Underground Utility Survey | PAS 128

Use of Electromagnetic Locator (EML) in PAS 128 utility survey

Another powerful technology we employ when performing utility detection survey is EML (Electromagnetic Locator). It is one of the primary tools used if conducing utilities survey. It operates by detecting and tracing the electromagnetic signals emitted by underground utilities. Our surveyors use advanced EML equipment to precisely locate buried utility lines, enabling us to provide you with accurate information about their depth, alignment, and type.

Please note that not all utility surveys must meet level A of PAS 128 industry standards (the most precise survey involving trial pits) if you choose to keep the cost of utility survey down.

What is a utility survey?

A utility survey is a crucial step in any construction or excavation project, as it allows for the identification and mapping of underground utilities. These utilities can include electrical cables, drainage, water pipes, gas lines, telecommunications infrastructure, and more. By accurately detecting and mapping these utilities, potential conflicts and hazards can be avoided, ensuring the smooth progress of your project.

Having utility survey completed in an early stage of the development might allow your company to extend the boundary of the project. That is especially important nowadays with the ongoing shortages of developable land.

Another important aspect of utility survey is increased site safety. Utility strikes during construction or excavation projects are serious risk. Damaging existing underground infrastructure can have substantial financial repercussions on your business, not to mention injuries to the workers and public or in the worst-case scenario on site fatalities.

According to UK DIRT Report, in 2023 there were 4 million holes and excavation dug across United Kingdom every single day with 60,000 utility strikes which costed UK economy £2.4 billion. The report observed that utility strike numbers over the recent years show increasing tendency.

Utility survey performed by trusted surveying company will greatly mitigate that risk and should be considered in your utility damage prevention plan.

Using Ground Penetrating Radar (GPR) in PAS128 utility survey

Ground Penetrating Radar (GPR) is one of the primary tools used if conducing utilities survey. By sending high-frequency electromagnetic waves into the ground and analysing their reflections, GPR allows us to create detailed subsurface images. This helps us identify not only the location of utilities but also detect additional features such as voids, rebar, and geological anomalies. By combining EML and GPR data, we deliver comprehensive utility survey that leaves no room for uncertainty.

Utilities located with geophysical methods, such as Ground Penetrating Radar (GPR) meet category B at PAS 128 accuracy levels. That means you will know horizontal and vertical position (depth) of any detected object.

Ground Penetrating Radar (GPR) vs. Electromagnetic Locator

As already mentioned, Ground Penetrating Radar (GPR) and Electromagnetic Location (EML) are totally different technologies:

GPR works by sending a signal into the ground and listening for reflections from that signal and contains its own transmitter and receiver within the unit.

EML works by detecting the electromagnetic signal that may be emitted by power cables, or the energy from low frequency communications signals which has ‘coupled’ onto metal services. Alternatively, in the case of a metal service which is not a power cable, it can be induced to emit a signal by accessing the service and clamping a separate transmitter onto it.

Let us explore the main differences between those two techniques as when conducting a survey, they need to be approached in a logical way and require training and experience to produce the best results.

Based on our experience and controlled experiments, Ground Penetrating Radar will recover the locations of more underground features compared to Electromagnetic Location methods (within the penetration and resolution limitations of a GPR) in a like for like test with no manhole access.

The missing information from a GPR survey is the information which is recovered by the site reconnaissance phase of a survey (by lifting manholes and identifying services) which is usually performed alongside an EML survey as part of the process of clamping onto services to induce a signal. The other difference is that EML works by inducing a known signal onto a specific utility which is then traced, therefor EML enables services to be positively identified and tracked compared to just located by GPR.

A summary of the advantages and limitations of each method:

-GPR provides its own transmitter and is able to detect underground features without a power source

-GPR is able to detect most types of utilities including plastic and other non-metallic services, as well as metal

-GPR will also detect other below surface features (rebar, voids, structural information)

-GPR does not require manhole access

-GPR cannot identify services

-EML can detect smaller diameter cables

-EML can detect metallic services which have had a signal induced onto them

-EML can track a specific, induced signal and therefor identify individual services

-EML requires manhole access

-EML cannot recover any other below surface information.

These differences mean that the final drawing from a GPR only survey will differ from the final drawing of an EML (and site reconnaissance) only survey with the GPR detecting underground features and recovering more below surface information but unable to identify the different utilities. Whilst in the EM survey results (combined with the crucial information from site reconnaissance) services will be identified, but the drawing will lack the additional details recovered by the GPR and would not be able to detect any non-metallic services or services for which there is no manhole access.

The best utility survey is one which combine the strengths of both methods as detailed both in PAS128 and the TSA utility survey guidelines.

What is PAS 128?

PAS 128 refers to the Publicly Available Specification 128, which is a widely recognized standard developed by the British Standards Institution (BSI) for all utilities survey providers. PAS 128 provides guidelines and recommendations for conducting utility surveys and determining the accuracy level of utility information. It aims to improve the quality and consistency of utility data collection and mapping processes.

Key features of PAS 128 specification

PAS128 defines four survey levels (D, C, B, and A), each representing different levels of accuracy and data quality. These levels guide utility survey practitioners in selecting the appropriate survey method based on project requirements.

Level D: Desk-based utility records and historical information are used for preliminary identification of utilities.

Level C: Site reconnaissance and visual inspection are conducted to verify the existence of utilities.

Level B: Detection technologies, such as Ground Penetrating Radar (GPR), are used to locate and map utilities.

Level A: Precise, high-accuracy methods, such as trial pits or vacuum excavation, are employed to expose and verify the location and attributes of utilities.

How is utility survey performed?

Our utility survey processes are strict and follow the steps below:

Desk-based research

The first step in a utility survey is to conduct a desk-based study to gather information about an area. This can include obtaining utility records and plans, site history, photographs, and any previous survey data.

If you cannot provide records information (STATS) we can obtain them ourselves. STATS records offer us valuable insight into which utilities and services are supposed to be present in the ground and roughly where they may be. The STATS information is also very helpful for ruling out what is not expected within a given area. Finally, having access to STATS records can help to identify services which would otherwise be classified as unknown.

Site reconnaissance

We begin by understanding your project requirements and objectives. Our team assesses the site and reviews available plans and records to gather essential information about existing utilities.

If STATS are available surveyor will attend site, with a copy of the records confirming the presence of underground utilities where possible from manholes, buildings, and street furniture. Identifying where buried services are supposed to be located.

Field Data Collection

The surveyor will systematically lift every manhole cover, identify the service located within and trace it where possible.

Using advanced GPR and EML equipment, our skilled surveyors conduct on-site data collection. We scan the area, mark utility locations, measure depths, and capture important information for accurate utility mapping. Electrically conductive services will be traced using EML (Electromagnetic Location) techniques, where the service is clamped, and a signal is induced into it from a signal generator. The surveyor will then follow along the service using his Electromagnetic Locator to trace the signal. The location of the service will be marked on the ground in real time.

All accessible street furniture will also be accessed – for example lamp posts, road signs, telephone poles, bus stops, exterior fittings of buildings. Where possible these will also be clamped and traced.

Services that cannot be traced electromagnetically (for example, drainage or non-metallic pipes) are traced using Ground Penetrating Radar (GPR) or using a sonde – which is a signal transmitting device which can be attached to a rod and pushed down a pipe, for example a drain. Where spurious signals are received, these are traced inductively using the generator box to induce a signal along its length.

After all the above is complete, the majority of the services will have been located, identified and traced. We then sweep the entire area using passive detection methods including EML in power and radio modes, and GPR, to detect any unreported lines. Where anything is located it is also traced and added to the drawing.

After all the above is complete, the utility survey should have located, identified and traced most of the detectable services within an area. The location of every pipe and utility is recorded using topographic survey methods, such as GPS or Total Station and sent back to the office for drawing.

Data Analysis and Mapping

To ensure that our results are as accurate as possible our team processes and analyses the collected data using specialized software. We interpret the GPR and EML results, identify utility locations, and create detailed utility maps and reports.

Deliverables

We provide comprehensive deliverables that meet your specific needs. This includes utility maps, CAD drawings, geospatial datasets, and detailed reports outlining the location and characteristics of detected utilities.

How much does utility survey cost?

Utility survey cost is estimated on the project-to-project basis.

When requesting a quotation, it would be helpful if you could include a clear survey boundary (something like a google maps image or a topographic survey, with a clear line showing the extents of the area), in addition it would be helpful if you can specify what kind of survey you would like (if you know).

If you are unsure exactly what you need, contact us and we can discuss your requirements in more detail.

For more information about utility surveys, or to request a quote. Please contact us at info@kbgprsurveys.co.uk or fill in the contact form below.

Why choose us?

	Our utility surveyors have extensive expertise and experience
	Our utility surveyors were involved in major airport and highways infrastructure projects
	We are trusted by large construction companies and local councils
	We are honest and reliable in dealing with all our clients