## 3 GPR Image Characteristics Analysis
### Model Parameters and Validation  
A layered highway model, including surface, base, and subgrade layers, was established based on the actual conditions. Different model parameters were set for each layer, as shown in Table 1, and Figure 4 illustrates the subgrade model, which includes loose subgrades, voids, and cavities.  
Figure 4: Subgrade defect model.  
Figure 3: YOLO target detection network architecture.  
Using the above parameters, a layered structure model of a highway subgrade without defects was established and subjected to forward simulation and rationality verification. The model's 2D cross-section and simulation results are shown in Figure 5. In the defect-free subgrade structure, only dielectric constant differences existed between the subgrade layers, with no abnormal bodies. Electromagnetic pulse waves passed through the air layer, surface layer, base layer, and subgrade in sequence, with the relative dielectric constants increasing progressively. The electromagnetic waves were reflected and refracted at the interfaces between the subgrade layers due to dielectric property differences, and the layer interfaces were clear. Strong reflection waves were generated at the air-surface layer interface, with an electric field intensity range of \(-443.1\) to \(318.62\) V/m. After passing through the air layer, an abnormal waveform appeared at a single travel time of \(2\) ns, with an electric field intensity range of \(-66.0\) to \(96.5\) V/m. The calculated propagation speed within the surface layer was approximately \(0.15\) m/ns, matching the \(0.3\) m surface layer structure. Subsequently, an abnormal reflection wave appeared at a single travel time of \(4\) ns, with an electric field intensity range of \(-6.2\) to \(7.6\) V/m. The calculated propagation speed was approximately \(0.1\) m/ns, consistent with the \(0.4\) m base layer thickness. The simulation results matched the theoretical calculations, confirming the reliability of the model calculations.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Left Lower** & **Upper Right** & **Thickness** & **Relative Dielectric** & **Conductivity** \\  & **/m** & **/m** & **/m** & **Constant** & **(S/m)** \\ \hline Model space & (0, 0, 0) & (\(6,2,0.002\)) & 2 & \(\backslash\) & \(\backslash\) \\ Surface of pavement & (0, 1.4, 0) & (\(6,1.7,0.00\)) & 0.3 & 4 & 0.001 \\ Base course of pavement & (0, 1, 0) & (\(6,1.4,0.00\)) & 0.4 & 9 & 0.05 \\ Subgrade & (0, 0, 0) & (\(6,1,0.002\)) & 1 & 10–30 & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Spatial parameters of highway subgrade pavement layer model.  
Figure 5: Forward simulation results of non-defect highway base model.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Space Size** & **Spatial** & **Time** & **Excitation** & **Frequency** & **Emission** & **Receiving** & **Step** \\ **/m** & **Discretization/m** & **Window/s** & **Source** & **/MHz** & **Source/m** & **Source/m** & **Distance/m** \\ \hline \(10\times 2\times 0.002\) & (0.002, 0.002, 0.002) & \(3\times 10^{-8}\) & Ricker & 1000 & (1.0, 1.75, 0) & (1.2, 1.75, 0) & (0.02, 0, 0) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Forward simulation parameters of GprMax.