Terahertz and millimeter wave transmission of soils

Transmission spectra were measured over the range 90-140 GHz and 300-4200 GHz for 20 soil samples that span a number of soil orders that have extensive worldwide distribution. A vector network analyzer equipped with 16 degree horn antennas covered the spectral range 90-140 GHz. Transmission measurements were also taken for some organic materials in the 90-140 GHz range. A Fourier spectrometer equipped with Hg arc lamp, pellicle beamsplitter, and Si bolometer collected transmission spectra over the range 300 to 4200 GHz. Transmittance ranged from 10-7 to almost 1. In all cases, transmission drops to zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. In samples of mixed particle size, low transmittance in the 90-140 GHz range was found to be caused by the coarse component. This work is relevant to mine detection using THz and millimeter wave (mmW) radiation.