A GPR survey uses ground penetrating radar to see beneath a surface without breaking into it. It sends short pulses of radio-frequency energy into concrete or ground, reads the signals that reflect back from buried objects, and builds a picture of where those objects are and how deep they sit. Surveyors use it to locate reinforcement, buried services, voids and other hidden features before anyone drills, cores or digs.
Because it works from a single face and uses non-ionising energy, GPR has become the standard first step wherever you need to know what is inside a slab or under the ground before committing to a penetration. This guide explains how the method works, what it can and cannot find, and how a GPR survey is used on real sites.
How ground penetrating radar works
A GPR unit has an antenna that transmits a pulse of radio-frequency energy into the material below it and then listens for the echoes. When that energy meets an object or a boundary between two materials, such as a steel bar in concrete or a pipe in soil, part of it reflects back to the antenna. The time the echo takes to return indicates depth, and the strength and shape of the reflection tell an experienced operator something about what caused it.
The surveyor moves the antenna across the area in a controlled pattern, and the individual reflections combine into a cross-section of what lies beneath. This is why interpretation matters as much as the equipment: GPR shows where reflections occur, not labels naming each object. Reading that data reliably is a skill, and it is the single biggest factor in whether a survey is worth the paper it is recorded on.
What a GPR survey can find
The same underlying method is used in two quite different settings, concrete elements and open ground, and it finds different things in each.
In concrete, a GPR survey typically locates:
- Reinforcement position, spacing and cover
- Post-tension cables and tendons before coring or drilling
- Electrical conduits and other embedded services
- Slab thickness and, where accessible from one side, the structural build-up
- Voids and anomalies within the element
In the ground, a GPR survey helps locate:
- Non-metallic buried services such as plastic pipes, ducts and fibre optics
- Voids, backfill and disturbed ground
- Buried structures, foundations and obstructions
For underground utilities, GPR is rarely used alone. It is paired with electromagnetic location, which finds metallic and conductive services that radar can miss, so that the two methods together give far more complete coverage. That combined approach is the basis of a PAS 128 utility survey.
GPR through concrete versus GPR through ground
The physics is the same, but the conditions are different. Concrete is a relatively consistent material, so radar can resolve closely spaced targets like rebar mats with good detail, though signal penetration is limited by the depth and density of reinforcement. Wet or heavily congested slabs are harder to read.
Ground is more variable. Clay soils, high moisture and made-up ground all absorb or scatter the signal and reduce the depth radar can reach. Sandy, dry and uniform ground gives the best penetration. A competent surveyor selects the antenna frequency to suit the job, higher frequencies for fine detail near the surface, lower frequencies for greater depth at coarser resolution, and manages expectations about what is achievable in the conditions on site.
Strengths and limits
GPR earns its place because it is non-destructive, non-ionising and needs access to only one face of the element or the ground surface. That means it can be used in occupied buildings, on live sites and over large areas quickly, without the exclusion zones and two-sided access that older radiographic methods require.
The limits are just as important to understand. GPR does not name objects, so results depend on interpretation. It struggles where reinforcement is very dense, where the ground is highly conductive, or where a target is too small or too deep for the antenna in use. No survey can promise that every object has been found, which is why GPR is treated as a tool for reducing risk, not eliminating it, and is combined with safe working practices at the point of drilling or digging.
What you get from a survey
On a concrete job, the core deliverable is usually a set of markings applied directly to the surface, showing reinforcement, detected services and the clear zones where it is safe to core or cut. Where a permanent record is needed, annotated drawings capture detected features, estimated depths and safe penetration points.
On a utility survey, the output is a coded drawing, normally in CAD, showing the located services with a survey classification and a method statement setting out how the work was done and its limitations. The right deliverable is agreed before work starts so you get a record matched to the decision you need to make.
Common questions
Is a GPR survey safe to carry out in an occupied building?
Yes. GPR is non-ionising and emits very low power, so there is no radiation risk to operators, workers or building occupants and no need to clear the surrounding area. This is a key advantage over X-ray methods, which require exclusion zones and access to both sides of the element.
How deep can ground penetrating radar see?
It depends on the material and the antenna. In good, dry, uniform ground a lower-frequency antenna can reach several metres, while in concrete or conductive clay the useful depth is much less. The surveyor selects the frequency to balance depth against the level of detail your job needs.
Can GPR tell the difference between rebar and a cable?
Not automatically. GPR shows the position of reflections, and distinguishing reinforcement from a conduit or a post-tension tendon relies on skilled interpretation of depth, pattern and layout, often combined with electromagnetic detection. This is why an experienced operator is central to a reliable result.
Do I need any other survey alongside GPR?
Often, yes. For underground utilities, GPR is paired with electromagnetic location so that both metallic and non-metallic services are covered. In concrete, cover meter or ferro scanning may be added to confirm rebar cover and bar size.
If you are planning to core, cut or drill and need to know what is inside a slab first, our concrete scanning service locates reinforcement, services and post-tension cables and marks the safe zones on site. If your work disturbs the ground, a PAS 128 utility survey maps the buried services before you dig. You may also find our guides on ferro scanning and concrete scanning versus X-ray useful when scoping the right approach.