FORWARD MODELING IN CROSS-HOLE SEISMIC TOMOGRAPHY USING RECIPROCITY

Seismic transmission tomography provides a means of direct estimation of compressional wave velocities using first arrival traveltimes from cross-hole data. Forward modelling in seismic-tomography involves comp utation of traveltimes at the receiver positions and estimation of ray travelpaths for a subsurface velocity model and source-receiver geome try. Conventional ray-tracing techniques face many difficulties when a pplied to real-life situations. These can be overcome to a great exten t by using a ray-tracing technique based on the principle of reciproci ty. In the present work, an algorithm is developed which makes use of the reciprocity principle and the Fermat's principle for estimation of raypaths. In this technique seismic traveltime calculation is based o n a two-dimensional dynamic programming approach, involving systematic mapping of traveltimes over a grid of constant-velocity cells. The al gorithm works for even highly complicated velocity models. First arriv al seismic energy can travel either as transmitted waves, diffracted w aves, or head waves, and this technique simulates all of them. The alg orithm is tested for various subsurface velocity models, some of which are presented in this paper. The results are represented as ray diagr ams, superimposed over the actual velocity model. The isotime lines de picting the wavefronts are also presented, which reveal the effect of velocity contrast on seismic wave propagation. (C) 1997 Elsevier Scien ce Ltd.