Study On Strain Monitoring Of Distributed Optical Fiber Sensor In Coal Mine Roof

In today's world,the demand of coal is still at the top of that of all energy,and the safety of coal mining is a major issue related to the national economy and people's livelihood.Accounting for a large proportion in the coal mine safety accidents is the roof accident,and the main reasons are the imperfect equipment monitoring,unreasonable roof support method.Therefore,taking a monitoring on the roof is urgent.Currently,the methods used in coal mine roof are using the geodesy monitoring technique,displacement sensor,etc.As the mining tasks increasing and the difficulty level of exploitation raising,the larger mining space,larger deformation,wider time span,larger range of strata movement have become key points in roof monitoring.In this way,this paper applies the distributed optical fiber sensing technology into the monitoring of coal mine roof.The innovation point of this paper is to apply the Brillouin Optical Time Domain Reflectometry technology(BOTDR)into the monitoring of the coal mine roof,to put forward the concepts of relative variation of frequency shift and average variation of settlement,and to establish the mathematical model of the relationship between the optical fiber strain and roof deformation through the derivation of certain formula.There are three major aspects of contents in the paper.Firstly,the author aims to study the deformation monitoring of coal mine roof through BOTDR technology,make a 1500mm×1000mm×1000 mm(length × width × height)type coal mine roof model to simulate the mining process of coal mine,apply BOTDR monitoring system into the three-division model test of coal mine roof,conduct tests of the frequency shift monitoring and subsidence monitoring by simulating the mining process,make fixed-point monitoring device of optical fiber to determine the location of sampling points in the test,make multi-point settlement measurement instrument,and conduct comparative test between traditional coal mine roof monitoring system and distributed optical fiber monitoring system.The results of test show that the distributed optical fiber monitoring technology is more reliable,accurate and real-time.Secondly,according to the relationship between frequency shift and strain of optical fiber,the author makes use of distributed optical fiber monitoring principle and the characteristics of strain variation to simulate the deformation variation of the roof and establish the mathematical model.In this paper,the concept of relative variation degree of frequency shift is put forward and the mathematical model is established.The frequency shift variation of the whole optical fiber is obtained by homogenizing the difference between the two frequency shift variations,which is used for characterizing the deformation and movement intensity of the rock mass structure in the coal mine roof.According to the strain formula,the mathematical model showing the positive linear relationship between the relative variation of frequency shift and the deformation is established.The concept of average variation degree of settlement is put forward and a mathematical expression is established,to characterize the average settlement of the whole measurement point.By analyzing the test results and comparing the data on settlement amount and shape variation,it is found that the variations of the two factors are consistent on the whole,but the results monitored by frequency shift have a short delay and high accuracy.Therefore,the feasibility and reliability of distributed optical fiber sensing technology for monitoring roof deformation are verified,and it is more suitable for engineering application and meets the application requirements of geological and engineering safety monitoring.Thirdly,through the analysis of experimental data,the frequency shift of optical fiber can characterize the deformation of coal roof.The simulated mining test shows that when the quarry face is closer to the laid fiber,the value of frequency shift is increasing,otherwise,decreasing,and it shows the positive correlation on the whole.The change of longitudinal frequency shift from bottom to top shows a trend of increasing firstly and then decreasing continuously in the form of ladder,that is,the deformation of the roof presents a step-like change with the increase of height.