BOTDR Based Detecting Technology For Water Flowing Fractured Zone

The water flowing fractured zone,including caving zone and fracture zone,is an important technical parameters to evaluate the safety and rationality of the waterproof pillars for mining under water.With the increase of coal seam mining under water,in order to ensure coal mine safety production and release the coal under water,accurate detection of the height of the water flowing fractured zone is of great significance.In this paper,the BOTDR(Brillouin Optical Time Domain Reflectometer),a kind of distributed optical fiber sensing technology,was applied to detect the water flowing fractured zone.The achieved results are as follows:(1)Based on BOTDR distributed optical fiber sensing technology,the distributed monitoring method was designed to detect water flowing fractured zone.The optical sensing cables were embedded into the overlying strata of the coal seam using a borehole.The strain distribution along the sensing cables was monitored periodically according to the mining progress.By analyzing the distributed data,it can be concluded that the caving zone is about 13 m high and the water flowing frractured zone is about 38 m high.(2)A numerical back-analysis was conducted using the strain distribution of the sensing cable.Two methods,including anchor element strain distribution comparison and rock strain distribution comparison were put forward to obtain the reasonable parameters of the numerical model.By numerical simulation,the water flowing fractured zone was determined according to the plastic zone.(3)Comparative analysis showed that the caving zone and fracture zone height obtained by numerical simulation and experience formula had highly consistence with those analyzed using distributed strain of optical sensing cables.It can be concluded that BOTDR distributed optical fiber sensing technology can be applied to detect the water flowing fractured zone accurately.In addition,BOTDR can be used to monitor the whole progressive deformation and failure of overlying strata during the mining of coal seam.The research provided a new approach to detect the water flowing fractured zone for under water mining.