Seismic data are sensitive to anisotropy on range of length scales and so can provide a wealth of information about rock properties. However, to interpret such data requires unravelling of the competing causes of anisotropy. Detailed analysis of rock cores allows the different contributions to the anisotropy to be assessed to constrain interpretation of field-scale seismic data. Here we present a method for separating the effects of anisotropy due to the preferred alignment of crystals and those due to inter-granular effects using multi-axial ultrasonic data and apply this to cores from a UKCS field. The approach is an extension of the work of Sayers (2002) and involves inversion of multi-axial ultrasonic velocity measurements to determine second- and fourth-order crack density tensors. The extensions to the inversion approach provide improved consideration of data uncertainties, by using all available P- and S-wave data, and also permit an orthorhombic background anisotropy to be included in the inversion (e.g., due to intrinsic crystal preferred orientation - CPO). The latter aspect yields estimates of extrinsic anisotropy, I.e., the quantified crack density tensors, that are “unpolluted” by the effects of the intrinsic anisotropy, thus permitting extrinsic and intrinsic anisotropies to be distinguished.
展开▼
