## 3 Results  
### Basic Data Processing and Interpretation  
The collected GPR data primarily depict the coupling of excitation and receiving antennas, known as \"direct wave\". In order to achieve a meaningful interpretation of underground media, data processing is often necessary based on the inherent characteristics of the data. We conducted fundamental data processing procedures on the gathered radar profiles, including direct wave removal (typically the initial step to mitigate air coupling effects), gain adjustment (usually the second step to amplify the signal for clearer target information representation), denoising techniques (such as band-pass filtering and median filtering, typically the third step to eliminate interference signals in the data), and deconvolution (generally the fourth step to compress radar wavelet and enhance resolution). The inline B-scan profiles and crossline B-scan profiles obtained after the initial GPR data processing are displayed in Figure 4a-c (inline 1-3) and Figure 5a-e (crossline 1-5), respectively. The legends show illustrations for various target bodies for reference. The radar waves in these profiles demonstrate consistent characteristics, revealing distinct layers such as a surface layer (approximately 10-30 ns), concrete base layer (approximately 30-60 ns), and a clay layer beyond 60 ns. GPR equipment excites electromagnetic waves to propagate downward in the form of spherical waves, and the shape function of the target body is a hyperbolic function in mathematics. Therefore, we generally observe the \"hyperbola\" in radar images to identify buried targets. However, due to the inhomogeneity of the underground medium, various deformations of the hyperbola form will occur in practical detection. Based on the surface observation records, we were able to identify the electromagnetic wave response in these profiles related to the majority of target bodies, as illustrated in Figure 2.  
Figure 3: Schematic view of survey line layout for GPR acquisition at the test site, with the legends providing illustrations of surface features for various target bodies for reference. We compiled a comprehensive list of target bodies, including those with markers or exposed on the surface, as detailed below. * **Manhole covers (square or round) and the rainwater catch basin** The presence of manhole covers of varying shape (square or round) and visible rainwater catch basins at ground level typically indicate the existence of underground pipeline networks for the collection and transportation of rainwater or sewage, as well as for access for maintenance and repair of underground pipe systems. The radar waves often illustrated chaotic multiples near the manhole covers over underground pipelines or rainwater catch basins, propagating downward multiple times and causing a discontinuity in events compared to adjacent sections in the GPR profiles. In Figures 4 and 5, we utilize rose dashed boxes to delineate the sections directly across the manhole covers, and employ black dashed boxes with a question mark at the bottom to indicate adjacent areas where underground pipes are suspected and which require further investigation. * **Cafeteria wall and the foundation below** In the profile of inline 1, there are horizontal events present (at approximately 35 to 40 ns, as shown in Figure 4a), and the inclined layers observed in all the crossline profiles in Figure 5 correspond to reflections from the wall and the foundation below. Figure 4: The inline B-scan profiles after basic GPR data processing: (**a**) inline 1, (**b**) inline 2, and (**c**) inline 3. * **Staircase**  
Near the staircase, at a distance of approximately 13 to 23.5 m from the starting point mentioned in inline 1, there is a lack of discernible reflections or anomalies in the shallower section compared to the previously mentioned horizontal events, indicating an absence of significant subterranean foundation beneath the staircase.