Ocean water column information profiles are essential for ocean research. Currently, water column profiles are typically obtained by ocean lidar instruments, including spaceborne, airborne and shipborne lidar, most of which are equipped with a 532 nm laser; however, blue wavelength penetrates more for open ocean detection. The researchers report that a Dual-Wavelength Ocean Lidar (DWOL) equipped with a 532 and 486 nm laser that can operate simultaneously was developed.

The laser wavelengths of the DWOL system were optimized to make the DWOL system compatible with coastal water and open ocean water. They reported that the vertical profiles of returning signals from a depth of approximately 100 m were obtained with the newly designed 486 nm channel. The results were published on Remote Sensing.

In June 1st 2019, they conducted the airborne experiment in the South China Sea, the airborne experiment was conducted from 7:30 (UTC+8) pm to 9:30 (UTC+8) pm to reduce the influence of background noise. They set the control voltage of the PMT detectors as 0.95 V. In this condition, the PMT detectors can detect the continuous returning signals and the discrete returning photon signals. In the data processing, they accumulate 500 frames of returning signals obtained within 5 seconds to extend the dynamic range of the DWOL system.

They also conducted a shipborne experiment in the same area in the South China Sea. Results show that—for a 500-frame accumulation—the 486 nm channel obtained volume profiles from a depth of approximately 100 m. In contrast, the vertical profiles obtained by the 532 nm channel only reached a depth of 75 m, which was approximately 25% less than that of 486 nm channel in the same detection area.

In the data processing, they inversed the lidar attenuation coefficient α(z) from the DWOL data; results show that the maximum value of α(z) ranged from 40 to 80 m, which was consistent with the chlorophyll-scattering layer (CSL) distribution measured by the shipborne instrument. Additionally, α₄₈₆(z) decreased for depth beyond 80 m, indicating that the 486 nm laser can potentially penetrate the entire CSL.

This work is funded by the Key R&D Projects of the State Key Laboratory of Satellite Ocean Environment Dynamics, the Second Institute of Oceanography, the Ministry of Natural Resources (2016YFC1400902), the Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology (2018SDKJ0102-4) and the General Project of NSFC for the Research on Precise Detection Technology of Airborne Lidar Ocean Sounding Signal (41876105).

Figure 1. Experiments in the South China Sea. (a) Distributed flight track and selected in situ sites; (b) photograph of the DWOL system in airborne experiment. (Image by SIOM)

Figure 2. (a–c) Comparison between the simulation results based on shipborne measurement data and the airborne results at altitude of 2000 m; (d) comparison between the simulation results and the airborne results at altitude of 2500 m. (Image by SIOM)

Figure 3. (a) Distribution of α₅₃₂(z) from Selected-A to Selected-B; (b) distribution of α₄₈₆(z) from Selected-A to Selected-B; (c) distribution of α₅₃₂(z) from Selected-C to Selected-D; (d) distribution of α₄₈₆(z) from Selected-C to Selected-D. Value of limited from 0 to 0.2 for better visualization. (Image by SIOM)

Article website:
https://www.mdpi.com/2072-4292/12/17/2844

Contact:
Mr. Wu Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn