Fully Automated Millimeter-Order Depth Estimation in Concrete Using Smartphone-Mounted Handheld GPR and Curve Fitting Techniques

Smartphone-mounted handheld GPR are mainly used for detecting rebar and are attracting attention in the field of infrastructure maintenance. However, the information obtained by electromagnetic wave radar is the round-trip propagation time of electromagnetic waves from the transmitting antenna to the receiving antenna, which requires estimation of the relative permittivity in the medium to be converted into distance information. In this study, we propose a method for estimating relative permittivity and depth that takes advantage of curve reactions generated from any reflective object, including rebar and cracks. First, we confirmed that the relative permittivity can be estimated with practicable accuracy by curve fitting, in which theoretical and measured curves are observed. Next, we proposed an algorithm that automatically extracts the range to be fitted, making it possible to obtain the relative permittivity and the depth to be estimated simultaneously. As a result, we succeeded in estimating the relative permittivity with high accuracy on the millimeter-order of less than 5% error in the 0.1 second range.