Wave-equation based traveltime seismic tomography – Part 1: Method

Abstract. In this paper, we propose a wave-equation based traveltime seismic tomography method with a detailed description of its step-by-step process. First, a linear relationship between the traveltime residual Δt = Tobs – Tsyn and the relative velocity perturbation δc(x) / c(x) connected by a finite-frequency traveltime sensitivity kernel K(x) is theoretically derived using the adjoint method. To accurately calculate the traveltime residual Δt, two automatic arrival-time picking techniques including the envelop energy ratio method and the combined ray and cross-correlation method are then developed to compute the arrival times Tsyn for synthetic seismograms. The arrival times Tobs of observed seismograms are usually determined by manual hand picking in real applications. Traveltime sensitivity kernel K(x) is constructed by convolving a forward wavefield u(t,x) with an adjoint wavefield q(t,x). The calculations of synthetic seismograms and sensitivity kernels rely on forward modelling. To make it computationally feasible for tomographic problems involving a large number of seismic records, the forward problem is solved in the two-dimensional (2-D) vertical plane passing through the source and the receiver by a high-order central difference method. The final model is parameterized on 3-D regular grid (inversion) nodes with variable spacings, while model values on each 2-D forward modelling node are linearly interpolated by the values at its eight surrounding 3-D inversion grid nodes. Finally, the tomographic inverse problem is formulated as a regularized optimization problem, which can be iteratively solved by either the LSQR solver or a non-linear conjugate-gradient method. To provide some insights into future 3-D tomographic inversions, Fréchet kernels for different seismic phases are also demonstrated in this study.