基于探地雷达的典型红壤区网纹红土层识别

Identification of Plinthitic Red Earth Layers in Red Soil Regions Typical of South China with Ground Penetrating Radar

网纹红土层的识别对于土壤学和第四纪环境等研究均具有重要意义。以位于南方红壤区的江西省鹰潭市孙家流域为例,通过多频率探地雷达图像解译、钻井探测和土壤含水量分析等方法相结合,研究使用不同天线频率雷达对网纹红土层的识别效果及最佳的雷达探测方法。结果表明:共中心点法(CMP)能够准确地计算出电磁波在不同土地利用类型、不同深度的传播速度,电磁波在网纹红土层的传播速度在0.052 m·ns^-1~0.065 m·ns^-1之间,平均速度为0.058 m·ns^-1;剖面深度上的含水量变化特征与土壤层次的分布具有协同性,网纹红土层内土壤含水量逐渐递增,至网纹红土层下界面出现最大值,风化层以下土壤含水量随着深度的增加不断减小;应用60 MHz和120 MHz天线的雷达均可识别网纹红土层的上、下界面,深度拟合结果的R^2分别为0.93和0.86;相比较于60 MHz单天线探测网纹红土层厚度的结果,采用组合天线的方法,即200 MHz和60 MHz天线分别获得网纹红土层上、下界面的深度,能够提高探测准确度,试验平均相对误差由16.2%缩小至6.8%;探地雷达技术不仅可以提高野外调查效率,还有利于推进土壤三维制图的发展。

【Objective】The existence of a plinthitic red earth(PRE)layer greatly affects the mass flow and energy flux in the red soil region of South China due to its unique physical structure,so it is of great significance to the study of Quaternary Environment and to precision agriculture as well.However,the costly traditional soil survey down to the bedrock or subsoil through dense sampling is destructive to soil,time-consuming and laborious.Meanwhile,it is unable to soil layers horizontally,which greatly affects scope and depth of the investigation.Therefore,ground penetrating radar(GPR)is a mature and widely used means to conduct shallow geophysical surveys that help overcome the shortage of the traditional soil surveys.【Method】The Sunjia mini-watershed of Yujiang Town,Yingtan City,Jiangxi Province was set as the study zone for the research.The zone under the subtropical monsoon climate varies in elevation between 48.1 and 66.9 m,and is covered mainly with red soil developed from Quaternary red clay and red sandstone.Meanwhile,a field survey was carried out in a watershed located in the red soil region with an area of 50.5 hm2,using indirect measurement(i.e.,GPR with three frequency antennas)and direct measurement(i.e.,hole-drilling investigation).An AKULA-9000C,a product of the Swedish Geoscanners Company,consisting of four parts,i.e.computer,transmitting antenna,receiving antenna and control unit was used,with the antenna working on frequencies,60 MHz,120 MHz and 200 MHz.Six boreholes were drilled deep to the soil–bedrock interface and photographed to document PRE layers.Water content analysis method was performed to validate thickness of the soil layers.Meanwhile,a linear regression model was established to fit and evaluate radiogram interpretation based on different radar measurements.【Result】Results show that the common midpoint method(CMP)can be used to accurately calculate propagation velocity of the electromagnetic waves in soil layers different in depth.Propagation speed of the electromagnetic waves in the PRE layer varied in the range of 0.052 m·ns^-1~0.065 m·ns^-1,with an average of 0.058 m·ns^-1.Variation of soil water content in the soil profile was concerted with distribution of soil layers.Soil water content increased gradually in the PRE layer and peaked at the bottom of the PRE layer.At the same time,soil water content decreased with soil depth below the weathered layer.The average soil moisture content of the PRE layers was 28.95%,higher than that of the weathered layer and bedrock.The radar working on 60 MHz and 120 MHz could easily and accurately distinguish upper and bottom interfaces of the PRE layer were accurately distinguished.Depth of the two interfaces of the layer was fitted with R2 being 0.93 and 0.86,respectively.Compared with the thickness of the PRE layer detected with the radar of a single antenna working on 60 MHz,the radar with a combination of antennas,working on 200 MHz and 60 MHz antenna greatly improved accuracy of the detection,by reducing its potential deviation from 16.2%to 6.8%.Notably,the GPR working on 60 MHz could also distinguish interfaces of the bedrock and groundwater tables.【Conclusion】GPR is an effective instrument to identify PRE layers in red soil regions because dielectric constant differs between different red soil layer.The radar working on 60 MHz and 120 MHz can relatively accurately distinguish the upper and bottom interfaces of the PRE layer,but is still not so good as that working o 200 MHz.Therefore,it could be concluded that antennas high in frequency have short wavelength that allows a higher resolution in characterizing subsurfaces.At the same time,higher frequencies At the same time,higher frequencies attenuate more vigorously in the medium,which reduces penetration depth of electromagnetic waves.Therefore,it is difficult to achieve a win-win situation between the resolution and penetration depth of GPR.The suggested combined antenna method can be used to identify PRE layers and improve accuracy of the detections.The utilization of the GPR technology is promising to improve field survey efficiency and to promote the technology of three-dimensional soil mapping.