Mapping the underworld

Ping Wang, Kevin Goddard, Paul Lewin, Steve Swingler19 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 UK’s 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 projectMapping 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: www.mappingtheunderworld.ac.uk

Aim of our research and general ideaAim: 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 cablesGeometry

CurrentMagnetic field

Estimating cable position from magnetic field

GeometryMagnetic field

Coil support

NI data acquisition module

Test equipmentEquipment(7 coils, 2 NI 9239 data acquisition modules, a laptop, and a battery)

An induction coil designed for this

experiment

Cable search methodMeasurement• 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

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-0.01

-0.008

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01Induced signal of the coil as a function of time

Time (s)

Vol

tage

(V

)

50 Hz signal with lots of harmonics

Results (University car park)

Signal waveform

Volt

ag

e (

V)

Time (s)

0 100 200 300 400 500 600 700 800 900 10000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

-3 Single-Sided Amplitude Spectrum of y(t)

Frequency (Hz)

|Y(f)

|

f 3f 5f 7f 9f

Am

pli

tud

e

(mV

)

Frequency analysis (fundamental f = 50Hz)

50Hz

harmonics

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

Dep

th (

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 University’s 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 mHorizontal distance is

1.4 m

3-phase search result (Z=2m)

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

- 0.2

-0.4

- 0.6

- 0.8

- 1.0

- 1.2

- 1.4

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

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