Maurizio Ferla

Constrained Anisotropy Estimation For PSDM Velocity Model Building And Refinement

Summary In this study a new stable and geologically plausible anisotropic inversion methodology is proposed for building and refining the velocity model for pre-stack depth migration workflow (PSDM). We describe an unconventional constrained inversion technique for the evaluation of the interval Thomsen anisotropic parameters e and δ. The main feature of this algorithm is the prevention of non-realistic and highly oscillating values which can be often obtained applying conventional unconstrained Dix transform methods, for small variations of the stacking or effective values. The new proposed approach is especially designed for robust anisotropy estimation, in terms of the anellipticity parameter η, which is fundamental for the vertical and lateral accurate positioning of seismic events, calibrated to the sonic well logs. The estimation of the anisotropic interval parameters is based on a combination of credible, internal, analytical trend function and a least-squares fit, matching the input RMS values. The benefits and the improvements respect to the standard methodologies are visible on the real dataset applications in terms of a better focusing and imaging, increasing the quality and consistency of the results, and adopting the velocity models built by the constrained inversion method.

Fracture Finite Difference Seismic Modelling

The objective of this study is to measure the seismic response of fluid filled fractures, trying to evaluate which acquisition survey parameters can better highlight this feature. The scope of fracture modelling is simulating the presence of a fracture system in the area of interest verifyin if it is detectable and describable by a 3D P-wave or PS-wave seismic data, for issues related to operative aspects but also for future support to the drilling activity. A system of aligned fractures can be described as an effective anisotropic medium when the dominant wavelength is long compared to the fracture scales. In order to model the behavior of seismic waves in fractured media, we can characterize the model with normal and tangential crack compliances, Coates and Schoenberg (1995). The main measurable effects on seismic data are the shear wave splitting and p-wave AVOaz anomalies. Shear-wave splitting due to the alignment of vertical cracks can be recognized by the polarization of the fast split shear-wave, which is usually parallel to the local strike of cracks and can be used to characterize fracture orientation. The time delay between fast and slow shear-waves is closely related to the intensity of crack-induced anisotropy in the medium (proportional to the fracture density). The main question to answer is if a 3D multicomponent survey can detect the travel time shift between parallel and perpendicular directions generated by the investigated formation. The second subject is to investigate the potentiality of fluid discrimination with PS waves but also PP waves. Modeled data is generated using elastic anisotropic finite difference code. The analogies between cracks systems and anisotropic media have been analyzed in order to infer interesting considerations about fractures characterization. The anisotropic parameters and the geophysical model have been assumed on the basis of inplace data measurements. The obtained results demonstrated the correlation between the seismic features and the fracture characteristics, such as crack density, fractures orientation, fluid content and AVO anomalies.