The Science Behind Utility Surveying – PAS 128 Surveys

Locating underground services is a complex and highly technical process. Utility surveying is a non-intrusive method of identifying and mapping underground services, providing accurate subsurface information without the need for extensive excavation.

At Survey Solutions, our utility surveys combine proven detection technologies, standardised workflows and PAS 128:2022 compliant methodology to give clients clear, reliable information before they break ground.

Electromagnetic Locating: Identifying Conductive Services

One of the core techniques used in utility surveying is electromagnetic locating, commonly referred to as EML.

EML is used to detect conductive buried services, such as metallic pipes and cables. This process typically involves the use of a CAT scanner, signal generator and a range of accessories, including clips, clamps and connection leads. These tools enable surveyors to apply or detect an electromagnetic signal associated with a buried service.

Where direct connection is possible, a signal can be induced into a pipe or cable, allowing the surveyor to trace its route and establish its horizontal and vertical position (depth of the service). This information is then recorded on site and processed into clear, easy-to-understand outputs, including mark-ups, CAD drawings and PDF deliverables.

Since our teams operate from offices nationwide, consistency is vital. Our standardised processing procedures ensure that the same rigorous approach is applied across every project, regardless of location, project size or delivery team.

The Role of Ground Penetrating Radar

While EML is highly effective for conductive services, there are limitations in using this technique solely, as not all buried utilities are conductive, meaning not all services can be accessed or traced using electromagnetic techniques.

This is where Ground Penetrating Radar (GPR), plays a crucial role. GPR works by transmitting radio signals into the ground. When these signals encounter a change in material properties below the surface, part of the signal is reflected to the receiver. By interpreting these reflections, trained surveyors can identify potential buried features, including pipes, cables, ducts, voids and other anomalies.

GPR is incorporated on all utility surveys that we undertake as it enables us to support and validate service locations to QL-B1 where appropriate. This level of verification provides valuable assurance prior to design, excavation or construction activity. This makes GPR valuable when investigating non-conductive services or areas where EML alone cannot provide sufficient confidence.

Understanding the Limitations of GPR

Although GPR is a powerful tool, it is important to understand that it is not infallible. There are several factors that contributes to detection success including ground conditions, soil composition, depth, material type and the contrast between the buried object and the surrounding ground.

The key factor is often the dielectric contrast between materials. Services with a strong contrast against the surrounding ground are generally easier to detect. Metallic services, such as cast iron or steel pipes, often produce clearer responses. Non-metallic materials, including plastic pipes and newer service types, can also be detected, but their visibility is more dependent on local ground conditions.

For example, clay-rich or highly conductive soils can limit GPR signal penetration, reducing the depth and clarity of results. Whereas dry, granular or more resistive ground conditions may allow for better signal performance.

Utility surveying goes beyond equipment, it requires scientific understanding of detection methods, site conditions, material behaviour, and data interpretation.

Building a More Complete Picture

The most reliable utility surveys are built from multiple sources of evidence. It is always recommended to incorporate statutory utility records alongside on-site detection techniques. These records can be supplied and overlaid onto final deliverables, helping to indicate services that may not have been detected during the survey.

When combined with EML and GPR data, statutory records provide an additional layer of context. They can highlight potential service routes, support interpretation and flag areas where further investigation may be required.

This integrated approach helps clients manage risk more effectively during the pre-construction phase.

Why PAS 128 Matters

PAS 128:2022 is the recognised specification for underground utility detection, verification and location. It provides a structured framework for utility surveys, helping clients understand the level of confidence associated with the data they receive. This is particularly important because not all utility information carries the same degree of certainty.

A PAS 128-compliant survey helps define the quality level of detected services, giving designers, engineers, contractors and asset owners a clearer understanding of risk before works begin. For clients, this means better decision-making, improved safety planning and reduced likelihood of unexpected service strikes, redesigns or costly delays.

Supporting Safer, Smarter Project Delivery

Accurate underground utility information plays a critical role in safe and efficient project delivery. It supports design development, reduces uncertainty, informs excavation planning and helps protect both site teams and existing infrastructure.

By combining EML, GPR, statutory records and PAS 128:2022-compliant processes, we provide clients with robust utility survey information that is clear, consistent and fit for purpose.

The science behind utility surveying is complex, but the objective is simple: to give clients the confidence they need to make informed decisions before works begin. For further information on EML techniques, GPR surveys, PAS 128 specifications and the limitations of underground utility detection contact Geospatial Consultant Adam Mellows on Adam.Mellows@survey-solutions.co.uk.