Unexpected Shear-wave Behaviour

Introduction Seismic anisotropy is often explained in terms of effective media where an ordered ensemble of sub-seismic elements leads to a directional variation of the materials properties. Examples of such systems include aligned cracks or fractures, fine layering and correlated pore space. Therefore, measurements of seismic anisotropy potentially allow insights to physical systems which would otherwise lie outside current seismic imaging capabilities. The case of aligned crack/fracture systems are possibly the most important to the hydrocarbon industry. This is because in many situations a reservoir’s performance may be critically dependent on fracture controlled permeability anisotropy. However, the interpretative step from the observations of seismic anisotropy to the underlying physical phenomenon may not be simple. One assumption that is often applied is that the fast shear-wave (@I) polarization is parallel to crack/fracture direction. However, a certain class of anisotropic materials with transverse isotropy (TI) can be identified with properties which may be inconsistent with this rule. Clearly, it needs to be established whether these ‘unusual’ materials can be realised in terms of realistic physical systems if any confidence is to be placed in such interpretations.