Monochromatic wave-equation traveltime inversion in vertical transverse isotropic media: Theory and application

Abstract

Wave-equation traveltime inversion (WTI) is a promising tool for reconstructing the intrinsic anisotropy properties of the earth’s interior. However, conventional time-domain implementation of anisotropic WTI involves crosscorrelation between source- and receiver-side time-domain wavefields for gradient calculation, which requires heavy memory consumption and computational cost for storing/recomputing the source-side time-domain wavefields. To overcome this problem, we develop a novel frequency-domain anisotropic WTI method using only monochromatic wavefields during inversion. Thus, the memory requirement and computational cost are significantly reduced, which makes the monochromatic anisotropic WTI practical for real large-size applications. We focus on transversely isotropic media with a vertical symmetry axis (VTI) and aim to invert the P-wave vertical velocity [Formula: see text] and anisotropic parameter [Formula: see text]. The subspace approach is adopted for calculating multiple step lengths to balance the updates of [Formula: see text] and [Formula: see text] values in our VTI WTI method. Synthetic numerical examples based on an inclusion model and the BP 2007 model prove that the proposed monochromatic VTI WTI method can reconstruct background VTI models as accurately as VTI WTI with multiple frequencies. Then, we apply our method to an ocean-bottom seismic data set acquired at the Qiuyue field in the East China Sea. The inverted vertical velocity [Formula: see text] matches well with the long-wavelength component of the sonic well-logging velocity. Compared with the inverted isotropic velocity model, the inverted VTI model produces more focused seismic imaging reflectors throughout the section and more accurately positions the reflector depths, which is confirmed by the sonic well logging.