We used a laboratory scale model to study the effects aligned fractures might have on seismic wave propagation at a larger scale in real Earth imaging. Our main objective was to investigate the effect of aligned fractures on seismic P-wave amplitude through the estimation of the induced attenuation. The physical model was constructed from a mixture of epoxy resin and silicon rubber, with inclusions designed to simulate two sets of inclined fractures at an angle of 29.2° with each other. Two-dimensional reflection data were acquired using the pulse and transmission method in three principal directions relative to the fracture strike azimuth with the model submerged in a water tank. We used the Quality Versus Offset (QVO) method, an extension of the classical spectral ratio method for determining attenuation to estimate the induced attenuation (inverse of the seismic quality factor) from the Common Mid Point (CMP) pre-processed gathers. The results of our analysis show that the induced P-wave attenuation is anisotropic, with elliptical (cos2θ) variations with respect to the survey azimuth angle θ. The minor axis of the Q ellipse corresponds to the fracture normal. In this direction, i.e. across the material grain, the attenuation is a maximum. The major axis corresponds to the fracture strike direction (parallel to the material grain) where minimum attenuation occurs. These attenuation results show consistency with the azimuthal anisotropy observed in the stacking velocities in the fractured-layer and are all consistent with the physical model, and thus provide a physical basis for using attenuation anisotropy to derive fracture properties from seismic data.
- fractured media
- seismic quality factor