Crosshole radar for underground structure defect detection: System design and model experiment

This paper presents a crosshole radar system and its model experiment for underground structure's defect detection. The system was built using a vector network analyzer (VNA), cylinder dipole antennas, and a radio frequency (RF) amplifier. The VNA enables the radar to work in a stepped frequency mode. The antenna, which has a carefully designed diameter of 32 mm, is compatible with most boreholes in common underground structures and is tested to work in the frequency range from 210 MHz to 560 MHz. The RF amplifier, set up at the output port of the VNA, amplifies radar signals by approximated 20 dB in the working frequency band. This system is tested by a reinforced concrete underground structure (diaphragm wall) model experiment, in which embedded defects are detected by the crosshole radar through two adjacent preset boreholes. MOG data were recorded and processed by a first-arrival travel time tomography method. The defective areas were reconstructed correctly from the inversion result, which validates the capability of the crosshole radar system and this method for underground structure defect detection.