Time-frequency Analysis of a Flexible RF Sensor for GPR Applications

The detection of utilities in complex underground using Ground Penetrating Radar (GPR) is still challenging due to electromagnetic attenuation and scattering effects. Radio-Frequency (RF) tagging of the buried pipes is a new approach to deal with these problems. In this paper, a flexible and passive tag is designed and tested in sand. Two indexes, i.e., the resonant frequency and amplitude, of the tag are extracted from the time-frequency spectrum based on Variational Mode Decomposition (VMD) with high frequency resolution to study the performance of the tag under different bending angles. The results show that large bending angles result in quick degradation of the tag's resonance performance, which mainly displays in two aspects, the big frequency shift and the weaken strength of the response signals. Such results demonstrate that the flexible conformal tag is able to enhance the detection, however it is of significance to design the tag which allows large bending angles while the performance degrades slowly.