Physical Modeling and Analysis of Seismic Data from a Simulated Fractured Medium

Faranak Mahmoudian

This thesis studies the physical seismic modeling of a simulated fractured medium to examine variations of seismic reflection amplitudes with source-receiver offset and azimuth (AVAZ). The intent is to extract information about the fracture orientation and magnitude of the anisotropy of a naturally fractured medium. The simulated fractured medium is constructed from phenolic LE-grade material which exhibits orthorhombic symmetry. For initial characterization of the phenolic model, its elastic stiffness coeffcients were determined from group velocities. The group velocities along various directions were obtained from three-component physical model transmission data. The phenolic model approximates a weakly anisotropic layer with horizontal transverse isotropy (HTI).

Three-dimensional (3D) physical model reflection data were acquired over a model consisting of the simulated fractured layer sandwiched between two isotropic plexiglas layers submerged in water. Interference between primary and ghost events was avoided with a careful 3D seismic survey design. After deterministic amplitude corrections, including a correction for the directivity effect of the physical model transducers, reflection amplitudes agreed with the amplitudes predicted by the Zoeppritz equations, confirming the suitability of the 3D physical model data for a quantitative amplitude analysis.

Linear AVAZ inversions for the fracture orientation and HTI anisotropic parameters (including shear-wave splitting parameter) were performed on P-wave reflection amplitudes from the top of the simulated fractured medium. Sensitivity analysis of the inversions results, including variations of the background velocity model and maximum incident angle used, confirms the accuracy of the amplitude analysis. The results reveal that the amplitude analysis of the P-wave data alone allows for extraction the information about the shear-wave anisotropy conbined in the P-wave multi-offset and multi-azimuth amplitude data, without any S-measurements.