Lithology characterisation of the roof and floor of the Moranbah measures coal seam using post-stack and pre-stack seismic inversion

Summary

3D seismic technology has become a popular exploration and development tool among coal mining companies in Australia in the last ten years. Traditionally the main use of these seismic data is to inform the mine of the structural features such as faults, rolls and dykes. There have been very few attempts to use quantitative seismic interpretation to help understand geological risks ahead of longwall mining. The purpose of this paper is to provide an example of roof and floor characterisation using post-stack and prestack seismic inversion.

First step in the study was to understand the relationship between elastic and rock properties using log data. Thus, when looking at the relationship between lithology/gamma ray response and compressional impedance I observed that massive, blocky sandstones have high velocity and density and hence high compressional impedance, while siltstones, claystones and fine-grained sands have low density, velocity and low compressional impedance. Lithology discriminators lambda-rho and mu-rho crossplots also showed good separation of blocky clean, sands and soft fine-grained sands. As a second step I ran pre-stack and post-stack inversions using a small subset of the boreholes. Thirdly, upon reviewing products of the inversions against all log data, I found the pre-stack results to be more superior than the post-stack.

The current work highlighted a few areas of blocky sandstone roof and weak floor. This information could be used to inform the future mine operations and design.

• coal
• seismic inversion
• QI
• longwall mining