Traditional transmission travel-time tomography hinges on ray tracing techniques.
We propose a PDE-based Eulerian approach to travel-time tomography so that we
can avoid using the cumbersome ray-tracing technique. We start from the eikonal equation, define
a mismatching functional and derive the gradient of the nonlinear functional by
an adjoint state method. The resulting forward and adjoint problems can be
efficiently solved by using the fast sweeping method; a limited memory BFGS method
is used to drive the mismatching functional to zero with quadratic convergence.
2-D and 3-D numerical results as well as Marmousi synthetic velocity model demonstrate the
robustness and the accuracy of the method.