Page 13 - 2018Fall

Megan S. Ballard
Postal:
Applied Research Laboratories The University of Texas at Austin PO Box 8029 Austin, Texas 78713-8029 USA
Email:
meganb@arlut.utexas.edu
Kevin M. Lee
Postal:
Applied Research Laboratories The University of Texas at Austin PO Box 8029 Austin, Texas 78713-8029 USA
Email:
klee@arlut.utexas.edu
The Acoustics of Marine Sediments
For biologically active sediments, understanding geoacoustic properties is a multidisciplinary undertaking, involving both the measurement of acoustic properties and the quantification of biological effects.
Introduction
In contrast to electromagnetic waves, which are highly attenuated by seawater, acoustic waves can propagate long distances through the ocean. For this reason, sound waves are used underwater for navigation, communication, and remote sensing. For shallow-water propagation environments, which include extensive regions on continental shelves, acoustic propagation can be described as a wave- guide bounded above by the sea surface and below by the seafloor. The geometry of the shallow-water waveguide is that of a broad, thin layer so that sound emitted from a source generally reflects from the waveguide boundaries many times before reaching a receiver. Because the distance that sound propagates from the source to the receiver can be equivalent to hundreds of water depths, the sound field is greatly influenced by the properties of the waveguide boundaries, and the acousti- cal properties of the seabed can be the dominant factor affecting propagation and scattering in shallow-water environments.
The geoacoustic properties of marine sediments have been studied for over a cen- tury, with papers covering this topic regularly appearing in The Journal of the Acous- tical Society of America for the past 60 years. This article begins by providing back- ground on the types of marine sediments, which can differ in their source (lithogenic: coming from land by erosion of rocks, vs. biogenic: derived from the hard parts of animals), predominate mineralogy (silicate vs. calcium carbonate), and grain struc- ture (solid vs. porous) as well as the approaches used to model their acoustic proper- ties. Next, the current state of measurement and modeling techniques is described and examples of applications are presented. The article concludes with a discussion of open questions and possible future directions for the field.
Types of Marine Sediments
Marine sediments are often classified according to grain size, with standardized defi- nitions for sand (median diameter greater than 62.5 μm), silt (median diameter be- tween 3.9 and 62.5 μm), and clay (median diameter less than 3.9 μm) (Wentworth, 1922). The composition of coarse-grained sediments (composed of sand- and silt- sized particles) differs greatly from that of fine-grained sediments (composed of clay- sized particles). The stress-strain behavior (how an elastic medium deforms under loading) of coarse-grained sediments is dominated by friction between the particles, which, along with viscous damping due to the thin layer of pore fluid (which may consist of freshwater or salt water as well as mucus and other animal byproducts) be- tween the grains, is a mechanism for the attenuation of acoustic waves (Buckingham, 2014; Chotiros and Isakson, 2014). Muddy sediments made up of clay-sized particles consist of a colloidal suspension of microscopic, irregularly shaped platelets, which carry a surface charge linked to their cation exchange capacity. These suspensions result in open structures that cause mud to have high porosity (indicating high wa-
©2017 Acoustical Society of America. All rights reserved. volume 13, issue 3 | Fall 2017 | Acoustics Today | 11