## 2 Methodology
### Dataset Description
#### 2.1.2 Constructing Dataset  
At present, most of the research work has only focused on a single type of highway distress, such as cracks [16], debonding [17], and moisture damages [18], which are relatively easy to identify. The task of categorizing interlayer distresses and intralayer distresses (i.e., structural loosening), as well as subdividing interlayer distress into interlayer debonding, interlayer loosening, and interlayer water seepage, posed a significant challenge in this study. From the perspective of road maintenance, the main reasons for subdividing highway distress into multiple categories are as follows:  
1. Assessing the severity of distress: different categories of distress affect the highway structure to different degrees, which ultimately affects the repair methods, measures, and costs;
2. Statistics and analysis: statistics on the frequency and distribution of various categories of distress can help formulate more effective maintenance plans and prevention strategies;
3. Data recording and management: establishing a detailed database of distress helps in distress prediction and highway maintenance. GPR uses electromagnetic waves to detect the internal structure of highways. Electromagnetic waves are reflected at the interface of different media due to changes in their dielectric constants. The amplitude of the reflection is stronger when the difference between the dielectric constants of the two media is greater. The image characteristics of the GPR B-scan reflect the health of highway structures and different categories of highway distress. By combining the radar images with the drilled sample cores, anomalous areas in the GPR B-scan images can be analyzed to achieve qualitative and quantitative interpretations of different categories of distress. The images and sample cores of the four categories of distresses are shown in Figure 2. The following is a brief description of the various distresses included in the field data:  
Figure 1: Map of the data collection area. Using the highways in Jiangsu Province as an example shows the highway structure in the area. * Interlayer debonding: There is a continuous separation between the layers of the road structure, and interlayer delamination occurs. This type of distress, known as interlayer debonding (abbreviated as poor_l), typically results from interlayer bonding due to natural aging of the highway, excessive loading, or other factors. * Interlayer water seepage: This type of distress (abbreviated as water_l) occurs when a highway that already has the interlayer debonding and other less serious distress is not maintained in time and is infiltrated by rain. Unlike other air-containing distresses, this type of distress contains water. The dielectric constants of water and air differ significantly (usually 81 for water and 1 for air). Therefore, Figure 2 shows a clear polarity reversal in the interlayer water seepage compared to the interlayer debonding. * Interlayer loosening: This kind of distress is characterized by nibbling and loosening of the interlayer bonding surface. Interlayer loosening (abbreviated as loose_l) occurs when interlayer debonding, interlayer water seepage, or other relatively light distresses are still not repaired in a timely manner and are constantly crushed by vehicles. * Structural loosening: This type of distress is characterized by fractures and looseness in highway structural layers. Structural loosening (abbreviated as loose_s) is the most serious class of these four categories of distress. To achieve accurate distress classification, experienced domain experts labeled the distress and verified the labels with drilled core samples.