Target strength of Sargassum plant with vesicles containing gas

Abstract

Information about the target strength (TS) of a vesicle is important to estimate the acoustic scattering of Sargassum plants. Experimental TS (TSe) of a vesicle was obtained at 70 and 200 kHz using quantitative echo sounders in a freshwater tank (10×10×15 m). The TSe of ten vesicles examined here were −56.88–51.68 dB at 70 kHz and −60.30–‐50.90 dB at 200 kHz. The TSe′s of small vesicles were 2.56−−3.70 dB higher at 70 kHz than at 200 kHz, suggesting that these small vesicles were affected by the airbladder resonance. Theoretical TS (TSt) was estimated by airbladder−scattering models to confirm this effect. The gas radii of 418 vesicles (containing the experimented ten vesicles) were measured using a soft x−ray system and applied to the theoretical models. Their radii were 0.52−−6.00 mm; calculated TSt matched with TSe. According to the theoretical model, the vesicles were not affected by the resonance when argument ka is ka>1 (where k is the wave number and a is the radius). However, the vesicles were affected by the resonance when ka<1. That fact indicates that small vesicles (a is 1.22−−2.38 mm) at 70 kHz were affected by the resonance.