Shallow seismic reflection investigations of coal in the Sydney Basin

Abstract

Surface reflection profiling with the Mini‐SOSIE technique successfully mapped shallow coal seam structure in the western Sydney Basin, New South Wales. Several minor faults and zones of fracturing were detected. In regions of thick Triassic sandstone cover, data quality was poor and unsuitable for geologic interpretation. Synthetic seismograms based on nearby borehole and petrophysical control show excellent agreement with the Mini‐SOSIE sections and illustrate the deleterious filtering effects of coal seams and sequences. To establish a phenomenological basis for seismic wave propagation in shallow coal measures, two vertical seismic profiles (VSPs) which used small explosive charges were recorded with high spatial and temporal sampling. Numerous multiple reflections were observed in the downgoing wave display. The isolated upgoing waves were migrated to yield blurred images of the main coal seams. The subsurface velocity function, also deduced from the VSP, shows broad correlation with the geologic log. The VSP seismograms are not simple because of the combined effects of wave absorption, scattering, and interference. Such problems impede recovery of fine structural detail from seismic data in the shallow environment, particularly when a surface energy source is used.