Shear and Scholte wave measurements on surficial intertidal mudflat sediments using medical sonoelastography techniques

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In low-energy environments, the water–sediment interface often contains a dynamic fluffy transition layer (FTL) of suspended organic and inorganic matter, which is transformed by complex physical, biological, and chemical processes. The FTL mechanical properties are important for understanding carbon cycle dynamics and for robust sediment acoustic characterization, but erodibility and size make it difficult to study non-invasively and at sufficient resolution. In medical ultrasound sonoelastography, shear waves are excited within the body either using the acoustic radiation force (ARF) or via external vibration at the skin surface. Sub-micrometer displacements are measured remotely by subsequent ultrasound imaging at up to 10 kHz frame rate with approximately 0.5 μm spatial resolution. In this study, sediment cores were extracted from the intertidal mudflats from the Great Bay Estuary, USA. Compressional wave speed and attenuation measurements were performed using a core logger. Shear and Scholte waves were excited in the FTL using ARF- and external-vibration-based techniques and the motion detected with clinical ultrasound imaging arrays. Upward refraction of the shear wave was observed, and shear and Scholte wave speed and attenuation were measured. Finally, scanning electron microscopy was used to image the microstructure. Comparison between wave properties and microstructure will be discussed.