Underground Utility Survey

A survey of the underground services affecting the project area and detection of the position and depth of all buried lines was conducted using a combination of the following techniques:

a) Passive Detection

Detection of the power signal radiated by loaded electric power cables and on nearby conductors. This is the signal that occurs naturally on a buried metal pipe or cable (e.g. 50/60Hz energy of VLF radio energy) used for locating unknown lines in a search area. Can be used in either power mode (to detect the 50/60Hz energy present on buried cables and other lines) or radio mode (to detect buried lines as they re-radiate very low frequency radio energy). The signal emitted by the target line forms a cylindrical shape around the line which can be detected by the receiver. It is suitable for the detection of most metal pipes and cables.

b) Active Detection

Detection of the signal originating from distant radio transmitters or a signal applied to a line using a transmitter that penetrates the soil and is re-radiated by buried conductors. The transmitter is used to apply an easily identified signal to a target line for it to be traced, identified and pin-pointed using the receiver. The transmitter can be connected directly to the target line or (if not accessible) can be set approximately above the target line. It is suitable for the detection of most metal pipes and cables.

The transmitter can either be used for:

a) Direct connection to a Transmitter, or

b) Induction.

Direct connection involves the application of an active signal to a conductor using a clamp.

Induction involves a signal being radiate from the internal antenna of the transmitter which is induced to any conductors in the vicinity and re-radiated. Generally 8, 33 or 56 KHz frequencies are used. It is often possible to determine the depth of the service in this mode up to an accuracy of ±10 centimeters.

GeoMak Survey Servicesis equipped with Radio Detection RD-7000 Precision Locators. The location of underground utilities is determined using the units operating on one of the following principles:

• Detection of the power signal radiated by loaded electric power cables and on nearby conductors;

• Detection of the signal originating from distant radio transmitters that penetrates the soil and is re-radiated by buried conductors.

The signal emitted by the conductor forms a cylindrical shape around the conductor. The unit then gives a response when it is over a conductor carrying some form of signal. The Radio Detection RD-7000 is used to pin point the utility more accurately and also to determine its depth below grade level.

For the most reliable determination of position, the optimum method is to clamp the Frequency Unit onto an exposed pipe or cable whereby a signal is then transmitted along the utility and can be detected using the Receiving Unit. A maximum signal is returned when the instrument is located directly over the conductor and this position or series of positions can then be marked on the ground. These positions can then be rapidly coordinated using either tacheometric observations (total station) or kinematic observations (GPS) and the data recorded and downloaded for computation and production of drawing files.

The Radio Detection RD-7000 is a Precision Locator comprising a receiver and transmitter for pinpointing buried utility lines in congested or complex situations. The receiver has a microprocessor driven display that provides information about the buried lines, including the amount of current received on the target line and depth measurements.