Fault and dyke detectability in high resolution seismic surveys for coal: a view from numerical modelling

Presented at the 22nd ASEG Geophysical Conference and Exhibition, February 2012.

Abstract

Modern underground coal mining requires certainty about geological faults, dykes and other structural features. Faults with throws of even just a few metres can create safety issues and lead to costly delays in mine production.

In this paper, we use numerical modelling in an ideal, noise-free environment with homogeneous layering to investigate the detectability of small faults by seismic reflection surveying. If the layering is horizontal, faults with throws of 1/8 of the wavelength should be detectable in a 2D survey. In a coal mining setting where the seismic velocity of the overburden ranges from 3000 m/s to 4000 m/s and the dominant seismic frequency is ~100 Hz, this corresponds to a fault with a throw of 4–5 m. However, if the layers are dipping or folded, the faults may be more difficult to detect, especially when their throws oppose the trend of the background structure. In the case of 3D seismic surveying we suggest that faults with throws as small as 1/16 of wavelength (2–2.5 m) can be detectable because of the benefits offered by computer-aided horizon identification and the improved spatial coherence in 3D seismic surveys.

With dykes, we find that Berkhout’s definition of the Fresnel zone is more consistent with actual experience. At a depth of 500 m, which is typically encountered in coal mining, and a 100 Hz dominant seismic frequency, dykes less than 8 m in width are undetectable, even after migration.

We use numerical modelling in an ideal noise-free environment and homogeneous layering to investigate the detectability of small faults by seismic reflection surveying. It is shown that the detection of faults relates not only to the seismic wave frequencies and wavelengths, but also to the presence of background structures.

Key words::

• coal mining, dyke detection, fault detection, seismic resolution, seismic surveying.

Acknowledgements

This study was supported by ACARP under Project C19013. The authors thank Associate Editor Dr Roman Pevzner and two anonymous reviewers for their constructive comments which significantly improved the manuscript.

Notes

Presented at the 22nd ASEG Geophysical Conference and Exhibition, February 2012.