Mapping the underworld Ping Wang, Kevin Goddard, Paul Lewin, Steve Swingler 19 January 2011 Detection and location of underground power cable using magnetic field technologies

Introduction There are around 4 million holes dug by utility companies annually, involving construction projects and works in the street across the UK Before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of the cables to ensure that they are not damaged during the work The problems associated with inaccurate or incomplete location of buried cables have been very serious for some years and are getting worse as a result of increasing traffic congestion in the UKs major urban areas These problems were highlighted by the first Mapping the Underworld (MTU) project. A second project (MTU2) aims to build a multi-sensor location tool for application to the buried utility service infrastructure

MTU project Mapping the Underworld (MTU) is a multi-disciplinary, multi-university, research project that aims to develop techniques and a single shared platform to locate and map in 3-D buried utility service pipes and cables without excavation Acoustics (University of Southampton) Ground Penetrating Radar (University of Bath) Magnetic Field Technologies (University of Southampton, our project ) Low-Frequency Electromagnetic Fields (University of Birmingham) Data Fusion (University of Leeds) A four-year research project funded by the EPSRC starting February 2009 Main research areas/technologies including : For information on the project please see:

Aim of our research and general idea Aim: Use a passive array of magnetic sensors together with advanced signal processing techniques to detect underground electricity cables and other metallic objects, and to develop the technique so that it can be integrated in the multi-sensor device General idea: Measure magnetic field due to cables Geometry Current Magnetic field Estimating cable position from magnetic field Geometry Magnetic field

Coil support NI data acquisition module Test equipment Equipment (7 coils, 2 NI 9239 data acquisition modules, a laptop, and a battery) An induction coil designed for this experiment

Cable search method Measurement The support frame with its 7 search coils is placed at a number of positions above the search area, and its position is recorded The voltages induced in the coils are measured, and Fourier analysis is used to extract the 50 Hz and harmonic signal components Analysis A least square error algorithm is then applied to the resulting data in order to estimate the cable currents and residual errors for various assumed cable positions The rms amplitude of the residual errors is then plotted against the assumed cable position to give an indication of the likely locations of a buried cable

50 Hz signal with lots of harmonics Results (University car park) Signal waveform Voltage (V) Time (s)

f 3f 5f 7f 9f Amplitude (mV) Frequency analysis (fundamental f = 50Hz) 50Hz harmonics

Depth (m) Horizontal distance (m) Three things support the view that the predicted location is correct: The maximum strength of the magnetic field measured using a field meter was found close to the indicated X value The Universitys utility map shows a cable buried at X = 1.4 m The minimum fitting error of 4% to 5% is much lower than that for other locations Cable position: Depth is around 0.6 m Horizontal distance is 1.4 m 3-phase search result (Z=2m)

Conclusion The principles of our cable locating programs are introduced Experimental testing has been conducted to test our cable location method with an operational cable in one of our campus car parks The results show the measurement system and cable location method give a very good prediction of target cable

New design 27 coils Large search area Continuous measurement Give more accurate cable position Future work coil installation areas LFEM CART