Underground Coal Mining
An installation of permanent surface and temporary underground seismometers communicates with a central seismic server located on surface for real-time seismic monitoring of the rock mass. The integrated GPS timing module is used so that recorded seismic data may be integrated with seismic data from another nearby system for information about regional seismic events. Any seismic stations on the surface use GPS modules to ensure common time with the seismic server and thus the underground seismic network. The intrinsically safe GSi units can be used in the potentially explosive underground environment. These units use a 2-pair cable for communication and power from a safe area.
Digital radios are used for communication between surface seismic stations and the central site. From the mining offices or safe area underground two twisted-pair copper cables pass through standard intrinsic safety barriers to service the underground GSi stations. Up to three GSi stations may use the same comms and power cables, provided these cables are not more than 2400m in length. Alternatively, a single GSi may use two twisted pairs, provided this cable is less than 7000m in length.
The low power consumption of the surface seismic stations (less than 0.6W/channel) allows use of an inexpensive solar panel and battery unit. The solar power together with digital radio communications means that the seismic station may be located in places that cannot be linked to the central seismic server by fibre-optic or copper cables. The underground GSi stations use less than 120 mW each, and are supplied with power from the central server or fresh air zone.
The surface stations would typically use geophones installed permanently into boreholes for the best signal quality. Underground, the fast pace of mining makes this approach expensive, and so sensors mounted to the tunnel surface are commonly used. These surface-mount sensors can be quickly moved to keep up with mining.
Each netADC can monitor and pre-process up to 8 seismic signals, in any combination of uni-, bi- or tri-axial seismic sensors. Both geophones and accelerometers (piezoelectric and Force-Balance) are currently supported, although geophones are commonly used in the low-stress-drop environment of most coal mines. The GSi units monitor up to 3 seismic signals from a single tri-axial or three uni-axial geophones. The geophones have been certified as Intrinsically Safe in the past.
SEISMIC DATA PROCESSING & INTERPRETATION
The seismograms of every seismic event that triggers multiple stations are transmitted to the seismic controller and recorded on a user-specific computer on the Local Area Network (LAN). The seismic event is automatically processed and seismic source parameters including the location, time, radiated seismic energy and co-seismic inelastic deformation are automatically calculated. This data is confirmed later by manual processing with the software packages Trace running on a Microsoft Windows or Linux computer. Spatial, temporal and source parameter trends made then be detected and analyzed using the Vantage package. If preferred, seismic data can be processed off-site by IMS, and the processed data uploaded back to the mine afterwards (usually <5 min turnaround time). IMS also offers routine analysis and reporting services.