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Limitations of EML

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    Electromagnetic location (EML) is an extremely valuable technique for tracing underground utilities. However as with Ground Penetrating Radar (GPR) it is subject to some limitations (including the skill of the operator). Some of these limitations are due to the technology and others may be a result of local conditions.

    Some of the limitations of EML include:

    EML works by detecting magnetic fields, where more than one metallic service are located alongside each other those fields can become distorted. This can cause the EML device to indicate the service in the incorrect position. Whilst there are techniques to overcome this distortion, the reality is that most are impractical and unlikely to work in a real life environment.

    Some services, such as triplex cables, can indicate the peak magnetic field a considerable distance from its true location.

    Local site conditions will influence and distort nearby electromagnetic fields, these can include parked vehicles and metal fences. These factors are outside the surveyors control.

    Balanced high voltage cables do not emit a magnetic field and cannot be detected using EML unless they can be accessed and clamped. Other cables that cannot be detected include pot ends and out of commission cables.

    Non-metallic services cannot be detected or traced using EML.

    The longwave communications signal which provides the signal detected in radio mode is sometimes randomly turned off for maintenance. In these cases the radio function will not produce any results.

    The electromagnetic signal emitted by cables under load which is detected in power mode is subject to random variation due to fluctuations of load and supply to the grid. As a result it is possible for a service to be undetectable on one occasion but be easily located on another. This is outside of the surveyors control.

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    What can a Ground Penetrating Radar survey detect?

    With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

    Will GPR compromise safety on my site?

    GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

    Is a GPR survey 100% accurate?

    While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

    Is GPR equipment difficult to use?

    The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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