  Ground-penetrating radar (GPR) has been an effective technology for locating
metal bars in civil engineering structures. However, the accurate sizing of
subsurface metal bars of small diameters remains a challenging problem for the
existing reflection pattern-based method due to the limited resolution of GPR.
To address the issue, we propose a reflection power-based method by exploring
the relationship between the bar diameter and the maximum power of the bar
reflected signal obtained by a wideband dual-polarized GPR, which circumvents
the resolution limit of the existing pattern-based method. In the proposed
method, the theoretical relationship between the bar diameter and the power
ratio of the bar reflected signals acquired by perpendicular and parallel
polarized antennas is established via the inherent scattering width of the
metal bar and the wideband spectrum of the bar reflected signal. Based on the
theoretical relationship, the bar diameter can be estimated using the obtained
power ratio in a GPR survey. Simulations and experiments have been conducted
with different GPR frequency spectra, subsurface mediums, and metal bars of
various diameters and depths to demonstrate the efficacy of the method.
Experimental results show that the method achieves high sizing accuracy with
errors of less than 10% in different scenarios. With its simple operation and
high accuracy, the method can be implemented in real-time in situ examination
of subsurface metal bars.
