Variations in ultrasonic transmission behavior along seagrass leaf blades

Seagrass is a complex multi-phase material, and an effective method for connecting acoustic propagation through seagrass meadows to internal or external characteristics of the seagrass would be beneficial for acoustic remote sensing applications. To investigate some of these connections, ultrasonic (1, 2.25, and 5 MHz) time-of-flight measurements through individual leaf blades of the endemic Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa are presented. Acoustic measurements were made at multiple points along the leaf blades and the sound speed and signal attenuation varied significantly within a single blade depending on measurement location. The measured acoustic variations are compared to external blade features such as discoloration, epiphyte coverage, and thickness. Microscopy images of blade cross-sections each taken at each acoustic measurement location are used to compare the void fraction to acoustic behavior. [Work supported by ONR, ONR Global.]