Weighted Adjoint-state Crosshole Traveltime Tomography

Abstract

Abstract The adjoint-state method of first arrival traveltime tomography, featured with implicit ray tracing calculation, was used to compute the gradient of the misfit function without introducing Fréchet derivatives. However, the majority of velocity gradients in the adjoint-state method focus on the high velocity area due to high raypath density. Thus, the current adjoint-state method faces challenges of imaging low velocity anomalies in the near-surface by crosshole seismic acquisition. In this paper, we proposed a weighted adjoint-state Eikonal equation-based crosshole traveltime tomography method. A preconditioning operator is provided to fix the ray path density influence. The weighted method was tested on a homogeneous model, a checkerboard model, and a reduced-scale Marmousi model. Both misfit cures and velocity logs are used to make a comparison between the weighted method and the traditional method. The results show the weighted method has better resolution, faster convergence speed, and lower final residual, which demonstrate the method can mitigate the ray path effect and reduce the inverted velocity error of low velocity anomalies.