Effect of Moisture Content and Particle Size on Extinction Coefficients of Soils Using Terahertz Time-Domain Spectroscopy

Use of ultrafast terahertz (THz) laser spectroscopy as a nondestructive way to characterize the physicochemical properties of natural soils is still immature because how and to what extent the soil type, particle size, porosity, and moisture contents affect the THz spectroscopic responses of natural soils remains poorly identified. Herein, we explored the effect of moisture content and particle size on extinction coefficients of soils at a frequency range of 0.2-2 THz using ultrafast THz time-domain spectroscopy (THz-TDS) for three representative natural soils at different moisture contents. In the absence of water, the porosity and particle size distribution were found to mainly affect the THz transmission and scattering. Particularly, the Mie scattering appeared to have a pronounced effect on extinction coefficients of sand. In the presence of water, the moisture content played the governing role in absorption over the other factors. It was found that the effective medium modeling using Lambert-Beer model well predicted the extinction coefficients of silt and clay for a frequency range of 0.1-1 THz. Whereas, the experimentally determined extinction coefficients of sand were significantly greater than our model over the tested frequency range, owing to severe Mie scattering. These results provide complied test data that support the feasibility of using THz-TDS to remotely identify the physical characteristics of soils.