How does a ground penetrating radar achieve subsurface imaging and what determines its depth resolution?

A ground penetrating radar (GPR) achieves subsurface imaging by transmitting electromagnetic pulses into the ground and recording the reflections from subsurface interfaces (boundaries between materials with different dielectric properties: soil layers, rock, pipes, voids, water table, buried objects). The depth resolution is determined by the bandwidth of the transmitted pulse: delta_d = v / (2 × BW), where v is the propagation velocity in the ground material and BW is the bandwidth of the pulse. The propagation velocity depends on the dielectric constant of the ground: v = c / sqrt(epsilon_r). For dry sand (epsilon_r approximately 5): v approximately 0.13 m/ns. For wet clay (epsilon_r approximately 25): v approximately 0.06 m/ns. For a GPR with 1 GHz center frequency and 100% fractional bandwidth (BW = 1 GHz): depth resolution in dry sand: delta_d = 0.13 / (2 × 1) = 0.065 m = 6.5 cm. In wet clay: delta_d = 0.06 / (2 × 1) = 3 cm. The trade-off: higher frequencies provide better resolution but: the ground attenuates higher frequencies more severely (attenuation increases with frequency and with ground conductivity), limiting the penetration depth. Low-frequency GPR (25-400 MHz): penetration 5-30+ meters; resolution 0.1-1 m. Used for geological surveys, water table mapping, and deep utility detection. High-frequency GPR (500 MHz-3 GHz): penetration 0.1-3 meters; resolution 1-10 cm. Used for concrete inspection, pavement evaluation, and forensic investigation.