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.

• 1.6 to 2.6 GHz (concrete scanning antennas). Typical depth in good concrete: up to 400–500 mm. Resolution is high enough to separate adjacent rebar.
• 800 MHz to 1 GHz. Useful for thicker concrete elements or shallow utility work. Depth up to about 1 m.
• 400–600 MHz. Standard for utility detection, archaeology, and ground investigation. Depth typically 2–3 m in clean ground.
• 200–270 MHz. Deeper subsurface work — depths of 5 m or more 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, the 1.6–2.6 GHz range is appropriate. For most utility detection, 400 MHz with array systems gives the best balance of depth and area coverage.

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. Still in range, 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.