Enhancing the detection of small coal structures by seismic diffraction imaging

Abstract

Faults are the most important geological structures that need to be detected in any modern underground coal mining project. Even a fault with a throw of a few metres can create safety issues and lead to costly delays in mine production. While seismic surveys are usually successful at locating faults with throws greater than 5-10m, reliable techniques for resolving more subtle faults, dykes and other minor features are yet to be developed.In seismic data, there are two types of seismic energy signatures that can be used for subsurface imaging: specular reflections and diffractions. Specular reflections are created by relatively smooth surfaces between layers of differing impedance contrasts such as at coal-rock interfaces. Specular reflections are used in conventional seismic reflection surveys. Diffractions are generated by local discontinuities caused by surface roughness, faults, dykes and fractures. In this paper, we describe the use of a moving average error filter which, along with a process of reflection event flattening for accommodation of dips and undulations of coal seam reflections, can be used to effectively extract diffraction signals from post-stack reflection seismic data. By identifying diffractions, small faults and other minor features that are elusive using conventional seismic reflection processing can be detected, suggesting that diffraction imaging may be a superior method to seismic reflection imaging. Both synthetic and real (2D and 3D) coal seismic data are used to illustrate the feasibility of diffraction imaging for small fault detection. It is demonstrated from synthetic examples that the extracted diffractions can be used to detect faults with a throw of 1m while a real data example shows that it is possible to detect igneous dykes with widths of just 4m.