That result is consistent with the sonic velocity recorded in well 3_9a-8, located at 32.83 km along the CMP axis on line 750. Figure presents the estimates of the interval vertical velocity obtained from surface seismic data, measurement of sonic velocity and checkshot. It illustrates that the vertical velocity estimated from surface seismic data -- under the assumption of isotropy -- is accurately estimated down to a depth of around 2 km and is overestimated deeper between the Balder and BCU horizons. This overestimation is due to the presence of anisotropic rocks under the Balder layer. The velocity measured from surface seismic data (VN) is equal to the vertical velocity (VV) under the assumption of isotropy and flat layers. However, in an anisotropic medium, for flat layers and a VTI model, the expression of the NMO velocity becomes , where is the first Thomsen parameter. Since the velocity measured from seismic data, VN, is larger than the velocity measured from sonic logs and checkshots, the anisotropic parameter probably can not be considered zero and takes significantly positive values.
Figure 13 Comparison of the vertical velocities at the location of well 3_9a-8. The different velocities are estimated from seismic surface data (assuming isotropy), measurement of sonic velocity and checkshots.
Finally, we computed anisotropic velocity spectra from the three different ADCIGs illustrated in Figure . The first series of velocity spectra are semblance panels of (Figure ). The velocity spectra are computed by trying to fit the RMO curves in ADCIGs with only . Figure illustrates that the VN was well estimated by CGG since high semblance values are centered around perturbation values close to zero.
Semblance panels of are presented in a second series of velocity spectra (Figure ). The second series of velocity spectra is are semblance panels of . The velocity spectra are computed by trying to fit the RMO curves in ADCIGs with only , assuming that VN is correct. Figure illustrates the presence of anisotropy at a depth greater than 2 km. More specifically, the fact that the energy is centered at negative values of the horizontal velocity perturbations indicates that the horizontal velocity we used for the migration is smaller than the true horizontal migration velocity. Since our migration was isotropic, we assumed that VH=VN=VV. As a consequence, we can infer that below 2 km depth, the anisotropic parameter probably should not be set to zero and will take significant positive values ().