Anisotropy of Seismic Attenuation in Fractured Media: Theory and Ultrasonic Experiment |
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Authors: | T I Chichinina I R Obolentseva G Ronquillo-Jarillo |
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Institution: | (1) Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Mexico D.F., 07730, Mexico;(2) Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences, Koptyug Prospect 3, Novosibirsk, 630090, Russia |
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Abstract: | 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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Keywords: | 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 |
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