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GPR Depth Calculator
Turn two-way travel time into depth for concrete, asphalt and soil. Or calibrate the dielectric from a bar at a known depth.
Depth from two-way travel time
Depth = velocity × time ÷ 2
Calibrate from a known depth
Measured a bar at a core hole or slab edge? Back out the real velocity.
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How it works
The formula behind the calculator
Radar travels slower in concrete than in air. How much slower depends on the material's relative permittivity, also called the dielectric constant.
Velocity
v = c / √εr, where c is the speed of light, 0.2998 m/ns or 11.8 in/ns.
Depth
d = v × t / 2, where t is the two-way travel time in nanoseconds.
Example
Typical concrete at 0.10 m/ns has a dielectric of about 9. A bar showing at 1.5 ns is 0.10 × 1.5 / 2 = 0.075 m deep, which is 75 mm or about 2.95 in.
Typical velocities
| Material | Velocity (m/ns) | Velocity (in/ns) | Dielectric |
|---|---|---|---|
| Concrete, dry and cured | 0.12 | 4.72 | 6.2 |
| Concrete, typical | 0.10 | 3.94 | 9 |
| Concrete, damp or green | 0.08 | 3.15 | 14 |
| Asphalt pavement | 0.10 | 3.94 | 9 |
| Dry sand or dry soil | 0.15 | 5.91 | 4 |
| Limestone | 0.12 | 4.72 | 6.2 |
| Granite | 0.13 | 5.12 | 5.3 |
| Silt | 0.07 | 2.76 | 18.3 |
| Wet sand or wet soil | 0.06 | 2.36 | 25 |
| Clay | 0.06 | 2.36 | 25 |
| Fresh water | 0.033 | 1.30 | 82.5 |
Velocities from the GPR notes for UBC EOSC 350, which reproduce an instrument manual table. Dielectric worked out as (0.2998 / v)².
GPR depth calculator FAQ
How do you calculate depth from GPR travel time?
Find the wave velocity in the material, v = 0.2998 / √εr metres per nanosecond, then depth = v × t / 2. You divide by two because the time on the screen covers the trip down and back.
What dielectric constant should I use for concrete?
Published GPR tables put concrete velocity around 0.08 to 0.12 m/ns, which works out to a dielectric of roughly 6 to 14. Dry, cured slabs sit at the low end. Green or wet concrete sits high. Calibrate on site when depth matters.
Why is my depth reading off?
Almost always the velocity. Moisture, mix, age and temperature all change it, so a default setting can be wrong by more than 10%. Check against a known depth, like a bar at a core hole or the slab edge, and use the calibration calculator.
Can I use this for utility locating in soil?
Yes, with a big caution. Soil velocity swings with moisture and clay content much more than concrete does, and wet clay can stop the signal altogether. Treat a soil depth as an estimate and expose the line before you dig near it.
Where do the material velocities come from?
They come from a GPR instrument manual table published in the University of British Columbia EOSC 350 course notes. They are typical values, not guarantees for your slab.
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