Mapping the Underworld: locating buried assets without excavation

Trenchless International — October 2008

It is estimated that up to four million holes annually are cut into UK roads to repair or install pipes, cables and other buried assets, at a cost of £7 billion per annum. England’s Mapping the Underworld project aims to develop the means to locate and record the position of 100 per cent of buried assets without the need for excavation.

The failure to accurately locate buried assets results in needless damage to the roadway, increased congestion, potential damage to third party property and puts both the construction workers and the general public at risk of injury. Damage to the road network and the disruption to society, has been estimated to cost the UK economy £1.5 billion in direct construction costs and as much as £5.5 billion in social costs.

Mapping the Underworld (MTU) is a multi-disciplinary, multi-university research project that aims to move current capabilities in asset detection and replacement to a sustainable set of procedures, over a 25 year period. Such procedures will minimise, and ideally remove, the use of excavation associated with geophysical surveying methods while at the same time ensuring that the assets are duly located and that newly installed or repaired pipes are clearly visible during future surveys.

The first phase of the MTU project (MTU1) was a study that investigated the feasibility of:

Locating all buried asset in all types of ground conditions;

Accurately mapping the position of the assets in 3-dimensions, even in crowded high rise urban environments (so-called ‘urban canyons’);

Integrating existing data sets and new survey data into a common framework;

Creating resonating radio frequency identification tags to make new or repaired pipes more visible to ground penetrating radar (GPR) so they can be relocated efficiently; and

Investigating national and international test sites for prototype testing and operator training of shallow surface geophysical techniques used for utility detection.

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Phase 2 of the MTU project (MTU2) will research a multi-sensor device that can detect all buried assets. This project aims to improve existing surveying techniques and introduce new ones, all incorporated into a single surveying device that is intelligently attuned to the ground and from which the data sets are fused to improve the efficiency of buried asset detection.

Improving geophysical location performance

Current geophysical techniques are unable to identify the location of all buried assets in all types of ground conditions, with fluid filled plastic pipes and saturated clay soils particularly problematic.

Ground Penetrating Radar (GPR) is the primary technology used in geophysical utility location, largely due to its flexibility in operating through a variety of ground conditions and due to the speed at which it can traverse a survey area. However, the depth at which it can detect utilities is limited by attenuation in clay soils. Attempts to mitigate this, such as through reduction in signal frequency, can make smaller targets much harder to identify.

In order to identify the location of the buried assets in these conditions, Phase 2 of the MTU project aims to research and develop existing geophysical technologies and combine them with novel techniques such as GPR, acoustic, low frequency electromagnetic and passive magnetic fields to form a single surveying tool.

Combining surface transmitters with in-pipe transmitters has the potential to improve GPR performance in poor ground conditions. The feasibility study funded under MTU1 demonstrated that the in-pipe transmitter can be placed well to the side of the target cluster. Thus, broad areas beneath a road can be illuminated from within a buried pipe located near its centre.

The configuration of the heads was modelled and the technique was found to be accurate in cases where there is a single uniform soil. In the case of a two layer soil/fill combination the error induced in the reported depth by assuming just one uniform layer was generally less than five per cent. Prior knowledge of the properties of the ground conditions will improve GPR capability. Novel arrays of GPR antennas and integration of the resulting data with those deriving from ground conditions will be researched to increase the probability of target detection and accuracy of location information.

Acoustic technology

MTU1 demonstrated that acoustic technologies have the potential to be effective as a method for the detection of buried assets. Two potential acoustic techniques for locating buried services were examined in the feasibility study:

An acoustic exciter attached to an exposed section of pipe, which proved particularly effective; and,

An acoustic exciter on ground and geophones on the surface.

The results indicated that both techniques show promise. In comparison with previous techniques the proposed method controls the input energy into the pipe at each frequency and uses phase rather than magnitude information to determine the location of the pipe. A series of laboratory and field trial experiments, and associated numerical modelling, will be undertaken to optimise the use of acoustic techniques to detect and accurately locate buried infrastructure.

If a current flow can be induced in the ground, then the flow must divert around a buried pipe. Low frequency electromagnetic fields may thus be used in utility location (particularly for the case of small, near-surface services) if the diverted current can be sensed above the ground. In the feasibility study, simulations from finite-element models showed that the horizontal electric field component provides the most valuable information. Results from small-scale experimental verifications were found to confirm the finite-element modelling, showing a good correlation between the two sets of results.

Additional research is required to make the transition from laboratory to field application and signal processing techniques are required to distinguish discrete objects from continuous targets, such as utilities, to minimise the number of ‘false alerts’ registered.

Detecting field distortion

Magnetic field lines, such as the passive magnetic field line generated by buried electricity cables, are distorted by the conductivity of the soil and by metallic pipes and cables. Buried assets can be located if the distortion in such fields can be detected. The MTU1 feasibility study used simple optimisation algorithms demonstrating that the location of a single linear source of magnetic field could be approximated. The next phase will utilise an array of sensors to develop a technique to identify multiple electricity cables and other metallic services in complex, close association. This faculty is currently beyond the capacity of available detection techniques, and yet it is one that is of vital importance to those operating in the streets.

In order to maximise confidence in the survey’s outcomes, data streams from the individual sensors will be fused with each other and combined with existing records to enhance the probability of asset location. A common data representation for existing buried asset records and new survey data is required, which is being developed in a parallel project.

The prototype multi-sensor device will be trialled at specialist testing facilities overseas and at well-characterised UK sites. Since the UK currently lacks a specialist testing facility, specifications for a national test site will be drawn up.

The ground conditions can greatly influence the effectiveness of the geophysical techniques considered herein. Therefore, it is advantageous to be able to characterise the ground conditions before the site is surveyed and optimise the survey strategy accordingly. During the feasibility study this process will be undertaken in laboratory conditions using clay soils, and an apparatus suitable for ground characterisation in situ will be developed in parallel with the multi-sensor device. The geotechnical and geological information of UK surface soils and shallow sub-surface geology held by the British Geological Survey (BGS) will be harnessed to produce a knowledge-based system that allows geophysical soil data to be predicted from geographically mapped BGS data sets.

Mapping the future

Geophysical surveying techniques are noninvasive and are well suited to the location of buried assets under a carriageway. However, these techniques cannot identify all buried assets in every type of ground condition, resulting in considerable damage to the road network and increased levels of congestions within urban areas when excavation is needed. The second phase of the MTU Project aims to develop a multi-sensor device that will be able to locate all of the buried assets, utilising prior knowledge of existing asset location data and of the properties of the ground to optimise the geophysical survey strategies.

The £1.2 million Phase 1 MTU project has been funded by the Engineering and Physical Sciences Research Council and UK Water Industry Research.

The Engineering and Physical Sciences Research Council has funded £3.5 million to Phase 2 of the project.

UK Water Industry Research commissioned a state of the art review, using a site containing 37 buried assets, to assess the detection capabilities of current technology. The location and type of the utilities were known and survey companies were invited to map them using their own equipment.

It was found that only an 80 per cent detection rate, at best, might be expected with technology available at the time of the study. Current technology does not appear to have advanced to a point where it can detect 100 per cent of all buried assets.

This article is a summary of a paper written by participants in the Mapping the Underworld project. The paper, titled Extending GPR Utility Location Performance – The Mapping The Underworld Project, was presented at the 12th International Conference on Ground Penetrating Radar, held at the University of Birmingham in June.

University of Birmingham; Rogers C.D.F., Royal, A.C.D., Thomas, A.M. Metje, N., Chapman, D.N., Atkins P.R., University of Bath; Pennock, S.R., Redfern, M.A., University of Leeds; Cohn, A. G., Fu, G., University of Sheffield; Saul, A.J., University of Southampton; Brennan, M.J., Muggleton, J.M., and Swingler, S.G.