What is the difference between GPR and ferro scanning?

GPR and ferro scanning sound similar, look similar on site, and even produce similar deliverables. They are not the same tool. The differences matter.

What each one is

GPR sends short electromagnetic pulses into concrete and records reflections from anything denser than the host material — steel, plastic conduits, post-tension cables, voids. It produces a layered map of detectable reflectors at depths from the surface to several hundred millimetres in concrete.

Ferro scanning uses electromagnetic induction to detect ferrous metal (steel reinforcement) close to the surface. It cannot see plastic conduits, voids, post-tension cables, or any non-ferrous reflector. What it does, it does very accurately: cover-depth measurement and bar-diameter estimation.

Where GPR wins

• Depth. Sees deeper than ferro. On thick slabs with both top and bottom mats, GPR is the only practical way to image the bottom mat.
• Non-ferrous targets. Plastic conduits, voids, and post-tension cables don’t show on a ferro scan. They show on GPR.
• Slab interfaces and voids. GPR measures slab thickness and identifies voids and honeycombing. Ferro can’t.
• Subsurface and ground. Outside concrete, GPR is the right tool. Ferro is concrete-only.

Where ferro scanning wins

• Cover depth accuracy. For shallow bars, ferro is more accurate than GPR.
• Bar diameter estimation. A calibrated diameter mode that GPR alone does not provide.
• Speed and density on shallow elements. A small, light probe — fast on walls and thin slabs.
• Cost on small jobs. Simpler kit, faster setup, often cheaper for a quick check.

How to choose

• Need to find post-tension cables before drilling? GPR.
• Need to verify cover-depth compliance? Ferro.
• Need full reinforcement mapping with depths? GPR (often paired with ferro for cover detail).
• Drilling into a thick reinforced slab and need to know what’s at the bottom? GPR.
• Quick cover check on a wall? Ferro.

Why we often use both

On any non-trivial reinforced-concrete job, GPR and ferro scanning are complementary, not competitive. A typical engagement runs GPR first for the layered map and any non-ferrous items, then ferro for engineering-grade cover and bar-diameter detail on the same areas. The combined deliverable is one report, one CAD plan, and the right tool for every part of the question.

The cost of running both on the same visit is much less than running them separately.

More detail

For a fuller side-by-side comparison, see our article GPR vs ferro scanning — which do you need.