22 Well seismic surveying and acoustic logging 1.2.1.1.1 Sources The standard ASTM D7400-08 (Standard Test Methods for Downhole Seismic Testing) clearly defines the most common S source. It utilizes a beam of 2.4 m long and 15 cm thick, on which a vehicle can be positioned to ensure good coupling. An SH wave-rich signal is then obtained by lateral strikes on the sides of the beam, with a hammer of 5 to 15 kg. The hammer can be mounted onto a pivoting axis as shown in the photos in Figure 1.4. Figure 1.4 Illustration of an S source used in downhole testing which complies with the ASTM standard (IgeoTest source on the left, IMG source on the right). To generate P waves, an anvil can be vertically struck, either next to the beam on the axis between the midpoint and the borehole head, or next to each end of the beam. Choice of source location The distance between the midpoint of the beam and the probe (offset) must be carefully selected to limit the impact of tube waves, refracted waves and the angle of incidence. This distance is generally between 1 and 3 m. If a larger offset is required (4 to 6 m) or if the medium has several different velocity layers in the first 10 meters, then the refracted waves and actual paths (Snell-Descartes law) must be taken into account in the analysis (see Figure 1.5). The impact of tube waves is a reality that must be taken into account. Figure 1.6 shows downhole data from a borehole full of water (blue signal), and then without water (black signal). This shows the complexity of the tube wave signal: a time of 60 ms at 30 m is not consistent with a simple signal reflected at the bottom and transiting through the water. The velocity corresponds more to that of a guided mode transiting at the water-casing interface (see Chapter 2, paragraphs on acoustic waves).
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