A double-grid algrithm for calculating traveltimes of wide-angle reflection waves
宽角反射地震波走时模拟的双重网格法

Layered and blocked homogeneous models are usually used to calculated travel times and ray paths of wide-angle reflection waves in interpreting wide-angle reflection data of deep seismic sounding. In order to make the model approach the Earth, the size of the model has to be large. On the other hand, for obtaining accurate travel times and rays calculated with the minimum travel time tree algorithm, the model elements have to be small, and thus the efficiency of calculation is decreased. In this paper, a double-grid scheme is employed in calculation of travel times and ray paths in order to fast and reliably interpret deep sounding data. The double-grid algorithm covers a model with sparse and dense grids. Nodes in the sparse grid are calculated in the same way as in the single-grid algorithm. As for nodes in the dense grid, they are calculated only if they are in the neighborhoods of source and geophone nodes, and ones where velocity gradients are not zero, and the propagation areas of secondary waves are larger for interface boundary nodes than those within homogenous media. The numerical example shows that the double-grid algorithm has much higher calculation efficiency than the single-grid one under the same accuracy condition for large layered and blocked homogenous models.