Investigation of surficial seabed heterogeneity and geoacoustic variability in the New England Mud Patcha)

Benthic biological processes influence seabed heterogeneity and contribute to variability in geoacoustic properties. To investigate these relationships, measurements were conducted to quantify spatial variability in the upper few decimeters of sediment near the water-seabed interface within a fine-grained sediment deposit on the New England continental shelf. At each measurement location, an acoustic multicorer was deployed to sample the seabed. Acoustic probes were inserted into the sediment to collect direct in situ measurements of sediment compressional wave speed and attenuation (30-100 kHz) under near-ambient conditions, after which cores were collected from the inter-probe propagation paths. Sediment physical properties, organic carbon, infaunal community composition, and ex situ compressional wave speed and attenuation spanning two frequency decades (104-106 Hz) were subsequently measured in the laboratory. The frequency dependence of sound speed ratio and attenuation was analyzed in the context of sediment acoustics models for mud based on the viscous grain shearing and extended Biot models. Sites with greater abundance of larger-bodied infauna (>1 mm) displayed higher variability in sound speed and attenuation. Correlation was found between sediment compressional wave modulus and total organic carbon, suggesting that organic matter in the sediment matrix also affects bulk acoustic properties.