56 Well seismic surveying and acoustic logging 2.2.2.3 Seismic source The seismic source must be the appropriate size for an optimal Signal/Noise ratio for the investigated well depth and for a suitable frequency content. The most used sources are: explosive (high energy but can cause a nuisance and authorization is limiting), gun (cartridge), hammer (practical but low energy) and weight drop (good compromise energy/bulk). These sources emit mainly compression-type volume waves. You can also use a mini-vibrator. The petroleum industry generally uses the seismic source for surface seismic data acquisition (vibrator). In water, an air gun is normally used. To emit S-waves, a hammer can be used to laterally strike a target anchored to the ground. This point is developed in Chapter 1. Figure 2.5 shows examples of seismic sources used in civil engineering. Source: gun Source: Weight drop (APEC document) Figure 2.5 Seismic sources used in civil engineering. 2.2.2.4 Acquisition parameters The VSP recording is a two-dimensional recording (time: vertical axis; depth or length: horizontal axis). The sampling interval is 0.25 ms, 0.5 ms or 1 ms. The listening time ranges from a few hundred ms to a few seconds. The vertical distance Δz between 2 measurement points (sampling according to depth, i.e. the horizontal axis) must be chosen to be less than the smallest half-wavelength encountered to avoid spatial aliasing phenomenon. Δz is calculated from the lowest propagation velocity Vmin of the seismic waves and the highest frequency Fmax likely to be recorded: Δz ≤ Vmin/2Fmax. If Vmin = 1500 m/s and Fmax = 150 Hz, we will choose Δz ≤ 5 m.
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