Physical and numerical modeling of tuning and diffraction in azimuthally anisotropic media

Fractured reservoirs are an important target for exploration and production geophysics, and the azimuthal anisotropy often associated with these reser voirs can strongly influence seismic wave propagation. We created a physica l model of a fractured reservoir to simulate some of these propagation effe cts. The reservoir is represented by a phenolite disk that is thin with res pect to the elastic wavelengths in the experiment, creating model dimension s that are representative of realistic reservoirs. Phenolite is strongly an isotropic with orthorhombic symmetry, which suggests that azimuthal amplitu de versus offset (AVO) effects should be obvious in data. We acquired both SH- and P-wave data in common-offset gathers with a near offset and a far o ffset and found that although the SH-wave data show clear azimuthal variati ons in AVO, the P-wave signals show no apparent changes with azimuth. We then applied numerical modeling to analyze the data. Because ray methods cannot model diffractions from the disk edge, we first used a ray-Born tec hnique to simulate variations in waveforms associated with such scattering. The synthetic seismograms reproduced variations in the SH-wave waveforms a ccurately, though the amplitude contrast between acquisition azimuths was o verestimated. Assuming a laterally homogeneous model, we then applied ray m ethods to simulate tuning effects in SH- and P-wave data and confirmed that in spite of the large contrasts in elastic properties, the tuning of the P -wave reflections from the thin disk changed so there was negligible contr ast in AVO with azimuth. Models of held scale reservoirs showed that the sa me effects could be expected for field applications.