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AN UNDERGROUND UTILITY DETECTION TECHNOLOGY – OUR EXPERIENCES

Dušan Petrovački1*, Aleksandar Ristić1*

1University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia

*Authors to correspondence should be addressed via email: petrovacki@uns.ns.ac.yu; aristic@uns.ns.ac.yu

Abstract: This paper describes modern, non-invasive data acquisition methods in underground utility detection. The work presents the possibilities of connecting data acquired with a Ground Penetrating Radar (GPR) with GPS (Global Positioning System) data and the joint use of these technologies through various examples. Also, the basic parameters, methods and their influence on the acquisition speed and data quality are defined.

Key Words: Geographic Information System (GIS)/ GPS/ GPR/ Pipeline route/ GPR scan

1. introduction

No dig utility mapping technology is important in data acquisition for computation, modeling and control of underground infrastructure systems. Classic mapping technologies (excavation, old maps) cannot follow the dynamics of development and repair of pipelines. Our work has shown that the use of GPR in combination with GPS is a more reliable method. A d

irect consequence of the classical approach is an expensive, uncoordinated process of establishing new pipelines accompanied with damage to existing utilities. Low quality information about condition and operation of, for example, city water supply system implicitly affects the flexibility of that system. This can be seen on the difference between the amount of processed and distributed water. Participants in the process of digital spatial data production, maintenance and management, as well as numerous users, who base their work on using spatial information, have the need to increase their efficiency. One of the ways to do this is the implementing the geo-information system. Considering the fact that spatial registers are extensive, the processes of producing data in digital form, maintenance and management are very complex tasks.

2. PHYSICAL CONCEPTS OF GPR WORK

The Ground Penetrating Radar (GPR) is a device used for non-invasive scanning and precise detection of underground utilities. GPR is composed of a receiver and transmitter antenna, a control unit with Win CE OS, battery supply and a survey cart. Survey cart is a tricycle equipped with incremental encoder. The incremental encoder is used for precise positioning of the center of the antenna above a pipeline route. The GPR also has a marker that is useful for marking interesting details on a radar scan. The GPR can be equipped with a GPS rover that is used for measuring spatial coordinates of the projection of the pipeline route on the site surface. The GPS rover can measure coordinates either independently or synchronized with the GPR scan. In the second case, the GPS rover measures all points on the scanned trajectory, or just the start and end coordinates. Synchronized work implies direct communication between GPS rover and the GPR device. Measurement of pipeline parameters with GPR and GPS measurement coordinates on the site surface are with centimeter accuracy. This measurement accuracy satisfies geodethic-mapping laws [1]. Figure 1 shows the schematic picture of the GPR equipment, its functional parts and the connections between them.

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Fig. 1. Functional parts of GPR

When the survey cart moves on the site surface the transmitting antenna sends polarized, high frequency electromagnetic (EM) waves in the ground. Because of different existing inhomogenities in the ground, e.g. soil layers, underground utilities, stones, gravel, cavities and other anomalies, part of the EM waves is reflected from the dielectric boundary between different materials and the other part is refracted and goes to the deeper layers. The described process is repeated until the EM waves become too weak. Reflection of EM waves from the dielectric boundary is the consequence of differences in the electric and magnetic properties of materials of infrastructural objects and soil layers [2].

Time necessary for the propagation of EM waves from transmit antenna to the boundary surface and its reflection back to the receiver antenna is defined as a two way travel tR [ns] time. The GPR measures tR, and finally calculates the relative depth of the underground object. Because each location has its specific soil structure, (R (dielectric permitivitty) has to be recalculated for each site. Usually, the GPR recalibration method is used on site. This method is based on a GPR scan of an underground object with known depth. Maximum penetration depth in pipeline detection is usually 3.5 to 7m (400MHz and 200MHz antenna, respectively). Vertical resolution in pipeline detection is usually 3 to 7cm (400MHz and 200MHz antenna, respectively). Methodology of radar scan generation is shown in Figure 2. A radar scan is a spatial section of the working area. The antenna's linear trajectory is shown on X axis, and Y axis shows the two way travel time tR i.e. the relative depth z from the surface to the underground object. The distance between transmit and receive antenna is very small. Because of this, the distance from transmit antenna to boundary surface is approximately equal to the distance from boundary surface to the receiver antenna. The distance from antenna to the underground object continuously changes. Distances r0,r1,...rN are projected ortogonally on the movement axis, see points x-N ... x0 ... xN (see middle section of Figure 2.). By sequentially connecting the ends of these segments, a geometrical hyperbola is formed [3].

All points on the scan include reflected wave amplitude data. Points on top of the segments have peak amplitude value. The peak on the shortest segment r0 (antenna center is above the pipe axis) is highest (positive or negative). This value is criteria for scan searching and determination of location and depth of underground utility.

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Fig. 2. Radar scan generation

Transmit antenna radiates a conical EM wave beam with a bandwidth (=35((45(. Based on these facts, it is not necessary for the center of the antenna to be above the underground object to detect it. Figure 2 shows an ideal one-pipe radar scan in a homogenous soil layer. Antenna moved ortogonally to the pipeline axis. In real conditions scan is with different noises and hyperbolical reflections, caused by other infrastructural objects. Postprocessing can eliminate this [3].

3. GPR PARAMETER ACQUISITION METHODS

Parameters are detected by 2D and/or 3D scans of the site. Complexity, type of pipeline network and amount of data determine which method is used for scanning. 2D scanning is useful for quick underground utilities location. The first step is the recalculation of (R based on a scan of an underground object with known depth. Orthogonal scanning is used to determine pipeline depth and direction. To determine pipeline direction, at least two scans are needed [4]. A regular hyperbola shows up on the scan when ortogonally crossing above the pipeline axis. In that case, rules explained in figure 2 are satisfied, and the reflected wave amplitude is maximal because of maximum radar cross section. When antenna crosses at a sharp angle above the pipeline axis, the hyperbola has a totally different shape, which is no longer hyperbolic. In an extreme case, when the antenna trajectory is along the pipeline axis, the hyperbola is distorted into a straight line [4]. Detection of pipe materials (metal or non-metal pipe) is possible by measuring differences between reflected waves (reflection strength) [4]. Taking this into account, 2D scanning is useful in determining depth, directions in the horizontal and vertical planes, pipe inclination, pipe length, fluid/void ratio, changes of pipe diameter, pipe material, detection of pipe leakage, pipe radius estimation etc. Pipe radius estimation requires additional software processing because it depends on hyperbolic reflection geometry. In 3D scanning the software connects a number of 2D scans in a predefined sequence, hence creating a 3D model of the site. It overcomes the obvious limitations of the common form of GPR data display, which is a 2D vertical cross-section. Voids between the 2D scans are filled with software interpolation methods. The 3D display has the advantage of looking at the entire survey site at once. Technology of 3D scanning is useful for complex processing. This is especially important in areas with multiple intersecting, dipping or layered targets (pipes, rebar, etc.) that may be hard to identify on single radar profiles. Postprocessing software RADAN is used for filtering, normalization and transformation of raw signals. RADAN has special functions for applications that require linear feature and hyperbolic signature recognition [5].

4. THE EFFECT OF THE MATERIAL AND ITS TYPE OF UNDERGROUND UTILITIES ON THE REFLECTION'S SHAPE

The shape of the hyperbola and the type of peaks (maximum or minimum) depends on the material of the utility [3]. By analysing the above data, it is possible to define the type and material of the utility. Figure 3 shows hyperbolas with various characteristics, measured on different locations in Novi Sad. By analysing the shape of the hyperbola, we can identify the type of the object: cable or pipe. Light segments of the hyperbola indicate positive peaks, while dark parts indicate negative peaks. If the positive peaks are the highest, it indicates a cable or a pipe filled with a liquid. The highest negative peaks indicate empty pipes. Figure 3A shows the reflection from an electrical cable, about 35mm in diameter. Figure 3B shows the reflection from a filled metal pipe, whose diameter is 100mm. Figure 3C shows the reflection from an empty concrete pipe, diameter 150mm. Figures 3C and 3D show the comparative view of an empty metal pipe and an empty PVC cable with an optic cable. Both pipes have a diameter of 110 mm.

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Fig. 3. Type and material detection of utilities

It is obvious, that the electric cables differ from empty PVC cables the most, and also that it can be difficult to differentiate reflections of concrete and PVC pipes. We also determined, that metal utilities have better reflections than those made of non-metalic (concrete, PVC) [4]. This difference is caused by various reflective capabilities of metals and non-metals. Metals reflect the most of EM waves, while PVC is transparent for EM waves. This allows us to detect metal cables in PVC cladding. Figure 4 shows a processed scan of an optic cable in PVC cladding, diameter 110 mm.

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Fig. 4. Optical cable in PVC pipe

Figure 4 shows a radar scan with marked positive and negative peaks. The negative peak shows the depth of the PVC pipe (h=1.00m), while the positive peak the depth of an optic cable (h=1.12m). PVC pipes have standard diameters, so it is possible to determine their diameters indirectly.

5. THE LONGITUDINAL SCAN

If the antenna moves along the pipeline axis – the longitudinal scan transforms the hyperbola into a straight line. Metal objects have weak reflections cause by minimal radar cross-section of longitudinal scans. Empty non-metal pipes (concrete, PVC, ceramics) have opposite polarisation, which cause negative peaks and good quality of longitudinal scans. Figure 5 shows the longitudinal scan of a PVC pipe in length of 12 meters (diameter 200 mm) taken in Novi Sad. It also illustrates the parameters which can be estimated: pipe inclination, length, junctions, reductions, etc [3].

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Fig. 5. Scan along PVC pipe

6. SOIL STRUCTURE ANALYSIS

The detection and mapping of existing, or planned utility routes, it is important to estimate the level of groundwater and leakages [6]. Figure 6 shows a radar scan of the soil with a clearly marked groundware depth. It was scanned in an industrial area in Novi Sad.

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Fig. 6. Detection of groundwater level

Pipeline leakages form cavities and voids. Analysing the scan taken above a leakage, it is possible to determine the depth and shape of the anomaly [6]. Figure 7 shows the radar scan of an anomaly caused by pipeline leakage. This can be used as the basis for preventive analyses of road structures. Alternative method of determining the location of a leakage is to determine the change of the soil's dielectric constant or the shape of the EM reflection. [7].

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Fig. 7. Void and pipe leakage detection

7. GPR AND GPS – TYPES OF PROJECTS

▪ Detection and mapping of existing pipeline route

o Real location of pipeline route

o Part or complete pipeline network

o Complexity: single pipeline tracking, crossing with other utilities

▪ Scanning and analysis of future pipeline route

o Along boundary lines of future pipeline trench

o Reconstruction and future projects

o Complexity: all types of pipes and cables, crossing with other utilities, at different crossing angles

▪ Scanning of area with complex underground structure

o Complete area scanning – factories, squares, crossroads etc.

o 2D or 3D analysis

o Complexity: all types of pipes and cables, crossing with other utilities, at different crossing angles

▪ Soil characterization

o Detection of groundwater level and movement

o Pipe leakage detection,

o Detection of underground tanks, shafts etc.

o Riverbed analysis and profiling

o Soil layers characterization

▪ Other applications

o Archeological and forensic analysis

o Concrete and bridge constructions analysis

o Road (asphalt thickness) and railroad assessment

o Another geological investigations

In following chapters the most interesting types of projects and their details will be presented.

7.1. Pipeline route mapping technology

One of the realized projects was the mapping of a regional mid-pressure gas line network in the city of Novi Sad done for the Serbian National Gas Company. The project included the mapping of 44 km of gas line network and additional elements of infrastructure like shafts, air-signs, markers, stations for gas measuring and flow regulation, cathode protection, gas outlets etc. One part of the route lies in urban environment, but the bigger part goes through suburban environments near Novi Sad. All detected parameters measured by GPR and GPS were recorded as attributes in a geo-referenced AutoCAD document [8]. All network gas line pipes were made from metal, with different diameters (from (25.4mm (1" diameter) to (508mm (20" diameter)) and on various depths (from 0.2m to 4.2m), depending on terrain configuration and gas flow direction. Shafts, markers, signs, gas outlets, cathode protections were placed on characteristic points along the route (cross-sections of gas lines, beside every station for gas measuring and flow regulation – for all network users, certain places for future users, special sections of gas line route). The acquisition process was finished in 30 days. The scans were recorded with a two GPR antennas. The GPR scans were verified with RD4000 [9]. It is transmitter - receiver system, in which electrical connection leads are plugged into the transmitter and attached directly to the pipeline, and the circuit is completed by connection of receiver to a ground stake (typically at 90º degrees to the pipeline). Parameters, which are measured from GPR scans are, exact gas line directions in all planes and gas line depth. Also, GPR scans are used for detection of isolated faucet (future users and gas line sections), exact location where the gas line crosses beneath certain streets and railroads, parallel gas line routes, crossings with other underground utilities, specific displacements of gas line route and finally software estimation of characteristic gas line diameter [10].

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Fig. 8. Network of permanent base stations in Serbia

Spatial coordinates of all characteristic points on the surface were measured with this GPS equipment: high precision (~1cm) GPS equipment (rover Trimble 5800 using RTK (Real Time Kinematic)). The measured coordinates have to be corrected to achieve even higher precision. Corrections come from a network of permanent base stations with fixed positions, and can be acquired in real time or offline [11] (Figure 8).

Real time corrections (RTK- Real Time Kinematic) can be received through GPRS or from a GSM modem. Offline PPK (Post Processing Kinematic) corrections can be downloaded from the website of the Center for geo-information technologies and systems [12]. A network of base stations covers the region Vojvodina since December 2004, and cover the all territory of Serbia since December 2005, enabling the measurement of GPS coordinates with a precision of 1cm [13]. Network is developed and maintained by Center for geoinformation technologies and systems.

Since there are no specified transformation parameters for the survey location, it was necessary to calculate them by measuring and processing certain geodethic points of state survey. Considering this, eight points inside and on the border of the survey location were measured. After that, seven datum transformation parameters were calculated. Figure 3 shows the survey area, gas line route and position of calibration points. All measured points and gas line routes are on a geo-referenced orthophoto map in scale 1:1000, with corresponding gas line parameters recorded with GPR. The AutoCAD document serves as a geo-referenced graphical database, holding information about the gas lines. In order to provide quick data review, all characteristic elements of traffic infrastructure are vectorised. Complete additional gas line infrastructure elements are cathegorized in separate layers (shafts, air-signs, markers, stations for gas measuring and flow regulation, cathode protection, gas outlets etc). Shafts and stations for gas measuring and flow regulation are measured and represented in corresponding scale. For most of measured points relative distances from fixed object (like houses, roads, lamp posts, fences, pavements, etc) are also given. Specific parts of gas line route (specified above) are clearly marked on geo-referenced map (Figure 9). All data was exported to Microsoft Excel datasheets and MS Access database.

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Fig. 9. Georeferenced graphical view of gas line route

7.2. Future pipe route scan technology

2D GPR method can be used for complete scanning of future pipeline routes, in order to prevent possible damage and collision with existing utilities. Two projects in different locations were finished in Novi Sad with analyses complete future route for new concrete sewer collector 3x3m and a water pipeline (500mm. Scanning of the future route for a sewer collector detected 25 new underground utilities and verified 47 objects from an old utility map. Figure 10 shows shape and position reconstruction of hot water pipes with diameter 400mm. These pipes are in protecting concrete duct.

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Fig. 10. Pipeline location and depth reconstruction

Scanning of future route for water pipeline detected 20 new undergound utilities and verified 24 objects from an old utility map. Beside that, on the second location GPR detected level of underground water table at 3.5m. The recorded data was used in deciding where future pipeline routes will be positioned [10].

7.3. Crossroad 3D scanning technology

Figure 11 shows a crossroad in Novi Sad. The emphasis was put on the atmospheric sewer junction under the crossroad. Crossroad coordinates measured with GPS rover Trimble 5800. This coordinates are used for crossroad map generation [3]. On figure 11 is marked GPR working area and scan sequencies. Space between scans is 1m. In right side of figure 11 are final results of the 3D processing exported into AutoCAD. The radar scan was processed with GSSI RADAN 6.0 software.

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Fig. 11. Georeferenced view of crossroad in Novi Sad with working area and pipe location

8. CONCLUSIONS

This paper presents the physical-theoretical principles of functioning of a GPR and GPS technology. The GPR can determine the lengths of straight pipeline segments, pipeline direction, directions in the horizontal and vertical planes, pipe inclination, pipe length, fluid/void ratio, changes of pipe diameter, pipe material, detection of pipe leakage, pipe radius estimation etc.

The presented of 2D/3D site scans allow 3D representation and complex analyses. Principles and technologies discussed above allow the detection of pipes and cables of different materials, groundwater level detection, the anlysis of road state, soil contamination levels, etc. Also, paper presents the basic principles and techniques of efficient mapping using a network of permanent base stations and high precision equipment. Underground pipe parameters were recorded with a two type of GPR antennas. Spatial coordinates of all characteristic points on the ground surface were measured together with high precision GPS equipment and additional information from a network of permanent base stations. All detected parameters measured by GPR and GPS were recorded as attributes in a geo-referenced AutoCAD project. Finally, further research is aimed at develop procedures for even better parameter estimation, and finding new areas of use.

9. LITERATURE

1] Geodethical Institute of Serbia: "Pipeline Cadastre Regulations", Belgrade, 1999. (in Serbian)

2] D. Daniels, "Surface penetrating radar", The Institution of Electrical Engineers, London, 2004.

3] D. Petrovački, A. Ristić, "Principles of using Ground Penetrating Radar and GPS technology for detection of underground utilities", 49-th ETRAN conference, Budva, S&M, 2005. (in Serbian)

4] J. Paniagua at all, "Test site for the analysis of subsoil GPR propagation", 10-th International Conference on GPR, 21-24 June, Delft, The Netherlands, 2004.

5] GSSI, "Radan 6 User Manual", North Salem, 2004.

6] Q. Lu, M. Sato, "Groundwater monitoring by GPR", 9-th International Conference on GPR, 21-24 June, Santa Barbara, California, USA, 2002.

7] M. Eiswirth, C. Heske, L. Burn, D. DeSilva, "New methods for pipeline assessment", World Water Congress, 15-19 Oct. 2001, Berlin, Germany

8] D. Petrovački, A. Ristić, "Application of GPS and remote sensing technologies for mapping of mid-pressure gas line network in area of Novi Sad", InterGEO East Conference, Belgrade, Serbia, 22-24 february 2006.

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10] A. Ristić, D. Petrovački, " Algorythm for radius estimation of cylindrical underground utilities ", 50-th ETRAN conference, Belgrade, Serbia 2006. (in Serbian)

11] W. Mansfeld, "Satellitenortung und Navigation", Vieweg, Wiesbaden, Germany, 1998.

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