Geoscience Reference
In-Depth Information
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 measure-
ments 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 drain-
age 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 gener-
ated using a Noggin
plus
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 Noggin
plus
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 rectan-
gular 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 diam-
eter 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 elec-
trical 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 Noggin
plus
unit with 250 MHz center
frequency antennas. The Figure 29.4a and Figure 29.4d GPR profiles were generated from measure-
ments 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 sur-
face 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 low-
ering 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
