141
Sustain. Environ. Res., 23(2), 141-152 (2013)
Identification and positioning of underground utilities using
ground penetrating radar (GPR)
Nga-Fong Cheng,* Hong-Wai Conrad Tang and Ching-To Chan
Department of Land Surveying and Geo-informatics
The Hong Kong Polytechnic University
Hunghom Kowloon, Hong Kong
Key Words: Ground penetrating radar, utilities, as-built positions, accuracy
ABSTRACT
Ground penetrating radar is one of the non-destructive methods useful in locating underground facilities.
Little researches had been investigated about the significances of measuring pipes and cables in Hong Kong.
In this research, general procedures on how to design the grids and to conduct the radar surveys were
reviewed. The main objective was to compare the radar results with the conventional survey methods in
three trial sites. Finally, it was crucial to evaluate the use of antenna frequency and the accuracy of the
obtained data.
.
INTRODUCTION
Utility surveying, according to the Survey Association [1], refers to the location, positioning and identification of buried pipes, cables and ducts irrespective
of their sizes, depths, material types and proximity to
other utilities using numerous techniques or technologies, so as to effectively facilitate planning, design and
excavation of work. Ground penetrating radar (GPR,
also known as ground probing radar, or georadar), one
of the non-destructive methods gaining importance
nowadays, is useful in locating underground pipes and
cables. Several researches had been done to detect the
existence and the depth of the underneath objects
[2-4], yet little pointed out the significances of getting
the positions of underneath utilities. Everyone may not
aware there are 7.1 x 104 km of underneath utility
systems installed around 1,900 km of roads in Hong
Kong, mostly in pavements, cycle tracks or amenity
strips [5]. Such subsurface facilities are progressively
becoming one of the important components of cosmopolitan infrastructure for engineering, environmental
and geotechnical purposes in this congested city. Many
incidents on damaging buried utilities such as the
Kwun Lung Lau event in July 1994 and the Shenzhen
gas leakage event in June 2005 had been reported [6].
To avoid the above accidents, there is an urgent need
for surveyors to accurately measure their respective
positions. In addition to that, few investigations related
to the shape and size of utilities had been mentioned in
*Corresponding author
Email: chengnfalice@yahoo.com.hk
.
Tong's studies [4], but not in Hong Kong.
This paper aimed to compare the radar survey results under certain circumstances types of utilities, site
conditions, and frequency antenna. To start with, some
introductions about the GPR and existing utilities in
Hong Kong were briefly discussed. General procedures
on how to design the grids and to carry out the surveys
were then examined in this study. As extracted both
as-built utilities as well as ground existing features
from engineering drawings supplied by the Civil
Engineering and Development Department (CEDD) of
the Hong Kong Government, the following parameters
like shapes, sizes together with positions can be easily
defined in post-processed radargrams, and the comparisons between the measurements and as-built positions of the utilities can be made. Finally, it is also
crucial to evaluate the use of antenna frequency and
the accuracy of the obtained data.
.
OVERVIEW OF GPR
The definition of GPR is “a range of electromagnetic techniques designed primarily for the location of
objects or interfaces buried beneath the earth's surface
or located within a visually opaque structure” [7]. It is
one of the geophysical techniques with electromagnetic waves to identify underlying structures, in particular plastic pipes or fiber cables. Its history can be
traced back to the 1920s when the GPR was used to
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
determine the ice thickness in Germany [8]. Thereafter,
the radar has been nearly forgotten. Not until 1950s
researchers once again continued to conduct the ice
research and people started to develop it for other
applications, e.g., soil and rock analysis [9,10]. The
first GPR system, designed and built by the National
Aeronautics and Space Administration was solely
served for the moon landing mission in 1967 [11]. In
1994, the data analyzing software was initially built in
Japan [11]. In the last few decades, it was recognized
by others [12,13] that GPR is applicable for
various disciplines such as archaeological survey,
geology, hydrogeology, utility detection, sand dune
study, sedimentology, landmine clearing and so on. .
ASTM [14] briefly gives a summary of the GPR.
Its components include a control unit, an antenna
(transmitter and receiver) and survey wheel(s) if
necessary (See Fig. 1). The control unit synchronizes
the antenna to generate radar waves and to receive
reflected signals. The resultant signals can be shown
either black and white, different colour transform,
filtered or amplified on the display. The antenna
transmits and receives radio frequencies, covered with
shielding and placed close to the ground surface.
Survey wheel(s) attaching to the antenna in contact
with the bark surface count(s) electromagnetic pulses
across a given distance, in order to calibrate a uniform
wheel speed. These waveforms are further stacked to
.
create two dimensional reflection profiles.
The characteristics of GPR lies on the Maxwell's
equation, two-way travel time and the electric and
magnetic properties of the materials itself. The first
equation describes the nature of electromagnetic wave;
the second one relates constant velocity of the medium
to the reflector; and the last one emphasizes the dielecDisplay
Record
Air
Timing
Transmitter
Antenna
Receiver
Antenna
Soil
Bedrock
Fig. 1. Components of GPR [13].
tric constant (also called relative permittivity) [11].
These have been discussed in many times, which are
both mathematically expressed in Eqs. 1 to 3. The
higher the dielectric constant (K), the lower the electromagnetic waves passing through the materials. Table 1
identifies the electromagnetic characteristics of different earth materials using GPR. The constant value
varies from 1 for air (the fastest propagation without
any energy loss) to 81 for fresh water (the slowest). .
Ä
(1)
E=-
Ä
where is the curl operator, E is the electric field
strength vector (V m-1), B is the magnetic flux density
vector (T), and t is time (s).
.
D=
tV
2
(2)
where V is the velocity of the material through which
the radar passes (m ns-1), D is the one way distance to
the object, and t is the two-way travel time to the
object.
.
(3)
K=
0
where 0 is the permittivity of vacuum (8.89 x 10-2 F
m-1), and is the permittivity of the target.
.
Table 1. Electromagnetic properties of various subsurface
materials [14]
Material
Dielectric constant (K)
Air
1
Fresh water
81
Sea water
70
Sand (dry)
4-6
Sand (saturated)
25
Silt (saturated)
10
Clay (saturated)
8-12
Dry sandy coastal land
10
Fresh water ice
4
Permafrost
4-8
Granite
5
Limestone
7-9
Dolomite
6-8
Quartz
4
Coal
4-5
Concrete
5-10
Asphalt
3-5
Sea ice
4-12
PVC, epoxy, polyesters
3
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Various selectable frequency antennas can be
chosen to best fit with the project requirements,
ranging from 10 MHz to 1.6 GHz. To determine the
appropriate centre frequency, Eq. 4 identifies certain
parameters (i.e., their depth ranges, dimensions and
electrical properties), which gives a negative relationship between the desired vertical resolution and the
frequency [15]. To design a suitable survey grid,
another important criterion is to determine size of
footprint (i.e., survey interval, A). Keeping K such as
chemical constituents, differences in retained moisture,
compaction and porosity throughout the radar medium,
the following approximate parameters such as angle
of the transmission cone, central frequency of the
antenna and its target depth are indicated in Eq. 5 and
shown in Fig. 2. To give a good spatial resolution in
the survey area, Eq. 6 is used for a limited range of
depth (D) and velocity (V) in the display window (W).
As referred to Table 2, high frequency waves can be
illustrated in narrower widths along with tighter spots.
As a general rule, the deeper the penetration, the lower
the antenna frequency, the lesser the resolution and
vice versa. Also, higher resolution is necessary for
smaller features.
.
ë=
150
MHz
W K
(4)
where ë refers the centre frequency of the antenna
(MHz) and W is the desired vertical resolution (m), .
A=
D
ë
+
m
4
K+1
(5)
where A refers the survey spacing of the grid (m).
W = 1.3
(2D
V ) ns
(6)
THE HONG KONG UTILITY SYSTEM
The underground utilities can be classified into 5
main categories - water, gas, drainage, power and
.
telecommunication cables.
The Water Supplies Department is in charge of the
Antenna
Ground
Surface
D
A
Footprint
Fig. 2. Illustration of the size of footprint of radar energy
[11].
supply of water. Several types of pipe materials with
standardized sizes are located in certain depths between 1 and 2 m (Table 3). Unplasticized polyvinyl
chloride (uPVC) less than 100 mm diameter is applied
to the salt water mains, lying in pavements with relatively shallow cover. Asbestos cement has been
discontinued for many years but many still exist in the
water mains. It varies from 100 to 450 mm in
diameter, which is internally durable but relatively
.
brittle when applying excessive external pressure.
The Hong Kong and China Gas Company Limited
is responsible for the classes of gas supply, ranging
from 2 kPa (low) to over 700 kPa (high) pressure.
Ductile iron is connected by mechanical flexible joint
system applied in both water and gas systems since
1970s, with a diameter ranging from 80 to 1600 mm.
Its material standard is based on BS4772/EN969. Unlined galvanized iron (GIU) and Lined galvanized iron
(GIL) are, less than 150 mm in diameter linking with
screw joints, used in the above two systems. Locating
them is relatively simpler than the other kinds because
of strong and obvious reflection signals. GIL pipes,
governing the standard under BS1387, have been
adopted for the replacement of GIU since 1995.
Polyethylene has widely used since 1998 to phase out
GIU, GIL and uPVC pipes. Its diameter is less than
400 mm, connecting to butt fusion, electrofusion
and transition fittings by heat melting. Its material
Table 2. Examples of capability of frequency [14]
Frequency
Typical applications
Maximum depth (m)
1.6 GHz
Structural concrete, roadways, bridge decks
0.5
900 MHz
Concrete, shallow soils, archaeology
1
400 MHz
Shallow geology, utility, environmental, archaeology
3
200 MHz
Geology, environmental
8
100 MHz
Geology, environmental
20
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
Table 3. Summary of respective depths of Hong Kong utilities [5]
Types of utilities
Examples
Standard burial depth (m)
Power cables
Street lightings in sidewalk and cycle track, Area traffic control
0.35
Low voltage in sidewalk and cycle track
0.45-0.90
High voltage in carriageway
1-2
Water mains
Drinking, flushing, watering pipelines
1-2
Drain mains
Storm and foul sewerage ducts
1.5-5
Gas mains
Towngas, other tubed gases
1-2
Telecommunication lines
Telephone lines, broadband, cable TV, military communication lines
0.5
standards are GBE/PL2, EN1555 and GB15558. The
advantages are corrosion free and strong resistance
from stress.
.
The Drainage Services Department is responsible
for flood prevention and wastewater treatment (i.e.,
stormwater drainage and sewerage building). Concrete
and vitrified clay are two major types of drain materials used today. Its pipes are placed much deeper than
other utilities, from 1.5 to 5 m.
.
The China Light and Power Company Limited and
the Hong Kong Electric Company Limited are the sole
service providers in the entire territory of Hong Kong.
The outer layers of cables, almost buried underground,
are appeared in green, red and black colours. Depending on the locations, the voltages (low or high
voltage) and its usage (transmission or distribution
purpose), the standard depths lay from 0.35 to 2 m. On
the subject of power and telecommunication cables,
PVC outersheath is appeared to those enclosed by
different colour sheaths at outer diameters from 16 to
.
95 mm buried beneath.
THREE TESTING SITES
To work with the above objectives, three locations
in Hong Kong were chosen to compare results between
as-built surveys and GPR surveys among different
utilities and materials in pavements. They were:
.
1. Drains along Kong Sin Wan Road, Cyberport (Sites
1A and 1B, KSWR);
.
2. Watermains at Nam Fung Path, Aberdeen (Site 2,
NFP); and
.
3. Electricity cables at Hong Tat Path, Tsim Sha Tsui
.
East (Site 3, HTP).
All are paved in flat surface without any irregularities. Site 1 was built for the purpose of accessing
facilities nearby in 2009. The drains were covered by
300 mm Type B compacted materials with 50 mm
brick on the top. These earthworks materials do not
exceed 75 mm maximum particle size at around 3%
moisture content, which complies with the requirements of Guidance notes No. 014B issued by the
Highways Department [16]. Similarly, Site 2 was
formed for the purpose of road facilities in 2008. Field
inspection revealed that the study area was close to the
construction site of the Mass Transit Railway south
island line. With regard to Site 3, it contained a brunch
of cables lying under pavements. It seemed to be
difficult in detecting several packed targets together
accurately. In situations some of survey lines had been
missed or not perfectly aligned where a complete
scan was obstructed by a tree or man-made ground
.
features.
It was also necessary to consider the following
parameters, such as ë, K, grid spacing and calibration
of survey wheel. Equipment available in the study was
the hand-pulled SIR-20 GPR unit with three groundcoupled centre frequency antennas - 100, 270 and 400
MHz (GSSI, USA). However, the use of 100 MHz
antenna was abandoned because of lack of precise
horizontal distance measurement without the usage of
survey wheel. To ensure transect lines easily and to
maintain a complete coverage of the targets, the grid
intervals designed for two different ë (270 and 400
MHz) in the vicinities were laid between 0.68 and 1 m
(Eqs. 4 and 5). As such, survey transects were spaced
1 m for Sites 1 and 2, and 0.5 m for Site 3 apart.
General survey information relevant to these sites is
described in Table 4 and Figs. 3 to 5. Besides, record
plans were requested by several utility undertakers to
.
fully understand the underneath site conditions.
Each plot was surveyed in two directions, but only
the longitudinal transects were used subsequently for
analysis because it was more difficult to distinguish
the reflectors along short horizontal profiles. Simply,
one or two reflectors could be possibly collected in
every two or three-metre horizontal profile (Fig. 6a),
whereas plenty of reflectors along the longitudinal
profiles could be identified as sample points (Fig. 6b)..
Prior to any start of every GPR survey, it is necessary to do the calibration which is divided into two
elements - survey wheel calibration and common
midpoint method. The former one is to correct the
horizontal scales on a measured baseline with respect
to the survey wheel in Fig. 7a. Without considering
any speed, the wheel has a distance encoder by count-
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
Table 4. Survey details of three sites
Site
Grid dimension
Total number of transects
(Horizontal & Longitudinal)
Types of utilities
Burial depths (m)
1. KSWR (A)
19 x 1 m, 1 m spacing
22 (20 + 2)
300 mm diameter concrete drains
1.93-1.96
KSWR (B)
24 x 3 m, 1 m spacing
29 (25 + 4)
225 mm diameter concrete drains
1.89-1.93
2. NFP
19 x 2 m, 1 m spacing
23 (20 + 3)
150 mm diameter water mains
0.49-0.64
3. HTP
26 x 2 m, 0.5 m spacing
58 (53 + 5)
25-58 mm diameter power cable
0.45
Fig. 4. NFP watermain site.
Fig. 3. KSWR drainage site 1A (a) and 1B (b).
ing the number of ticks per metre (Preset is 407 ticks
per metre). The survey wheel distance error does not
exceed ± 2% (Fig. 7b) under certain favourable conditions (e.g., smooth ground, proper procedures, no
slippage, etc.) [17], which is in agreement with the
findings from various sites. On the other hand, the
latter one is to determine the radar wave velocity as
well as K for each site. Using the above formula
mentioned above, the signal-to-noise ratio together
with the velocity could be analyzed by measuring
.
several known target depths.
As mentioned before, the underneath site conditions existed almost in concrete and compacted materials. Therefore, K were set all 8 among four sites as
referred to Table 1 (K between 5 and 10). Table 5
illustrates the relationship between the calculated
figures of K in various locations and their differences.
The values of velocities, which were deduced from the
depths and times of different utility points collected
and presented in the primitive radar profiles among
three sites, were calculated by Eq. 2. The results of the
calculated K (Kcalculated) were determined using Eq. 7.
For instance, K is 2.25 while both velocities of utility
points and light are 0.2 and 0.3 m ns-1 respectively. As
stated beforehand, the reliable positions of the utilities
in these sites were surveyed and fixed during the
construction stage (Table 4). By comparison, the
results of the calculated figures were found steady and
consistent in different material complexities, ranging
from 6.35 to 9.96. Hence, the antennas were successfully calibrated on site with adoption of K of 8. Radar
profiles were collected in 50 ns vertical range of
display window.
.
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
V=
C
K
(7)
where c denotes as the velocity of light (0.3 m ns-1). .
The presence of available surface features and
utilities was firstly identified. The reliable positions of
the utilities in Sites 1 and 2 were then extracted from
as-built CEDD engineering drawings. Also, the cable
depths in Site 3 had just been estimated by tie measurements during excavation. At the same time, individual radar profiles were examined for indicating
parabolic reflector signals of utilities. The positions of
these sample points were marked as in m either threedimension or one-dimension coordinates in HK1980
local coordinate system. To estimate the positional
accuracy of the GPR, the root mean square error
(RMSE) is a statistical measure which was used to
examine the difference between GPR results and the
as-built results, i.e., the magnitude between two
results.
.
RESULTS AND DISCUSSION
(a)
(b)
Fig. 7. (a) Survey wheel calibration, and 7 (b) Number of
ticks per unit for distance estimation [17].
Fig. 5. HTP electricity cable site.
(a)
(b)
Fig. 6. Comparison of a horizontal profile (a) and a longitudinal transect (b).
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
Table 5. Dielectric constant analysis (the calculation results are corrected to 1 decimal place)
Venue
NFP
HTP
KSWR Site 1A
KSWR Site 1B
400 MHz 270 MHz 400 MHz 270 MHz 400 MHz 270 MHz 400 MHz 270 MHz
Centre freq.
Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max.
V (m ns-1) 0.12 0.10 0.12 0.10 0.12 0.10 0.12 0.10 0.11 0.10 0.11 0.10 0.11 0.10 0.11 0.10
Kcalculated
6.8 9.5 6.4 10.0 6.6 9.4 6.4 9.9 7.7 8.3 7.6 8.3 7.5 8.3 7.3 8.6
Kadopted
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
K diff.
-1.2
1.5
-1.6
2.0
-1.4
1.4
-1.6
1.9
-0.3
0.3
-0.4
0.3
-0.5
0.3
-0.7
0.6
Fig. 8. Time zero correction in Site 2 using 270 MHz antenna (The vertical line indicates 1.61 ns as the first peak
displayed in horizontal axis and the vertical axis is the displacement in m).
Table 6. Time zero correction of 3 sites with 2 antenna frequencies
Frequency/Site
Site 1A, KSWR (ns)
Site 1B, KSWR (ns)
Site 2, NFP (ns)
Site 3, HTP (ns)
270 MHz
1.61
0.88
1.61
1.61
400 MHz
0.29
0.49
0.78
0.39
With the use of application software Radan
Version 6.5.3.0, further processing techniques were
performed to improve the clarity of reflector signals,
such as time-zero correction, infinite impulse response
(IIR) filtering, finite impulse response (FIR) filtering,
deconvolution, migration, and so on. To illustrate
some examples, time-zero correction is to determine
the antenna-ground separation and to correct the initial
depth of the ground interface in terms of ns (Fig. 8),
which is usually set as the first peak wave when the
electromagnetic wave hits the ground [8]. The correction of 3 sites in ns was described in Table 6, from
which a larger gap had been found in 270 MHz result. .
In addition to eliminate the human interference or
the system noise for the enhancement of radargram
interpretation, either IIR or FIR method or both can be
effectively used in the filtering process. The IIR
method is that the filters decay the signal exponentially
towards zero when coming across a feature, in order to
reduce the background noises in the whole picture. As
shown in Fig. 9, users can define the low and/or the
high frequency from 75 to 700 MHz, horizontally
and/or vertically together with the time interval in the
IIR filter parameters box [17]. Figures 10 and 11 are
the examples indicating the changes in different filter
applications and their effects. For instance, the unwanted low frequency features appeared in “snowlike” noise in Fig. 10 could be eliminated using the
vertical high pass 500 MHz filter. Similarly, the high
frequency signals in Fig. 11 could be excluded using
.
vertical low pass 200 MHz filter.
On the other hand, the FIR filters contain vertical
and horizontal filters as well as spatial 2D filters.
These filters have resulted in symmetrical nature and
linear phase characteristics to remove some restricted
features which would not be shifted in time or position
[17]. Two other kinds of calculation including boxcar
averaging filter and triangle weighting filter are available in the software. A simple running average with
equal weight is the prime process of boxcar filter
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
Fig. 9. IIR filtering (the horizontal axis indicates the radar signal in ns and the vertical axis is the displacement in m)
[17].
Fig. 10. A vertical IIR high pass 500 MHz filter (band-pass) in Site 2 - 400 MHz antenna in transect 22. Upper half of
spilt screen is original raw data. Lower half is processed data.
Fig. 11. A vertical IIR low pass 200 MHz filter (band-pass) in Site 2 - 400 MHz antenna in transect 22. Upper half of
split-screen is original raw data. Lower half is processed data.
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
applied to the data along all profiles. By contrast, the
triangular filter is a weighted moving average like a
triangular shape which highly focuses the centre of the
filter rather than the end of the filter [17]. For the
above filters, some repetitive trials are essentially
performed to check whether the target(s) is (are) undoubtedly reflected to the profiles. For most of our
study sites, it was found that the filtering processes
were employed between 100 and 500 MHz to all
samples, which is subject to certain conditions such as
the level of moisture content, soil composition and
material complexities, and so on. The interpretation of
radar profiles is also operator subjective.
.
Another example is the deconvolution. Both Sites
1 and 2 were surveyed on top of metal surface (e.g.,
manhole cover, u-channel cover) and resulted in
ringing waves' effect (Fig. 12). Since GPR is pulling
onto the surface, the multiple reflections are then
resolved using pre-whitening step. This step mathematically alters the autocorrelation function by
boosting the white noise (zero delay) component, and
also stabilizes the filter by smoothing the output and
reducing noise as illustrated in Figs. 13 and 14. Values
between 0.1 and 5% are the good start [17]. Gain value
is finally adjusted to increase the visual contrast of the
.
target wavelets [17].
We were able to detect these three kinds of utilities
and to identify them in the radargrams. Other than that,
three extra observations were pointed out in the study.
The first one is to identify the thickness of Type B
surface compacted material (300 mm) in Site 1A. A
clear layer on the top as shown in Fig. 15 was quite
consistent with that in the record plan. Also, two
nearly parallel lines (Fig. 16) between 0.7 and 0.83 m
indicated that there was a strong reflection of 150 mm
outer diameter watermains in Site 2, which agreed
with the result shown in the engineering drawing. At
the same time, more than 300 GPR sample points had
been taken into account of the comparison of the as.
built surveys as seen below.
Serving the as-built data as the reference sample to
assess the accuracy of two dimension locations and
depth values, both RMSE and standard error (ó) were
adopted for the comparison of the GPR results, as
given by Eqs. 8 and 9. The RMSE is defined as the
root of the sum of squares of 2D position residuals
while the ó is the root of the sum of squares of depth
.
residuals.
(8)
(9)
Fig. 12. Deconvolution in Site 2 - 270 MHz antenna in
transect No. 2 (left: its outlook; middle: before
deconvolution; right: after deconvolution).
where ÄN is the northing difference between the asbuilt value and GPR value, ÄE is the easting
difference between the as-built value and GPR value,
Fig. 13. The pre-whitening step 0.1%. Upper half of split-screen is original raw data. Lower half is processed data.
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Fig. 14. The pre-whitening step 5%. Upper half of split-screen is original raw data. Lower half is processed data.
penetration with the lower ë”, it was shown that the
270 MHz measurement was much better than the 400
MHz one in detecting utilities 2-m below (i.e., Sites
1A and 1B). On the contrary, the 400 MHz one was
good at distinguishing underlying objects less than 2-m
(i.e., Sites 2 and 3). For instance, Figs. 17 and 18
indicate both RMSE and ó in Site 2. By comparison,
the 270 MHz antenna data had better results in terms
of RMSE and ó.
.
CONCLUSIONS
Fig. 15. Type B compacted material in KSWR Site 1A
using 400 MHz antenna.
Fig. 16. Watermains in NFP Site using 400 MHz antenna
(transect no. 22).
ÄZ is the depth difference between the as-built value
and GPR value, and n refers to number of sample
.
points.
As referred to Table 7, Site 2 gave a model example in determining the positional accuracy of GPR.
Generally speaking, the outcome proved the capability
of nondestructive GPR techniques to obtain the reliable
utility positions average to dm level, some better to cm
level dependent on favourable site conditions. By
applying the preceding rule of thumb “the deeper
The objective of the research was to evaluate the
positions of several types of underneath utilities using
the non-destructive radar results. The as-built measurements extracted from the engineering drawings surveyed by conventional land surveying methods during
site formation stage were treated as reliable references
to make comparison with the radar results. To begin
with, GPR and its characteristics were briefly reviewed
and the five main types of utility system in Hong Kong
were summarized. Also, general survey considerations
were made such as choice of ë, K, grid spacing and
calibration of survey wheel. The study found that both
270 and 400 MHz antennas were calibrated well on
site with adoption of K of 8. Several GPR surveys had
been successfully carried out in these three pedestrian
trial sites - Site A and Site B of Kong Sin Wan Road,
Nam Fung Path and Hong Tat Path. They are repre.
senting three different kinds of utilities.
Post-processing techniques including time-zero
correction, IIR and FIR filters, and deconvolution were
performed to enhance the interpretability of original
radar signals. The identifications of shape and size of
available surface and subsurface features (i.e., manhole
cover, u-channel cover and Type B compacted
material) and subterranean utilities (i.e., Drains,
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Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
Table 7. Positional accuracy of the study areas
Site/RMS and ó
270 MHz RMS (m)
400 MHz RMS (m)
270 MHz ó (m)
400 MHz ó (m)
1A, KSWR
0.136
0.623
0.116
1.441
1B, KSWR
0.093
0.185
0.203
0.861
2, NFP
0.028
0.197
0.165
0.035
3, HTP
Inapplicable
Inapplicable
0.150
0.110
2D positions in Site 2, Nam Fung Path
812533
As-built watermain positions
400 MHz GPR result
270 MHz GPR result
812531
812529
Northing (m)
812527
812525
812523
812521
812519
812517
812515
836330
836332
836334
836336
836338
Easting (m)
Fig. 17. Correlation between the positions of 270, 400 MHz and the target watermains in Site 2, Nam Fung Path.
Depths in Site 2, Nam Fung Path
Radargram
1
3
5
7
9
11
13
15
17
19
0.9
0.8
Depth (m)
0.7
0.6
0.5
0.4
0.3
0.2
As-built depth
270 MHz GPR depth
400 MHz GPR depth
Fig. 18. Correlation between the depths of 270, 400 MHz and the target watermains in Site 2, Nam Fung Path.
152
Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)
watermains and electric cables) were identified among
these processed radargrams. To estimate the positional
accuracy of the GPR, RMS of as-built data and GPR
data were then examined. The study revealed that there
was a strong correlation in the RMSE and ó in Site 2
(Nam Fung Path site), especially for 270 MHz result.
In other words, the site was proved the capability of
GPR to obtain the reliable utility positions average to
decimetre level, some better to centimetre level. It was
also achieved that the 400 MHz antenna was good at
distinguishing underlying objects less than 2-m,
whereas 270 MHz one was 2-m below. However, these
were the straight forward cases and still have room for
improvement. Some special attentions are crucial for
the surveyors in conducting GPR surveys, for example,
weather conditions, irregular or rough topography,
heavy vegetation, and grid clearance for attaching
.
survey wheel to the antenna respectively.
ACKNOWLEDGEMENTS
The heartfelt thanks to the survey division of the
CEDD and other utility undertakers were highly appreciated by supplying as-built engineering drawings and
utility record plans correspondingly. Special thanks
also go to the two anonymous referees who provided
very useful comments on an earlier version of the
manuscript.
.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
.
Manuscript Received: July 19, 2012
Revision Received: November 5, 2012
and Accepted: December 10, 2012