Imaging of subsurface faults by walkaway VSP waveguiding

Abstract

Hydrocarbon reservoirs sometimes occur within low velocity channels, thus forming potential seismic waveguides. The waveguide might be excited at its discontinuities (faults) by seismic waves travelling downward from a surface source. Given that a waveguide would confine scattered energy to the horizontal plane, it should be possible to map fault scatterers at large range by means of a downhole triaxial geophone placed within the reservoir. A simple laboratory scale seismic model of a Southern North Sea gas reservoir was constructed to study the problem. Multicomponent seismic measurements were taken in both walkaway VSP and offset VSP shooting geometries. The results validate the presence of strong waveguiding in a low velocity layer disrupted by faulting. The diffracted arrivals can be imaged to locate the fault 'source'. The model results, when scaled to the field situation, suggest that faults within a reservok can be detected at horizontal distances greater than several kilometres from the drill hole.