A novel VSP method for fault proximity detection using low velocity waveguides

Abstract

A potentially useful seismic technique for fault detection has been tested by finite-difference modelling of elastic wave propagation in heterogeneous media. It employs a surface-to-borehole vertical seismic profiling (VSP) geometry. Geophones are lowered into the borehole and seismic sources are fired at or near the ground surface. Small disruptions to a low-velocity zone (LVZ) bounded by higher velocity media can be detected by the technique with the analysis of seismic arrival patterns recorded at geophones in a nearby borehole. A disruption in the LVZ will act as a secondary source when an incident wave strikes it. Some of the energy radiated from the secondary source can be captured by the LVZ and will propagate as guided waves. Both offset VSP and walkaway VSP recording geometries are considered for the numerical simulation. In the offset geometry, a spread of geophones is deployed in the borehole around the coal seam while a single source is fired. Both disrupted and continuous LVZ models have been considered for numerical simulations. Explosive sources (P-wave energy only) are used. Synthetic seismograms show that channel waves can be excited by the scattered energy at a LVZ disruption. The maximum amplitude of the channel waves is comparable to those of direct body waves, which clearly indicates that the disruption in the LVZ can act as an efficient source to generate guided waves. In the walkaway VSP geometry, a single geophone is fixed in the LVZ and a spread of sources is fired on the surface away from the borehole. Synthetic seismograms demonstrate that the apex of the channel-wave arrivals pinpoints the location of the disruption of the LVZ for a layered model. Although this technique has parallels with in-seam seismic coal exploration, it does not require that the source be placed within the low-velocity channel. This method is equally applicable to any exploration target which relates to a LVZ.