history of electromagnetic surveying

Electromagnetic induction has always been the most widely used technology for locating buried totality pipes and cables.

The phenomenon of electromagnetic induction was discovered by Michael Faraday. In 1831 he presented a paper to the Royal Institution in London describing the characteristics of the phenomenon and it is evident that he realised at least a part of its potential in the modern world. There is a story that a 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. The photograph at the top of the page shows a cable locator made from a coil wound round a wooden truss; the coil is just visible at the front of the truss.

More portable locators were made over the next years. The Sharman Main Finder is 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.'

An American and a German school of design developed over the years leading up to the Second World War. Roads in North America had wide cables hung on poles and 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. Germany, with cables as well as pipes buried under narrow streets, developed elaborate low frequency and high power locators that required considerable expertise to obtain results.

Dr Gerhard Fisher of California who obtained the first patent ever issued for an aircraft radio direction finder 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 produces the M-scope, an up-to-date descendent of the original Metallascope.

One of the engineering sections of the Bell Laboratories studying the problem of accurate location of their newly buried cables recognised that an antenna with twin sensing aerials would give more positive plan definition and also measure the depth of a target cable. The design, which was called the Depthometer was engineered and manufactured by the Western Electric Company for use by the Bell Operating companies 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 company, which later became Radiodetection Ltd developed the twin aerial system without being aware of the earlier Bell Labs developments.

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.

Push button electronic calculation of pipe or cable depth was an obvious advantage and so was a completely new way of locating buried cables as they re-radiate VLF radio energy.

The introduction of the twin aerial antenna coupled with miniaturised electronic circuitry coincided with a programme of extending and upgrading utility distribution systems: digging in the street became a national industry in many countries and this led to an increasing need for locating buried pipes and cables.

Growing demand and technical progress resulted in a series of advances and new features to make locating more certain and more simple. Among the more interesting advances:

• Combination of active and passive signal reception to locate a target line and make a quick sweep to check if other utilities are present.
• 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.
• A range of accessories and equipment to increase the usefulness of the locator and widen its scope.

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 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.