## 3 Results and Discussion
### Measured GPR Data  
In order to further verify the application of the above methods in practical engineering, several sets of laboratory tests were carried out in this study. The IDS-RIS GPR produced in Italy was used in the test. The antenna was selected as the ground-coupled TR-HF antenna and the frequency was selected as 1.6 GHz, which was chosen to provide higher resolution. Concrete specimens were used to simulate the base. The size of the specimens was 60 cm \(\times\) 10 cm \(\times\) 20 cm. Cylindrical cavities with diameters of 4 cm, 6 cm, 8 cm, and 10 cm were present. GPR detection was performed on each of these specimens. The test site is shown in Figures 12 and 13. Taking the defect model with a diameter of 6 cm as an example, the detected original B-Scan image and the processing results of the preprocessing and BP imaging are shown in Figure 14, where (a) is the original image, (b) is the preprocessing image, (c) is the image processed by the BP algorithm, and (d) is the result of the three-dimensional reconstruction. Figure 12: Laboratory test model. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Model No.** & 1 & 2 & 3 & 4 & Mean \\ _IoU_ & 0.650 & 0.687 & 0.721 & 0.735 & 0.698 \\
**Model No.** & 5 & 6 & 7 & 8 & Mean \\ _IoU_ & 0.624 & 0.668 & 0.677 & 0.697 & 0.667 \\
**Model No.** & 9 & 10 & 11 & 12 & Mean \\ _IoU_ & 0.674 & 0.696 & 0.715 & 0.729 & 0.704 \\
**Model No.** & 13 & 14 & 15 & 16 & Mean \\ _IoU_ & 0.564 & 0.591 & 0.654 & 0.705 & 0.629 \\
**Model No.** & 17 & 18 & 19 & 20 & Mean \\ _IoU_ & 0.534 & 0.593 & 0.612 & 0.628 & 0.592 \\
**Model No.** & 21 & 22 & 23 & 24 & Mean \\ _IoU_ & 0.563 & 0.587 & 0.609 & 0.644 & 0.601 \\
**Model No.** & 25 & 26 & 27 & - & Mean \\ _IoU_ & 0.636 & 0.675 & 0.704 & - & 0.672 \\
**Model No.** & 28 & 29 & 30 & - & Mean \\ _IoU_ & 0.621 & 0.643 & 0.685 & - & 0.650 \\ \hline \end{tabular}
\end{table} 
Table 5: _IoU_ of simulated defects. Figure 14: B-Scan images and processing results of laboratory test. Figure 13: IDS-RIS GPR. The central position and _loU1_ of the reconstructed defects are shown in Table 6. Compared with the results of the software simulation, the accuracy of the 3D reconstruction was somewhat reduced in the actual measurement process due to the more complex media composition and more significant interference from environmental noise. However, it was still within an acceptable range. Then, the above method was applied to the actual road for verification. The test road was an asphalt road located in Jinhua, Zhejiang, China, and the length of the section tested was about 1.07 km. The radar used for detection is shown in Figure 15, which had a detection depth of 8 m and a detection speed of 60 km/h. According to the field test results, the main structural defects of this road included cavities, cracks, and looseness. According to the 3D reconstruction algorithm proposed in this paper and relevant municipal data, the location and size of the structural defects in this road were further judged and analyzed. Taking the second lane on the right as an example, some results obtained from the detection and calculation are summarized in Table 7. An example of the 3D reconstruction results is shown in Figure 16.  
Considering the future traffic demand of the road, we drilled the core of only part of the area, as shown in Figure 17. Through these limited core samples, we found that due to the complexity of the actual road conditions, compared with the real defects, the algorithm found it more difficult to recover the shape of the defects, but the position and volume of the defects calculated by the algorithm were basically consistent with the actual situation, and the _loU_ could reach above 0.5.