Calibration of a Stationary Multichannel GPR Monitoring System Using Internal Reflection Measurements

Advanced and extensive processing of ground-penetrating radar (GPR) data, for example, needed for full-waveform inversion approaches, requires a reliable temporal calibration of the system. Usually, the calibration of GPR systems is performed with a known medium between the transmitting and receiving antennas. Thereby, the observed time difference between the expected and measured signal arrival times, termed as time-zero, can be accounted for as a system-specific time delay. For measurement configurations where the antennas are permanently positioned around an object for monitoring purposes, time-consuming additional measurements where parts of the system need to be deinstalled would be required. This is not feasible for the proposed system. Therefore, novel calibration methods for such stationary monitoring systems are required to capture the temporal drift of time-zero. In this article, we present a novel calibration approach that uses internal signal reflections in the measurement system to derive the system-specific time delay without the necessity of knowing the medium between the antennas. We demonstrate that parasitic reflection and coupling signals can be used for accurate in situ calibrations. The presented approach is capable of identifying and correcting for the differences in hardware fabrication, while also correcting the temporal changes in time-zero during experiments. The presented approach is able to reduce the error in time-zero to below 25 ps, enabling high-resolution soil research. The presented approach is characterized by requiring no additional calibration setups or measurements since all the necessary data can be acquired during the original soil measurement.