Geoscience Reference
In-Depth Information
tAble 29.1
Summary of GpR drainage pipe detection Results for fourteen test plots across ohio
Average Soil
electrical
Conductivity (mS/m)
Amount
of pipe
located (%)
test
plot
Region within ohio
(Closest City or town)
Surface Soil
textural Class
1
1
Central Ohio (ESL-Columbus)
Silty clay
15.6
100
2
Central Ohio (Columbus)
Clay to silty clay
30.6
75
3
Central Ohio (Columbus)
Clay loam
20.5
50
4
Central Ohio (Columbus)
Clay loam
16.2
100
5
Central Ohio (Columbus)
Silty clay loam
9.6
50
6
Central Ohio (Columbus)
Silty clay
12.1
90
7
Southwest Ohio (Washington Courthouse)
Clay
28.0
0
8
Northwest Ohio (Defiance)
Clay
76.9
75
9
Northwest Ohio (Defiance)
Silty clay
76.8
100
10
Northwest Ohio (Defiance)
Sandy loam
15.9
100
11
Northwest Ohio (Delta)
Clay loam
22.6
100
12
Northwest Ohio (Delta)
Sandy clay loam to sandy
loam
12.0
100
13
Northwest Ohio (Lima)
Silty clay to silty clay loam
—
100
14
Northwest Ohio (Hoytville)
Silty clay
31.6
0
of 30°. The GPR response to the southeast-northwest drain line shown on the Figure 29.5b south-
to-north profile is highlighted with an upward pointing arrow and is represented by a reflection
hyperbola that laterally is slightly stretched compared to other reflection hyperbolas within the pro-
file representing drain lines oriented perpendicular to the measurement transect. The GPR response
to the southeast-northwest drain line shown on the Figure 29.5a west-to-east profile is again high-
lighted with an upward pointing arrow and is represented by a reflection hyperbola that has been
substantially stretched laterally. Consequently, as the orientation of the GPR measurement transect
relative to the drain line changes from 90° (perpendicular) to 0° (along trend), the GPR profile
drainage pipe response changes from a tight, narrow reflection hyperbola, to a laterally stretched
reflection hyperbola, to a banded linear feature.
The overall effectiveness of GPR drainage pipe detection was assessed at fourteen test plots
in central, southwest, and northwest Ohio using a Noggin
plus
unit with 250 MHz center frequency
antennas. The location, surface soil texture, average soil electrical conductivity, and the GPR drain-
age pipe detection effectiveness for each test plot are provided in Table 29.1. The textural class of
the surface soil (2.5 to 15 cm depth) was determined by particle size analysis (Wray, 1986). Average
soil electrical conductivity was calculated from 14,610 Hz electromagnetic induction measurements
taken at the test plots with a Geophex, Ltd. GEM-2 ground conductivity meter. Table 29.1 indicates
that GPR was successful in finding on average 74 percent of the total amount of pipe present at the
fourteen test plots. In seven test plots, 100 percent of the pipe was located, and in two test plots, none
of the pipe was found. All in all, the GPR method worked quite well in finding clay tile and cor-
rugated plastic tubing drainage pipe down to depths of around 1 m within a variety of different soil
materials. GPR was even proven capable of locating buried drainage pipes in silty clay and clay soils
having extremely high average electrical conductivity values above 75 mS/m, which would typically
be expected to severely limit radar signal penetration depth (test plots 8 and 9). It is still unclear as
to why GPR detected none of the drainage pipe in test plots 7 and 14.
