Attenuation of ground penetrating radar signal in Kuru granite

Ground penetrating radar (GPR) is a non-destructive electromagnetic method widely used in civil engineering surveys and investigations. GPR applications can be used to analyse rock mass integrity and to detect geological and geometric features such as lithological unit interfaces, fractures, shear zones and voids. Typically, GPR studies have relied on laboratory values of relative dielectric permittivity for calibration. These are effective (homogeneous) medium values for a system that differs from the instrument that is actually used in the field, measured from small samples. In order to investigate GPR signal propagation in the rock mass, a slab of 100 x 90 x 30 cm (width x height x depth) was sawcut from a larger intact rock block of largely homogenous grey granite from Kuru, Tampere, Finland. Point like GPR measurement with central frequency of 1600 MHz were carried out in a grid on both 100 x 90 cm slab surfaces. Altogether 63 points per side were measured with approximately 250 scans per point. Average relative dielectric permittivity of the rock slab was 4.82, which was also the median value. Relative dielectric permittivity varied from 4.74 to 4.90, giving a range of 0.16 (3.3 % of the mean value). Average attenuation of the signal in the rock slab was -16.39 dB/m, with a range from -16.08 dB/m to -16.67 dB/m, or 0.59 dB/m (3.6 %). This study provides information on true attenuation and dielectric properties, as measured with a device to be used in the field, in an actual, relatively homogeneous rock mass. They can be used to obtain more accurate information on the location and size of e.g. fractures, fracture zones, inclusions or altered zones within the rock mass. Furthermore, the process described here can be applied elsewhere to obtain similar, localised and device specific attenuation and relative dielectric permittivity values.