Ground Penetrating Radar
A Practical Guide for Project Managers, Engineers, and Site Teams, by Aidan Waszaj, Zara Omidi, and Daniel Beech.
Purpose of this Guide
Ground Penetrating Radar (GPR) is a nondestructive method widely used in construction, infrastructure, and subsurface investigations to support decision-making. However, its capabilities and limitations are often misunderstood. This guide provides practical advice on what GPR can and cannot do, factors affecting results, how to interpret outputs, and how to use findings safely and realistically.
What GPR Is and How It Works
GPR sends a short electromagnetic pulse into the ground or a structure, recording reflections from changes in material properties (dielectric constant), such as concrete-to-air, soil-to-pipe, or rebar-to-concrete. It does not "see" objects like a camera does, but detects signal responses that require interpretation with experience and context. What you get is an interpretation of subsurface conditions, supported by site knowledge, drawings, and verification.
Common Applications of GPR
Concrete scanning
Locating reinforcement, post-tension cables/ducts, conduits, slab thickness checks, and congested areas.
Utility locating and mapping
Identifying possible buried services and alignments, often best combined with EM locating and records.
Void/ground loss checks
Detecting areas that may indicate voiding, loss of support, or disturbed ground.
Roads & civil infrastructure
Measuring pavement layer thickness, subgrade changes, moisture-related anomalies, and subsurface condition mapping.
Geophysical/environmental/archaeological investigations
Mapping stratigraphy changes, buried features, targets of interest, and anomalies.
Factors Controlling GPR Performance
Wet, clay-rich, or salty materials absorb radar energy quickly, reducing depth and clarity. Dry, resistive materials allow deeper penetration.
Congested top layers in reinforced slabs can mask deeper features.
GPR detects boundaries; if the target's dielectric properties are similar to the surroundings, the reflection may be weak.
Higher-frequency antennas provide more detail at shallower depths; lower-frequency antennas reach deeper but with less detail. The targets being located also need to be bigger with lower frequencies.
Key Limitations and Expectations
GPR is a risk-reduction tool, not a guaranteed locator. It improves understanding but has limits.
Depth estimates are approximate, as they depend on material properties that vary across a site.
Detection does not equal identification: GPR highlights responses that may represent objects or boundaries, but cannot confirm what the object is without supporting evidence.
Typical Deliverables
B-scan
A vertical profile along a line showing reflections versus distance and travel time, often converted to depth. Hyperbolic shapes indicate discrete reflectors like rebar.
C-scan / depth slice
A plan-view interpretation from multiple lines, visualising patterns and zones of response. It is not a literal photo of what's below.
Recommendations and Cautions for Using GPR Results
Share drawings, records, and known constraints. Clarify objectives (e.g., locating PT ducts vs. confirming slab thickness).
Expect reduced confidence in wet/clay soils, highly congested slabs, and near edges/walls (no-scan zones).
- Use GPR as a risk-reduction input, not a guarantee.
- If the consequences of hitting something are high, plan for verification (potholing, scanning from multiple directions, complementary methods).
- Treat "no response" as "not detected," not "nothing exists."
Frequently Asked Questions (FAQ)
How deep can GPR see?+
Why does it see deeper in some slabs than others?+
Can GPR detect water and gas lines?+
Why can't you scan on metal surfaces like Q-deck/Bondek?+
Does it say on the screen that it's a conduit?+
Why is there a no-scan zone close to walls, and what does it mean?+
Can GPR tell rebar size (diameter)?+
Can GPR locate utilities under reinforced concrete?+
Final Note
GPR is a powerful tool when used with realistic expectations and good survey practice. The best outcomes occur when GPR results are interpreted within the context of site conditions and when high-risk decisions are supported by appropriate verification steps. GPR should therefore be regarded as a complementary technology that forms part of a broader investigative approach, which may also include review of available plans and records, conventional electromagnetic locating methods, and, where necessary, physical verification such as potholing or vacuum excavation.
Not sure what's under your slab?
Get in touch with South East Scanning to plan a GPR survey for your site.
