What Is a GPR Survey and How It Fits Into PAS 128 (UK Guide)
There's a phrase we hear on almost every new enquiry: "can you do a GPR survey for us?" And the honest answer is — yes, but that's probably not quite what you need.
Ground Penetrating Radar is one of the main tools used in utility detection. But it's a method, not a standard. When contractors and designers ask for a GPR survey what they almost always need is a full PAS 128 investigation — with GPR as one part of it.
This guide explains what GPR actually does, where it falls short, and how it sits within the PAS 128 framework that governs utility surveys across the UK.
AKN Engineering carrying out a GPR survey on a highways project in Northamptonshire — the cart-mounted antenna builds a continuous picture of buried services before excavation begins.
What a GPR Survey Actually Does
Ground Penetrating Radar works by sending electromagnetic pulses into the ground and measuring how they reflect back from different materials and interfaces. On a typical utility survey, a GPR antenna — usually cart-mounted and pushed across the surface — builds up a continuous picture of what's below.
It can identify:
Non-metallic services such as plastic water pipes and telecom ducts
Metallic services
Voids and obstructions
Changes in ground composition
Approximate depth and position of buried features
Unlike a desktop records search, GPR is a physical detection method. That's what makes it essential before excavation — it's telling you what's in the ground now, not what a drawing from 1987 suggests might be there.
Further technical detail on how radar interacts with different ground conditions is published by the British Geological Survey — which outlines how electromagnetic pulses behave across varying soil types and saturations.
What GPR Can't Do On Its Own
GPR is a powerful tool — but it has limitations that matter in practice, particularly on congested urban sites.
A GPR survey in isolation:
✗ Cannot guarantee every service will be located — no single method can
✗ Can produce reduced accuracy in saturated, clay-heavy, or reinforced ground
✗ Does not provide utility ownership or record information
✗ Cannot on its own determine a PAS 128 confidence level — that is assigned based on the full combination of methods used
✗ Should not replace electromagnetic location (EML) or desktop records research
That's why on highways and civils projects, GPR is deployed as part of a multi-method PAS 128 investigation — combining:
Electromagnetic Location (EML)
Desktop utility records research
Site reconnaissance
PAS 128 reporting and confidence classification
Together, these methods allow a genuinely high confidence level to be achieved. Radar alone leaves gaps. The combination closes them.
How GPR Fits Inside PAS 128
PAS 128 is the BSI specification that sets out how underground utility surveys should be planned, executed, and reported in the UK. It defines four investigation types:
→ Type D — desktop records search
→ Type C — site validation
→ Type B — physical detection (GPR + EML)
→ Type A — physical verification (trial holes)
GPR sits within Type B — the detection phase. It's used alongside EML to build the most complete picture of what's below ground before any design or excavation decisions are made.
The full specification is published by the BSI (British Standards Institution) — and sets out how these methods should be combined and reported, replacing guesswork with graded confidence levels usable in design and risk management.
So when a client says "we need a GPR survey", what they usually need is: PAS 128 Type B detection using GPR as one of the investigation methods.
When GPR Is Particularly Useful
GPR delivers most value where non-metallic services are likely, or where service density is high:
✔ Plastic water and telecom duct detection
✔ Congested urban highways corridors
✔ Pre-excavation checks ahead of bulk earthworks
✔ Drainage and service runs
✔ Reducing the number of trial holes needed
It's less reliable in:
✗ Very wet clay or waterlogged ground conditions
✗ Areas with significant reinforced concrete cover
✗ Deep metallic services in electrically noisy environments
What Good Outputs Should Include
A proper PAS 128 GPR-based survey isn't just radar data printed off a machine. Done properly, the deliverable should include:
Detection drawings showing located services
Confidence levels assigned to each service
Method statements and equipment records
Limitations and caveats clearly stated
CAD plans compatible with design software
Marked-up site photographs
Without those elements, you don't have a PAS 128 survey. You have radar data — which is not the same thing.
How Long Is a GPR Survey Valid For?
There's no fixed legal expiry date. Validity depends on risk, ground activity, and where you are in the project programme. That said, sensible industry practice follows broadly consistent rules:
Desktop Utility Search (Type D)
Normally treated as valid for 3–6 months, provided no major works have started, the area hasn't been excavated, no new connections are known, and the project hasn't moved from design to construction.
Practical rule: if your desktop search is more than 6 months old by the time you break ground, refresh it. Asset owners update drawings on their own timetables, and not every change makes it back into the record set you've been issued.
PAS 128 Type B Detection Survey
Usually treated as valid for 3 months — sometimes less. After any trenching, new utility installations, heavy trafficking, significant programme gaps, or design changes, it should be treated as requiring update.
Practical rule: if anything intrusive has happened since the survey, assume it needs repeating.
Why This Matters for CDM and Insurance
Using out-of-date utility information doesn't just create practical problems — it creates liability. A utility strike on a highways project can trigger HSE investigation, third-party claims, and CDM Principal Contractor liability. Stale data can undermine risk assessments, method statements, permit-to-dig processes, and insurance positions.
Most principal contractors treat utility information as a living record — not a one-off document filed and forgotten.
Frequently Asked Questions
Is GPR the same as a PAS 128 survey?
No. GPR is a detection instrument. PAS 128 is the specification that governs how utility investigations should be carried out and reported. A PAS 128 Type B survey uses GPR as one of its methods — but GPR data alone does not constitute a PAS 128 survey.
Do I need GPR if I have utility records?
Records alone are not sufficient for excavation safety on most UK construction projects. Drawings are frequently out of date, incomplete, or inaccurate. Physical detection is required to verify what's actually present — and PAS 128 exists precisely because records-only approaches have led to utility strikes.
How much does a PAS 128 GPR survey cost?
Cost depends on site size, access, reporting requirements, and whether EML and records searches are included. Budget for GPR + EML + records as a combined package rather than individual line items. Contact us for a project-specific quotation.
How We Deliver GPR & PAS 128
We provide:
PAS 128 Type B surveys using GPR and EML
Type C site validation
Type D desktop records searches
CAD outputs with confidence level classification
Integration with topographic survey and control networks
The aim is simple: actionable information before you break ground.
Read more about our Utilities Surveys below:
Visit www.aknengineering.co.ukor ☎️ call us on01279 927 033 to discuss your next project.