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FIGURE 29.3 Ground-penetrating radar (GPR) profiles from ElectroScience Laboratory (ESL) test plot showing the drainage pipe detection effect of antenna frequency; (a) and (b) 500 MHz center frequency antennas, (c) and (d) 250 MHz center frequency antennas, and (e) and (f) 100 MHz center frequency antennas.

FIGURE 29.3 Ground-penetrating radar (GPR) profiles from ElectroScience Laboratory (ESL) test plot showing the drainage pipe detection effect of antenna frequency; (a) and (b) 500 MHz center frequency antennas, (c) and (d) 250 MHz center frequency antennas, and (e) and (f) 100 MHz center frequency antennas.

to a greater or lesser extent in Figure 29.3a and Figure 29.3c with refection hyperbola apexes (top of pipe position) found at travel times between 20 and 25 ns. The Nogginplus unit with 500 MHz center frequency antennas barely detected three of the four pipes, of which one is highlighted with an upward pointing arrow (Figure 29.3a). The Nogginplus unit with 250 MHz center frequency antennas detected all four pipes (Figure 29.3c), one of which is highlighted with an upward pointing arrow; although, the response to the one farthest west was fairly subdued. The pulseEKKO 100A unit with 100 MHz center frequency antennas detected all four pipes (Figure 29.3e); however, the GPR response to the buried drainage pipes was not the typical reflection hyperbola expected, but rather a rectangular block-like extension of the top white band down into the black band beneath it (one is highlighted with an upward pointing arrow). The difference in GPR drainage pipe response of the pulseEKKO 100A unit with 100 MHz center frequency antennas compared to the other two systems is at present unclear but may be related to reduced object size resolution and interference with radar pulses traveling directly through the air and along the ground surface.

The Figure 29.3b, Figure 29.3d, and Figure 29.3f GPR profiles were produced using measurements obtained from the same ESL test plot line, which was oriented directly along the trend of one of the buried north-south CPT drainage pipes (the second drain line from the east in Figure 29.2). The typical GPR response in this scenario is a banded linear feature representing the buried drainage pipe. The position of the top of the banded feature corresponds to the top of the buried drain line. The banded linear feature highlighted by the arrows is somewhat subtle on the profile generated using a Nogginplus unit with 500 MHz center frequency antennas (Figure 29.3b). In comparison, the banded linear feature for the buried north-south drainage pipe shows up quite well (highlighted by the arrows) on the Figure 29.3d profile generated from data collected using a Nogginplus unit with 250 MHz center frequency antennas. Figure 29.3d also shows a strong reflection hyperbola on the south end of the banded feature and a subtle one on the north end, both of which represent the CPT main pipe connected at each end of the drainage line (see Figure 29.2). Finally, the response of the pulseEKKO 100A unit with 100 MHz center frequency antennas was again different. Instead of a distinct banded linear feature, the arrow highlighted drain line position is shown by a long rectangular extension of the top white band down into the black band directly beneath it (Figure 29.3f).

Choosing the proper antenna frequency based on the subsurface depth and size (diameter) of the drainage pipe is an extremely important consideration. Overall, taking into account different drain line orientations with respect to a GPR transect, the 250 MHz center frequency antennas appeared to work best for detecting buried agricultural drainage pipe at depths of up to 1 m. For larger diameter pipes at greater depth, perhaps antennas with a 100 MHz center frequency are the best option.

As the water content of a soil increases, so too does its electrical conductivity, and as soil electrical conductivity increases, radar signal penetration depth decreases. However, with regard to drainage pipe detection, this adverse GPR impact due to increased soil wetness, could potentially be offset if there is a greater amount of radar energy reflected from the drainage pipe due to the nature of the dielectric constant contrast between the soil outside the pipe and the air and water inside the pipe. Large rainfall events in the Midwest United States increase wetness within the soil profile by increasing water contents near the surface and sometimes causing a rise in the shallow water table.

The influence of shallow hydrologic conditions on GPR drainage pipe detection is displayed in Figure 29.4. The data for Figure 29.4 were collected using a Nogginplus unit with 250 MHz center frequency antennas. The Figure 29.4a and Figure 29.4d GPR profiles were generated from measurements obtained along one line, which was oriented perpendicular to the four clay tile and CPT north-south trending drainage pipes at the ESL test plot (Figure 29.2). The Figure 29.4b and Figure 29.4e GPR profiles were produced from data obtained from one ESL test plot line measurement transect, which was oriented directly along trend over a buried north-south CPT drainage pipe (second drain line from the east in Figure 29.2). The ESL test plot GPR amplitude maps, Figure 29.4c and Figure 29.4f, represent the amount of reflected radar energy from a 15 ns time window bracketing the drainage pipe positions. Lighter shaded linear features shown on the GPR time-slice amplitude maps indicate drainage pipe patterns.

Figure 29.4a through Figure 29.4c correspond to shallow hydrologic conditions with a wet surface from a recent (<18 h) rainfall of 7.8 mm and a water table raised 0.5 m above the drainage pipes. Figure 29.4d through Figure 29.4f correspond to shallow hydrologic conditions with a very moist soil profile and pipes totally drained of water. The shallow hydrologic conditions for Figure 29.4d through Figure 29.4f were obtained by continually pumping water from the drainage pipes and lowering the water table for 24 h prior to the GPR field survey. Essentially, Figure 29.4a, Figure 29.4b, and Figure 29.4c are representative of shallow hydrologic conditions with a wet soil profile and water-filled pipes, and Figure 29.4d through Figure 29.4f are representative of shallow hydrologic conditions with a wet soil profile and air-filled pipes.

Figure 29.4a through Figure 29.4c indicate that the poorest shallow hydrologic condition in regard to GPR drainage pipe detection occurs with a wet soil surface and a static water table located

0.0 2.5 5.0 7.5 10.0 12.5 15.0 Line Distance (m) (a)

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