Ground Penetrating Radar (GPR) Surveys

Radiodetection techniques are unable to detect non-metallic buried services such as plastic water and gas pipes, fibre-optics and drainage pipes that we cannot introduce a sonde into. To overcome this potentially hazardous situation, we combine the use of Radiodetection with Ground Penetrating 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.

We have also been commissioned to used GPR on a number of occasions now to locate the extents of the roots of mature trees, this is invaluable for a site where there are TPO's in force in the vicinity of the development.

Click here to view the results of a survey of tree roots that we have recently carried out.

We have also used GPR to locate voids, as well as RSJs in platforms and walls, and determine the thickness of concrete slabs and asphalt.

GPR does NOT trace services, but indicates the presence of voids, disturbed ground, trenches, pipes and cables under the surface. 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.

Laser Surveys hold an Ofcom Licence which is a legal requirement in order to use GPR equipment.

The benefits of using Ground Penetrating Radar

GPR is able to detect metallic and non-metallic services such as polyethylene water and gas pipes as well as utilities such as fibre-optic cables which are not normally traceable using any other method.

Using GPR alone we cannot guarantee finding or identifying all buried services, but when used in conjunction with Radiodetection the most complete answer available is produced.

All GPR survey scans are recorded on-site and post-processed and analysed in the office before inclusion on existing digital site drawings or OS mapping.

The European GPR Association is the regulatory body for GPR and its remit is 'to promote the understanding and use of GPR throughout the world'.
EuroGPR provide guidelines for the use of GPR and have a Policy Statement which outlines GPR Survey Strategies for the location of underground utilities and also outlines the advantages and disadvantages of 'on-site' and 'off-site' interpretation of GPR scans recorded.
Click here to view a copy of the EuroGPR Policy Statement for Utility Detection. For more information on the brief and aims of the Association visit - www.eurogpr.org

Survey Methodology

Firstly using Radiodetection, a survey is carried out of the services that we can identify from the presence of inspection covers on and around the area of interest. Secondly using Radiodetection, we would carry out a scan of the survey area searching for passive signals that may exist in buried cables. Finally we would carry out a GPR Survey with 5m cross sections using a 500mhz antenna in order to locate services up to a depth of 2m. A further scan using a 250mhz antenna would be carried out on a 10m grid in order to locate services up to 3.5m deep.

GPR is an echo sounding method where a transducer (transmitter/receiver) is passed over the ground. The transmitter sends out low powered radio energy and reflections from material boundaries and embedded features such as metal, voids or buried cables are picked up by the receiver. The results are viewed on-screen and recorded onto disk for later analysis in the office. Radar is effective on rough surfaces and through multi-layered media, but penetration and resolution is hampered when "looking" through highly conductive materials such as multi-layered reinforcement, ash or clinker, saline saturated material and wet clay.

Survey Effectiveness:

The material surrounding, and particularly above buried services can affect whether the service can be resolved by GPR. Reinforced concrete, buried metallic objects, the presence of moisture or clay and changes in the material construction can lead to poor data resolution.

Depth results using GPR are typically accurate to within +/- 10% with a horizontal accuracy of +/- 150mm. Radiodetection is able to penetrate far deeper than GPR and has an accuracy of +/- 10% of depth for positional accuracy.

The effectiveness of GPR is reduced on slopes of greater than 1:3 and in woodland areas where the signal is denigrated by tree roots.

It is imperative that the area to be surveyed is reasonably level as it is not possible to scan deep vehicle ruts, etc.

It is not possible to carry out a GPR survey in the areas of shrubbery, undergrowth and flowerbed unless the site is cleared.

Deliverables:

Deliverables would comprise an annotated drawing (DWG & PDF formats) illustrating the service location and a short form written report.

Health and Safety:

All work would be conducted within the guidelines of Laser Surveys Limited's Health and Safety Policy and standing instructions.

A site specific method statement and risk assessment will be prepared before commencement of the work.

Click here to view our 'Practical Guide to Underground Services Location & CCTV Drainage Surveys'.