Open Cut » Coal Extraction
This report summarises work on Phase III of the Top of Coal project to develop a system to detect the top of coal during overburden blast hole drilling. The project aims to develop a system capable of displaying to operators reliable and accurate stand-off distances from the bottom of the drill hole to the top of coal seam in real time.
Goal of Phase III
The goal of this phase was the collection of field data which would enable selection of the optimum location for the remote sensor. Two options were investigated, the co-location of the sensor with the drill rig and the location in an adjacent empty blast hole. The choice of an empty blasthole for the sensor would require the relocation of the sensor once per shift.
The results obtained clearly meet the technical requirements of the programme and demonstrate the ability to progress the system to a commercially viable product of great benefit to the coal industry. It is recommended that the programme advance to the next phase and that the work encompassed should include:
- Validation of the system on different overburdens at mine sites in both NSW and Queensland.
- Manufacture of a real time system.
- Development of user friendly displays.
It is also recommended that information from the existing drill rig sensors be accessed and incorporated into the system for improved reliability.
Reducing coal loss in a surface coal mine operation leads directly to increased production and profitability of the mining venture. Coal loss can come from many parts of the operation but blast damage to the top of the coal is a cause. The major contributor to blast damage to the top of the coal seam is incorrect stand off distance. If the stand off distance is too close to the coal seam then coal is inadvertently removed during the blast and if it is too far from the coal seam then insufficient overburden is removed requiring additional effort to expose the top of the coal seam.
As a result of this problem a need was identified by the Australian coal industry to detect the top of the coal seam to an accuracy of +/- 0.5m during blast hole drilling.
Thomson Marconi Sonar Pty (TMS) responded by proposing an integrated four phase programme to develop a commercial system which would meet this requirement. The system would comprise sensors, real-time data acquisition, signal processing and an interactive seismic display of drilling in progress.
Two sensors only are required. The first sensor is located on the drill stem and monitors the drill vibrations. The second sensor is remotely located in an adjacent blast hole and monitors the drill bit through both the direct and reflected seismic path. The remote sensor signal is transmitted to the drill rig by a wireless FM telemetry. The testing to date has indicated that the remote sensor can be located up to 40m from the drill rig and still achieve acceptable signal levels for monitoring the seismic path. Operationally, this means that the sensor would probably require relocating every shift.
Drill Stem Sensor
TMS has monitored acceptable signal levels from twenty seven different blast holes involving three different geological stratigraphies. Phase 2 data were recorded on a site dominated by a massive sandstone overburden. An extensive analysis was undertaken to develop signal processing techniques which would enable the drill's proximity to the seam to be determined. The conclusion drawn from this analysis was that as the drill approached the coal seam, the vibration levels recorded by the sensor mounted on the drill stem reduced dramatically and more importantly, that these levels could be directly related to the thickness of the overburden remaining under the drill.
The theory behind this phenomenon is briefly described in the report.
If one assumes the simple case of an homogeneous overburden, then when the drill is close to the surface it acts as a point source radiating into an unbounded medium. The resilience levels recorded on the drill stem sensor are high as there is not an efficient transfer of seismic energy into the rock. This is similar to impacting energy into a thick concrete block.
When the drill approaches the coal seam, however, the medium's reaction to the drill impact changes and more energy is transmitted into the rock. The change occurs at a specific distance from the coal seam when the transmission mode changes to a transverse wave propagation (as is found when impacting a thin plate). When this change in the transmission mode occurs, the distance to the coal seam can be predicted from theory.
Further confidence of the position of the coal seam can be obtained independently by a specific signal processing technique, the narrowband auto correlation function, of the remote geophone data. The data from this sensor can be used to predict the speed of sound of the overburden as the drill moves through the overburden. This is seen as a slope on the time delay / depth trace of the sensor. As the drill moves through the overburden the relative distance between the drill bit and sensor increases and the slope of the time delay change is directly related to the speed of sound through the overburden. When the drill approaches the coal seam the interference effects of the direct and reflected path manifest themselves on this display.
In consequence, when the drop in power levels recorded by the drill stem sensor coincide with the interference effects on the remote geophone trace, then the operator has a high confidence level of the distance to the coal seam.
The following conclusions can be drawn from the analysis:
The proximity of the drill bit to the coal seam can be determined by two complementary processing techniques. The first requires a drill stem sensor and the second a geophone.
The geophone can be either co-located to the drill rig or remotely deployed. The remotely deployed sensor has been the most reliable in detecting the coal seam. The reliability of the coal seam detection would be increased by using both a remotely deployed and co-located geophone.
Coal seams were detected when the remotely deployed geophone was up to 40m from the drill, which infers the need to relocate the sensor generally every shift.
Co-locating the geophone to the drill rig ideally requires the monitoring of the vibration levels on each of the elephant feet.
It is recommended that:
the system be improved to include the equal time outputs from existing drill rig sensors. [In particular the drill bit rpm sensor, pull down force sensor and depth sensor would improve the reliability of the system].
the data be fused through a neural network and the operator be shown a less complex display than the one used in this report.
that the system be trialed at other mine sites.
that the drill stem sensor be designed and manufactured by TMS.