117 4. Near-surface reflection surveying a b Figure 4.23 Example of deconvolved and pseudo velocity depth sections (in-line section 31). (a) Depth section after deconvolution and depth tying (top). Depth pseudo velocity section (bottom). (b) Velocity functions at well C1: velocity function derived from acoustic measurement (black curve), velocity function derived from seismic trace (red curve). In the in-line direction, the abscissa zero indicates the location of the source line. The abscissa of the reflecting points varies between –120 m and 120 m in the in-line direction. The distance between two reflecting points is 2.5 m. In the cross-line direction, the distance between two reflecting points is 5 m. The pseudo velocity sections of the 3D block thus obtained were merged with those obtained by refraction tomography (see “Refraction surveying” chapter) to create a 3D extended velocity model from the surface (Figure 4.24). Figure 4.24 (top left) shows the results obtained for the in-line 31 seismic section extracted from the 3D extended velocity model. It also shows the velocity map at a depth of 87 m (Figure 4.24, top right). The 3D velocity model shows the large heterogeneity of the aquifer reservoir in the horizontal and vertical planes. To quantify the porosity variations within this aquifer, the seismic interval velocities were first converted into resistivity values. For this purpose, the empirical relationship between seismic velocity and resistivity proposed by Faust (1953) was used. Resistivity values were then converted into porosity values, using Archie’s law (1942). Figure 4.24 (bottom) shows the pseudo velocity and porosity seismic sections for the in-line 21 and crossline 24.
RkJQdWJsaXNoZXIy NjA3NzQ=