# Analysis of Small-Loop Electromagnetic Signals to Detect Subsurface Anomaly Zones
## 3 Test Plan  
### Exploration Targets in the Testbed  
In 2017, the Korea Expressway Corporation constructed a testbed with a width of 5-6.6 m and a total length of 120 m in its road safety facility to evaluate nondestructive exploration techniques for road pavement subsurfaces. Various types and sizes of objects were embedded under asphalt pavement, concrete pavement, and an approaching slab in the testbed. Subsurface exploration was performed on eight asphalt pavement sections. The sections were selected as asphalt pavement is the main urban pavement type. The asphalt pavement section was comprised of the following layers: 50 mm of surface asphalt, 70 mm of intermediate asphalt, 150 mm of an asphalt base, 200 mm of an aggregate subbase, and 130 mm of an anti-freezing aggregate [17].  
The objects embedded in each testbed section are listed in Table 1. Cavities, areas of ground softening, and underground facilities were selected as the target objects as they represent the main causes of sinkholes. The ground where the target objects were placed was compacted until the degree of compaction returned to the original state. The use of polystyrene to simulate cavities in the ground was deemed appropriate as its dielectric constant is similar to that of air [18]. An empty plastic bottle was also used to simulate a cavity as it has similar insulation characteristics to air. Asphalt and concrete blocks were used to represent objects in the ground with insulation characteristics. In addition, surveys were performed on conductive objects, such as a bottle filled with water, metal, and reinforced concrete drainage pipe.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Section No.** & **Facility** & **Abbreviation** & **Size (m)** & **Shape** & **Depth (m)** \\ \hline \hline
1 & Natural Ground & - & - & & - \\ \hline
2 & Sand and Polystyrene Particles & SP & \(2\times 1.8\times 0.5\) & Cuboid & 0.55 \\ \hline
3 & Polystyrene Hemisphere & PH & R 0.5 & Hemisphere & 0.45 \\ \hline
4 & Polystyrene Hemisphere & PH & R 0.5 & Hemisphere & 0.45 \\ \hline \hline \multirow{4}{*}{5} & Bottle Filled with Water & BW & \(0.5\times 0.5\times 0.1\) & Cuboid & 0.35 \\ \cline{2-6}  & Empty Bottle & EB & \(0.5\times 0.5\times 0.1\) & Cuboid & 0.35 \\ \cline{2-6}  & Concrete Block & CB & \(0.5\times 0.5\times 0.1\) & Cuboid & 0.35 \\ \cline{2-6}  & Asphalt Block & AB & \(0.5\times 0.5\times 0.1\) & Cuboid & 0.35 \\ \hline
6 & Steel Plate & ST & \(0.5\times 0.5\times 0.1\) & Cuboid & 0.55 \\ \hline \multirow{4}{*}{7} & Empty Bottle & EB & \(1\times 1\times 0.1\) & Cuboid & 0.55 \\ \cline{2-6}  & Concrete Block & CB & \(1\times 1\times 0.1\) & Cuboid & 0.55 \\ \cline{2-6}  & Asphalt Block & & \(1\times 1\times 0.1\) & Cuboid & 0.55 \\ \hline
8 & Drainage Pipe & & \(3\times 0.9\times 0.9\) & Cuboid & 1.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Facilities installed in the ground of the testbed.  
Figure 2 shows the side views of the eight target sections, each of which was 5 m in length. The natural ground under the asphalt pavement of Section 1 did not have any cavities, areas of softening, or underground facilities. Objects of various sizes and materials were used to simulate the cavities, areas of softening, and underground facilities and were installed at various depths in Sections 2-8.