Inversion of walkaway VSP data in the presence of lateral velocity heterogeneity

Multi-azimuth walkaway vertical seismic profiling is an established technique for the estimation of in situ slowness surfaces and inferring anisotropy parameters.Normally,this technique requires the assumption of lateral homogeneity,which makes the horizontal slowness components at depths of downhole receivers equal to those measured at the surface.Any violations of this assumption,such as lateral heterogeneity or nonzero dip of intermediate interfaces,lead to distortions in reconstructed slowness surfaces and,consequently,to errors in estimated anisotropic parameters.In this work,we relax the assumption of lateral homogeneity and discuss how to correct vertical seismic profile data for weak lateral heterogeneity.We describe a procedure of downward continuation of recorded traveltimes that accounts for the presence of both vertical inhomogeneity and weak lateral heterogeneity,which produces correct slowness surfaces at depths of downhole receivers,noticing that sufficiently dense receiver coverage along a borehole is required to separate influences of vertical and lateral heterogeneity on measured traveltimes and obtain accurate estimates of the slowness surfaces.Once the slowness surfaces are found and a desired type of anisotropic model to be inverted is selected,the corresponding anisotropic parameters,providing the best fit to the estimated slownesses,can be obtained.We invert the slowness surfaces of P-waves for parameters of the simplest anisotropic model describing dipping fractures(transversely isotropic medium with a tilted symmetry axis).Five parameters of this model,namely,the P-wave velocity V0 in the direction of the symmetry axis,Thomsen's anisotropic coefficients e and d,the tilt n,and the azimuth b of the symmetry axis,can be estimated in a stable manner when maximum source offset is greater than half of receiver depth.

Upper-Mantle Velocity Heterogeneity of Eastern Tibetan Plateau from Teleseismic P-Wave Tomography and Its Tectonic Implications

An attempt has been made to reveal the upper mantle velocity structure of the eastern Tibetan Plateau using 628 teleseismic events recorded from 2003 to 2009 at 95 stations.A total of 8 532 Pwave arrival time residuals were inverted by using the FMTOMO(fast marching tomography) software package.Tomographic results show upper mantle velocity heterogeneity in many aspects.In the southern part visible high velocity anomaly is denoted as the Indian lithosphere.This part seems to be affected by slab tearing at 940E longitude as it is located on the eastern Himalayan syntaxis(EHS).The high velocity zone down to 500 km depth in the northern part could be the Asian lithosphere.At the central part some high velocity anomalies can be identified as detached patches of the lithosphere,surrounded by low velocity anomalies.These anomalies are the potential to create thermal convection and trigger plateau uplift or plateau growth.Sudden velocity change occurs on both sides of patches where low velocity anomaly is visible in between patches and Bangong-Nujiang suture even in between Songpan-Ganzi terrain and Asian Plate.In both cases intense low velocity zone spread down to 500 km.The depth range of low velocity anomalies in between two plates observed from 200 to-500 km.Hence the low velocity anomalies detected in our results may reflect either the hot asthenosphere upwelling or the mantle wedge due to the presence of the cold lithosphere.