VSP`s in complex structures in Columbia

Vertical Seismic Profiles (VSP`s) are used to provide high resolution images around deviated wells in complex geology in Colombia. The VSP images resolve faults and dipping reflectors. Key reflectors below a major reverse fault system, at depths in excess of 12,000 feet can be correlated unambiguously with the conventional 2D seismic data. VSP`s also show reflections below the well which are used to establish correlation of the target reflector with the conventional seismic. Where there are no logs, the VSP provides a way to check the depth prognosis. Key markers are tracked across logs to the VSP with the same depth index. On a workstation the VSP images are compared with the surface seismic. Knowing the VSP phase and polarity, and the log correlation, can result in a refinement of the surface seismic interpretation. A variety of sources and source offsets have been used. The wells in this area are usually highly deviated and access along the track of the well is restricted by security and environmental regulations, plus surfaces conditions. This paper discusses a vibroseis VSP and an airgun VSP. Vibroseis times are checked at various levels using dynamite or airgun checkshots. The surface seismic source is dynamite.