Fault-scattered seismic guided waves in a shallow coal seam

Abstract

Over the past two decades guided waves ? propagating within low-velocity/low-density coal seams ? have regularly been exploited by the coal industry to detect geologic disturbances ahead of longwall mining. In these standard in seam transmission surveys, channel waves are generated by firing shots within the coal seam; the seam waves are detected by placing array of geophones within the coal. In this paper we investigate a novel low-cost 2D method for locating faults and dykes in coal seams. The method entails running a simple one-pass walk-away source VSP survey over a suspected lateral discontinuity in the coal panel and imaging the scatterers by multicomponent recording of the channel waves. The channel waves are excited at the discontinuity (fault) as a result of scattering of seismic waves travelling downward from the surface source. The borehole geophone, at the seam level, and laterally displaced from the fault, detects some of the forward scattered channel wave energy captured by the waveguide. Simple numerical and laboratory-scale seismic models studies were undertaken to simulate typical shallow coal seam field situations found in Australia. Different source types were to run 2D profiles over the coal/host-rock geological sequence. In these studies, both offset VSP and walkaway VSP recoding geometries were employed. Substantial forward scattering from incident S-wave to channel wave propagation was observed on the borehole receiver. Our results suggest that the waveguide walk-away survey can be an effective method to reveal small faults (less than one-tenth of a wavelength) and other seam disruptions. It is therefore possible to map small seam faults in the field ? with a resolution that is otherwise unattainable ? at a long horizontal distance from a drill hole.