• Changes in reflected signals from the same underground objects according to the arrangement of transceiver antennas used in ground penetrating radar survey are analyzed.

• We demonstrated that the transmitter and receiver antennas perpendicular to the survey line are the most effective ways to show the underground structure.

Ground-penetrating radar (GPR) is a geophysical survey technique, in which radar pulses propagate through the subsurface and are reflected back at the boundaries where subsurface physical properties change, or geophysical research method which uses reflection and diffraction properties of electromagnetic waves caused by the difference in permittivity between two media. Since the GPR antenna has a high frequency range of 25 MHz to 1 GHz and uses a short wavelength, it gives high-resolution details and thus enables high-resolution subsurface imaging. It is therefore an effective tool in examining pipes (Allred, 2013) and artificial structures buried underground or shallow underground structures (Baek et al., 2017; Kim et al., 2017). It can also be used for research in various fields such as sediments (Bristow et al., 2003), high-resolution mapping of soil and rock stratigraphy (Davis et al., 1989), mining (Kemp et al., 2009), dam safety analysis (Kim et al., 2007), glacial hazard assessment (Reynolds, 2006), and shallow subsurface survey in contamination areas (Reynolds et al., 1992).

The higher the water content of soil, the higher its dielectric constant (Ercoli et al., 2018; Schmalz & Lennartz, 2002). This paper assumes that the higher the dielectric constant, the faster the radar wave velocity (m/ns). Accordingly, if a drainage pipe is saturated with water and its dielectric constant is larger than that of the surrounding soil (Reynolds & Taylor, 1992), the GPR response gets stronger when the antenna is oriented parallel to the direction of the survey direction, and clearer results can be obtained. Most farmland is saturated with water and drainage pipes are often partially filled with water, and better results can thus be obtained if the GPR antenna is configured parallel to the drainage pipe.

The most widely used antenna configuration for GPR survey is the perpendicular-broadside (PR-BD) configuration where the antenna and the survey line are orthogonal (Allred, 2013; Seol et al., 2000). In this configuration, however, sensing becomes difficult when the survey line and reflecting surface have the same dominant orientations, whereby the reflecting surface has a high incident angle (≥ 60 degrees). In this case, the reflecting surface can be detected by using the XX configuration in which the antenna and the survey line are configured in parallel (see Figure 1). Moreover, when the reflector is relatively horizontal (≤30 degrees), higher amplitudes were yielded in the YY configuration compared with the XX configuration when the survey line and the reflector were configured parallel to each other. This explains why the YY configuration is most widely used in a GPR survey. Consequently, in the XX and YY configurations, the slope of the reflector has a great effect on reflection energy, and in the XY configuration, the reflection energy is influenced more strongly by the dominant orientation of the reflector than by to the slope of the reflector. Therefore, in order to achieve higher accuracy in the survey of underground structures, it is necessary to use two or more antenna configurations rather than a single antenna configuration. Apart from this, it is necessary to choose a survey method best-suited for the situation (Allred, 2013; Seol et al., 2000). A GPR antennas usually has radiation pattern and polarity. This explains why the energy reflected from the same reflector varies depending on the direction of the transmitting/receiving antenna configuration. In the case of the sewer targeted for this experiment, the slope is less than 30 degrees, so theoretically, and YY configuration is the most suitable antenna configuration, which was confirmed by the experi-mental results.

Figure 1. Typical GPR antenna configurations (Allred, 2013). (a) Both perpendicular to the survey line (perpendicular-broadside, PRBD), (b) both parallel to the survey line (parallel-broadside, PL-BD), (c) serially perpendicular to the survey line (parallel-endfire, PL-EF), (d) serially parallel to the survey line (perpendicular-endfire, PR-EF), and (e) intersecting each other at the right angle (cross polarization, XPOL).

In this study, comparative analysis was conducted on the changing patterns of the signals reflected back from underground objects in GPR survey depending on the transmitting/receiving antenna configuration.

The experiment was conducted with the pulseEKKO PRO GPR system (Sensors & Software Inc., Mississauga, ON, Canada), which can be used with an antenna with a wide frequency range (12.5 MHz–1000 MHz) and has depth-dependent high spatial resolution and a low noise level. Data can be collected directly from the transmitting and receiving antennas. Furthermore, it shows low energy loss. As mentioned earlier, this system can be applied in many different fields such as stratigraphy, mining, drainage pipe survey, construction site, ice thickness measurement, and concrete or pavement surfaces. Antenna used with pulseEKKO PRO can be divided into two categories of interchangeable antennas with bandwidths of 12.5, 25, 50, 100, 200 MHz and broadband antennas with 250, 500, 1000 MHz. We used interchangeable 200 MHz transmitting and receiving antennas. Antenna frequency determines the resolution for its physical length and observable resolution. In general, 1/4 of the antenna length is regarded as the maximum resolution. The length and spatial resolution of a 200 MHz antenna are 0.5 m and 0.125 m, respectively. Antenna separation and antenna step size best-suited for showing adequate results are preconfigured, but the configurations can be modified. Antenna separation should exceed the antenna size, and excessive information influx results from a smaller separation. Accordingly, we configured the antenna separation at 0.5 m, which is the initial value of a 200 MHz antenna. We also used the recommended basic antenna step size (i.e., 0.1 m).

GPR experiment was conducted in a road located at Chungnam National University, Yuseong-gu, Daejeon, South Korea (Figure 2). We measured along a 10 m line on the asphalt in the direction perpendicular to the sewer pipe buried underground (see red line in Figure 2b), and a 5 m line parallel to the sewer pipe (black line in Figure 2b). Given that the main purpose of this experiment is to observe the different signal patterns reflected from the same underground object depending on the antenna configuration, the experiment was conducted in five different antenna configurations (Figure 1) along the horizontal and vertical lines, respectively, in the same place.

Figure 2. Test survey site located at Chungnam National University, Daejeon, Korea: (a) Road view, (b) Schematic diagram of the survey lines.

Among many different survey modes used for GPR survey, typically adopted methods are reflection, common mid-point (CMP) (Feng et al., 2009), wide-angle reflection and refraction (WARR) (Annan & Jackson, 2017), and transillumination. In the reflection method, which is most widely used for stratigraphy, transmitter and receiver are placed at a fixed interval and survey is done at regular step size. CMP and WARR are useful methods for estimating the information of depth-dependent wave propagation velocity, whereby CMP survey is conducted by spreading at a regular distance from the mid-point and WARR survey is conducted by moving the receiver gradually away from the transmitter at a fixed position. The transillumination method, which is similar to the reflection method using transmitted and reflected waves, respectively, is useful for inspecting, for example, cracks in a building. We conducted GPR survey using reflection method (Prego et al., 2017).

For the experiment, we configured the transmitting and receiving antennas as five different models (Figure 1), which were utilized in the directions parallel and perpendicular to the sewer pipe, respectively, and hence resulting ten data sets. The collected data were subjected to post-processing correction using EKKO_Project V3 R2 (Sensors & Software Inc.). Since the data are influenced by the electrical properties of the ground and the proximity of the transmitting and receiving antennas, signaling can induce “wow”, low-frequency elements, under the influence of high frequencies. Therefore, we removed unnecessary noises using the De-wow filter as pre-processing of most GPR data. Then we optimized the data for analysis by increasing the dipping events at the center of a hyperbola and removing strong reflected waves and horizontal reaction line at the same time, using a background average subtraction (BAS) filter (Forte & Pipan, 2017).

Figures 3 and 4 show post-processed GPR subsurface images in five antenna configurations in the directions perpendicular and parallel to the sewer pipe. The images obtained from the survey perpendicular to the sewer pipe (Figure 3) show multiple hyperbolic curves which vary in width and slope depending on the radar wave velocity in the subsurface medium. Whereas the topmost black curves are slightly different from image to image, the black curves underneath show similar shapes. This allows the interpretation that the radar wave velocity was measured similarly along the upper layers of the sewer pipe. In general, the radar wave velocity on an asphalt pavement surface was measured in a very narrow range of 0.101–0.109 m/ns. Unlike all other models, the XPOL model did not show anything noticeable.

Figure 3. Processed radargram acquired along the perpendicular line to the underground pipe.

Figure 4. Processed radargram acquired along the parallel line to the underground pipe.

The surveys parallel along the sewer pipe (Figure 4) using the same five antenna configurations yielded clearly different results from those yielded by the survey perpendicular to the sewer pipe, but the differences were insignificant except for the XPOL model as was the case with the survey perpendicular to the sewer pipe.

In order to investigate the reason for the different topmost black hyperbolic curves for the same sewer pipe in the image data from the survey perpendicular to the sewage pipe, we performed simulation using gprMax2D program (Giannopoulos, 2005) (Figure 5) considering two cross-sections of sewer pipe: circle and square. As a result, it was found that the deeper-layer curves have similar shapes with our processed observation and the those of upper layer showed broader and flattened shapes in the case of square cross-section. Similar curve shapes were shown in the PL-EF and PR-EF models with the square cross-section, i.e., broader and flattened upper-layer curves. The flattened parts may be interpreted as errors or as reflecting the slightly angular shape of the upper part of the sewer pipe.

Figure 5. Simulated radargram. (a) Circular and rectangular bodies for simulation. (b) Simulated radargrams using gprMax2D (Giannopoulos, 2005).

The most widely used transmitting/receiving antenna configuration is the PR-BD model. In general, reflected energy patterns vary depending on the slope and main orientation of the object buried underground in GPR survey as well as the antenna configuration. In this experiment, 200 MHz antennas were used to detect an almost horizontally installed sewer pipe. The XPOL model was outperformed by all other configuration models, and PL-BD yielded better results than PR-BD. The underground depth of the sewer pipe was calculated to be 1.2 m by applying the radar wave velocity using the measured hyperbolic curves.

The measurement results in the GPR survey of the sewer pipe in the direction parallel to the sewer pipe revealed that the sewer pipe was observed as a boundary surface, not as a standalone obstacle. As shown in the experiment in the perpendicular direction, a sewer pipe was observed at the depth of 1.2 m. Likewise, no clear reflected pattern appeared in the XPOL model. In the PL-EF model, the sewer pipe appeared in a slightly lower position compared with other models. This is presumably due to the fact that the antenna configurations of PR-BD, PL-BD, and PR-EF measured the distance along the survey line over the topmost center part of the sewage pipe, whereas the PL-EF configuration measured the distance along the side of the survey line, resulting in reflection from the side area of the sewer pipe with circular cross-section and displaying the position of the sewer pipe at a slightly lower place than in other models.

The XPOL configuration model (Figure 6) showed the same results when the directions of the transmitting and receiving antennas were switched, regardless of XY or YX, because they mutually satisfy the reciprocity principle. The XPOL model did not yield any results along both perpen-dicular and parallel GPR survey lines. According to a previous study (Seol et al., 2000), in the XPOL configuration, in which the transmitting and receiving antennas intersect each other at the right angle, reflected energy is not influenced by the slope of the underground object, but has specific characteristics according to the main orientation of the reflector. Whereas no noticeable reflected energy was measured if the survey lines are set perpendicular or parallel to the targeted underground object, as in the above experiment, maximum reflection can be obtained at the incident angles of 45° and 135°.

Figure 6. Survey configuration of XPOL at the incident angles of 45° and 135°.

Figure 7 displays the results obtained by GPR survey at the incident angles of 45° and 135°, which support the hypothesis presented in previous research. All data underwent De-wow filtering and optimization process using background average subtraction, as did all initial data. The depth of the sewer pipe was measured at ~1.2 m and the hyperbola-based radar wave velocity calculation was in the range of 0.101-0.109 m/ns, similar to that measured for asphalt pavement. The processed radargrams from the XPOL con-figuration exhibits the distinct hyperbola signals associated with the buried pipes and easily discernable from the background, compared to the other configurations (Figures 3 and 4). This indicates the maximum reflection from the buried target was obtained along the given survey lines (Figure 6).

Figure 7. Radargrams acquired using XPOL configuration with (a) 45° and (b) 135° angles with respect to the buried pipe.

We investigated the characteristics of radar reflected waves that vary depending on antenna configuration, employing five different experimental models. The perpendicular-broadside (PR-BD) configuration is the most widely used transmitting and receiving antenna configuration in ground-penetrating radar (GPR) survey, and the PR-BD model showed the clearest subsurface images in our experiment. In the case of the cross polarization (XPOL) configuration, reflected energy was not influenced by the slope, but showed specific characteristics according to the main orientation of the reflector. Therefore, better results can be obtained by employing all three configurations, PL-BD, parallel-broadside (PR-BD), and XPOL, when conducting GPR subsurface survey.

This work was supported by Chungnam National University. We thank two anonymous reviewers for their thorough and constructive comments that improved the manuscript greatly.