Underground » Strata Control and Windblasts
The trend of longwall coal mining goes to longer and wider longwall panels for more production. The exhaustion of coal resources near the surface forces the panels to go deeper into higher stressed and more geologically complicated environments. Implementing tougher safety regulations requires semi to full mining automation to reduce the risks to underground hazards. All of these changes pose greater challenges for mine operators striving to achieve high production while maintaining favourable mining conditions. Poor roof conditions in a coal longwall panel may result in rock collapse when a problematic zone is mined through. This can cause significant interruption to mine production. The ability to image roof conditions - stress and degree of fracturing - ahead of the face gives the mine the ability to proactively respond to such problems. There has been a requirement from the mining industry for technologies that are capable of mapping the stress distribution and geological structures across the entire panel ahead of the working face, without any interruption to the production.
This project investigated the feasibility of using the longwall shearer as a seismic source to tomographically image roof stress conditions ahead of a mining face. A longwall panel is a rectangular block. This geometry is particularly good for using seismic tomography techniques. Using the shearer as the source enables continuous mapping of the strata condition while mining.
The goals of this project were:
- To conduct field trials to record seismic signals from the shearer and other noise sources; to examine the seismic radiation from the longwall shearer; and to address the issue of whether such radiation is suitable for stress and fracture imaging ahead of the face.
- To test the application of the new navigation technologies in the seismic imaging context.
- To investigate cost-effective procedures for geophone installation.
This project consisted of two field trials and the development of data processing techniques for continuously generated seismic signals.
The results from the field trials have shown that the longwall shearer can generate strong seismic energy that can be adequately detected by geophones deployed along the gateroads as far as 300m from the face. Using noise filtering and signal cross-correlation techniques, the seismic arrival times associated with the shearer cutting can be reliably determined. It is demonstrated that the seismic velocity ahead of the face can be mapped out for each shearer return using tomographic techniques while mining is in progress.
It was demonstrated that the installation of vertical component geophones in shallow holes in the roof along the gateroads can provide more reliable seismic data and prove more cost effective for longwall tomographic mapping than the installation on roofbolts and dynabolts.
A couple of issues were encountered during this project, which need to be addressed in order that the seismic tomographic technology can be fully applied in underground mines.
Only IS-approved geophones can be used near the longwall face. As IS-approved geophones cannot be sourced in Australia, we had to place geophones in the non-hazardous zone (at least 100m ahead of the longwall face) in the trials. This greatly reduced seismic ray coverage near the face and significantly decreased image resolution in this region. In order to solve this problem, CSIRO researchers have conducted extensive discussions with Simtars and have developed a R&D plan to develop IS geophones conforming to Australian regulations.
The time synchronisation between the seismic data acquisition and the shearer navigation system proved to be very important for real-time tomographic mapping. The seismic recording should be controlled by the longwall automation system in order to ensure that seismic records are always associated with shearer cutting operation (not idling or stop time). This can be resolved by integrating the triggering control unit of the seismic recording as part of the longwall automation system.
The objective of this first phase was to determine the feasibility of imaging conditions ahead of the face using the shearer as a source for tomographic imaging. We have shown that this can be done, and obtained some initial tomographic images of rock velocity. The next phase, would develop this tool for routine use:
- Development of IS-approved geophones for use in Australian coal mines, so that sensor arrays can be installed in both gateroads, and close to the face.
- Comparison of tomography results with face conditions and other geotechnical information.
- Imaging changes in stress as the abutment moves forward with the face.
- Processing techniques that are completely automatic.
- Augmenting the velocity tomography with attenuation tomography, to better allow distinguishing geology, stress, and fractured rock.