Ground Penetrating Radar

The recorded reflections are analyzed to identify:

• Embedded objects (e.g., reinforcement, post-tension cables, utilities)

• Layer boundaries and thicknesses

• Voids, anomalies, and zones of deterioration

The travel time and amplitude of reflected signals are used to infer depth and material characteristics.

Basic Principles of GPR

Key physical principles governing GPR include:

• Wave propagation: EM waves travel through materials at velocities controlled by dielectric permittivity.

• Reflection: Signal reflections occur at interfaces where dielectric properties change.

• Attenuation: Signal strength decreases with depth due to material conductivity and signal scattering.

• Resolution vs. penetration trade-off: Higher frequencies provide better resolution but shallower penetration.

GPR data are commonly displayed as radargrams, showing signal amplitude versus time (or depth) along a scan line.

GPR Antenna Frequencies and Typical Uses

The choice of antenna frequency depends on the target size, required resolution, and investigation depth.

Typical Frequency Ranges

• High frequency (1.5–2.6 GHz)

  • Shallow penetration, high resolution

  • Used for concrete scanning and structural elements

• Medium frequency (400–900 MHz)

  • Moderate penetration and resolution

  • Used for pavements, slabs-on-grade, and shallow subsurface investigations

• Low frequency (50–400 MHz)

  • Greater penetration, lower resolution

  • Used for soil profiling, utilities, bedrock depth, and geotechnical studies

Common Applications by Material

• Concrete scanning: 1.6–2.6 GHz

• Pavements and bridge decks: 400–900 MHz

• Subsurface soils and utilities: 100–400 MHz

Dielectric Properties and Their Importance

The dielectric permittivity (dielectric constant) of a material controls how fast EM waves travel through it. It is a critical parameter in GPR interpretation.

• Low dielectric materials (air, dry concrete) → faster wave velocity

• High dielectric materials (wet concrete, clay, water) → slower velocity and higher attenuation

Dielectric properties influence:

• Depth estimation accuracy

• Reflection strength

• Signal penetration

In concrete investigations, dielectric variations can also indicate:

• Moisture ingress

• Delamination

• Deterioration or voids