## 3 GPR System and Data Processing  
Ground-penetrating radar is a system that operates by emitting electromagnetic (EM) waves into the subsurface. The computer system then generates an image by analyzing the radar pulses reflected from objects inside the media being analyzed. These waves are transmitted by the transmitting antenna and captured by the receiver when embedded objects reflect them. Civil engineering applications typically have a wave frequency between 20 MHz and 2.5 GHz. Two crucial physical properties influence the propagation of the EM waves in the subsurface: the dielectric constant of the EM waves and the electrical conductivity of the subsurface.  
The conductivity of the subsurface plays a pivotal role in the propagation and attenuation of the EM pulses. Low-conductivity materials, such as dry sand, ice, and dry concrete, are the perfect candidates for GPR. They facilitate the EM waves to travel to greater depths with minimal energy loss. On the other hand, high-conductive materials, like saltwater and wet concrete, absorb radar energy before the EM pulses can penetrate deep into the subsurface. This precision makes GPR an ideal method for examining materials like dry concrete, wood, sand, and asphalt, instilling confidence in its capabilities.  
Along with the materials' conductivity, the dielectric constant is a critical physical property in the analysis of the GPR data. The dielectric constant determines the speed at which EM pulses travel into the subsurface. Given the velocity of electromagnetic waves, multiplying the two-way travel time from the transmitter to the receiver by the electromagnetic speed provides the object's depth in the material. Electromagnetic waves propagate through air at a speed nearly equal to the speed of light, corresponding to a dielectric constant of 1, while, in water, their velocity is approximately one ninth that of light, with a dielectric constant of 81. For most construction materials, wave velocities range from 3.5 to 7.0 inches per nanosecond, corresponding to dielectric constants between 12 and 3 [36].  
The GPR data are collected and stored in a two-dimensional matrix, representing the signal reflection based on depth and the horizontal position of the radar. In the radargram, the signal is visualized as a color spectrum transitioning from dark blue to dark red on the jet-color scale, corresponding to signal amplitudes ranging from minimum negative to maximum positive.  
When an electromagnetic wave is emitted into the material, it travels at a constant dielectric speed until it encounters a material with a different dielectric constant, such as rebars, moisture, delamination, and air voids. This difference in material properties is reflected in the receiver and detected by the system. The more exponential the difference between the two materials is, the stronger the reflected waves will be. Rebars in concrete,for instance, show intense reflections because of the conductivity difference between steel and concrete.  
The GPR system (GSSI SIR-4000) (Geophysical Survey Systems, Inc. (GSSI), 40 Simon Street, Nashua, NH 03060-3075, USA) used in this research is a comprehensive setup, comprising a 1.6 GHz antenna with transmitter and receiver spacing (T-R offset) of 2.3 in. The system used in this paper is illustrated in Figure 1. To ensure data accuracy, a \"background removal\" method was employed to eliminate surface deflection noise (i.e., direct coupling) pulses.