GPR Limitations Explained: Why It Detects Disturbances (Not Every Utility)
When it comes to locating buried services, Ground Penetrating Radar (GPR) is often seen as the magic tool that can “see” everything underground. In reality, GPR doesn’t detect pipes or cables directly — it detects disturbances and metallic reflections beneath the surface. Understanding this distinction is vital for project managers and site engineers who rely on utility survey data to plan excavation or design works safely.
GPR identifies subsurface disturbances and metallic reflections — helping engineers map hidden utilities before excavation.
How GPR Works in Simple Terms
GPR uses high-frequency radio waves that travel into the ground. When those waves hit something with a different material property — for example, a concrete duct, void, or metallic pipe — they bounce back to the surface and are captured by the radar unit. These reflections indicate changes or anomalies in the subsurface, not the exact object itself. That’s why the interpretation of GPR data is just as important as the scan itself.
As explained by the Institution of Civil Engineers (ICE), GPR data requires careful processing and expert judgment to determine what those signals actually represent — especially in complex urban environments.
What GPR Can Detect Reliably
Metallic objects: copper, steel, aluminium, and other conductive materials show clear, strong signals.
Changes in soil composition or moisture: for example, trenches backfilled after previous works.
Voids or buried structures: chambers, ducts, or unrecorded foundations.
These readings give valuable insights for pre-construction planning, as they highlight potential risks or service routes that may need further verification.
What GPR Cannot Detect
Despite its usefulness, GPR has its limits — and that’s where some misunderstandings arise. GPR cannot detect:
Plastic pipes or fibre-optic cables (unless surrounded by disturbed soil or concrete encasement).
Live electrical current or non-metallic utilities without conductive wrapping.
Accurate depth or size without correlation from other detection methods.
As noted in the HSE guidelines for avoiding underground services, no single technique can guarantee 100% detection. That’s why survey data should always be interpreted within the wider context of existing utility records, site history, and complementary tools like EML (Electromagnetic Locators).
Why Layered Surveys Are More Reliable
At AKN Engineering, we never rely on GPR alone. Our utility surveys follow a layered approach — combining GPR scanning, EML detection, and desktop records to cross-verify all available data. This process aligns with PAS 128 standards for utility detection and classification, helping reduce the likelihood of missed services or unexpected obstructions.
By combining multiple detection methods, we can build a more complete and reliable picture of what lies underground — essential for safe excavation, accurate setting out, and efficient project planning. Learn more in our recent post: Topographical + Utility Surveys = Seamless Site Start.
Setting the Right Expectations
Clients often ask whether a GPR survey can guarantee “full detection”. The honest answer is — it can’t. But when interpreted correctly by experienced engineers and supported by other data, it dramatically reduces uncertainty. The key is knowing what the results mean and how to use them responsibly in the design or construction phase.
For project managers, this means reviewing both the raw data and the interpretation summary, and understanding that GPR identifies potential anomalies rather than giving a definitive “map” of every buried service.
In Summary
GPR detects disturbances and metallic objects, not utilities themselves.
Plastic and fibre-optic lines may remain invisible.
Combining GPR with EML and records provides the most accurate results.
Always work with experienced surveyors who understand how to interpret radar data correctly.
👉 Visit www.aknengineering.co.uk or call 01279 276030 to book your next PAS 128-compliant Utility Survey.