Identification and positioning of underground utilities...

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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

*Corresponding authorEmail: [email protected]

ABSTRACT

INTRODUCTION

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. .

Utility surveying, according to the Survey Associ-ation [1], refers to the location, positioning and identi-fication of buried pipes, cables and ducts irrespective of their sizes, depths, material types and proximity to other utilities using numerous techniques or technolo-gies, 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

4aware there are 7.1 x 10 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 cosmo-politan 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

Tong's studies [4], but not in Hong Kong. This paper aimed to compare the radar survey re-sults 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 com-parisons between the measurements and as-built posi-tions 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.

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OVERVIEW OF GPR

The definition of GPR is “a range of electromag-netic 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 electromag-netic waves to identify underlying structures, in partic-ular plastic pipes or fiber cables. Its history can be traced back to the 1920s when the GPR was used to

Sustain. Environ. Res., 23(2), 141-152 (2013)

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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 dielec-

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Fig. 1. Components of GPR [13].

Display

Record

Timing

Transmitter Receiver

Antenna Antenna

Soil

Bedrock

Air

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 electro-magnetic waves passing through the materials. Table 1 identifies the electromagnetic characteristics of differ-ent 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). .

where V is the velocity of the material through which -1the radar passes (m ns ), D is the one way distance to

the object, and t is the two-way travel time to the object. .

Table 1. Electromagnetic properties of various subsurface materials [14]

Material

Air

Fresh water

Sea water

Sand (dry)

Sand (saturated)

Silt (saturated)

Clay (saturated)

Dry sandy coastal land

Fresh water ice

Permafrost

Granite

Limestone

Dolomite

Quartz

Coal

Concrete

Asphalt

Sea ice

PVC, epoxy, polyesters

Dielectric constant (K)

1

81

70

4-6

25

10

8-12

10

4

4-8

5

7-9

6-8

4

4-5

5-10

3-5

4-12

3

where is the curl operator, E is the electric field -1strength vector (V m ), B is the magnetic flux density

vector (T), and t is time (s). .

Ä

(1) E = -Ä

(2) D =tV2

(3) K =0

-2where is the permittivity of vacuum (8.89 x 10 F -1m ), and is the permittivity of the target. .

0

Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)

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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 relation-ship 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. .

where ë refers the centre frequency of the antenna (MHz) and W is the desired vertical resolution (m), .

where A refers the survey spacing of the grid (m).

Fig. 2. Illustration of the size of footprint of radar energy [11].

Table 2. Examples of capability of frequency [14]

Frequency

1.6 GHz

900 MHz

400 MHz

200 MHz

100 MHz

Typical applications

Structural concrete, roadways, bridge decks

Concrete, shallow soils, archaeology

Shallow geology, utility, environmental, archaeology

Geology, environmental

Geology, environmental

Maximum depth (m)

0.5

1

3

8

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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

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supply of water. Several types of pipe materials with standardized sizes are located in certain depths be-tween 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 rela-tively 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. Un-lined 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

.

(4)ë =150

W KMHz

(5)A = + më4

DK + 1

(6)W = 1.3 ( ) ns 2DV

Ground Surface

Antenna

D

Footprint

A


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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 materi-als 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. De-pending 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.

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Table 3. Summary of respective depths of Hong Kong utilities [5]

Types of utilities

Power cables

Water mains

Drain mains

Gas mains

Telecommunication lines

Examples

Street lightings in sidewalk and cycle track, Area traffic control

Low voltage in sidewalk and cycle track

High voltage in carriageway

Drinking, flushing, watering pipelines

Storm and foul sewerage ducts

Towngas, other tubed gases

Telephone lines, broadband, cable TV, military communication lines

Standard burial depth (m)

0.35

0.45-0.90

1-2

1-2

1.5-5

1-2

0.5

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 irregular-ities. 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 require-ments of Guidance notes No. 014B issued by the Highways Department [16]. Similarly, Site 2 was

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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 ground-coupled 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 onlythe 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 neces-sary 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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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 con-ditions (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 condi-tions existed almost in concrete and compacted mate-rials. Therefore, K were set all 8 among four sites as

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Table 4. Survey details of three sites

Site

1. KSWR (A)

KSWR (B)

2. NFP

3. HTP

Grid dimension

19 x 1 m, 1 m spacing



26 x 2 m, 0.5 m spacing

22 (20 + 2)

29 (25 + 4)

23 (20 + 3)

58 (53 + 5)

Types of utilities

300 mm diameter concrete drains

225 mm diameter concrete drains

150 mm diameter water mains

25-58 mm diameter power cable

Burial depths (m)

1.93-1.96

1.89-1.93

0.49-0.64

0.45

Total number of transects(Horizontal & Longitudinal)

Fig. 3. KSWR drainage site 1A (a) and 1B (b).

Fig. 4. NFP watermain site.

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 (K ) were determined using Eq. 7. calculated

For instance, K is 2.25 while both velocities of utility -1points and light are 0.2 and 0.3 m ns 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 success-fully 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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Fig. 5. HTP electricity cable site.

(7) parabolic reflector signals of utilities. The positions of these sample points were marked as in m either three-dimension 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. .

Fig. 6. Comparison of a horizontal profile (a) and a longitudinal transect (b).

(b)(a)

-1where c denotes as the velocity of light (0.3 m ns ). 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 meas-urements during excavation. At the same time, indi-vidual radar profiles were examined for indicating

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Fig. 7. (a) Survey wheel calibration, and 7 (b) Number of ticks per unit for distance estimation [17].

RESULTS AND DISCUSSION

V =CK


(a)

(b)

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Table 5. Dielectric constant analysis (the calculation results are corrected to 1 decimal place)

Venue

Centre freq.Min.

0.12

6.8

8.0

-1.2

Max.

0.10

9.5

8.0

1.5

Min.

0.12

6.4

8.0

-1.6

Max.

0.10

10.0

8.0

2.0

400 MHz 270 MHz

Min.

0.12

6.6

8.0

-1.4

Max.

0.10

9.4

8.0

1.4

Min.

0.12

6.4

8.0

-1.6

Max.

0.10

9.9

8.0

1.9

400 MHz 270 MHz

Min.

0.11

7.7

8.0

-0.3

Max.

0.10

8.3

8.0

0.3

Min.

0.11

7.6

8.0

-0.4

Max.

0.10

8.3

8.0

0.3

400 MHz 270 MHz

Min.

0.11

7.5

8.0

-0.5

Max.

0.10

8.3

8.0

0.3

Min.

0.11

7.3

8.0

-0.7

Max.

0.10

8.6

8.0

0.6

400 MHz 270 MHz

-1V (m ns )

Kcalculated

Kadopted

K diff.

KSWR Site 1A KSWR Site 1B NFP HTP

Table 6. Time zero correction of 3 sites with 2 antenna frequencies

Frequency/Site

270 MHz

400 MHz

Site 1A, KSWR (ns)

1.61

0.29

Site 1B, KSWR (ns)

0.88

0.49

Site 2, NFP (ns)

1.61

0.78

Site 3, HTP (ns)

1.61

0.39

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).

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 correc-tion 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

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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 un-wanted low frequency features appeared in “snow-like” 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 availa-ble in the software. A simple running average with equal weight is the prime process of boxcar filter

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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.

148 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) un-doubtedly 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 mathe-matically alters the autocorrelation function by boosting the white noise (zero delay) component, and

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Fig. 12. Deconvolution in Site 2 - 270 MHz antenna in transect No. 2 (left: its outlook; middle: before deconvolution; right: after deconvolution).

Fig. 13. The pre-whitening step 0.1%. Upper half of split-screen is original raw data. Lower half is processed data.

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.

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.

(8)

(9)

where ÄN is the northing difference between the as-built value and GPR value, ÄE is the easting difference between the as-built value and GPR value,

149Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)

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 exam-ple 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

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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.

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).

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.

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CONCLUSIONS

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 measure-ments extracted from the engineering drawings sur-veyed 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 pedestriantrial 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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Table 7. Positional accuracy of the study areas

Site/RMS and ó

1A, KSWR

1B, KSWR

2, NFP

3, HTP

270 MHz RMS (m)

0.136

0.093

0.028

Inapplicable

400 MHz RMS (m)

0.623

0.185

0.197

Inapplicable

270 MHz ó (m)

0.116

0.203

0.165

0.150

400 MHz ó (m)

1.441

0.861

0.035

0.110

Fig. 17. Correlation between the positions of 270, 400 MHz and the target watermains in Site 2, Nam Fung Path.

Fig. 18. Correlation between the depths of 270, 400 MHz and the target watermains in Site 2, Nam Fung Path.

2D positions in Site 2, Nam Fung Path

Easting (m)

As-built watermain positions 400 MHz GPR result 270 MHz GPR result

Nor

thin

g (m

)

812533

812531

812529

812527

812525

812523

812521

812519

812517

812515836330 836332 836334 836336 836338

Depths in Site 2, Nam Fung Path

Radargram

Dep

th (

m)

As-built depth 270 MHz GPR depth 400 MHz GPR depth

151Cheng et al., Sustain. Environ. Res., 23(2), 141-152 (2013)

1 3 5 7 9 11 13 15 17 190.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

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.

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ACKNOWLEDGEMENTS

The heartfelt thanks to the survey division of the CEDD and other utility undertakers were highly appre-ciated 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. .

REFERENCES

TSA, Guidance Note Utility Surveys - Detailed Guidance Notes for Specifying a Utility Survey. The Survey Association, Nottinghamshire, UK (2010). Annan, A.P. and S.W. Cosway, Ground penetration radar survey design. 53rd Annual Meeting of European Association of Exploration Geo-physicists. Florence, Italy, May 26-30 (1991). Bristow, C.S. and H.M. Jol, Ground Penetrating Radar in Sediments. Geological Society, London, UK (2003). Tong, L.T., Application of ground penetrating radar to located underground pipes. Terr. Atm. Ocean Sci., 4(2), 171-178 (1993). Wong, K., Utility Systems Part 1. Lecture 6 of Utility Systems and Design (LSGI 3352). Depart-ment of Land Surveying and Geo-informatics, the Hong Kong Polytechnic University, Hunghom, Kowloon (2010).

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Discussions of this paper may appear in the discus-sion section of a future issue. All discussions shouldbe submitted to the Editor-in-Chief within six monthsof publication. .

Manuscript Received: Revision Received:

and Accepted:

July 19, 2012November 5, 2012

December 10, 2012

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