3-D traveltime computation using second-order ENO scheme

We consider a second-order finite difference scheme to solve the eikonal equation. Upwind differences are requisite to sharply resolve discontinuities in the traveltime derivatives, whereas centered differences improve the accuracy of the computed traveltime, A second-order upwind essentially non-oscillatory (ENO) scheme satisfies these requirements. It is implemented with a dynamic down 'n' out (DNO) marching, an expanding box approach. To overcome the instability of such an expanding box scheme, the algorithm incorporates an efficient post sweeping (PS), a correction-by-iteration method. Near the source, an efficient and accurate mesh-refinement initialization scheme is suggested for the DNO marching, The resulting algorithm, ENO-DNO-PS, turns out to be unconditionally stable, of second-order accuracy, and efficient; for various synthetic and real velocity models having large contrasts, two PS iterations produce traveltimes accurate enough to complete the computation.