ABSTRACT
Ground-penetrating radar (GPR) is one of the newer geo-physical methods, and one of the most successful in detecting shallow caves (Chamberlain et al., 2000). Pulses of lowfrequency (10 to 1500 MHz) electromagnetic energy penetrate the ground and are partially or totally reflected from rock or soil boundaries with contrasting electrical properties (notably their dielectric constants, or permittivities). Air-filled voids and layers of water-saturated sediment are strong radar reflectors. The reflected signals are detected on the ground surface and are collated by computer to produce the ground profiles; a series of parallel profiles can be combined to generate gridded 3-D images of the subsurface. GPR has been used to map joints enlarged by dissolution under thin soils, and may have the potential for mapping the cross-sectional shape of caves that are not too deeply buried by conductive overburden. The selection of radar frequency is probably the most important choice when undertaking a GPR survey, since lower frequencies (10-300 MHz) allow depth penetration to several tens of metres, but they will fail to detect small diameter anomalies. Frequencies of 500 MHz and greater provide excellent resolution, but they limit the subsurface depth penetration of the GPR to less than 5 m. Radar signals penetrate well into dry sandy soils and into carbonate rocks to map features at depths of several tens of metres. In contrast, clay-rich soils attenuate GPR signals and can restrict depth interrogation to depths of just 1 m. Lateral reflections limit the use of GPR in rugged terrain. Roadbeds are penetrated by GPR, and a transmitter/receiver towed by a car can be used to detect karstic voids developing under the highway. GPR is a relatively new technology that should become increasingly functional as field geophysicists discover ways to enhance its capabilities.
