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| A History of Underground Electromagnetic Surveying
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Article Author:
Josh Whiten
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The phenomenon of electromagnetic induction was first discovered by Michael Faraday in 1831, and it is evident that he realised at least a part of its future potential in the modern world. A contemporary politician asked him about the usefulness of the discovery; he answered 'at present I do not know, but one day you will be able to put a tax on it.'
The earliest record of using electromagnetic technology to locate buried cables dates from around 1910. More portable locators were made over the next years and the Sharman Main Finder was just one example. The user instructions give a tinge of envy to anyone trying to trace gas pipes .. ..'just clip the generator to a gas bracket in the nearest house or onto a street lamp.'
American and German schools of design emerged during the years leading up to the Second World War. In North America roads were wide and wide cables hung on poles so the main requirement for a locator was to locate widely spaced buried pipes. The result was a simple, high frequency, low power and low cost locator.
In Germany, cables as well as pipes were buried under narrow streets, so elaborate low frequency and high power locators were developed that required considerable expertise to obtain satisfactory results.
Dr Gerhard Fisher of California designed the Metallascope, the first high performance buried pipe and cable locating set. His system made use of the latest scientific developments and his company exists today and still produces the M-scope, an up-to-date descendent of the original Metallascope.
One of the engineering sections of Bell Laboratories studied the problem of accurate location of newly buried cables and recognised that an antenna with twin sensing aerials would give more positive plan definition, and also measure the depth of a target cable. The subsequent design, called the Depthometer, was engineered and manufactured in 1964. It was another 12 years before the first commercial twin aerial antenna locator was made by the Electrolocation company in Bristol England.
The twin aerial system was found to have substantial advantages over single aerial locators. Twin sensing aerials combined the seemingly contradictory qualities of discrimination with sensitivity. For the first time it was possible to locate buried cables below an overhead power line and to sort out crowded utility services under a city street intersection.
The introduction of the twin aerial antenna coupled with miniaturised electronic circuitry coincided with a programme of extending and upgrading utility distribution systems. This growing demand and technical progress resulted in a series of advances and new features to make locating more certain and more simple. Some of these advances included:
• Combination of active and passive signal reception
• Multi-frequency locating sets enabling the user to select the most suitable frequency for each application
• Electronic depth measurement.
• Current measurement along the length of a pipe or cable to detect coating or insulation defects.
• Current direction recognition to verify the identity of a target line.
• Permanently installed signal transmitters to apply a signal tone to a telephone cable over distances up to 150km/100 miles.
Today, electromagnetic locators are the worldwide standard for locating buried pipes and cables. A number of specialised manufacturers offer a choice of locators ranging from simple equipment used to detect the presence of buried cables to sophisticated instruments for pinpointing, identifying and fault finding buried pipes and cables in the most complex situations.
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Written by Select Surveys, one of the UK’s leading independent surveying companies specialising in using electromagnetic, CAT and ground penetrating radar equipment to detect underground cables and utilities. |
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