GPR vs ferro scanning — which do you need

Ground penetrating radar and ferro scanning sound similar, look similar on site, and even produce similar deliverables. They are not the same tool. Picking the wrong one — or paying for both when one would have done — is a common mistake. Here is the difference, in plain English.

What each one is

GPR sends short electromagnetic pulses into the concrete and records reflections from anything denser than the host material. It can see steel reinforcement, plastic conduits, post-tension cables, voids, and slab interfaces. It works at depths from the surface to several hundred millimetres in concrete (more in clean ground) and produces a layered map of detectable reflectors.

Ferro scanning uses electromagnetic induction to detect ferrous metal — that is, steel — close to the concrete surface. It does not see plastic conduits, voids, or non-ferrous reflectors. It only detects steel. But it does so with high accuracy on cover depth, useful estimates of bar diameter, and very dense, very fast coverage of an element.

The two technologies overlap in their job — both are used to find rebar — but they differ in what they detect, how deep they go, and what they tell you about what they find.

Where GPR wins

• Depth. GPR sees deeper than ferro. On a thick slab with both top and bottom reinforcement layers, GPR is usually the only practical way to image the bottom mat.
• Non-ferrous targets. Plastic conduits, voids, and post-tension cables don’t show up on a ferro scan. They are part of the picture GPR gives.
• Slab interfaces and voids. GPR can tell you the slab thickness and identify voids and honeycombing. Ferro can’t.
• Subsurface and ground. Outside concrete, GPR is the appropriate tool. Ferro scanning is concrete-only.

Where ferro scanning wins

• Cover depth accuracy. For shallow bars, ferro gives more accurate cover-depth measurements than GPR. If you need to verify cover to a code requirement, ferro is the right tool.
• Bar diameter estimation. Ferro scanning has a calibrated diameter mode that estimates bar diameter at shallow cover. GPR does not.
• Speed and density on shallow elements. A ferro probe is small, light, and quick. On a wall or a thin slab where the question is “where are all the bars and how deep are they?”, ferro is faster than GPR.
• Cost on small jobs. Ferro scanning gear is simpler, calibration is quicker, and a small job can often be done in a fraction of the time of an equivalent GPR survey.

Where they overlap

For routine reinforcement mapping on a typical reinforced concrete element with reinforcement at normal depth, either tool will do the job. In practice, the surveyor’s decision is driven by:

• The depth of the deepest reinforcement.
• Whether non-ferrous items (plastic conduit, post-tension cables, voids) are in scope.
• The accuracy of cover-depth measurement required.
• The size and access of the element.
• The downstream use of the data.

For a “find me everything that’s in this slab” brief on a typical commercial floor, GPR is the default. For a “verify cover and spacing on this reinforced wall” brief, ferro is the default. For a “find the post-tension cables before we drill” brief, GPR is the only safe answer.

Why we often use both

On any non-trivial job, GPR and ferro scanning are complementary, not competitive. A typical engagement runs GPR first to get the layered map and identify any non-ferrous items. Where cover depth or bar diameter is needed at engineering accuracy, ferro is then used on the same areas. The combined deliverable is one report and one CAD plan, but the data underneath is from both methods.

The cost of running both on the same visit is much less than running them on separate visits. If the brief might benefit from cover-depth accuracy or bar-diameter estimation, it is almost always worth scoping both into the original quote.

How to decide

A short decision rule for the contractor or engineer commissioning the work:

1. If you need to drill, core, or cut, and you don’t have full as-built drawings — start with GPR. The cost of a struck cable or PT cable dwarfs the cost of the survey.
2. If post-tension construction is in any doubt — start with GPR. Ferro alone cannot find PT tendons reliably.
3. If the question is “is the cover compliant?” or “what diameter are these bars?” — add ferro to the GPR brief.
4. If the element is shallow and lightly reinforced and the question is just “where is the steel?” — ferro alone may be enough.

The surveyor on the quote call should be able to talk you through the choice in a few minutes. If they cannot, they are the wrong surveyor.