Crack density tensor inversion for analysis of changes in rock frame architecture

This paper presents a development of the use of multi-axial ultrasonic data for the quantification of extrinsic, crack or grain-contact induced elastic anisotropy in core samples with application to a UKCS oil field. An approach for inversion of multi-axial velocity measurements is presented, which extends the previous work by Sayers (2002) for the determination of secondand fourth-order crack density tensors from inversion of multi-axial ultrasonic velocity data. The extensions to the inversion approach provide improved consideration of data uncertainties, by using all available Pand S-wave data and also permit the inclusion of an orthorhombic background anisotropy in the inversion [e.g. due to intrinsic lattice preferred orientation(LPO) effects]. The latter aspect leads to estimates of the extrinsic anisotropy, that is, the quantified crack density tensors, that are 'unpolluted' by the effects of the intrinsic anisotropy, thus permitting extrinsic and intrinsic anisotropies to be distinguished.