Top of Coal Program: Phase 4 - Validation of Different Overburden Types

The Top of Coal project sought to develop a system, which would allow the standoff distance to be monitored in real-time during a blast hole drilling.

The main source of damage to the surface of the coal seam during overburden blast hole drilling is incorrect standoff distance. If the standoff distance is too close to the coal seam then the coal seam is damaged during the blast. On the other hand, if the standoff distance id too far from the coal seam then insufficient overburden is removed requiring additional methods to expose the surface of the coal. The necessity of determining the standoff distance to the surface of the coal seam to an accuracy of ?0.5m during blast hole drilling is identified by the Australian coal industry.

The seismic energy generated by the rotating drill bit can be considered as a signal source. The sensor on the drill stem (which has a very high speed of sound) provides a reference signal of the emitted sound source for subsequent correlation processing. Sensors located the ground level can also be used to detect subsequent returns of the emitted signal. The signals received by the remote sensor consist of both the direct path from the drill bit to the sensors and reflected path from the coal seam. Time and frequency domain signal processing techniques can be used to extract these time delay information from these signals. Therefore, the delays in those multi-path signals can be used to calculate the distance to the surface of the coal seam.

As this project progressed and advances were made it became apparent that the complexity associated with reliably locating the top of coal in the variable geology in real time was greater than originally perceived. This final report details advances made through the project.

The ACARP technical committees elected to discontinue work in this area due to the scientific complexity of the task.