How to Read a GPR Radargram

Updated: 23 October 2026

What is a GPR radargram?

Before learning how to read a GPR radargram, it helps to know what one is: the picture a ground penetrating radar builds as its antenna moves over the surface. Every few centimetres the system fires a short radio pulse into the ground and records the echoes that come back. One such record is called a trace. Placed side by side, hundreds of traces form a grey-scale or coloured image of the ground below the line you walked.

Two things decide how that image looks:

• The horizontal axis is distance along the scan line, usually measured by a wheel encoder on the cart.
• The vertical axis is two-way travel time in nanoseconds – the time a pulse takes to go down to a reflector and return. Software converts it to depth once a signal velocity is chosen.

Strong echoes appear where the dielectric properties of the ground change sharply: soil to metal, soil to an air-filled pipe, dry fill to wet clay. Reading one well is mostly a matter of learning which of those changes made each mark on the screen.

Why does correct interpretation matter on site?

The radar does not label anything. It shows reflections, and a person decides whether a reflection is a water main, a cable, a stone or a buried kerb. A wrong call has direct costs: a trench opened over a phantom service wastes a day, while a missed cable can be cut by the next excavator or drill head.

Good interpretation also tells the site team what the image cannot show. A blank zone may mean nothing is buried there, or it may mean the ground absorbed the radar signal. Those two readings lead to very different decisions, and only an interpreter who checks signal strength section by section can tell them apart. Our note on GPR depth accuracy explains why depth figures always carry a tolerance.

What types of patterns appear on a radargram?

Hyperbolas from pipes, cables and other small targets

When the antenna approaches a buried pipe, it starts receiving the echo before it is directly above it, because the pulse spreads out as a cone. The echo path is longest at the edges and shortest overhead, so the target draws an inverted U. The apex is the true position; the arms are only the antenna seeing it from the side. A clean, symmetric arch usually means the line was crossed close to a right angle.

Flat or gently sloping bands

Continuous bands across the image come from boundaries that run along the scan line: the base of a road crust, a change from fill to natural soil, a gravel layer or, in some ground, the water table. These are not services, but they help explain where depth reach drops.

Disturbed ground

An old trench often shows as a zone where layer bands break, sag or become patchy, sometimes with a service hyperbola at the bottom. Even when no pipe is visible, a backfilled trench is a clue that something was laid there.

Ringing and multiples

Metal pipes and cables can produce a stack of repeated arches below the first one. Only the top arch is the object; the rest are echoes bouncing between the target and the antenna.

Noise and surface clutter

Speckled grey texture deep in the image is usually noise where the signal has faded. Strong slanting lines can come from walls, vehicles or overhead structures picked up through the air rather than from anything underground.

Which radargram terms should you know?

Depth is calculated as velocity multiplied by two-way time, divided by two. That simple formula is why the velocity choice matters so much.

How to read a GPR radargram step by step

1. Check the header first. Note antenna frequency, time window, scan direction and the line's start point. A higher-frequency antenna shows finer detail at shallow depth; a lower one sees deeper with less sharpness.
2. Confirm time zero. The first strong band near the top is the direct wave and ground surface. Depth should be counted from the surface, not from the top of the screen.
3. Find where the signal fades. Scroll down until arches and bands stop and only noise remains. That is the effective penetration depth for this line, and it belongs in the record.
4. Pick the hyperbolas. Mark each apex, its distance along the line and its time. Ignore the lower ringing copies of metal targets.
5. Calibrate velocity. Fit a hyperbola shape to a clear arch, or use a service of known depth such as a pipe seen in an open chamber. A wider arch means a faster, usually drier medium.
6. Link lines together. A pipe should appear at matching positions on neighbouring parallel lines. Points that join into a straight or gently curving run are a service; isolated arches may be stones or roots.
7. Cross-check with EM locating. Metallic utilities that radar suggests can often be confirmed with an electromagnetic cable locator. See our comparison of EM locators and GPR.
8. Write down confidence. Every pick should carry a note: clear, probable or uncertain.

What mistakes do new readers make?

• Treating every arch as a utility. Boulders, tree roots, rebar in a slab and debris in fill all make hyperbolas. Continuity across lines is the test.
• Missing a line that runs along the scan. A pipe parallel to the scan direction shows as a flat band, not an arch, and is easy to mistake for a soil layer. Scanning in two directions avoids this.
• Using a default velocity. Factory settings rarely match local soil. A wrong velocity can move a depth estimate by a large margin.
• Over-gaining the image. Too much gain turns noise into convincing-looking features.
• Reading a blank area as empty ground. If the signal died above that depth, the image proves nothing below it. Our guide to soil conditions and GPR limits covers this.
• Deciding from one line. A single pass cannot show direction or continuity. For dense junctions, a grid or 3D survey is more reliable.

How should radargram records be kept?

Keep the raw data files, not only screenshots. Raw files can be reprocessed later with a better velocity or filter; a picture cannot. Store with them a line log listing each file name, its start point, direction and length, plus the processing settings used for the final picks.

The interpreted results – positions, estimated depths, confidence notes and the depth reach on each section – go into the survey report, which is the document site teams actually use. Where a depth has to be proved, the report should point to locations for a trial pit; our article on GPR versus trial pits and boreholes explains when that step is worth it.

If you would rather have trained operators scan and interpret your route, our GPR survey service covers route and site work across India, and the GPR survey price list sets out the metre rates and minimums.