[A-OS16] Coastal ocean circulation and material cycle

[AOS16-P08] Three-dimensional acoustic imaging of internal waves using a multibeam echosounder – an example across the shelf edge in the East China Sea

*Ryosuke Nagasawa¹, Ryoichi Horinouchi ¹ (1.Japan Coast Guard)

Keywords:echosounder, internal wave, pycnocline, underwater acoustics, acoustical oceanography

Acoustic imaging of oceanographic structures has long been performed using single-beam echosounders, which form a single acoustic beam to observe ocean interiors (e.g., Proni and Apel, 1975). In recent years, as multibeam echosounders (MBES) have become widely used, it has also been applied to the observation of internal waves and oceanic microstructures (Colbo et al., 2014; Zwolak et al., 2021). MBES can more easily acquire high-resolution, high-density, three-dimensional acoustic images in the ocean by forming short pulses and highly directional multiple receiving beams. In this presentation, we discuss a method for three-dimensional acoustic imaging of internal waves from MBES data and present a detailed analysis method using an example of internal gravity waves observed in October 2022 over the shelf edge in the East China Sea (Horinouchi et al., JpGU, 2024).
The observation was made by using MBES with an operating frequency of 70 – 100 kHz in a continuous round-trip cruise across the slope of the shelf edge. The echogram drawn using the method described in the previous report (Nagasawa and Horinouchi, 2023) shows scattering layers with internal solitary wave-like undulations over an area of more than 10 km horizontal distance. The vertical profiles of temperature and electrical conductivity obtained along the survey line were used to estimate the vertical variation of acoustic impedances, and that showed good correspondence with the observed scattering layers. The vertical oscillation of pycnoclines, which characterize the internal waves, was visualized with spatial and temporal continuity. In addition, as a consideration for estimating wavelength and other characteristics of internal waves from mobile observation data, the movement of the observation platform and the geometry of the acoustic beams were considered in the analysis, and a three-dimensional visualization of the internal wave was attempted.