How to prepare a site for GPR scanning

A GPR scan is most useful when the surveyor walks straight onto a clean surface, with the right access, the right information, and a brief that has been agreed in advance. Preparation makes the difference between a half-day job and a half-day plus a lot of friction. Here is what to do before the survey team arrives.

Surface condition

GPR is rolled across the surface, so the surface needs to be clean enough for the antenna to maintain contact.

• Sweep loose debris off the slab.
• Remove tape, packaging, and loose finishes from any area to be scanned.
• If the slab has been recently poured and is still curing, agree the timing with the surveyor — newly poured slabs scan differently from cured ones.
• For walls, mark up any furniture or fixings that need temporary removal.

Surface preparation is usually a job for the contractor’s own labour, not the surveyor’s. Building a half-hour of slab cleaning into the programme on the day saves more time than it costs.

Access

Plan access in advance. The surveyor needs:

• Clear access to the area being scanned.
• Sufficient working space around it (typically 1–2 m).
• A power outlet within reach for some equipment, although most modern systems are battery-powered.
• Lighting good enough for chalk markings to be visible.

For floors above ground level, plan stairs and lift use. For confined spaces, agree the access permit and any required PPE. For high-level work, agree the platform — MEWP, scaffold, mobile tower — and book it.

Information the surveyor will ask for

Before attendance, the surveyor will want:

• The drawings, even if they are old or incomplete.
• The known structural type — RC slab, post-tension floor, RC wall, masonry, etc.
• The thickness of the element if known.
• The history of the structure — age, any major works, any known defects.
• The brief — what question is the scan answering?

Sending these in advance lets the surveyor pick the right equipment and the right method. A surveyor who arrives on a post-tension slab without being told it is post-tension is a surveyor who will be slower than necessary.

Programme integration

Schedule the GPR scan into the programme so it sits in the right place:

• Pre-drill scans must be done before any drilling. That sounds obvious; it is the most commonly missed.
• Pre-cut and pre-core scans need the planned cut lines marked on the slab before attendance, so the surveyor knows where to focus.
• Structural assessment scans benefit from having the engineer on a phone or on site, so questions can be answered the same day.
• Refurbishment as-builts are best scanned before any soft-strip starts.

If the scan is on the critical path, agree the access window and the survey duration with the contractor and the surveyor. A slot of half a day for a typical pre-drill scan, full day for larger jobs, is a reasonable starting point.

Marking up the brief

Before the surveyor arrives, mark up on the surface or on the drawing:

• The areas that need scanning.
• The proposed drilling, coring, or cutting positions.
• Any obvious hazards — known service runs, fragile finishes, known defects.
• Reference points the surveyor can tie their scan to (column lines, edges, openings).

The clearer the brief on the surface, the faster and more useful the scan will be.

What not to do

• Don’t ask the surveyor to “scan the whole building, just in case”. Scope drives quality. A scoped scan tied to a question gives a useful deliverable; a fishing-expedition scan gives a stack of data that nobody can act on.
• Don’t drill before the scan and then ask for a scan to verify. The point of GPR is to inform the work, not to forensically explain it after the fact.
• Don’t give the surveyor a 2 m square area when they need a 4 m square area to tie the scan into reference points. A bit of extra access is almost always worth it.
• Don’t expect the surveyor to also act as the structural engineer. The scan provides data; the engineer interprets that data into design or construction decisions.

On the day

Set the surveyor up with:

• A site induction, signed off in advance if possible.
• A point of contact who can answer questions about the structure or the brief in real time.
• Clean, accessible surfaces.
• Time to mark up findings before they leave.

A well-prepared GPR job typically runs cleanly in a single visit. A poorly-prepared job extends, costs more, and produces lower-quality data. Half an hour of preparation in advance pays for itself many times over.