A-Scan GPR Trace: Reading One Radar Pulse at a Single Point

What is an A-scan in GPR?

An A-scan is the raw record from one radar pulse, or the average of several, at one point on the ground. Time runs down the axis and amplitude across it, so each reflection appears as a short wavelet. The first big wavelet is the direct wave from transmitter to receiving antenna; later wavelets are echoes from layers, pipes, voids or rebar.

Ground penetrating radar (GPR) is a geophysical survey method for looking below the surface. Its antenna fires short pulses of electromagnetic waves into the ground, and every pulse yields one such trace.

What do engineers look for in a single trace?

Mainly where the useful signal is, where it fades, and whether the wavelet is clean.

• First break: the start of the direct wave, used to fix time zero.
• Echo time and polarity: when a return signal arrives and whether it starts positive or negative, which hints at the type of contrast.
• Ringing: repeated wavelets that carry on after a reflection, often below metal or from poor antenna coupling.
• Noise floor: the time after which the trace is only random wiggle; beyond this nothing can be interpreted.

Gain curves are chosen by looking at A-scans: deep echoes are weak, so the processor boosts them progressively with time. Too much gain lifts noise into false targets.

A-scan, B-scan and C-scan compared

The letters describe how many dimensions are shown.

Field time is spent collecting these traces along each line; the GPR cost calculator turns that line length into a price.

Line after line of traces forms a section, as described in B-Scan in GPR. A full grid of them gives the plan maps in C-Scan and Depth Slice in GPR.

Setting gain from one trace on a city road

A single A-scan tells the processor where to boost and where to stop. Take a trace from a typical bitumen road in Faridabad. It might show the direct wave at time zero and a sharp reflection from the base of the road crust at about 4 ns. A soil echo follows near 18 ns, and the noise floor starts at about 45 ns. At 0.10 m/ns the usable record therefore ends near 2.25 m. The gain curve is shaped to lift the 15–40 ns band, where most utility lines lie, and left flat beyond 45 ns. Boosting pure noise would only paint false targets into the image.

What goes wrong when traces are read carelessly?

The usual faults are a misread first wavelet and a clipped signal.

• The direct wave is mistaken for a shallow target, and a phantom service appears near the surface.
• Gain is set so high that the trace clips at full scale, flattening the peaks and hiding the polarity that hints at metal or air.
• Stacking too many pulses while the cart moves fast smears the wavelet along the line.
• One trace is treated as proof, yet a reflection seen at only one position is rarely a real subsurface object.

Where single traces sit in our workflow

Clients seldom see them, but every file of data collected by Mezux is checked trace by trace before interpretation. Our processing log records the first-break time used for time zero, the noise-floor time and the gain applied, so a reviewer can reproduce the image from the raw file. ASTM D6432-19, the general guide to surface GPR work, treats such record-keeping as part of sound practice. The A, B and C labels themselves come from ultrasonic testing, which uses the same three display names.

What polarity in a trace can and cannot tell you

The sign of the first lobe of an echo helps separate metallic and non-metallic objects, but only as a clue. When radar passes from soil into something faster, such as an air-filled duct or a void, the reflected wavelet flips relative to the direct wave. Metal and water-filled pipes return a wavelet of the same sense, much stronger. On a single A-scan that difference can be seen plainly, provided the trace is not clipped and gain has not been applied unevenly. Corrosion, partial filling and nearby clutter all blur the pattern, so we treat polarity as supporting evidence for a test hole, not as an identification on its own.

A practical example

Over a buried steel pipe, a trace typically shows the direct wave, then a strong, sharp wavelet at, say, 20 ns, then ringing. At 0.10 m/ns that wavelet places the pipe top about 1.0 m down. A neighbouring trace a metre away shows the same echo later and weaker, the beginning of a hyperbola. Stacking, or averaging several pulses at each position, cleans random noise from such a trace before it is displayed. The GPR Radargram Library explained pages show how these traces combine into images.

To detect and map services with processed, checked data, ask our engineers for your project.

Frequently asked questions

Why is it called an A-scan?

The letters label the display type: A for a single amplitude trace, B for a section, C for a plan view. Ultrasonic testing uses the same three names for its equivalent displays.

Do you deliver A-scans to clients?

Normally no. Clients receive marked drawings and selected radargram images; raw files can be supplied on request.

How do I see A-scans inside a radargram?

Most software can show wiggle traces over the image. Read How to Read a GPR Radargram to start.

What is clipping on an A-scan?

Clipping happens when the signal exceeds the recorder's range and the peak is cut flat. Shape and polarity are lost, so gain is reduced and the line re-run.

How many pulses are stacked for each trace?

It varies with antenna and speed, commonly from a few to several dozen. Slow, careful lines allow more stacking and cleaner traces.

Can one trace give the depth of a pipe?

Only if the velocity is already known, and even then it cannot show whether the echo is a pipe or a layer. Neighbouring traces settle that.

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