Spectral shift and absorption of GPR signals in a wetted sand column

Spectral shift and absorption of dispersed GPR signals corresponding to a downward wetting front in a sand column simulated in laboratory were studied. The non-stationary GPR signals (making use of a 1000MHz antenna) were processed with short time-fourier transform (Lai et al. 2012) and wavelet transform (WT) in a time-frequency domain. Distributions of peak frequency, bandwidth and associated spectral absorption obtained by the STFT and WT are mapped and compared at (1) the direct wave across the transmitting and receiving antennae, (2) reflected wave of the downward wetting front. There are two important findings. First, the WT-processed results show a better representation of the wetting front reflections than the STFT-processed results because of its capability of multi-time and multi-frequency resolution accounting for both high-frequency wetting front and low-frequency direct wave signals. Second, the spectral content of high frequency component is continuously absorbed over time with propagation of downward wetting front. This paper shows the possibility to study the subsurface in the time-frequency domain, which gives additional information to the well-established time/spatial-domain mapping.