VTI documented

Shear-wave (S-wave) seismic data indicate that rocks of McElroy oil field o n the Central Basin Platform of West Texas are transversely isotropic with a vertical symmetry axis (VTI). Although geophysicists from time to time ha d anticipated or assumed that sedimentary rock was VTI, no one had shown th at any sedimentary section actually was VTI. The proof at McElroy comes fro m a nine-component, near-offset vertical seismic profile (VSP) combined wit h a ring of 15 offset VSPs spaced about 24 degrees apart at offsets of abou t 460 In. S-wave splitting at VSP frequencies was negligible for vertical p ropagation to a depth of 885 m but reached about 12 ms for nonvertical prop agation from the offset source locations. Crossed-dipole log data supported the VSP result for vertical propagation but found two layers of vertically birefringent rock at depth whose thicknesses were below VSP resolution.