A physical model study of microcrack‐induced anisotropy

Abstract

A laboratory study of the effects of oriented pennyshaped inclusions embedded in a solid matrix on the propagation of seismic shear waves shows good agreement with theoretical predictions for some polarizations and poor agreement for polarizations at large crack densities. The models are constructed of solid matrix of epoxy resin with inclusions of thin rubber discs of approximately equal cross-sectional areas. The theoretical basis for these experiments is the theory of Hudson, in which the wavelength is greater than the dimensions of the individual cracks and their separation distance, and the cracks are in dilute concentration. By a pulse transmission method, seismograms were gathered in models free of inclusions and models with inclusions. Seismic measurements of velocity anisotropy, for variations in both a polarization and propagation direction, were performed on physical models with inclusions (cracks) representing five different crack densities (1, 3, 5, 7, and 10 percent). Variations in velocity anisotropy at different crack densities have been evaluated by using Thomsen’s parameter (γ) which relates velocities to their elastic constants, [Formula: see text]. Comparisons between experimental and theoretical results indicate that with the waves polarized parallel to the aligned inclusions, [Formula: see text] agree well with the theoretical model. However, shear waves for the same propagation direction but polarized perpendicular to the plane containing the inclusions [Formula: see text] produced results that agree well with the theory for crack densities up to 7 percent, but disagree for higher crack densities. The deviation of γ at 10 percent crack density suggests that crack‐crack interaction and their coalescence may be observable and could lead to seismic techniques to differentiate between microcracks and larger macrocracks.

Experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles

“…Theoretical models are needed to interpret seismic data in terms of fracture properties and fluid properties. The combined effect of fluids (100% gas or liquid saturation) and aligned fractures have been studied theoretically and experimentally (e.g., Thomsen, 1995, Hudson, 1981, Hudson et al, 2001, Chapman, 2003, Ass'ad et al, 1992, Rathore et al, 1995. The fluid independent relationship between fracture density and shear wave splitting for wave propagation at 90 o to the fracture normal has been observed in laboratory experiments (e.g., , Rathore et al, 1995 in line with theoretical predictions (e.g., Hudson, 1981).…”

“…A major limitation is that idealised fracture geometries (e.g., penny shaped cracks) often used in theoretical models as a mathematical approximation do not resemble natural fractures in rock, and it is difficult to control the introduction of aligned natural fractures in laboratory samples. Controlled fractured rock experiments have therefore required the use of synthetic rocks, leading to the use of synthetic materials to construct physical models, such as an epoxy matrix with embedded rubber discs to represent fractures (e.g., Ass'ad et al, 1992, de Figueiredo et al, 2013. These studies provided very useful results on elastic wave behaviour in fractured rocks, but they did not capture important waveinduced flow mechanisms (e.g., squirt flow; see Thomsen, 1995, Chapman, 2003.…”

Comparison of experimental velocity measurements with theoretical results in a solid‐solid composite material

“…When a two-phase medium is constructed of a solid matrix with solid inclusions, most studies address cracked solids with aligned penny-shaped inclusions, so that the medium becomes anisotropic (Hudson, 1981;Hudson and Knopoff, 1989;Sayers and Kachanov, 1991;Ass'ad et al, 1992Ass'ad et al, , 1993. The discussions are not generally on random embedded solid inclusions mixed in another solid matrix, and the experimental data for such a medium are scarce.…”

Detection of coalbed fractures with P-wave azimuthal AVO in 3-D seismic exploration

“…The thought of predicting fracture with seismic exploration has come from the observation of the S-wave splitting phenomenon [2 4] . The S-wave splits into the fast one parallel to the fracture strike and the slow one perpendicular to fracture strike when it travels through the fracture medium.The strike and intensity of fractures are predictable by component rotation and time difference of the fast S-wave and slow S-wave [5,6] . The methods of predicting fracture by S-wave splitting have been mature in the theoretical and physical model in the past years [7 9] , but less practicable in the seismic exploration case.…”