Underground » Coal Burst
In this project, the feasibility of using distributed optic fiber sensing (DOFS) technique was investigated for microseismic monitoring in coal mining. The investigation involved in theory study, laboratory tests and a mine site experiment.
Results from the laboratory experiments have demonstrated that DOFS can acquire high quality microseismic signals associated with microseismicity and the differences of seismic arrival times can be reliably obtained at high accuracy along an ordinary fiber cable. These differences allow a seismic event to be reliably located. The quality of the seismic data recorded by the fibers is comparable to that of the geophones. It has also been proven that DOFS can achieve high spatial resolution for seismicity sensing and high accuracy for locating events. These characteristics are particularly useful for using DOFS to monitor ground vibrations associated with rock fracturing events induced by mining.
In the field experiment, a DOFS system, consisting of a commercial fiber interrogator and a 3,000 m long optic fiber cable were tested. The cable was installed in two sections, with one section cemented in a deep borehole and the other buried in shallow notches at the ground surface around a longwall panel being mined. Four geophones were installed along with the fiber cable for comparison of seismic characteristics recorded by the two sensing devices. The monitoring program captured high quality microseismic data for the investigation. This field experiment has demonstrated that DOFS can be used as a new and cost-effective tool for recording microseismic events created by mining activities. The application of DOFS can enable mine operators to obtain detailed information about ground failure processes and locations that are useful for identifying precursory microseismicity for warning or predicting geodynamic risks induced by mining.
It has also been found that DOFS can achieve higher sensitivity for microseismic event detection and greater accuracy for event location than a geophone network due to its dense spatial sampling. Installation of a fiber cable in three perpendicular directions is very helpful for applying the phase tracking method for rapid seismic event detection and location. Using the phase tracking method, there is no requirement for picking the first seismic arrival times and constructing a reference seismic velocity model for event location. This improvement allows many events, particularly tensile seismic events that do not present clear first arrival times, to be located.
The results have shown that both P- and S-waves can be reliably captured by DOFS and some other seismic signals, in additional to the P- and S-waves, can also be identified on the seismic records. These additional phases may be attributed to seismic refraction, reflection or diffraction from unknown structures, and hence they may be used for exploration of these structures.
The fiber interrogator tested in this study is currently commercially available. The ordinary single mode fiber cable has been proven to be sufficient for capturing microseismic signals.
It was found that the influence of background noise (mechanical vibrations, moving vehicles and animals) on the seismic records was insignificant for seismic event discrimination and phase tracking.
Several challenges were identified that must be addressed before DOFS can be applied in mining for real-time microseismic monitoring.
- The greatest challenge is that commercially available interrogators are not able to handle real-time data.
- To use DOFS for real-time microseismic monitoring, software development and hardware improvement must be conducted.
- The fiber cable was damaged several times during the experiment period, causing data missing for many days.
- Temperature control for the fiber interrogator is another concern, particularly when the instrument is housed in the field during summer.