Seismic wave propagation through surface basalts – implications for coal seismic surveys
Weijia Sun 1 2 4 Binzhong Zhou 1 Peter Hatherly 3 Li-Yun Fu 21 CSIRO Exploration and Mining, Kenmore PO Box 883, QLD 4069, Australia.
2 Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, China.
3 Coalbed Geoscience Pty Ltd, 10 Waiwera Street, McMahons Point, NSW 2060, Australia.
4 Corresponding author. Email: swj@mail.iggcas.ac.cn
Exploration Geophysics 41(1) 1-8 https://doi.org/10.1071/EG09015
Submitted: 28 February 2009 Accepted: 14 December 2009 Published: 19 February 2010
Abstract
Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basalt flows are needed.
In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that: 1) basalts with high impedance contrasts and multiple flows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.
Key words: coal seam, finite difference, forward modelling, inhomogeneity, long-offset recording, low frequencies, sub-basalt imaging.
Acknowledgements
This work was supported by the Australian Coal Association Research Program (ACARP), BHPB Illawarra Coal, Xstrata Coal, BMA Coal and Anglo Coal Australia. Comments by the two anonymous reviewers were appreciated and led to the investigation of the models involving basalts with different velocities and multiple flows.
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