Abstract：Cross-borehole seismic tomography is an effective and accurate global optimum inversion method. It is used to make cross-borehole dynamic hydrocarbon monitoring，and to find some subtle structures and minor faults between boreholes. To cope with the limitation of cross-borehole observation angle，reflection wave is introduced in tomographic inversion. In terms of similarity between VSP observation system and cross-borehole observation system，a new seismic observation system is designed in which seismic wave is generated on surface but received both in boreholes and on surface with variable offset. As a result，the deficiency that ordinary VSP data are not enough is removed，and observation angle is enlarged in quite big degree.The numerical modeling of this observation system is made desirably. Transmissive and reflective tomography methods are used to achieve satisfactory inversion of both velocity and structure shape between boreholes.
