Data-matrix asymmetry and polarization changes from multicomponent surface seismics

We present methods for interpreting data-matrix asymmetry and polarization changes with depth from multicomponent surface seismics. There are two main sources of data matrix asymmetry in four component shear-wave seismics: that arising from the acquisition geometry caused by source and receiver misorientation, misalignment, imbalance, and cross-coupling, and that arising from the medium caused by variations in the geological structure, lithology, or stress. The asymmetry caused by acquisition geometry is more significant than that from the medium. Two asymmetry indices are used to quantify these medium and acquisition asymmetries separately Their behavior may be used to identify the origin of the asymmetry. The asymmetry caused by the medium is studied by deriving approximate normal-incidence, plane-wave reflection coefficients for an interface separating two anisotropic media with differently oriented symmetry axes. The degree of asymmetry in the reflectivity is proportional to the product of the degree of anisotropy in the layers above and below the reflector, and is thus small for most realistic cases. Consequently, the reflection coefficients can be approximated by a similarity transform of the principal reflection coefficients using the expected polarization difference. These equations can then be used to formulate a singular-value decomposition (SVD) in the time-domain to recover both the principal reflectivity and the changes of polarizations with depth. Applications to field data in south Texas reveal the potential of the technique, and zones of polarization changes in the Austin Chalk are identified that may be correlated with fracture swarms.