Ray-free and wave-equation-free seismic traveltime tomography

Abstract:

Seismic traveltime tomography is a routine technique to investigate structural heterogeneities and geodynamics of the Earth’s interior. Ray-based seismic traveltime tomography is simple and easy to be implemented. But ray tracing may be inaccurate even in mild heterogeneous model. Wave-equation-based seismic traveltime tomography is physically more correct and maintains its accuracy in complex media. However, a good knowledge of the earthquake source is an essential requirement for the accurate measurement of the misfit between synthetic and observed waveforms. In addition, wave-equation-based seismic tomography is computationally expensive. We present a third form of seismic traveltime tomography method, which is neither ray-based nor wave-equation-based. The traveltime tomography is formulated as an eikonal equation-constrained optimization problem, which is solved by the adjoint-state method. For the isotropic media, we discuss absolute traveltime adjoint tomography, common-source double-difference traveltime adjoint tomography, common-receiver double-difference traveltime adjoint tomography, Vp/Vs ratio tomography, and eikonal equation-based earthquake location. Eikonal equation-based seismic anisotropy tomography will also be discussed for the azimuthally and radially anisotropic media.