Hyperspectral Imaging Tera Hertz System for Soil Analysis: Initial Results

Abstract

Analyzing soils using conventional methods is often time consuming and costly due to their complexity. These methods require soil sampling (e.g., by augering), pretreatment of samples (e.g., sieving, extraction), and wet chemical analysis in the laboratory. Researchers are seeking alternative sensor-based methods that can provide immediate results with little or no excavation and pretreatment of samples. Currently, visible and infrared spectroscopy, electrical resistivity, gamma ray spectroscopy, and X-ray spectroscopy have been investigated extensively for their potential utility in soil sensing. Little research has been conducted on the application of THz (Tera Hertz) spectroscopy in soil science. The Tera Hertz band covers the frequency range between 100 GHz and 10 THz of the electromagnetic spectrum. One important feature of THz radiation is its correspondence with the particle size of the fine fraction of soil minerals (clay < 2 µm to sand < 2 mm). The particle size distribution is a fundamental soil property that governs soil water and nutrient content, among other characteristics. The interaction of THz radiation with soil particles creates detectable Mie scattering, which is the elastic scattering of electromagnetic waves by particles whose diameter corresponds approximately to the wavelength of the radiation. However, single-spot Mie scattering spectra are difficult to analyze and the understanding of interaction between THz radiation and soil material requires basic research. To improve the interpretation of THz spectra, a hyperspectral imaging system was developed. The addition of the spatial dimension to THz spectra helps to detect relevant features. Additionally, multiple samples can be scanned in parallel and measured under identical conditions, and the high number of data points within an image can improve the statistical accuracy. Technical details of the newly designed hyperspectral imaging THz system working from 250 to 370 GHz are provided. Results from measurements of different soil samples and buried objects in soil demonstrated its performance. The system achieved an optical resolution of about 2 mm. The sensitivity of signal damping to the changes in particle size of 100 µm is about 10 dB. Therefore, particle size variations in the µm range should be detectable. In conclusion, automated hyperspectral imaging reduced experimental effort and time consumption, and provided reliable results because of the measurement of hundreds of sample positions in one run. At this stage, the proposed setup cannot replace the current standard laboratory methods, but the present study represents the initial step to develop a new automated method for soil analysis and imaging.

Application of a Fusion Attention Mechanism-Based Model Combining Bidirectional Gated Recurrent Units and Recurrent Neural Networks in Soil Nutrient Content Estimation

“…During the indoor spectral measurement, the soil sample was prepared by filling and flattening the processed air-dried soil in a blood container with a diameter of 10 cm and a depth of 2 cm. The soil surface was leveled using a straight edge to ensure uniformity [54]. A standard DC tungsten quartz halogen lamp served as the light source for the measurement.…”

Editorial: Application of Terahertz Frequency in Substance Detection and Recognition

“…Frequencies in the terahertz range, from 100 GHz (GHz) to 10 THz (THz), are endowed with many uniquely attractive features and assumed to be widely used in various fields, such as wireless communication [1], aerospace, security and biomedicine [2]. In laboratory demonstrations, terahertz frequency can transfer large amounts of data quickly [3], obtain high-resolution images [4,5], identify explosives and reveal hidden weapons without ionizing atoms and molecules in human tissue [6][7][8]. Indeed, located between microwaves and infrared, the THz band has a number of relevant benefits when applied to sensing and imaging [9].…”

Hyperspectral Image Reconstruction Based on Reference Point Nondominated Sorting Genetic Algorithm

“…According to the discretization method of the ðs, u, υ, ωÞ, all atoms are generated to form the initial feasible solution set Z int , the size of Z int is N × N atom , and the element in Z int is expressed in formula (9), where ðs, u, υ, ωÞ is the n atom th time-frequency parameter in set Γ.…”

“…The theory assumes that the information flow of continuous-time signals should be significantly lower than the signal's maximum frequency and that the degree of freedom of discrete-time signals should be significantly lower than the signal's length [8]. Therefore, the compressed sensing theory believes that the information characteristics of the signal can be directly collected, completing the reconstruction of the signal [9].…”