Zero-offset profiling using frequency cross-hole radar in a layered embankment test site: Antenna design, simulation and experimental results

摘要

Road surface quality is very sensitive to the mechanical properties of an underground structure and particularly to its water content. Depth sounding using boreholes is generally used for geotechnical testing and particularly oven dried measurements. In parallel, electromagnetic measurements based on a cross-hole radar to perform zero-offset profiling are proposed in this paper. Thus, two specific large band dipóle geometries, a 3D folded dipole and a planar blade dipole have been designed to work in the frequency band [0.5;1.5] GHz in order to estimate the vertical water content of a multilayered clay sand embankment test site. The design of both antenna geometries and their integration in a transmission link considering a cross-hole radar with a separation distance less than 1 m, has been performed using the finite difference time-domain (FDTD) commercial software EMPIRE? The picking of the time arrivals has allowed to estimate the real permittivity and the volumetric moisture content using Malicki's relation. The comparison of measurement and simulation results has allowed to highlight the contrasts of real permittivities induced by the volumetric water content and the embankment compaction level. Moreover, in this study the dissipative properties in the soil have been evaluated as they represent complementary depth properties closely linked to the accuracy of the first arrivals picking used for the permittivity estimation, The intrinsic attenuation evaluation is based on a straight analysis in the frequency domain of successive transmission coefficients using the centroid frequency downshift method. Afterwards the electrical conductivity profile has been recovered using a full-wave FDTD analysis and a comparison between measurement and simulation data.