Fig. 1. Schematic diagram of terahertz spectral detection（The inset shows the terahertz time-domain spectrum of the sample and the reference）

Fig. 2. Terahertz spectral signal and principle of analysis

Fig. 3. The attenuation coefficient associated with the sand size（The error line is 10% error）

Fig. 4. Absorption coefficients of different sand samples in the frequency range from 0.05 THz to 0.42 THz