The effect of a fracture on the propagation of seismic waves can be represented in terms of the normal compliance BN and tangential compliance BT of the fracture. If BN/ BT = 1 for all fractures, the effective elastic stiffness tensor of an isotropic background containing an arbitrary orientation distribution of fractures is orthotropic (i.e., has three orthogonal planes of mirror symmetry) in the long-wave limit. However, deviations from orthotropy may occur if BN/BT differs significantly from unity and this can cause the principal axes of the P-wave NMO ellipse and of the variation in the PP-reflection amplitude as a function of azimuth, to deviate from the fast and slow polarization direction of a vertically propagating S-wave. Simple models of a fracture in terms of a planar distribution of cracks suggest that BN/BT ≈ 1 for dry fractures. However, naturally occurring fractures often exhibit mineralization in the form of bridges between opposing faces of the fracture. The presence of such bridges leads to significant departures of BN/BT from unity.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology