B-Scan GPR Section: How a Radargram Cross-Section Is Read

What is a B-scan in GPR?

A B-scan is a two-dimensional GPR image made of many A-scans recorded at regular spacing along a straight line. Each column is one trace; amplitude is shown as grey shades or colour. Point-like objects crossed by the line show as hyperbolas, continuous boundaries as horizontal or sloping bands, and zones with no return signal as dull areas.

In ground penetrating radar (GPR) surveys the antenna sends a burst of electromagnetic waves into the soil at each step. The B-scan stacks every recorded echo into one picture of the ground below the line.

How is a B-scan used on site?

The operator reads it live on the screen while walking the line and marks the ground over every apex. A survey wheel measures distance, so a mark at 15.2 m on the screen is 15.2 m along the chalk line. Scan lines are laid across the expected direction of utility lines, at intervals of a few metres on roads and tighter at junctions. When the target direction is unknown, a grid of lines lets the team detect and map services in plan, as explained in C-Scan and Depth Slice in GPR.

What do the patterns mean?

A long road is many such images end to end; to calculate metres × rate yourself, add up the line lengths.

An example across a city road

A 30 m B-scan across a sector road might show the road crust at 0.35 m and a cable at about 0.6 m near the kerb. A water main lies near 1.1 m under the carriageway, with a drain deeper still. The figure above draws that case. Each arch is then checked on the neighbouring lines. A feature that lines up from line to line is a service, while a lone arch is often a stone or other small buried object.

Reading the crossing angle from two sections

Parallel B-scans also tell you which way a service runs. On a service road scanned with lines 5 m apart, a pipe apex sits at 6.0 m along line 1 and 6.8 m along line 2. The 0.8 m shift over a 5 m spacing means the pipe runs about 9° off square to the lines (arctan 0.8 / 5). If the shift grows to several metres between lines, the service is running almost along the road. The lines should then be turned to cross it properly before any depth is trusted.

Common misreadings of a section

Most misreadings come from forgetting how the image is scaled and where the line ran.

• Ignoring vertical exaggeration. Most sections are stretched downward, so gentle slopes look steep and arches look narrower than they are.
• Trusting an uncalibrated wheel. If the survey wheel has not been checked against a tape, every horizontal position drifts with distance.
• Reading a line run along a pipe as a soil layer. The pipe then appears as a flat band, easily taken for a pavement base.
• Missing surface clutter. Manhole covers, kerbs and parked vehicles leave strong marks that are not underground at all.

How sections are presented in a Mezux report

Selected B-scans go into an appendix, annotated so they match the drawing. Each image carries its line number, start and end chainage, velocity, and labels on every pick using the same service IDs as the CAD layers. A reader can then move from a line on the plan to the section that produced it. ASTM D6432-19 is the general reference for collecting such profiles, and PAS 128:2022 in the UK likewise expects interpreted results to be traceable to the data collected in the field.

What resolution can a B-scan offer?

Horizontal detail depends on trace spacing; vertical detail depends on antenna frequency. With traces every 2 cm, objects a few centimetres apart can in principle be separated along the line. In depth, though, the radar wavelength limits how close two services can be in depth before their echoes merge. As a rough rule, mid-frequency utility antennas separate targets about a quarter of a wavelength apart, which in moist ground conditions works out at roughly five to ten centimetres. A high frequency antenna sharpens that vertical detail but gives up depth. Two cables lying almost on top of each other in one trench may therefore show as a single arch, and the drawing should say "possible multiple services" there.

How is it processed?

With a short, standard chain. The steps are time-zero correction, dewow to remove low-frequency drift, background removal to suppress flat ringing, gain with depth, band-pass filtering and, where exact geometry matters, migration.

Background removal can also erase genuine flat layers, so it is applied with care. Migration is covered in Migration in GPR Data Processing. Many processed sections are collected in the GPR Radargram Library.

For a line-by-line survey of underground utilities on your road or plot, ask our engineers for your project.

Frequently asked questions

Is a B-scan the same as a radargram?

Yes, in everyday use; strictly, a radargram is any GPR image, but most of them are B-scans.

Can a B-scan show the direction of a pipe?

Not on its own: one line shows where a pipe crosses; two or more lines, or a grid, show its direction.

How do I start reading B-scans myself?

Follow a guided example; more on How to Read a GPR Radargram.

Why is the vertical scale stretched on most B-scans?

Because a few metres of depth would otherwise be a thin strip under tens of metres of distance. Exaggeration makes arches visible but distorts angles.

How long can one B-scan be?

Technically hundreds of metres, but we break long routes into manageable lines so positions can be checked against marks on the ground.

What is wheel calibration?

Running the cart along a taped distance and correcting the wheel's count so on-screen metres match real metres. It is repeated when surfaces change.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026