GPR
Ground Penetrating Radar
A short radio pulse is radiated into the ground and the reflections from dielectric contrasts are timed. It is the only geophysical method routinely delivering centimetre resolution, but conductive clay and saline ground absorb the signal quickly.
Crew
1-2 operators (single operator for cart surveys)
Mobilisation
Half a day: antenna check, wheel-encoder calibration, grid pegging
Daily coverage
6-15 line-km with a cart; 1-3 km for dense 3D grids
Equipment and specifications
| Control unit | 16-bit, 100+ scans/s, GNSS and encoder inputs |
|---|---|
| Antennas | Shielded 100 / 250 / 500 MHz for geology; 1-2.6 GHz for concrete and utilities |
| Odometer | Wheel encoder or robotic total station for sub-5 cm trace positioning |
| Positioning | RTK GNSS for open ground; local grid and tapes indoors |
| Calibration targets | Buried pipe or metal plate at known depth for velocity checks |
Setup sequence
- 01
Pick the antenna
Highest frequency that still reaches the target depth — resolution and penetration trade off directly.
- 02
Set the time window
Two-way travel time to 1.5x target depth, plus 20% headroom.
- 03
Calibrate velocity
Fit a hyperbola over a known reflector, or run a CMP to get the true velocity.
- 04
Lay out the grid
Line spacing under a quarter wavelength for 3D; mark line ends and direction.
- 05
Collect
Constant speed, antenna in ground contact, note every surface obstacle in the log.
- 06
Field QC
Review each line on-screen for ringing, saturation and encoder slip before moving on.
Safety
Live buried services
Cable locator sweep and permits before any intrusive follow-up.
Traffic on road surveys
Traffic management plan, cones, high-vis, spotter.
Manual handling
Cart handles at hip height, rotate operators on long shifts.
Sun and heat
Shade for the control unit, hydration schedule, screen hoods.
Field vs lab
| Aspect | In the simulator | In the field |
|---|---|---|
| Signal-to-noise | Clean synthetic response; noise added only if you ask for it. | Cultural noise, wind and instrument drift set the usable dynamic range. |
| Ground coupling | Perfect antenna-ground coupling assumed. | Air gaps, grass and gravel cost amplitude and shift the direct wave. |
| Clutter | One clean target reflection. | Roots, rebar, boulders and services all reflect; interpretation is pattern work. |
| Moisture | Permittivity is a single slider value. | Varies through the day and with depth — velocity must be re-checked. |
| Repeatability | Identical parameters return an identical curve every time. | Coupling, moisture and temperature move the response between repeats. |
Field QC checklist
- · Velocity confirmed against a known target within 10%
- · No clipped traces at the surface reflection
- · Encoder distance within 1% of taped line length