Surface-to borehole fault imaging in a buried horizontal waveguide; an experimental study in the southern North Sea

One fifth of the United Kingdom’s energy is derived from the gas fields of the southern North Sea. The area is divided into large fault blocks, which are laced with minor faults. Surface reflection seismic fault maps of the main producing horizon are distorted by salt rollers or pillows in the overlying high velocity Zechstein evaporites. We have developed an inexpensive method of confirming the presence of minor faults in the relatively flat base of the Zechstein around a well that would simplify production planning. Deep well sonic logs indicate that seismic velocity drops abruptly beneath the base Zechstein and then rises gradually with depth. This profile is characteristic of a whispering gallery, or a buried, variable‐index beam waveguide. If base Zechstein faults scatter energy efficiently into the waveguide, then the faults might be mapped by moving a seismic source over the surface and recording the mode converted arrivals at a geophone within the guide. To test this possibility, over 3000 shots w...