CDP mapping and image reconstruction by using offset VSP data

Becausezero-offset VSP (Vertical Seismic Profile) data can only provide the information of rockproperties and structure in the area around the Fresnel zone within the well, the schemeof VSP with offset was developed to acquire the reflection information away from theborehole in order to widen the range of VSP survey and to improve the precision ofimaging.In this paper, we present a new CDP (Common Depth Point) mapping approach to imagethe reflecting structure by using offset VSP data. For the processing of offset VSP data,we firstly separated the up-going and down-going wave-fields from VSP data by means of F-Kfiltering technique, and we can calculate the mapping conditions (position and reflectingtraveltime for CDP point) in homogeneous media, and then reconstruct the inner structureof the earth.This method is tested by using the offset VSP data which are used to simulatethe case of super-deep borehole by means of finite-difference method. The imaged structurematches the real model very well. The results show that the method present here couldaccurately image the inner structure of the earth if the deviation of initial velocitymodel from the true model is less than 10%. Finally, we presented the imaged results forthe real offset data by using this method.