Underground Utility Mapping

Overview

Underground utility mapping uses the principles of geophysics to detect, locate, identify and investigate buried services.

Stratascan undertakes underground utility mapping to update utility plans to ensure that our clients can carry out safe intrusive work. We employ the techniques of Radiodetection (electromagnetics) and ground probing radar to locate utilities. The depth of investigation will depend on the conditions of the site. In general, Radiodetection and ground probing radar techniques will detect services up to 3m below the ground surface. The depth of penetration of radar can be increased but often at the expense of resolution.

Radiodetection uses the principles of electromagnetics to detect underground utilities. This principle works on the basis that a current flowing along a conductor creates a magnetic field which extends around the conductor in concentric circles. A receiver coil can be used to detect the amplitude of this magnetic field. The amplitude varies depending on the position and orientation of the receiver within the field. The amplitude is maximum when the receiver is in line with the field and directly above the conductor. Moving the receiver from side to side it is possible to follow the maximum signal response and, therefore, the line of the buried service.

Both active and passive methods are used to ensure all possible detectable services are located.

Active Methods

An active signal is a signal that has been artificially generated by an external source. Active radiodetection 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

Passive Methods

A passive signal is a signal that occurs 'naturally' on a buried conductor. Passive radio detection methods will provide an indication only that services are present underground. Passive methods use the electro-magnetic fields already present around the underground utility to locate them. Two modes are generally used:
Power -detects 50-60Hz energy present on most buried conductors
Radio - detects re-radiated radio energy often present on conductors -it is not possible to determine depth in this mode

Surveying in the traced services

As the survey progresses the positions of the detected services are marked unobtrusively onto the ground surface using chalk or spray paint. The coordinates of these points are surveyed using a total station allowing quick and accurate transfer into AutoCAD.

Ground Probing Radar (GPR)

Radiodetection techniques are unable to detect non-metallic buried services including plastic water and gas pipes and clay drainage pipes. To overcome this potentially hazardous situation, we often combine the use of Radiodetection with Ground Probing Radar (GPR). GPR is able to detect non metallic as well as metallic pipes. It will also give an indication of the location and depth of buried services but will not identify them.

GPR works by emitting a short pulse of energy into the ground. Echoes are returned from the different interfaces between different materials in the ground. As the antennae emit a "cone" shaped pulse of energy an offset target showing a perpendicular face to the radar wave will be "seen" before the antenna passes over it. resultant characteristic diffraction pattern is thus built up in the shape of a hyperbola. A classic target generating such a diffraction is a pipeline when the antenna is travelling across the line of the pipe. However it should be pointed out that if the interface between the target and its surrounds does not result in a marked change in velocity then only a weak hyperbola will be seen, if at all.

WARNING - AS NOT ALL SERVICES ARE DETECTABLE, EXTREME CAUTION SHOULD BE USED WHEN EXCAVATIONS ARE UNDERTAKEN.

Levels of Survey

Typical Application

Limitations

Level 1
Desk top survey to include a search of existing utility records and consolidation onto one plan in CAD

Low cost assessment of site to demonstrate extent of services. Often carried out by client

Low level of accuracy - may totally omit some types of utilities and recent additions/alterations

Level 2
Radiodetection survey in passive and active mode with located services being marked onto ground surface

A quick low cost site assessment mainly useful for locating cables and ferrous pipes. Useful for siting trial trenches or boreholes

Will not detect non-metallic services. No record retained

Level 3
Level 2 with located services marked onto ground surface and co-ordinates picked up with total station. Depth assessed in active mode. Plot results onto digital site plan in CAD

As above but results plotted into CAD for record use

Will not detect non-metallic services

Level 4
Level 3 & lifting of all manhole and inspection chamber, gully and valve covers. Rodding sonde or flexitrace through gravity pipes & unoccupied ducts to assess depth. Mark line of service onto ground surface and pick up co-ordinates with total station. Note contents of each chamber and levels of pipes and cover. Plot all results in CAD and produce table of MH/IC details

A "value added" Radiodetection survey particularly useful in understanding drainage and telecom ducts etc.

Will not detect plastic gas & water mains and fibre optic cabling etc.

Level 5
Level 4 & GPR survey at 1m orthogonal grid (or optional 0.5m) over targeted sections of site identified from Level 1 survey.  Collect data, process, analyse and interpret results and mark line & depth onto digital site plans in CAD

A "lower cost" GPR survey using this additional technique to understand complex areas

Some areas still not covered

Level 6
Level 4 & GPR survey at 1m orthogonal grid (or optional 0.5m) over whole survey area

The most comprehensive level of survey

Highest expectation of picking up all services and other underground obstructions

Level 7
Levels 4, 5 & 6 & Selected trial holes over targeted salient services to assess accuracy of plan and depth

A verification of the quality of the report. Normally sub-let to contractor used to excavating in various site and ground conditions

Part of the results of a typical survey of underground utilities. This shows a particularly complex section beneath a road.