How deep can GPR scan?

GPR depth is one of the most common questions on any scanning enquiry. The honest answer depends on three things: antenna frequency, host material, and what you are looking for.

Antenna frequency drives depth

High-frequency antennas give better resolution but shallower penetration. Low-frequency antennas penetrate deeper but resolve less detail.

• High-frequency concrete scanners. Rated depths vary by instrument: Hilti rates the PS 1000 X-Scan to 300 mm and Screening Eagle rates the Proceq GP8100 to 80 cm. Resolution is high enough to separate adjacent rebar.
• 800 MHz to 1 GHz. Useful for thicker concrete elements or shallow utility work.
• 400–600 MHz. Standard for utility detection, archaeology, and ground investigation.
• 200–270 MHz. Deeper subsurface work in favourable conditions.
• Below 200 MHz. Specialist deep-subsurface applications.

Host material matters

Depth is not just antenna-driven. The material being scanned absorbs the signal:

• Dry, dense concrete: good penetration.
• Wet or saturated concrete: much shallower.
• Heavily reinforced concrete: dense top mat shadows lower layers.
• Clean sand and gravel: good penetration.
• Saturated clay: poor penetration; conductivity attenuates the signal quickly.
• Made ground with metal debris: noisy returns, limited useful depth.

For most concrete scanning, a high-frequency concrete scanner is appropriate. For most utility detection, frequencies around 400 MHz are a common balance of depth and resolution.

What this means for your job

For typical UK concrete elements:

• Standard slab with reinforcement at 25–50 mm cover and bottom mat at 200–250 mm. Comfortably within range.
• Thick transfer slab with reinforcement at 100 mm and bottom mat at 400 mm. Within range of deeper-reaching instruments, but interpretation gets harder.
• Mass concrete with reinforcement deeper than 500 mm. May need lower-frequency antennas.

For ground work:

• Service detection in typical UK ground at 0.5–1.5 m depth. Standard utility-frequency systems handle this.
• Voids or features deeper than 3 m. Lower-frequency systems required.

Combining methods

Where the depth required exceeds the practical range of a single system, the answer is usually multiple frequencies on the same job. Modern multi-channel arrays and dual-frequency systems make this routine.

Honest limits

GPR is not a magic see-through tool. Some materials (heavily reinforced concrete, saturated clay) limit penetration sharply. A defensible surveyor states the achievable depth before the job and acknowledges it in the deliverable. A surveyor promising guaranteed deep penetration in difficult conditions should be questioned.

For a deeper explanation of how the technology works, see our article How does ground penetrating radar work.