24 Well seismic surveying and acoustic logging Figure 1.7 shows that air waves also exist in a borehole without water, but they are significantly less energetic (95 ms reflection at 30 m). The effect can be reduced by placing a plug at the borehole head. Figure 1.7 Illustration of a tube wave in a borehole without water. Depending on the velocity of the medium, the tube wave in the water can mask P or S arrivals (see Figure 1.8). To avoid this it is essential to empty the borehole as much as possible or to increase the distance between the source and the borehole head. However, this last option will facilitate the generation of refracted waves (see Figure 1.5). Figure 1.8 Illustration of tube wave impact (InterPacific Project), with a 3 m offset and borehole that has been emptied of water in the first example; with a 5 m offset and a borehole with water for the second example; and finally a third example showing a 3 m offset and borehole with water. This last example shows tube waves (InterPacific Project).
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