P-wave attenuation anisotropy in fractured media - Results from a laboratory 'scale-model'

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 effects of aligned fractures on seismic wave amplitude through the estimation of fracture-induced attenuation and relating these effects to the fracture properties. The physical model comprised two fracture models constructed from a mixture of epoxy resin and silicon rubber and designed to simulate two sets of intersecting fractures. 2D data were acquired using the pulse-transmission method in three principal directions. The QVO method, an extension of the classical spectral ratio method, was used to estimate the quality factor from the pre-processed data. The results of the study reveal attenuation anisotropy in the modelling data sets. Attenuation was observed to increase systematically away from the fracture strike, with a maximum at the fracture normal. Azimuthal variations in the fracture-induced attenuation were elliptical to a good approximation, enabling fracture orientations to be obtained quite accurately from the axes of the ellipse. We concluded that attenuation anisotropy may be used in fracture characterization to complement the use of travel-times, velocity, amplitude and AVO gradient attributes.