Conventional fast marching method (FMM) is improved for traveltime computation. A non-equidistant difference scheme is used to carry local traveltime computation near surface and interface. New grid node type is used to realize narrow band technique under irregular boundary conditions. Analysis on accuracy, efficiency and numerical tests shows that the new method has high-accuracy and accuracy of the reflection wave is higher than that of the first-arrival wave. The new method does not need too more additional computing capacity in dealing with irregular boundaries. The new method can treat strong topographical change, near-surface and subsurface complex media effectively and flexibly. The results consist with laws of wave propagation under complex topographical conditions.

For interpolation of velocity and velocity derivative of non-grids in wavefront construction, interpolation algorithms, such as the nearest, bilinear and linear bicubic interpolation algorithms, are compared and analyzed. With characteristics of wavefront construction, path of rays and computational efficiency of the algorithms are compared. It indicates that the bicubic interpolation algorithm is more accurate and faster.