The characterization of fractures is of vital importance for studies in many industries. In the present work, we used terahertz reflection measurements to identify simulated gaps of different widths that resemble fractures. The time interval between pulses (Δt) was proportional to the distance between the interfaces. In addition, fast Fourier transform (FFT) corresponding to the waveforms was also employed to distinguish the gaps. The widths in frequency were found to be inversely proportional to that of the gaps. In general, both terahertz time-domain spectroscopy (THz-TDS) and terahertz frequency-domain spectroscopy (THz-FDS) can achieve the measurement of the gaps of micro size.

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The study of fluid inclusion is one of the important means to understanding the evolution of mineral crystals, and can therefore provide original information of mineral evolution. In the process of evolution, outside factors such as temperature and pressure, directly affect the number and size of inclusions, and thus are related to the properties of crystals. In this paper, terahertz time-domain spectroscopy (THz-TDS) was used to detect sodium sulfate crystals with different growth temperatures, and absorption coefficient spectra of the samples were obtained. It is suggested that the evolution of sodium sulfate could be divided into two stages, and 80 C was the turning point. X-ray diffraction (XRD) and polarizing microscopy were used to support this conclusion. The research showed that THz-TDS could characterize the evolution of mineral crystals, and it had a unique advantage in terms of crystal evolution.