The project aims to create a prototype multi-sensor device, and undertake fundamental enabling research, for the location of underground utilities by combining novel ground penetrating radar, acoustics and low frequency active and passive electromagnetic field (termed quasi-static field) approaches. The multi-sensor device is to employ simultaneously surface-down and in-pipe capabilities in an attempt to achieve the heretofore impossible aim of detecting every utility without local proving excavations. For example, in the case of ground penetrating radar (GPR), which has a severely limited penetration depth in saturated clay soils when deployed traditionally from the surface, locating the GPR transmitter within a deeply-buried pipe (e.g. a sewer) while the receiver is deployed on the surface has the advantage that the signal only needs to travel through the soil one way, thereby overcoming the severe signal attenuation and depth estimation problems of the traditional surface-down technique (which relies on two-way travel through complex surface structures as well as the soil). The quasi-static field solutions employ both the 50Hz leakage current from high voltage cables as well as the earth's electromagnetic field to illuminate the underground infrastructure. The MTU feasibility study showed that these technologies have considerable potential, especially in detecting difficult-to-find pot-ended cables, optical fibre cables, service connections and other shallow, small diameter services. The third essential technology in the multi-sensor device is acoustics, which works best in saturated clays where GPR is traditionally problematic. Acoustic technology can be deployed to locate services that have traditionally been difficult to discern (such as plastic pipes) by feeding a weak acoustic signal into the pipe wall or its contents from a remote location. The combination of these technologies, together with intelligent data fusion that optimises the combined output, in a multi-sensor device is entirely novel and aims to achieve a 100% location success rate without disturbing the ground (heretofore an impossible task and the 'holy grail' internationally).The above technologies are augmented by detailed research into models of signal transmission and attenuation in soils to enable the technologies to be intelligently attuned to different ground conditions, thereby producing a step-change improvement in the results. These findings will be combined with existing shallow surface soil and made ground 3D maps via collaboration with the British Geological Society (BGS) to prove the concept of creating UK-wide geophysical property maps for the different technologies. This would allow the users of the device to make educated choices of the most suitable operating parameters for the specific ground conditions in any location, as well as providing essential parameters for interpretation of the resulting data and removing uncertainties inherent in the locating accuracy of such technologies. Finally, we will also explore knowledge-guided interpretation, using information obtained from integrated utility databases being generated in the DTI(BERR)-funded project VISTA.
GPR is used to ‘see through’ the ground, either to establish the structure of the ground or to find buried objects. Here we seek to detect pipelines that might be made of a variety of materials (e.g. metal, plastics, ceramics, concrete) with a variety of contents (e.g. water, gas, optical fibre cables). The radar signal, an electromagnetic wave, is transmitted into the ground. Reflections, whether from sub-soil interfaces or the buried objects, are captured by a nearby surface-mounted receiver and the data interpreted.
GPR operates on very short ranges, often over 1 or 2 metres range, unlike radars used for navigation of ships and planes. This research looked into more novel technological approaches to accurately discern the targets, in particular looking at Orthogonal Frequency Division Multiplexing (OFDM), Frequency Modulated Continuous Wave (FMCW) and Step Frequency Continuous Wave (SFCW) techniques. OFDM is used in Digital TV, Digital Radio and Wireless Local Area Networks and provides very good, and adaptable, control of the radiated pulse shapes and frequency content. FMCW and SFCW also possess this advantage, but to a lesser degree. A technique for improving FMCW GPR was developed where linearising the frequency sweep more accurately, allowed clearer resolution of targets.
In addition the notion of placing the transmitter or receiver in a buried pipeline was investigated. This gives a one-way signal propagation path that can potentially greatly increase the range of measurement through the ground. To realise these schemes research was also undertaken into long, thin ultra-wideband antennas that can fit into pipes. This in-pipe scheme was seen to quite easily identify the permittivity of the ground surrounding the pipes and targets. This information is very useful in improving our ability to focus GPR data into images of the sub-surface.
The MTU research has also improved our understanding of propagation effects that can cause the well known problem of poor GPR detection of cast-iron pipe targets. This came about due to the wide range of expertise within the group on electromagnetic propagation and device modelling combined with expertise on decay processes of pipes within soils. Extensions of this research indicate that the propagation effect is also likely to be applicable to bitumen-coated or leaking gas pipes, which can also be problematic to detect with GPR.
Part of the research is concentrated on signal processing and focussing techniques to remove the clutter of unwanted reflections, hence making the pipes more clearly visible. Moreover, it produces images that are more easily understood than traditional data presentation methods. The signal processing techniques developed also allow data from different sensor systems, such as acoustic and magnetic field data, to be combined in a manner that reinforces the identification of the targets.
A significant focus of the MTU research was investigating ways to make a step change in utility surveying practices with a truly integrated approach to analysis and interpretation taking advantage of all possible information. In combining sensor packages their mutual interaction was minimised by appropriate physical layout. As expected, combining data from the four sensors has been shown to produce a more reliable assessment of buried targets and therefore increased the confidence in utility location. This is most apparent when the sensors produce target signatures that agree in plan and depth, and therefore reinforce each other. The diversity of approaches means that where one or more of the sensors are performing poorly due to ground conditions, the remaining sensors can still providing target information. Thus combining data from several sensors produces a more resilient system that can operate in a wide variety of situations and detect a wide variety of targets.