# Recognition of Rebar in Ground-Penetrating Radar Data for the Second Lining of a Tunnel
The dielectric inhomogeneity of the rebar buried in the concrete leads to different variations in the reflected wave response. This variation is the basis for the formation of a hyperbola in the B-scan of the rebar. As the GPR moves in the direction of the survey line, the two-way travel time and electromagnetic wave distance from the receiving antenna to the rebar will change. The reflected signal of the rebar is recorded in each A-scan. Each A-scan is then arranged and combined in chronological order to form a B-scan and to give the reflected signal of the rebar a curved shape, which is the main reason for the curved shape of the rebar in the B-scan [14]. Thus, based on the difference in electromagnetic properties between the rebar and the concrete, rebar recognition can be provided to offer a basis. When electromagnetic waves pass through different media, they are partially absorbed and reflected by the media [15; 16]. This phenomenon can cause significant changes in the waveform and electric field strength of the reflected waves from the medium. Therefore, the differences in electromagnetic properties between media are used to recognize buried objects through the time-frequency analysis of reflected waves. [PERSON] et al. [17] calculated the attenuation coefficients for different layers based on GPR signal attenuation and the electromagnetic property of the medium. The resistivity values were then assigned to the other layers based on the attenuation coefficient, resistivity and dielectric constant. In addition, envelope functions of traces previously corrected for geometrical effects were created to detect the interface of the medium and to generate local attenuation curves and radioactive fraction bands before calculating the attenuation coefficients. [PERSON] et al. [18] determined the position of the tunnel lining by using the attenuation and reflection coefficients. When an electromagnetic wave is transmitted from the air (or second lining) to the second lining (or first lining), the reflection coefficient is negative (i.e., the reflected wave is in the opposite phase of incidence). In the A-scan, the lining is located at inflection points determined by the reflection and attenuation coefficients of the electromagnetic waves in the transmission medium. The recognition of the tunnel lining layer is achieved by using these inflection points. [PERSON] et al. [19] studied the propagation of electromagnetic pulses in wind-formed sands with different water contents because GPR signals respond specifically to changes in water content during propagation. The results showed that the wind-formed sand's dielectric constant and conductivity increased nonlinearly with an increasing water content. The attenuation coefficient of the EMP increased parabolically. At the same time, the time waveform fluctuated significantly at the boundary of the different media. The two-way travel time increased nonlinearly with an increasing water content, and the pulse velocity decreased nonlinearly. In addition, the peak frequency of the spectrum of the signal propagating through the wind-formed sand decreased with an increasing water content. The reflection waveform could therefore be analyzed by using the attenuation and reflection coefficients [20; 21]. The rebar was then identified by reconstructing the hyperbolic features of the rebar in the B-scan. In this paper, we have determined in an A-scan that the intersection of rebar and concrete is located at the peak point, based on the fact that there are differences in the electromagnetic properties of concrete and rebar within the tunnel's lining structure. The reflection coefficient at the rebar-concrete interface is negative (i.e., the phase at the peak point is the opposite of that of the incident wave). The hyperbolic shape of the rebar is then reconstructed in a B-scan from these peak points containing the rebar location characteristics.