In Practice

As can be seen from above the GPR is not looking for a pipe or a service it is purely looking for significant contrast in dielectric properties. The type of ground that the service is laid in is therefore of great significance. Broken and
granular ground makes it more difficult for the GPR as does the size of the service. The smaller the service the more difficult it is to detect. Generally the larger the service and the more uniform the surrounding ground the better the detection rate. Features close to the surface can make detection with GPR impossible. Root infestation can prevent any significant waves passing through creating “noise” in the radar and preventing detection below them. The same is true when reinforcing mesh is placed in concrete roads and yards. When services are laid close to each other the reflection from the uppermost service may mask the services below as the tail of one hyperbola
masks the peak of another thus preventing detection.

In Summary

GPR is the last tool employed in utility surveying and is used to try and verify services detected thus far and to try and locate any other services that are suspected to be in the area. If the ground conditions are such that significant contrasts in dielectric properties between the ground and the services are not achieved then services will not be detected using GPR. Services running close to each other may mask other features. Surface features such as roots and reinforcing may prevent any detection below them. Large diameter services in ground that has a significant contrast in dielectric properties to the service are most easily located. Depth can be detected, and is within +/- 10% of the depth, ie at 2m depth the accuracy should be in the range of 1.8 to 2.2m from the top of the pipe or duct to the ground level.

* Insulator or non conductor of electricity, such as rubber, glass, and paraffin wax

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Passive signals therefore enable conductors to be located, but not identified, because the same signals may appear on any conductor. An active signal is a signal that has been artificially generated by an external source. Active radio detection methods can allow different services such as BT and electric to be individually identified. Active methods involve the use of a transmitter.

The transmitter can either be used for direct connection or for induction. Direct connection involves the application of an active signal to a conductor using a clamp. Induction involves a signal being radiated from the internal antenna of the transmitter which is induced to any conductors in the vicinity and re-radiated. Generally 8, 33 or 65KHz frequencies are used. It is often possible to determine the depth of the service in this mode.

■ Direct connection - a signal from signal generator is connected to a pipe or cable at an access point such as a valve.

■ Induction - a signal is induced into a buried conductor by placing a signal generator over a known section of the conductor.

If it is possible to gain access to the interior of a non-metallic gravity pipe or duct, a sonde can be entered and pushed along on a rod. The signal from the sonde is typically 33KHz frequency and can be located on the surface, so tracing the line and depth of the pipe or duct.

In Practice

A passive signal is of limited use in identifying services for the reasons described above, its primary use is for cable avoidance and providing a “general sweep” of an area.

An active signal using direct connection is always the preferred method of detection and identification because in theory it is unique. This can obviously only be used if there is a surface feature that can be connected to such as a street light or if an

access chamber is present that will allow direct connection to its contents. Direct

connection however still relies on the conductivity of the service and complications can arise when services are in close proximity, cross or change material type along their length. The connected signal will always opt to take the easiest route ie follow the most conductive material. Should the service that has been made active say a ductile iron main be crossed or be in close proximity to say a street light cable or redundant

communications cable then there is a reasonable chance that the signal will “jump” to the cable. This will only be apparent when the operator and detector reach the street light!

An active signal may be induced into the ground without a direct connection. However as can be seen from the scenario above there can be no certainty as to what is being traced as the signal will follow the most conductive material and it is again possible for signals to jump from one service to another when they cross or are in close proximity.

The introduction of a sonde into a pipe or duct generally proves to be very reliable. Obviously it can only be used if surface manholes or duct chambers are present. It is not possible to place a sonde into ducts carrying communications or other supplies for risk of entanglement and drainage is the most frequent application. Y connections within a run will not be detected and CCTV would need to be employed for this unless access there is rodable access to all Y runs within the survey area. The use of the sonde is also limited by blockages and distance between access points.

In Summary

In Summary

Radio detection is a reliable means of detection and identification of services, high end equipment can identify depth as well as location. Location typically within 150mm and depth as good as 5% of the depth so at 1m within 0.95 to 1.05m is achievable. An active signal by direct connection is the preferred means of detection although complications can still occur when services cross or run in close proximity. Non conductive services are unlikely to be traced with radio detection although a sonde can be used if entry to the pipe or duct is possible.