f More reliable shear-wave data from VSP using CIPHER technique

It has long been recognized that fracture systems in rocks can significantly influence the flow of fluids in the subsurface. Geophysical fracture characterization techniques based on seismic anisotropy and related shear-wave splitting have been under investigation for many years. It is now established as an observable effect. To keep rig time costs low, a 3-C multilevel downhole sonde has been introduced many years ago. Additional horizontal vibrators and additional survey time and processing costs for shear-wave data, however, are still important constraints. The CIPHER method (Edelmann, 1992) described below aims both at the reduction of costs and at an improvement of VSP shear-wave data.