Characterization of surficial mudflat sediments using shear wave elastography, core-logging, and microscopy techniquesa).

Surficial sediments are highly susceptible to physical, biological, and chemical processes, which can create significant heterogeneity, affecting the transmission and scattering of elastic waves. Non-invasive medical shear wave elastography (SWE) can potentially resolve shear speed heterogeneity in this delicate surficial layer. Samples were extracted from two mudflats in New Hampshire, USA, where sound speed and attenuation were measured 1 cm below the water-sediment interface using the core and resonance logger (200 kHz-1 MHz). Subsequently, propagating shear waves (50-200 Hz) were generated using acoustic radiation force and imaged with high-frame-rate ultrasound, yielding two-dimensional shear speed maps in the top 1 cm. Finally, subsamples were imaged using a suite of microscopy techniques. SWE revealed stiff millimeter-scale inclusions with shear speeds up to 16 m/s suspended in a lower-shear-speed medium with values around 2 m/s. In the 1000-20,000 m-1 band, shear speed spatial spectral strengths are between -42 and -38 dB re 1 m and the spectral slope is 3.5. Positive shear speed depth gradients between 189 and 332 s-1 are observed, where microscopy revealed that pore and grain connectivity decreased and increased with depth, respectively. Finally, the grain shearing model was fit to measured wave parameters. Results are consistent with highly lubricated and compliant grain contacts in the presence of large volume fractions of organo-clay.