the crack-sets are more compliant in compression than in shear). For one case it is found that B N ≥ B T (i.e.
CRYSTAL IMPACT MATCH 2 CRACK CRACK
Additionally, three samples show significant pressure sensitivity of the vertical crack sets indicating that loading-induced azimuthal anisotropy might also exist in some layers in situ and be detectable using azimuthal AVO type analyses.Īnalysis of the fourth-order crack density tensor allows insight into the relative sizes of the normal and tangential crack compliances, B N and B T. These horizontal cracks in the mica-rich samples often show moderate-to-strong variations in crack density with loading that might be detected in situ with non-hyperbolic moveout. In fact, it is shown that increased horizontal (grain parallel) crack densities are associated with higher mica content. Furthermore, a strong correlation is observed between the degree of VTI anisotropy and the modal content of micas in the samples, which cannot be explained solely by the LPO effect. However, the LPO and extrinsic anisotropies are in general aligned, which indicates a linked origin. 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.įor the samples considered, the extrinsic crack-induced anisotropy is strong relative to that of the intrinsic LPO effects, and the pre-dominant crack-set is commonly aligned parallel to the depositional fabric (which is generally horizontal). The extensions to the inversion approach provide improved consideration of data uncertainties, by using all available P- and S-wave data and also permit the inclusion of an orthorhombic background anisotropy in the inversion.
An approach for inversion of multi-axial velocity measurements is presented, which extends the previous work by Sayers (2002) for the determination of second- and fourth-order crack density tensors from inversion of multi-axial ultrasonic velocity data. 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.