Effects of intrinsic anisotropy on seismic dispersion, attenuation and frequency-dependent anisotropy

Interlayer wave-induced fluid flow is an important mechanism for seismic dispersion, attenuation, and frequency-dependent anisotropy in fluid-saturated porous layered media. The previous models assume that the layered medium is composed of isotropic layers, whereas the layer itself can exhibit anisotropic properties (i.e., intrinsic anisotropy). To study the effects of intrinsic anisotropy, based on the Biot's theory of poroelasticity, we propose an approximate theoretical model for seismic dispersion, attenuation, and frequency-dependent anisotropy in a layered medium which is composed of transversely isotropic fluid-saturated porous layers. To validate the approximate theoretical model, we compare the theoretical model with the numerical simulations, which show good agreement between each other. Using the theoretical model, we analyze the effects of intrinsic anisotropy in four cases (different fluid properties, different matrix properties, and their two different combinations). The results show that, for the layered medium composed of alternating gas-saturated and brine-saturated layers, the intrinsic anisotropy of the brine-saturated layers have the largest effects on the seismic dispersion and attenuation, whereas the influence of the gas-saturated layers is much smaller. When the brine-saturated layers have the property of transversely isotropy, the seismic dispersion and attenuation are more notable. Contrarily, for the layered medium containing highly porous thin layers that is saturated with a single phase of fluid, the seismic dispersion and attenuation are more notable when the background layer is isotropic. In such a layered medium, if the background layer and the highly porous thin layer are saturated with different phases of fluids, the effects of intrinsic anisotropy depend on the fluid distribution. Different fluid distribution can enhance or diminish the interlayer wave-induced fluid flow and the corresponding seismic dispersion and attenuation. The effects of intrinsic anisotropy on seismic dispersion and attenuation thus vary with the fluid distribution. In terms of the frequency-dependent anisotropy, the effects of the intrinsic anisotropy of the brine-saturated layers are also the largest in the layered medium composed of alternating gas-saturated and brinesaturated layers. For the layered medium containing highly porous thin layers, the intrinsic anisotropy primarily affects epsilon and epsilon(Q), but has little effects on delta and delta(Q). This means that the difference between the P-wave velocities (attenuation) in the directions perpendicular and parallel to the layers is greatly affected by the intrinsic anisotropy, whereas the variations of P-wave velocities (attenuation) with wave incident angles in the vicinity of the layer normal are little affected by the intrinsic anisotropy. The model proposed in this paper is concise and easy to use, which has a great potential to be applied in the shale and tight sandstone that have distinct layering features.