What Depth can you Locate a Utility Service?

Subsurface Mapping
Severn Partnership have developed its in-house utility detection capabilities in line with demand and now of- fer bespoke Subsurface Mapping surveys and consultations to meet any client objective. Severn Partnership’s ability to provide a complete mapping service of above and below ground infrastructure, will offer clients multiple benefits that include cost savings, greater accuracy and one point of contact for all your Topographic and Utility survey needs.

We are often asked by clients “To what depth can you detect utility services down to”. Unfortunately there is not one simple answer. It depends on multiple variables that include:

• Detection technique used (GPR, EM location, geophysical technique etc
• What services can be detected by which method (eg. plastic pipes can not be located by Electromagnetic (EM) location)
• The service (what is it made of, how large is it, what is inside it etc)
• Ground conditions and obstructions - local site conditions can affect certain techniques
• Multiple services in a close proximity can mask one another
• Below is a summary of the main techniques used in utility surveys with a brief description of depths you can expect to achieve

Ground Penetrating Radar (GPR)
Most high end GPR systems used to locate utility services have a dual frequency antenna. The frequency used controls how deep the GPR can image. Low frequency radar systems can see deeper than high frequency antennas but have a poorer resolution due to a larger wavelength. This means smaller utility services may be missed. Using a dual frequency antenna means you have the best of both worlds.

Typically in the UK GPR can image to approximately 1.5 and 2.5m below ground level, although this can be much less in certain conditions such as clay rich soils. On the other hand, large features in good conditions may be detected to 3 or 4 meters below ground level. Larger diameter services are detectable to a much great- er depth than small diameter services.

The material of the service will also affect the depth that it can be detected to. Metal pipes will generally have a greater contrast to the ground material than plastic or clay pipes meaning they can be detected to a greater depth.

Main Advantages

• Very accurate
• Can detect anything
• Can post process data - higher quality assurance levels achievable

Main Disadvantages

• High cost
• Slow data collection
• Can only be collected on clear open ground

Electromagnetic (EM) Location
EM locators, such as an RD8000 or RD8100 have a maximum line detection depth of 6m. The depth a buried conductor can be located to is proportional to the strength of the signal it is able to carry. It is very important to understand what types of services EM locators are capable of detecting and what they are not.

There are multiple modes that the EM locator can be used in. All of which should be utilised during a util- ity survey. The modes do not dictate the depth a service can be detected to but do affect which ones can be located.

The main 3 modes are:

1. Passive - Receiver only. Will only locate buried conductors carrying a background signal (for example when street lights are not switched on the circuit is incomplete and the power cable will not be detectable)
2. Induction - Transmitter used to induce a current on a service which is then located using the receiver.
3. Direct Connection - using transmitter to directly place a current on a service. Most accurate way to locate and measure depths.

Main Advantages

• Fast data collection
• Can be used over almost any terrain
• Low cost

Main Disadvantages

• Less accurate than GPR
• Can only detect services capable of carrying a current (no plastic or clay pipes)
• Data collected in field by operative. No check on how well surveyed

Sondes
Sondes are a device that emit an electromagnetic signal at a specified frequency. The Sonde is directly insert- ed into a service such as a sewer or culvert at an access point (manhole, gully, headwall etc).

The Sonde can be attached to either rods or a flexible carbon fiber reel and pushed along the service. The Sonde can then be located from the surface using an EM locator. A standard Sonde can locate to a depth of approximately 5 m. A super Sonde can locate to a depth of 15 m but has a minimum detection depth of below 3 m. Can only be collected on clear open ground.