Detection Of Railway Subgrade Moisture Content By GPR

With the rapid development of China's economy,high-speed rail has entered our life.In the China northeast area and the Qinghai Tibet railway construction,the underground ice in winter with high Moisture content area has become one of the major risks of frost heaving,railway running speed and safety,so the railway subgrade moisture problems can not be ignored.Ground penetrating radar(GPR)is a shallow geophysical technique using electromagnetic wave for nondestructive detection.It is a hotspot in recent years to use GPR to detect Moisture content of railway subgrade.The main content of this paper is to accurately detect the Moisture content of railway subgrade by GPR,and to prove the effectiveness and feasibility of the method.Ground penetrating radar(GPR)for detecting near surface moisture content has been successful in many fields both at home and abroad.At present,there are 4 main methods to measure soil moisture content by using ground penetrating radar: reflected wave method,borehole radar method,ground wave method and reflection coefficient method,among which reflection wave method is a common method.CMP mid-point(Common)is a measuring method of reflected wave method.This method is used to measure the original data of railway subgrade.The principle of velocity analysis is the main theoretical basis for extracting formation velocity information from CMP data.Because the railway subgrade is a horizontal layered medium,the stack velocity obtained by velocity analysis is the root mean square velocity.The Dix formula can be used to convert it into layer velocity,so as to obtain the distribution of underground dielectric constant.The Topp formula represents the relationship between the dielectric constant and Moisture content,and then obtains the subsurface Moisture content.After obtaining the underground Moisture content of some columns,an underground water cut profile is presented by interpolation.Because the thickness of each layer of the railway subgrade is thinner than that of the radar signal,it shows shallow stratum,which makes it difficult for the conventional velocity analysis to extract the subgrade information accurately.In order to extract the accurate rate of water cut of railway subgrade,the railway subgrade based on GprMax platform based on numerical simulation and analysis of different models under the radar response,and the modeling results are interpreted and analyzed.From the refraction multiples and interference problems found in the analysis process,in order to solve this problem and puts forward the method of analysis of optimal gathers speed,this method can effectively solve the above problems and picking accurate strata information.In order to make the optimal Lane set method applicable to more different railway subgrade conditions,a large number of numerical simulations are used to sum up the optimal angle,which is 75 degrees.In the course of velocity analysis,the effect of refraction and multiples on velocity analysis can be reduced as long as the reflection angle is smaller than the optimum angle.The optimum angle is suitable for the analysis of velocity under different thickness and different dielectric constant.In this paper,the influence of antenna frequency,time correction and antenna moving step length on the accurate detection of Moisture content of railway subgrade is further analyzed.It is found that the high frequency antenna can eliminate the influence of some multiples;the time correction has a great influence on the result of velocity analysis;the step length of the antenna provides a reliable basis for data acquisition.Polarization method is a reliable method to reflect the abnormal location and relative size of Moisture content.Using the processing experience of analog data to guide the processing of the measured radar data,the processing results of several measured data are compared with the corresponding polarization results,and the results show that the two anomalies are basically the same.Therefore,the method of GPR for measuring moisture content of railway subgrade is proved to be effective and reliable.The method has the advantages of no damage,high resolution and low cost,and has certain popularization value.In order to popularize this method and to improve the efficiency of data processing in practice,a software system for rapid processing of measured data is developed.The software is based on the Matlab GUI environment,with fast,efficient and accurate processing of data and imaging capabilities.