基于探地雷达的田块尺度下不同深度土壤含水量监测

Monitoring Specified Depth Soil Moisture in Field Scale with Ground Penetrating Radar

为确定田块尺度下探地雷达对不同深度及相邻反射层间土壤含水量的反演精度、有效反演深度、最佳反演深度及最优反演模型,本研究采用1000 MHz中心频率探地雷达设备,分别在无降雨偏干旱土壤和降雨后湿润土壤两种条件下,在选定农田区域基于共中心点法采集雷达波数据,提取有效地表波与反射波数据,通过双曲线拟合法分别获取不同深度反射层雷达波的传播速度,计算得出土壤的相对介电常数,最后根据土壤体积含水量和介电常数之间的经验模型计算获得不同深度的土壤体积含水量。通过Topp、Roth、Herkelrath和Ferre四种经验模型分别进行土壤体积含水量反演测定,同时以利用烘干法获取的代表性测定土壤含水量实测值为指标进行精度验证。田间试验结果表明,1000 MHz探地雷达的有效反演深度范围为0~50 cm;土壤偏干旱和偏湿润条件的最佳反演深度分别为50 cm和40 cm;Roth模型相关性最好,决定系数R2最高为0.750,且Roth模型反演土壤含水量值最稳定,在土壤偏干旱和偏湿润条件下均方根误差(Root Mean Square Error,RMSE)平均值分别为0.0401 m;/m;和0.0335 m;/m;相对误差(Relative Error,RE)最低为3.0%。探地雷达具备对定量深度田间土壤含水量快速、精准监测的能力,但其反演模型需根据不同土壤类型等条件进行相应参数校正。

Ground-penetrating radar(GPR)is one of the emerging technologies for soil moisture measurement.However,the measurement accuracy is difficult to determine due to some influence factors including radar wave frequency,soil texture type,etc.The GPR equipment with 1000 MHz center frequency and the measurement method of common midpoint(CMP)were adopted in the research to collect radar wave raw data in the selected field area under arid soil and moist soil conditions.The transmitter and receiver antennas of the GPR equipment were moved 0.01 m respectively in opposite directions on each radar wave raw data collection.Therefore,a CMP radar image consisted of 100 pieces of radar wave raw data by increasing the antenna distance from 0 m to 2 m.Each radar wave raw data indicated that the radar waves were reflected in the reflective layer with different dielectric constant under the same antenna distance.And the reflected and refracted radar waves were acquired by the receiving antenna at different two-way travel time respectively,and recorded in the computer.The collection of CMP soundings aimed to determine the inversion accuracy,optimum inversion depth,effective inversion depth and optimal inversion model of soil moisture content at different depth ranges and adjacent reflective layers by GPR at field scale.The reflected and refracted radar wave data were extracted from the raw data.The velocities of the surface waves and reflected waves were obtained respectively from the line slope of the surface wave data and the hyperbolic curves fitting of the reflected wave data.In addition,the relative dielectric constant of the soil at specified depth were deduced according to the soil dielectric constant and its reflected wave velocity.Moreover,4 different models including Topp,Roth,Herkelrath and Ferre were used to figure out the soil volumetric water content inversion.Meanwhile,the measured data of soil volumetric moisture content obtained by oven drying method were used to verify the accuracy of the inversion results.The results showed that the effective inversion depth of 1000 MHz GPR ranged from 0 to 50 cm.The best inversion depth was 50 cm in arid soil and 40 cm in moist soil.The Roth model had the best correlation and stability with the highest R2was 0.750,the Root Mean Square Error(RMSE)was 0.0114 m3/m3and the lowest Relative Error(RE)was 3.0%.The GPR could possess the capacity of quick,precise and non-destructive measurement of specified depth soil moisture in field scale.The inversion model of soil moisture content needs to be calibrated according to different soil conditions.