Real-time Seismic Roof Condition Mapping Ahead of Longwall Mining

Poor roof conditions in a coal mine longwall panel may result in rock collapse. 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 opportunity 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 production.

This project is a continuation of two previous ACARP projects, namely:

· ACARP Project C15023, 2006-2007 - Roof condition monitoring ahead of a longwall face - Phase one - initial assessment and experimental. This project involved theoretical study of the tomographic techniques for longwall mining and field trials at Beltana Mine in NSW. The results have shown that the longwall shearer can generate seismic energy that can be adequately detected by geophones for roof condition imaging;

· ACARP Project: C18018, 2009-2010 - Design, certification and manufacture of IS geophones for underground coal mining. A major impediment to achieving longwall seismic tomography was the lack of a commercially available intrinsically safe (IS) geophone in Australia. This project led to the development of two types of IS geophones certified to IEC standards (certification IECEx SIM 10.0011X).

The success of the two previous projects led to this stage of the research and development - integration of seismic tomography and longwall shearer navigation systems. In 2012, ACARP extended its support to this project, Real time seismic roof condition mapping ahead of longwall mining (ACARP Project C21020) that had the following objectives:

· To integrate seismic tomographic mapping techniques with the LASC longwall shearer navigation system and develop a software package for mapping roof condition ahead of the face in real-time; and

· To conduct field trials using the newly developed IS geophones and test the application of the new navigation technologies in the seismic imaging context.

This project was conducted in three phases:

· Phase 1: Development of methods to synchronise seismic data acquisition and the shearer navigation systems and extract the shearer location for real-time tomographic mapping. It is required that seismic records are always associated with shearer cutting operation (not idling or stop time);

· Phase 2: Test of the reliability of the new methods developed in the first stage at Grasstree Coal Mine, and improve the techniques if required after the test;

· Phase 3: Field demonstration of the seismic tomographic imaging techniques at an operating longwall panel of Grasstree Coal Mine.

This project has achieved the following:

· Integration of the seismic tomographic technology with the LASC server (longwall shearer navigation data server) is successful. The shearer position can be reliably obtained from the server in real-time;

· Time synchronisation between the seismic data acquisition and the shearer navigation systems was achieved with timing accuracy better than 100ms, sufficient for the application of longwall seismic tomography;

· Real-time tomographic images of roof conditions ahead of the face, against each return of the shearer cutting, were successfully obtained. The images can be visualised in real-time at any mine location that has a PC with network connection;

· Tomographic images (seismic velocity anomalies) obtained at Grasstree Mine, ahead of the face, have shown certain correlation with chock pressure data obtained at the face. As the images are located in front of the face, they can be used to infer stress anomalies in the roof in front of and at the longwall face, induced by mining processes;

· The CSIRO seismic tomographic technique has been successfully demonstrated in an Australian underground coal mine and is ready for mine application.