The impact of salinity diffusion, poroelasticity, and organic carbon in sediment acoustics

Date

2019-05

Authors

Venegas, Gabriel Ricardo

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Abstract

To optimize the use of sound in waters on the continental shelf for naval, commercial, and environmental monitoring applications, the acoustic properties of the ocean bottom must be well understood. The effects of 1) pore water salinity variability on acoustic reflectivity, 2) poroelasticity on geoacoustic inference, and 3) organic carbon on sediment properties were formerly-considered insignificant in sediment acoustics, but due to advancements in other areas of underwater acoustics systems and modeling, have now become significant. Three separate but related studies were conducted to begin to quantify these effects. 1) A high-frequency acoustic reflection experiment was performed on a water-clay interface, while varying the salinity of the water. Results demonstrated significant changes in reflectivity at high incident angles, as well as a transient effect explained by a new coupled salt diffusion/reflection model. Using the model, the effective diffusion coefficient of salt in clay was inferred from the experiment, and reflectivity was then simulated at lower frequencies and longer time-scales. From this modeling effort, at a given time-scale of fluctuation, a characteristic frequency was identified, below which the reflectivity should not be assumed temporally invariant. 2) A model geoacoustic inference procedure was performed on a layered waveguide consisting of water and water-saturated glass beads contained within a glass tube. The resonance frequencies of the system were measured and compared with simulations of the experiment. Within each simulation, various sediment acoustics models were used. The only model that allowed for self-consistency between the inference and an independent set of high-frequency sound speed measurements, was a model that accounted for poroelastic effects. 3) A sediment constituent that has great value to the planet and is ubiquitous in natural marine sediment, organic carbon, has been ignored in sediment acoustics models. To begin to explore this relationship, sediment cores were extracted from a T. testudinum seagrass meadow in the Lower Laguna Madre, Texas, USA. A strong correlation between organic carbon and the primary-wave modulus was identified using a custom-built automated broad-band core and resonance logger and an elemental analyzer. The sediment properties attained from the cores were compared, and a theory explaining the correlations was developed. The acoustic sensitivity to organic carbon in a seagrass meadow has demonstrated promise toward developing an acoustic tool to more rapidly quantify marine organic carbon stores, which is needed in climate science. However, a larger-scale study is required to determine its applicability across a broader range of seagrass meadows and sediment types

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