Underground » Geology
Seismic reflection is widely recognised as a significant geophysical tool for remote imaging of subsurface coal deposits. The technique is now often an integral part of mine planning, with positive contributions to mine productivity and safety. However, the desire to image increasingly subtle features is stimulating research into new seismic technologies which have the potential to enhance geological characterisation of the sub-surface.
One such novel technique is converted-wave (or PS-wave) seismic reflection. Theory and numerical modelling suggest that when a compressional (P) wave from a conventional seismic source (e.g. dynamite, Vibroseis, MiniSOSIE) strikes a coal seam, a significant fraction of the energy can be reflected back toward the surface as a shear (S) wave. Conventional seismic reflection ignores the occurrence of such P-to-S mode conversion, and records only the vertical component of ground motion to detect reflected P-wave energy. Consequently, this single-component mode of acquisition disregards the potential contaminating effects of PS waves, and ignores the additional information contained in the mode-converted energy.
Converted-wave seismic reflection aims to detect and exploit the available complementary S-wave information. The conventional vertical geophone is replaced with a multi-component geophone, capable of recording all incoming energy in a true vector sense. This provides the capability to discriminate between arriving P and PS waves. Over the past five years, converted-wave technology has been embraced by the petroleum seismic industry, with several striking successes including improved structural imaging, lithological classification, and fracture characterisation. To date there has been no significant practical assessment of the technology in the coal environment.
This ACARP project represents the first examination of converted-wave technology in the Australian coal context. Two field data trials have been conducted at sites having contrasting geological character. These multi-component seismic experiments have used a conventional dynamite source with purpose-built, high-resolution, multi-component sensors replacing the conventional arrays of vertical geophones. Specialised processing algorithms, designed to compensate for the highly variable S-wave velocities in the near surface and the asymmetric raypaths of PS waves, have been implemented to yield PS images in parallel to conventional P images.
The overall objective of this research has been to demonstrate the viability of using converted-wave seismic technology in the coal environment, and to assess its potential in terms of enhanced subsurface imaging. The results achieved in this first study provide a compelling argument that converted-wave reflection can evolve to be a cost-effective enhancement to conventional seismic imaging. From an acquisition viewpoint the technology is attractive in that relatively minor adjustments to current practice are needed. The development of a robust and cost-effective processing methodology is seen as a greater challenge. Prior geophysical history would, however, suggest that this will be achieved with further experience.
Future research should be aimed at understanding PS-wave propagation behaviour, tuning processing algorithms to improve PS image resolution, and demonstrating the practical implications of integrated P and PS interpretation. Such research will stimulate additional methodological advances, and ultimately lead to the use of converted-wave seismology as a standard coal-imaging tool.