How does ground penetrating radar work and what frequencies are used for different applications?

Ground penetrating radar (GPR) works by transmitting short electromagnetic pulses (typically 0.5-5 nanoseconds duration) into the ground or structure and recording the reflected signals from subsurface interfaces where the dielectric constant changes (such as boundaries between soil and rock, concrete and rebar, or dry and saturated material). The transmitter and receiver antennas are moved along the surface, and the collection of reflected waveforms is displayed as a cross-sectional image (radargram) showing subsurface features versus depth and position. GPR frequencies range from 10 MHz to 4 GHz depending on the application: low frequencies (10-100 MHz) penetrate deeper (10-50 meters in dry soil) but have coarser resolution (0.5-2 meters), while high frequencies (1-4 GHz) have limited penetration (0.1-1 meter) but provide fine resolution (1-5 centimeters). Common center frequencies include 100 MHz for geological surveys and deep utility mapping (3-15 m depth), 250-400 MHz for utility detection and shallow geology (1-5 m depth), 900 MHz-1.5 GHz for concrete inspection and rebar mapping (0.3-1 m depth), and 2-4 GHz for high-resolution pavement and bridge deck assessment (0-0.3 m depth). The depth of penetration depends on the soil or material conductivity; highly conductive materials (wet clay, salt water) severely attenuate the signal and limit GPR effectiveness.