2.3 left). The source is located in one of the wells, and the receiver in the other. In the example presented, the source is a weight drop generating S-waves. After processing, the resulting S-wave section shows a better vertical resolution than that obtained with a P-wave VSP acquired in the receiving well. In civil engineering, well-to-well seismic surveying is commonly used to determine the S velocities of formations. An example is presented in Chapter 1. 2.4 Processing sequence Conventional VSP processing enables time-depth relationship and velocity logs to be obtained at the well, along with the VSP stacked trace, which is comparable to a very high-resolution seismic trace, without multiples. We assume here that the source and receiver are located on the same line perpendicular to the layers. The processing sequence includes: 1. Editing (elimination of poor quality recordings) 2. Correlation by sweeps for a vibrator source and if processing was not carried out at acquisition 3. Correction of signature fluctuations (time and amplitude) using the reference geophone 4. Summing of same depth records 5. Component sorting, if a multi-component receiver is used 6. Picking of first arrival times and calculation of the time-depth relationship, and then of velocity logs (interval velocity, average velocity and RMS velocity) 7. Separation (by apparent velocity filter) of volume and guided waves, upgoing and downgoing waves 8. Flattening of upgoing and downgoing volume waves (P or S) 9. Deconvolution of upgoing volume waves by downgoing waves 10. Generation of the VSP stacked trace. Deconvolved and flattened upgoing waves are stacked within a corridor immediately following the first arrival times 11. Optional analysis of the amplitudes of the downgoing volume waves and calculation of attenuation log 12. Optional guided wave amplitude analysis (VSP with hydrophone) In the different separation methods, the separation of upgoing and downgoing waves is based, explicitly or implicitly, on the fact that both wave types have positive
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