The imaging of offset VSP data in local phase space can improve the image of the subsurface structure near the well.In this paper,we present a migration scheme for imaging VSP data in a local phase space,which uses the Gabor-Daubechies tight framebased extrapolator（G-D extrapolator） and its high-frequency asymptotic expansion to extrapolate wavefields and also delineates an improved correlation imaging condition in the local angle domain.The results for migrating synthetic and real VSP data demonstrate that the application of the high-frequency G-D extrapolator asymptotic expansion can effectively decrease computational complexity.The local angle domain correlation imaging condition can be used to weaken migration artifacts without increasing computation.
In this paper,we image the subsurface reflectors by interferometric imaging using primary and downgoing first-order free-surface related multiple reflections in walkaway VSP data.By analyzing the stack fold distribution,we find that primary-direct interferometric imaging has a smaller image range,but its stack fold is higher near the well while ghostdirect interferometric imaging is the opposite.We try to solve this problem by the joint interferometric imaging of walkaway VSP data,combining primary-direct interferometric imaging with ghost-direct interferometric imaging.In this way,we can effectively widen the imaging range,simultaneously increase the fold（especially near the well）,suppress spurious interference,and improve the image SNR,so that we can get a more credible image.Test results on synthetic walkaway VSP data and field data show that joint interferometric imaging is very effective.
