How to read a GPR scan report

A GPR scan report is the deliverable that turns hours of careful site work into something a contractor or engineer can act on. Done well, a GPR report is a tight document — a few pages, clearly labelled, with depth-accurate plans you can hand to the drilling team. Done badly, it is a stack of unannotated screenshots that nobody can use without ringing the surveyor.

This is what to look for, in order, when a GPR report lands in your inbox.

1. The cover page

The cover page tells you what was scanned and by whom. It should name:

• The site address.
• The element scanned (slab, wall, column reference).
• The date of the survey and the date of the report.
• The surveyor who did the work.
• The equipment used and the calibration record.

If any of these are missing, ask. A report without a clear scope is hard to defend if anything goes wrong on the work that follows.

2. The brief

Every report should restate the brief in the surveyor’s words. This matters because GPR data alone is meaningless without a question — the difference between “find every reflector” and “confirm the absence of post-tension cables before drilling” is the difference between a useful report and an academic one.

Read the brief and check it matches what you asked for. If it does not, the rest of the report may not answer the question you needed answered.

3. The method

A defensible report includes a short method section that says what equipment was used, what frequency, what scanning pattern, and how depth calibration was set. You do not need to be a GPR specialist to read this — but if you ever need to defend the work to a structural engineer or an insurer, this section is what they will look at first.

4. The findings

This is the substance of the report. There are usually two parts:

• Plan view drawings, showing the position of every target identified, with depths annotated. The plan should be referenced to features you can identify on site (column lines, edges, drawings).
• Narrative interpretation, explaining what was found and any anomalies that the surveyor wants to flag.

When you read the plan, look for these things:

• Is the orientation clear? North arrow, level reference, column grid.
• Are bars annotated with depths, or just shown as lines? Engineering work needs depths.
• Are non-ferrous items clearly distinguished from rebar? Conduits and post-tension cables are usually shown with a different symbol.
• Are voids or anomalies highlighted with a separate symbol?
• Is the scale clear and correct?

5. The deliverable formats

A modern GPR report should ship in two formats: a PDF for stakeholder review and a CAD-ready DXF or DWG for design use. The CAD plan should be drawn at the same scale as the underlying site plan and referenced to the same coordinate system. If you find yourself trying to align the CAD output to your drawings by eye, something has gone wrong.

For pre-drill work, the on-slab markings made during the survey are part of the deliverable too. Photographs of the marked-up areas, taken before the surveyor leaves site, should be in the report.

6. Limitations and assumptions

A good report has a limitations section. GPR has real physical limits — depth, attenuation through dense reinforcement, response to saturated concrete — and a defensible report acknowledges them. Limitations are not a weakness in the report; they are a sign that the surveyor knows what they are doing.

A typical limitations note might read: “GPR signal attenuates through dense top reinforcement, and the bottom mat below 180 mm should be treated as approximate. Where critical, intrusive verification is recommended.”

If a report has no limitations section, ask why.

7. The recommendation

Many reports end with a short recommendation — “drill in the marked positions only”, “core extraction recommended for verification”, “structural engineer to review prior to alteration”. This is the surveyor giving you the benefit of their interpretation. Read it carefully. It is part of the deliverable.

8. The sign-off

The surveyor who did the work should sign the report — by name, by qualification, and by date. An anonymous report is a red flag. The person who interpreted the data is the person whose name should be on the document.

Questions to ask

Before you sign off the report, three quick checks:

1. Does the report answer the question you asked? If not, raise it before any drilling starts.
2. Is the deliverable in a format your downstream users (engineer, contractor, drilling team) can actually use?
3. Does the recommendation sit with the rest of the project documents — RAMS, method statements, drawings — without conflict?

A well-written GPR report saves time and money on every project that follows. A poorly written one creates ambiguity exactly where you cannot afford it. Take five minutes to read it properly.