Ground penetrating radar services and electromagnetic locating are the two main ways to find underground utilities without digging, and they detect different things. Electromagnetic locating traces metal and anything that carries a signal.
Ground penetrating radar services and electromagnetic locating are the two main ways to find underground utilities without digging, and they detect different things. Electromagnetic locating traces metal and anything that carries a signal. Ground penetrating radar, or GPR, sees changes in the ground itself, including plastic and concrete pipes. Neither finds everything, which is why a proper locate uses both.
How electromagnetic locating works
An electromagnetic locator detects the magnetic field around a conductor carrying an alternating current. The locator either picks up a field that is already there, such as a live power cable, or puts one there with a transmitter.
There are three ways to apply the signal:
- Direct connection. The transmitter is clipped to the service at a valve, meter or pit. This gives the cleanest trace.
- Clamp. A ring clamp induces the signal onto a cable without touching the conductor.
- Induction. The transmitter sits on the surface and broadcasts into the ground. Quick, but the signal lands on everything nearby.
Once the signal is on the line, the locator follows it along the surface, marks the alignment and reads an estimated depth.
What electromagnetic locating misses
It needs a conductor. PVC and polyethylene water and gas pipes, vitrified clay sewers, asbestos cement mains, concrete stormwater and empty conduits carry no signal at all. Some plastic gas mains were laid with a tracer wire for this reason, and where that wire is broken the pipe is invisible again.
The other weakness is congestion. In a footpath with six services side by side, the signal bleeds from the target onto its neighbours. The locator still shows a strong, confident reading. It may be the wrong service, and the depth may be distorted. Experienced operators check from more than one connection point for exactly this reason.
How ground penetrating radar works
GPR sends a short radio pulse into the ground and records the reflections. A reflection occurs wherever the material changes: soil to pipe, soil to void, soil to rock. As the antenna rolls across a pipe, the reflection forms a curved shape on the screen that the operator reads as a target.
Because it responds to a change in material, not to conductivity, radar can detect plastic, clay and concrete pipes as well as metal ones. It also finds things that are not utilities, such as underground tanks, old footings, voids and trench edges.
Where GPR struggles
This is the part radar brochures leave out. GPR performance depends on the ground more than on the machine.
- Clay and wet soil absorb the signal. Much of Western Sydney sits on reactive clay, and penetration there can be shallow. Sandy ground in the Eastern Suburbs and along the coast is far kinder.
- Depth and resolution trade off. A high frequency antenna sees small targets but only at shallow depth. A low frequency one goes deeper and loses the small ones.
- Rubble and fill create clutter. Old demolition fill produces reflections everywhere.
- Small, deep targets disappear. A thin conduit a couple of metres down in clay is unlikely to show.
GPR also does not identify what it has found. It shows that something is there. Working out whether it is a water service or an old drain takes records, pit inspection and judgement.
GPR vs electromagnetic locating at a glance
| Electromagnetic | Ground penetrating radar | |
|---|---|---|
| Finds metal pipes and cables | Yes | Yes |
| Finds plastic, clay, concrete pipes | Only with tracer wire or sonde | Yes, in suitable ground |
| Identifies which service it is | Yes, when directly connected | No |
| Affected by soil type | Slightly | Heavily |
| Finds tanks, voids, footings | No | Yes |
Why a proper locate uses both
The two methods cover each other's blind spots. Electromagnetic tracing identifies a service and follows it. Radar picks up the non-conductive assets and the unknowns in between. A sweep with one method only is a partial answer.
That is the standard approach for utility locating to quality level B under AS 5488, the classification published by Standards Australia. Results from both methods are checked against Before You Dig Australia plans and against what is visible in pits.
Even combined, detection gives an estimated depth, not a measured one. Where a bore, pile or trench passes close to an asset, the only way to know the clearance is to expose it by non-destructive excavation.
GPR in concrete is a different tool
The radar used on slabs is a different instrument from the one pushed across a car park. Concrete scanners use much higher frequencies and image the top few hundred millimetres in fine detail, showing reinforcement, post-tension tendons and embedded conduits before coring or cutting.
If your question is about a slab, wall or suspended floor, you want concrete scanning, not a utility locate. The principle is the same and the equipment, method and deliverable are not.
What to ask when you book
Three questions separate a thorough locate from a quick one:
- Will both electromagnetic and radar methods be used on my site?
- What will you tell me about services you could not find?
- Is the result classified to AS 5488, and what do I receive in writing?
Next step
If you have plans for the site, send them with a description of the ground and the work through our contact page. We will tell you which methods suit the conditions and where potholing is likely to be needed.
Common questions
What can ground penetrating radar detect?
Ground penetrating radar detects changes in subsurface material, so it can find metal, plastic, clay and concrete pipes, conduits, underground tanks, voids, old footings and trench lines. Detection depends on soil conditions, target size and depth. It shows that an object is present but does not by itself identify what the object is.
How deep can GPR see?
It depends on the soil and the antenna. In dry sandy ground, utility radar can image targets a few metres down. In wet clay the useful depth can drop to well under a metre. A locator will tell you after the first few passes how the ground on your site is behaving.
Can GPR find plastic water and gas pipes?
Yes, in suitable ground. Radar detects the boundary between the soil and the pipe, so plastic is visible where electromagnetic locating would miss it. Small-diameter plastic pipes in clay are the hardest case and may need to be confirmed by vacuum potholing.
Is electromagnetic locating more accurate than GPR?
For horizontal position on a directly connected metal service, electromagnetic locating is very reliable. Neither method gives a guaranteed depth. They are best seen as complementary: one identifies and traces conductive services, the other finds what does not conduct.
Is ground penetrating radar safe?
Yes. GPR emits low-power radio waves, not ionising radiation, so there is no exclusion zone and no need to clear the site. It can be used in occupied buildings and public areas during normal hours.