Ultrasonic Transmission Behavior in Posidonia oceanica Rhizomes

The roots and rhizomes of seagrass can form a complicated multi-phase layer within sediments. Gas channels in the rhizomes, known as aerenchyma, give rise to a complicated acoustic response. A model for acoustic propagation through such rhizomes would be beneficial for acoustic remote sensing communities. Ex situ measurements of the ultrasonic sound speed through the dense woody structure of the rhizomes of the seagrass Posidonia oceanica are presented. Ultrasonic (1–5 MHz) time-of-flight measurements were made with the rhizomes segments aligned both lengthwise and crosswise to the propagation direction of the acoustic pulse. The acoustic behaviors of plants collected from different locations in the Mediterranean are compared to each other and to the behaviors of the associated leaf blade tissues. Two measurements were also made of rhizomes before and after degassing the aerenchyma, to quantify the effects of entrained gasses on acoustic behavior. [Work supported by ONR and ONR Global.]