*2.2. Surveying Equipment*

A KRRI (Korea Railroad Research Institute, Uiwang-si, South Korea) GPR device with an operating frequency of 500 MHz, developed for ballast track surveys, and commercial GSSI (Geophysical Survey Systems, Inc., Nashua, NH, USA) devices, with operating frequencies of 400, 900, and 1600 MHz, were used for laboratory tests. The KRRI GPR device was developed for field surveys, especially of ballast tracks. Accordingly, the KRRI GPR device was used for verification tests on the Gyeongbu high-speed railway [11].

In order to avoid the boundary effects of the ballast boxes, the centerlines of the boxes were surveyed. The test setup and operation using GSSI and KRRI devices are illustrated in Figures 3 and 4, respectively. The KRRI device has eight channels, four on the left four on the right. Thus, in order to scan the centerline only one side antenna (channel-0) was used during the lab test (pleases see Figure 4b).

**Figure 4.** (**a**) KRRI GPR device; (**b**) KRRI ballast survey in the lab; and (**c**) KRRI ballast survey on the Gyeongbu HSR line.
