期刊
TRANSPORT IN POROUS MEDIA
卷 79, 期 1, 页码 1-14出版社
SPRINGER
DOI: 10.1007/s11242-008-9233-9
关键词
Q-anisotropy; Attenuation; Seismic anisotropy; Fractures; Ultrasonic experiment; Transversely isotropic (TI) medium; Linear-slip model; Seismic quality factor Q versus offset (QVO); Q(S)/Q(P)-ratio
资金
- Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia
This is a study on anisotropy of seismic attenuation in a transversely isotropic (TI) model, which is a long-wavelength equivalent of an isotropic medium with embedded parallel fractures. The model is based on Schoenberg's linear-slip theory. Attenuation is introduced by means of a complex-valued stiffness matrix, which includes complex-valued normal and tangential weaknesses. To study the peculiarities of seismic attenuation versus wave-propagation direction in TI media, numerical modeling was performed. The model-input data were the complex-valued weaknesses found from the laboratory ultrasonic experiment made with a Plexiglas plate-stack model, oil-saturated (wet) and air-filled (dry). The laboratory experiment and the numerical modeling have shown that in the vicinity of the symmetry axis, in the wet model, P-wave attenuation is close to S-wave attenuation, while in the dry model, P-wave attenuation is much greater than S-wave attenuation. Moreover, the fluid fill affects the P-wave attenuation pattern. In the dry (air-saturated) model, the attenuation pattern in the vicinity of the symmetry axis exhibits steeper slope and curvature than in the wet (oil-saturated) model. To define the slope or the curvature, a QVO gradient was introduced, which was found to be proportional to the symmetry-axis Q (S)/Q (P)-ratio, which explains the differences between dry and wet models. Thus, depending on the Q (S)/Q (P)-ratio, the QVO gradient can serve as an indicator of the type of fluid in fractures, because the QVO gradient is greater in gas-saturated than in liquid-saturated rocks. The analysis of P-wave attenuation anisotropy in seismic reflection and vertical seismic profiling data can be useful in seismic exploration for distinguishing gas from water in fractures.
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