Eikonal Tomography of Short-period Ambient Noise on a Seismic Array in Karamay

摘要

Eikonal tomography is a new array-based method applying eikonal equation and accounting for bent rays.It becomes one of the most effective methods to reveal shallow earth structure.Under the premise of high-frequency approximation,the eikonal equation is derived by neglecting the amplitude term of Helmholtz equation.New processing techniques,including the phase front tracking,computation of gradients and averaging over phase velocities at all azimuths,have been used in place of the traditional methods such as the ray tracing and matrix construction and inversion.There are three significant improvements of the method as follows: firstly,the meaningful uncertainty information for both the isotropic and anisotropic phase velocity,secondly,more reliable information about azimuthal anisotropy without assuming its functional form,and thirdly,no requirement of a priori information about the medium.According to the eikonal equation,the gradient is the direction of propagation of waves,which is equivalent to use of off-great-circle geometrical rays and avoids the iterative computation.In this study,A dense seismic array recording ambient noise data has been deployed in Karamay,Xinjiang.The array consists of 35 broadband seismometers with averaging spacing near1Km.We use cross-correlation of two months of ambient noise data to acquire 890 empirical Green's functions.For each station pair,the phase velocity dispersion between 0.6 to 6Hz is determined by frequency time analysis (FTAN).We apply the eikonal tomography to compute the isotropic phase velocity and the phase speed uncertainty at each grid node in the survey region.Our results show that phase velocities are a clear reflection of geological feature such as two ore bodies separating in shallow layer and linking up in deep structure.