153 6. Hybrid seismic imaging coefficient computation. Thus, this reflectivity section displays the reflection coefficients associated to the interfaces, filtered in the seismic frequency bandwidth. The third and last step of this hybrid approach focuses on extending the reflectivity section upwards through the depth velocity model of the uppermost region, obtained by tomographic inversion. The depth velocity model was converted to time and used to estimate the reflectivity according the definition of RVi = (Vi+1 –Vi)/(Vi+1 + Vi) where Vi is the velocity at cell i and Vi+1 is the velocity at cell i+1. The section obtained was then filtered in the same frequency bandwidth as defined for the section in the previous processing step. Before they could be gathered into a single time reflectivity section, the two time reflectivity sections required a scale factor (k): the reflectivity section derived from the velocity model (RV) is related to the reflectivity section derived from acoustic impedance contrasts (RZ) by the equation RZ = kRV. The scale factor was computed by the amplitude ratio between the reflectivity sections in a time-distance window, where the reflected wave on the bottom of the weathering zone is visible. In practice, the time-distance window is defined as follows: time window (between 0.025 s and 0.050 s) for the short offsets (between 0 m and 25 m). Figure 6.5-bottom right shows the final time reflectivity section obtained by gathering the two reflectivity sections also shown in Figure 6.5, where a noticeable reflection at approximately 40 ms is associated with the bottom of the weathering zone. Figure 6.5 Reflectivity section for 10EST04 profile: Left: Single fold section derived from reflection processing of off-end shots. Bottom right: Upward continuation of single-fold section using reflectivity derived from tomographic velocity model. Top right: Time converted velocity model obtained by tomographic inversion. Adapted from Mendes et al. (2014).
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