Page 14 - 2018Fall
P. 14

The Acoustics of Marine Sediments
 Figure 1. Examples of bottom loss (BL) calculated for “fast” and “slow” bottoms. BL is calculated as 10 log10|R|2 , where R is the com- plex reflection coefficient. For the coarse-grained sandy sediment (fast bottom: the sound speed in the sediment is faster than that in the overlying water), nearly all of the incident sound wave is reflected from seabed for grazing angles less than the critical angle (here about 20° relative to the horizontal direction). Much less sound is reflected from the fine-grained muddy sediment (slow bottom: the sediment sound speed is lower than that in the overlying water) because total sound transmission into the sediment occurs at the angle of intromis- sion, which is at a grazing angle of about 22°.
ter content and low bulk density) and exhibit gel-like behav- ior. For pure clay muds, the intrinsic attenuation can be very small (Holland and Dosso, 2013).
Coarse-grained sediments are characterized by a sound speed faster than that of the overlying water, and the angu- lar dependence of the refection coefficient (the ratio of the amplitude of the reflected wave to the incident wave) at the water-sediment interface possesses a critical angle. For graz- ing angles smaller than the critical angle, there is total re- flection of the incident wave (see Figure 1). For propagation environments with sandy seabeds, sound waves that reflect from the bottom with small angles relative to the horizon- tal direction can propagate for long distances with relatively little energy loss due to bottom interaction. On the other hand, fine-grained sediments are characterized by a sound- speed ratio (sound speed in the sediment compared with that of the overlying water) of less than one, giving rise to a reflection coefficient with an angle of intromission, which is the angle for which total transmission of sound into the sea-
bed occurs. As a result, comparatively less sound is reflected from fine-grained sediments.
Although coarse- and fine-grained sediments have distinct acoustic properties, most naturally occurring marine sedi- ments contain a mixture of sand-, silt-, and clay-sized par- ticles in varying proportions. An example of sand grains coated with increasing quantities of clay platelets are shown in Figure 2. The addition of clay particles to a sandy sedi- ment can have disparate effects on porosity and wave speed depending on the relative volume of clay particles. For low concentrations of clay particles (less than 20% by weight), the clay particles are located in the sand pore space and act to stiffen the pore-filling material. As a result, porosity de- creases and velocity increases. In contrast, when the volume of clay particles is high (greater than 40% by weight), sand grains are suspended in the clay matrix. Therefore, porosity increases and velocity decreases with increasing clay content (Marion et al., 1992). Many natural marine sediments also contain free or trapped gas and/or organic content that can further alter the effective bulk properties of the sediment.
Figure 2. Scanning electron mi- croscope (SEM) images showing the variable extent of attached clay coats observed in surface sediment samples. a: Complete absence of clay coats; b: 1-5% at- tached clays on less than half of the grains; c: every grain exhib-
its 5-15% clay coats coverage; d: clay coats observed on every grain, with the majority exhibiting extensive 15-30% coverage; e: extensive, >30% clay coats coverage observed on every grain. Adapted from Woolridge et al. (2017).
    12 | Acoustics Today | Fall 2017

   12   13   14   15   16