Development of a Multimode Field Deployable Lidar Instrument for Topographic Measurements of Unsaturated Soil Properties: Instrument Description

Abstract

The hydrological and mechanical behavior of soil is determined by the moisture content, soil water (matric) potential, fines content, and plasticity. However, these parameters are often difficult or impractical to determine in the field. Remote characterization of soil parameters is a non-destructive data collection process well suited to large or otherwise inaccessible areas. A ground-based, field-deployable remote sensor, called the soil observation laser absorption spectrometer (SOLAS), was developed to collect measurements from the surface of bare soils and to assess the in-situ condition and essential parameters of the soil. The SOLAS instrument transmits coherent light at two wavelengths using two, continuous-wave, near-infrared diode lasers and the instrument receives backscattered light through a co-axial 203-mm diameter telescope aperture. The received light is split into a hyperspectral sensing channel and a laser absorption spectrometry (LAS) channel via a multi-channel optical receiver. The hyperspectral channel detects light in the visible to shortwave infrared wavelengths, while the LAS channel filters and directs near-infrared light into a pair of photodetectors. Atmospheric water vapor is inferred using the differential absorption of the on- and off-line laser wavelengths (823.20 nm and 847.00 nm, respectively). Range measurement is determined using a frequency-modulated, self-chirped, coherent, homodyne detection scheme. The development of the instrument (transmitter, receiver, data acquisition components) is described herein. The potential for rapid characterization of physical and hydro-mechanical soil properties, including volumetric water content, matric potential, fines content, and plasticity, using the SOLAS remote sensor is discussed. The envisioned applications for the instrument include assessing soils on unstable slopes, such as wildfire burn sites, or stacked mine tailings. Through the combination of spectroradiometry, differential absorption, and range altimetry methodologies, the SOLAS instrument is a novel approach to ground-based remote sensing of the natural environment.

Multi-Channel Optical Receiver for Ground-Based Topographic Hyperspectral Remote Sensing

“…The reflectance spectrum collected via Channel 1 was characteristic of a Lambertian reflector across the range of wavelengths (reflectance values close to 1.0). Although Channel 2 was designed to deliver light to a pair of near-infrared photodetectors used for the LAS measurements, as discussed previously in this paper and in [11], the specifications of the ASD FieldSpec 4 instrument were well suited for also assessing the functionality of the Channel 2 optical design across the near-infrared wavelength range. This also enabled direct comparison between receiver channels.…”

“…Atmospheric attenuation, due to absorption and scattering by water vapor and other aerosols along the receiver path, will necessitate corrections to derive exact reflectance measurements. These corrections will be achieved using the LAS measurement system of the SOLAS instrument, as described previously [11], and will be addressed in future work.…”

“…The aforementioned light on Channel 2 is delivered to a pair of near-infrared photodetectors as part of a topographic LAS measurement system. The LAS measurement system is described in further detail in Salazar et al [11]. A labeled photograph of the receiver is presented in Figure 1 and a schematic of the receiver is presented in Figure 2.…”

“…The receiver was developed to enable simultaneous range-resolved hyperspectral measurements of hard targets and differential laser absorption measurements for atmospheric corrections of the hyperspectral measurements. The receiver is part of a new ground-based remote sensing instrument, called the soil observation laser absorption spectrometer (SOLAS), previously described in the literature by Salazar et al [11]. The instrument was developed for rapid characterization of bare soil, rock surfaces, and/or vegetation.…”

“…The atmospheric measurements will be used in the future to correct the hyperspectral reflectance from long-range targets. Although the SOLAS instrument was described previously [11], a more detailed discussion of the development and testing of the multi-channel receiver portion of the instrument, as used to collect the backscattered energy, is discussed in the following sections.…”