195 7. Integrated seismic study variation of the seismic amplitude. The amplitude anomaly corresponds to the presence of a porous layer, between CMP 600 and CMP 400 at the top of the Dogger formation. Figure 7.23 (top) shows the specific surface and Ik-Seis distributions in depth. The interpretation is confirmed by a very low value of the specific surface and a high value of the Ik-Seis attribute. The high values of the specific surface are associated with the Callovo-Oxfordian (550-720 m depth interval). The pseudo gamma ray section computed from the Biot Shear modulus is shown in Figure 7.23 (bottom). We noted the lateral variation of the shale content in the Callovo Oxfordian. 7.8 Conclusion The different steps involved in building a geo-model in depth can be summarized as follows: • pre-processing must include QC on static corrections for the detection and compensation of long-wavelength anomalies, • QC on seismic amplitudes and velocity models. Filtering of footprint anomalies. Quantification of amplitude quality (SQI factor) before pre-stack migration and inversion, • processing: pre-stack time migration. At least three angle migrated stacks must be generated, • elastic inversion to compute Ip and Is sections, • computation of an a priori velocity model Vp, assuming that the P-wave velocity Vp and the acoustic impedance Ip of a formation vary in a consistent way and a relationship between the two variables Ip and Vp exists. A single relationship or a relationship per range of impedances or per geological unit can be computed, • update of the a priori Vp model, using the depth conversion of seismic horizons by a geo-statistical method which simultaneously uses Bayesian co-kriging and a multilayer model and which handles sources of uncertainty: Vp-update, • depth conversion of the seismic blocks (amplitude, Vp-update, Ip, Is) using the updated velocity model Vp-update, • computation of the density distribution in depth: ρ = Ip / Vp-update, • computation of the shear velocity distribution: Vs = Vp-update x (Is / Ip). Knowledge of the density (ρ) and velocity (Vp-update, Vs) distributions allows the computation of mechanical dynamic modules such as the shear modulus, Young’s modulus, bulk modulus and the Poisson’s ratio. The conversion of dynamic Young’s modulus into static Young’s modulus was conducted. We have described a method to build a geo-model in depth, using a 2D seismic line (07Est10) and 3D seismic lines (IL405 and XL-217) as examples.
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