A physical model study of shear-wave splitting and fracture intensity

Abstract

Observations of seismic wave velocity anisotropy have been associated with the presence of fracturing in rocks. Thus, such observations may potentially be exploited as a fracture detection technique. Both P-wave anisotropy, where P-wave velocity varies with propagation direction, and S-wave anisotropy, where S-wave velocity varies with polarization as well as propagation direction, have been observed in fractured rock. These observations apply to both laboratory studies and actual field experiments. The questions remain as to how intense fracturing must be to be seismically detected and can we estimate the degree of fracture intensity above that threshold. With these questions in mind, a physical model study was initiated to observe shear-wave anisotropy for constant wave forms in simulated fractured media, where known variations in fracture intensity could be established. The study required the construction and calibration of shear-wave transducers, as well as the construction of the physical model. In a series of models, fracturing is simulated by stacks of thin Plexiglas sheets clamped tightly together to form a block. For each model, sheets of constant thickness are used, but the intensity of fracturing between the different models is simulated by using different thickness of Plexiglas. Observation of direct shear-wave arrivals through the stack, with propagation parallel to the sheets and polarization of particle motion allowed to be parallel to, normal to, or at any arbitrary angle to the sheets, definitely demonstrated the existence of anisotropy and shear-wave splitting. For Plexiglas sheets 1/16" thick, representing a fracture intensity of about 16 fractures per wavelength, strong anisotropy and shear-wave splitting is clearly observed. Observations for 1/8" sheets, about 8 fractures per wavelength, indicate that the anisotropy is more difficult to observe. These preliminary results suggest a threshold for reliable detection of fracturing and an experimental relation between fracture intensity (in fractures per wavelength) and the degree of anisotropy (expressed as a relative time delay) observed as shear-wave splitting. The threshold of detection appears to be at least 10 fractures per wavelength and the relation between fracturing and time delay appears to be nearly linear.