130 Geophysics in Geothermal Exploration frequency content of well seismic data, being generally wider (up to 150–175 Hz) than that of surface seismic data, the time sampling interval does not exceed 2 ms (between 0.5 to 2 ms). The distance between 2 adjacent sensor locations must be chosen to be less than the smallest half-wavelength encountered to avoid spatial aliasing phenomenon (Mari, 2015), usually between 5 and 20 m. For Offset VSP or seismic walkaway acquisition, the offset D of the source relative to the wellhead depends on the depth H of the objective. An offset D < 3/4 H allows to obtain VSP sections, with reflected events for which angles of incidence do not exceed 30 degrees, recommended for amplitude analysis versus angle or offset. For a VSP recorded in a vertical well, crossing geological layers with small dips, wave field, emitted by a source located at a small offset from the well head, propagates at normal incidence. In these conditions, if the seismic source generates P-waves, VSP records are composed of down-going and up-going P-waves and Stoneley waves. There is no phenomenon of conversion from P-wave to shear wave (S-wave). The P-wave field is composed of primary waves and multiples. Stoneley waves, more commonly known as 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. However, tube waves, created by conversion of P-wave, are very useful to detect layers of high permeability. In case of wave propagation at normal incidence, VSP can be recorded using a vertical geophone or a hydrophone. Figure 3.10 is an example of VSP recorded with a vertical geophone and a hydrophone, in a reconnaissance borehole of about 400 m depth drilled to determine the geothermal parameters of the geological formations crossed, as part of a deep (a) (b) Figure 3.10 VSP sections (a: vertical geophone, b: hydrophone) and wave identification (After Mari et al., 2024).
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