Estimation of Relative Permittivity of Shallow Soils by Using the Ground Penetrating Radar Response from Different Buried Targets

Ground penetrating radar (GPR) for landmine detection has reached the stage where portable equipment for ¯eld operations is commercially available. Dual sensor systems in which high performance metal detectors (MD) are combined with GPR have been extensively trialled. The operating conditions for the GPR are strongly a®ected by the electromagnetic characteristics (magnetic susceptibility and complex relative permittivity) of the soil. These can change in space (soil inhomogeneities) and also in time (environmental factors like moisture, temperature). These variations are the main reasons why the GPR systems need either manual or auto-calibration before their use as a mine detector. This paper describes an assessment of methods that can be used by operators in the ¯eld for the estimation of the relative permittivity ("r) of the soil at shallow depth. The estimation of "r is obtained indirectly by the propagation velocity v = c= p "r, where c is the speed of light in vacuum. Experiments were carried out using the MINEHOUND dual sensor system jointly developed by Vallon GmbH and ERA Technology at the test site of the latter. Di®erent metal targets were buried at di®erent depths in a soil de¯ned as ballast. The time-of-°ight was estimated from data acquired in a 10 second scan above the target. The MINEHOUND antenna uses two linearly polarised parallel elements separated by 8.3 cm, one transmitting and the other receiving and has a centre frequency of 1 GHz. The received signals also contain another re°ection from the air-soil interface that, with the bistatic antenna con¯guration, corresponds to the following time-of-°ight equation. By suitable processing of the experimental data the di®erence of the various times-of-°ight (Eq. (1){Eq. (2)) can be evaluated and the value of "r can be estimated. Note that Eq. (1) is non-linear for the unknown "r. The e®ect of the variability of h during the sweep also needs to be considered, because it impacts on the uncertainty of the estimate of "r. In this paper we assess which of several di®erent metal targets provides the best calibration target for a measurement procedure in the ¯eld: 1. metal pipe 2. metal sphere 3. metal planar re°ector The analysis includes the following points: ² Radar response ² In°uence of burying procedures on soil properties ² Cost and availability