57 2. Well seismic surveying In practice, vertical distances of between 2 m and 5 m are chosen in geotechnical engineering. For a seismic walkaway acquisition, the offset D of the source relative to the wellhead depends on the depth H of the objective. For reflection imaging, the angles of incidence should not exceed 30 degrees. As a rule of thumb, D < 3/4 H. 2.2.2.5 Security Site security must be ensured by the Head of Mission, in accordance with the Quality System of the provider. Access to the measurement area must be secured. Only professionally qualified personnel are permitted to use explosives. 2.2.2.6 Quality Control At each depth interval, the operator controls the quality of the recordings (well and surface receiver, noise level, good coupling of the well receiver…). 2.2.2.7 Production On average, for a standard VSP: 5 mn/depth. 2.3 Seismic waves The VSP recording is composed of downgoing and upgoing P and/or S volume waves as well as of interface/guided waves related to the presence of the well and the well fluid. Guided waves are Stoneley waves, more commonly known as tube waves. The downgoing volume waves are direct waves emitted by the source. They result in direct arrivals and all the multiple events created by seismic markers located above the well receiver. Upgoing volume waves are primary and multiple reflected waves. Downgoing waves have positive apparent velocities, upgoing waves have negative apparent velocities. Tube waves are created when the particles of the sludge column that fills the well are set in motion. Surface waves are the main source of tube waves, which are considered as organized noises that disrupt VSP recordings and interfere with interpretation. There are, however, tube waves that may be useful to the geophysicist. These are the tube waves created in situ in areas of high permeability via compression waves that cross the permeable zone and give rise to two tube waves, one upgoing, the other downgoing as shown in Figure 2.6. The amplitude of these two tube waves is directly proportional to the permeability of the area where they were created and
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