TRANSMISSION-REFLECTION TOMOGRAPHY - APPLICATION TO REVERSE VSP DATA

A multiphase tomographic algorithm is presented that allows 2-D and 3- D slowness (inverse of velocity) and variable reflector depth to be re constructed simultaneously from both transmission and reflection trave ltimes. We analyze the ambiguity in the determination of velocity and depth in transmission and reflection data and realize that depth pertu rbation is more sensitive to reflection traveltime anomalies than slow ness perturbation, whereas the reverse is true of transmission travelt ime anomalies. Because of the constraints on velocity and depth provid ed by the different wave types, this algorithm reduces the ambiguity s ubstantially between velocity and depth prevalent in reflection tomogr aphy and also avoids the undetermined problem in transmission tomograp hy. The linearized inversion was undertaken iteratively by decoupling velocity parameters from reflector depths, A rapid 2-D and 3-D ray-tra cing algorithm is used to compute transmission and reflection travelti mes and partial derivatives with respect to slowness and reflector dep th, Both depth and velocity are parameterized in terms of cubic B-spli ne functions. Synthetic examples indicate the improvement in tomograph ic results when both transmission and reflection times are included, T he method has been applied to a reverse vertical seismic profile (VSP) data set recorded on the British coal measures along a crossed-linear array. Traveltimes were picked automatically by the simultaneous dete rmination of time delays and stacking weights using a waveform matchin g technique. The tomographic inversion of the observed reverse VSP ima ges two fault-zones of lower velocity than the surrounding media. The location of the faults was confirmed by near-by reflection liner, The technique can be applied to offset VSPs or reverse VSPs and coincident VSP and surface reflection data.