A Method Of Monitoring Mining-induced Strain In A Road Pavement Using The Distrubuted Optical Fiber Sensor

The highway is the blood of national economics and plays an important role in the social development.Mining underground coal resources has an adverse effect on the roads above the coal.Therefore,it is very important to monitor the roads health.Here,we did research on factors affected measurement accuracy of BOTDA sensing system based on laboratory experiments and theoretical analysis.Meanwhile,we established two image restoration methods based on spatial domain and frequency domain respectively,to reduce signal noise and improve the performance of the instrument.Furthermore,we did an experiment on monitoring mining-induced strain in a road pavement.The main research contents and conclusions are as follows:(1)We designed the fixed-length stretching device to study the Brillouin frequency shift demodulation accuracy,position accuracy,range accuracy and event quantization accuracy,which are related to parameters of the BOTDA sensor.The experimental results show that:a).the increase of pulse width(that is the reduction of spatial resolution)can improve the quality of data,while the spatial resolution is larger than the range of events,the measured range of events will be enlargered and the position accuracy will be lowered.b).The increase of the average times can improve Brillouin frequency shift demodulation accuracy,position accuracy,range accuracy and event quantization accuracy.Because the relationship between frequency accuracy and average times conforms to the ExpAssoc model,the amplification of frequency accuracy will be reduced when average times arrive to the certain level.c).Frequency sweep range should be symmetrical and minimum/maximum sweeping frequency should be 1.0 linewidth smaller/more than the Brillouin central frequency shift,which can improve the frequency accuracy.d).The increase of frequency step will reduce demodulation accuracy.(2)We analyzed theoretically the effects of layout method,cable type and construction process on measurement accuracy.Firstly,we illustrated the transfer error of embedded and surface-bonded snsors.Secondly,we calculated parameters of the cable through experiments in advance.Moreover,in the process of cable installment,the bending radius should be larger than the limited bending radius of 43mm to prevent the signal loss caused by bending.Meanwhile,it is necessary to reduce the power loss caused by optical fiber connection.(3)In order to reduce signal noise and improve the performance of instrument,we proporsed two filters,including Kuwahara filter and self-adaptive constrained least square filter.The results show that:The Kuwahara filter can improve the SNR by 6.7dB and the Brillouin frequency shift error decreases by 0.58MHz;the self-adaptive constrained least squares filter,which is adaptive and does not need a time-consuming process to select parameters,can increase the SNR by 12.2 dB and increase the temperature accuracy by 1.3?.(4)We designed the experimental schemes of monitoring strain distribution of the road above 33_?04 working face in Nantun coal by using the BOTDA sensor,and studied the deformation and strain of the roads based on experimental results.The maximum deformation was observed in the tensile zone,and the peak tensile strain appeared around the crack.The road located in compression area has the smaller deformation,which is in form of both tensile deformation and compression deformation;and the road effected by both forms of deformation alternate.It was worthy to mention that the distribution of strain is different in different parts of roads above compression area:as for the part outside the mining face,the strain of cracks is compressive and the strain of both ends of cracks is tensile;as for the part inside the mining face,the strain of cracks is tensile and caused by raised road surface.