Geoscience Reference
In-Depth Information
C PT M a i n p i p e
PVC
Riser
4.6 m
4.6 m
4.6 m
16.8 m
N
fIGURe 29.2 The ElectroScience Laboratory (ESL) test plot utilized for studying ground-penetrating radar
(GPR) drainage pipe detection. GPR surveys were conducted within the dashed boundary.
by grain size analysis (Wray, 1986), is classified as silty clay. Figure 29.2 is a schematic showing
the layout of the ESL test plot, which was constructed with both clay tile and CPT drainage pipe
placed in 0.5 m wide trenches. Due to land slope, depth to the drainage pipe system on its northwest
corner was 1 m, and 0.6 m on the southeast corner. Shortly following backfill of the trenches where
the 10 cm diameter pipes were placed, the test plot was tilled down to a depth of 20 cm so that typi-
cal agricultural field conditions could be replicated. Two 10 cm diameter riser pipes (Figure 29.2)
connect the buried drainage pipe system to the surface, thereby allowing a shallow water table to
be maintained at any desired level through use of a water supply hose connected to a Hudson valve
suspended inside one of the riser pipes. The overall effectiveness of GPR for detecting buried agri-
cultural drainage pipe was evaluated at fourteen test plots (including the one at ESL), which ranged
in size from 200 to 12,000 m 2 , had different soil textures, and were located throughout central,
southwest, and northwest Ohio.
29.3 ReSUltS And dISCUSSIon
The radar signal penetration depth beneath the surface is governed to a large degree by the GPR
antenna center frequency. Radar signal penetration depth decreases as the GPR center frequency
is increased. Furthermore, as GPR center frequency increases, object resolution improves (better
imaging of smaller subsurface objects). Consequently, determining the proper GPR antenna fre-
quency based on a buried target's depth and size is extremely important.
Figure 29.3 shows the GPR response based on different antenna frequencies. The Electro-
Science Laboratory (ESL) test plot data for Figure 29.3 were collected under dry surface conditions
with the water table below the drainage pipes (based on field observations and measured monitor-
ing well water levels). Figure 29.3a and Figure 29.3b were obtained with a Noggin plus unit and 500
MHz center frequency antennas. Figure 29.3c and Figure 29.3d were obtained with a Noggin plus
unit and 250 MHz center frequency antennas. Figure 29.3e and Figure 29.3f were obtained with a
pulseEKKO 100A unit and 100 MHz center frequency antennas.
The Figure 29.3a, Figure 29.3c, and Figure 29.3e GPR profiles were produced from measure-
ments obtained along the same line, which was oriented perpendicular to the four north-south trend-
ing clay tile and corrugated plastic tubing (CPT) drainage pipes at the ESL test plot (Figure 29.2).
An upside-down U-shaped feature called a “reflection hyperbola” is the typical drainage pipe
response depicted on a GPR profile generated from data collected along a transect oriented perpen-
dicular to the drain lines. This reflection hyperbola response to the buried drainage pipes is shown
Search WWH ::




Custom Search