Two-Way Travel Time (GPR): How Radar Time Becomes Depth

What is two-way travel time in GPR?

Two-way travel time (TWT) is the delay between transmission of a radar pulse and arrival of its reflection, counted for the full journey down to the reflector and back. GPR records every trace against TWT in nanoseconds; depth is a calculated value, d = v × t / 2, and is only as good as the velocity used.

A ground penetrating radar survey sends electromagnetic waves into the soil and logs when each echo comes back; that clock reading is the whole raw measurement. A nanosecond is a billionth of a second, in which radar covers 30 cm in air and perhaps 7 to 10 cm in moist soil.

Which settings depend on it?

The time window, time zero and sample rate are all set in TWT.

• Time window is how long the receiver listens after each pulse. A 100 ns window at 0.10 m/ns covers 5 m of depth. A longer window only adds noise if the signal has already died, which is where GPR depth of penetration comes in.
• Time zero is the instant that corresponds to the ground surface. Processing shifts each trace so the first arrival lines up, otherwise depths drift with cable delay and antenna height.
• Samples per trace divide the window into slices; more samples give a finer time step.

Each recorded trace at a single position is an A-scan; read A-Scan in GPR to see how the time axis looks on one.

Converting time to depth

To find the depth of a buried object, halve the time and multiply by the velocity. The table shows why velocity matters as much as the reading itself.

Once you know the length of the lines to be scanned, estimate your survey bill in a minute.

With separated transmitter and receiver, a small correction applies for shallow targets: d = √((vt/2)² − (s/2)²), where s is the antenna spacing. For most utility antennas it changes the answer by only a centimetre or two below half a metre.

Choosing a time window for a deep drain

Work backwards from the deepest target, then add a margin. Say a storm drain is expected at 2.4 m in moist alluvium with a velocity near 0.08 m/ns. Its echo should arrive at 2 × 2.4 / 0.08 = 60 ns, so we set the window to about 80 ns to see the ground beneath it as well. With 512 samples across 80 ns, each sample spans about 0.16 ns, roughly 6 mm of depth at that velocity, which is finer than the antenna can resolve. A window of 40 ns on the same site would have stopped the record at 1.6 m and missed the drain entirely.

Mistakes that corrupt travel-time depths

Most are bookkeeping errors, not physics.

• Not halving the time. This basic slip doubles every depth and appears in spreadsheets more often than expected.
• Time zero set on the wrong peak. Picking the second lobe of the direct wave shifts all depths by several centimetres.
• Ignoring antenna height. A cart that rides high over kerb stones adds air travel time, which then masquerades as extra soil depth.
• Trusting the depth ruler on screen. It is drawn with whatever velocity was last entered, which may belong to a different site.

How travel time appears in our deliverables

Radargram figures carry both scales: time on the left, depth on the right, with the velocity printed beneath. The depth schedule that accompanies each drawing lists, for every service crossing, the line, chainage, two-way time, velocity and resulting cover. Keeping the raw time with the data collected means a later reviewer can re-check a depth without the original files. ASCE 38-22 treats such depths as estimates at quality level B until a test hole confirms them. ASTM D6432-19 is the usual general reference for how time readings are collected.

An example from a footpath scan

A cable echo arrives at 30 ns, and hyperbola fitting on the same line gives 0.08 m/ns. Depth is 0.08 × 30 / 2 = 1.2 m, and on the drawing we record the depth alongside the velocity used, so anyone can recheck it later. Labelled examples of time scales can be seen when you read GPR Radargram Library pages.

If you need utility lines located with stated depths, request a written estimate for your project.

Frequently asked questions

Why is the time divided by two?

Because the recorded time includes the journey down to the target and the journey back up, and depth needs only one leg.

What is a typical time window for utility scanning?

Often 50 to 100 ns with mid-frequency antennas, enough for a few metres in average soil, where most underground utilities lie. Concrete work, done with high frequency antennas, uses much shorter windows.

Is the vertical axis on a radargram time or depth?

Raw data is in time; depth scales need a velocity. See: How to Read a GPR Radargram.

What happens if time zero is set wrongly?

All depths shift by the same amount. Over a long project this is easy to miss, so we check time zero against a surface reflector on each file.

How many samples per trace are typical?

Often 512 or 1,024. More samples give a finer time step but larger files; the window length matters more.

Does antenna height change the travel time?

Yes: any air gap adds time at 0.3 m/ns. Ground-coupled antennas are kept on the surface so the gap stays small and constant.

Why do two surveys of the same pipe give slightly different times?

Changing ground conditions, antenna height and the exact crossing point all shift the echo by a nanosecond or two. Converted to depth that is a few centimetres, which is why we quote depths as estimates with a stated tolerance.

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