How does sound propagate through sediment?

The medium through which a sound travels affects the behavior of the sound. Sound behaves differently in air than in water, and likewise sound behaves differently in ocean bottom sediments. Sound can move into sediments from the ocean or originate within them. Common sources of sound that can propagate through the sediments are from natural sources, such as earthquakes, volcanic eruptions, and marine mammal calls, as well as anthropogenic sources, such as pile-driving, seismic surveys, dredging, and explosives.

Most ocean sediment is made of particles with sizes ranging from 1µm to 2mm. Marine sediment is usually a mix of particle compositions including terrigenous material, clay, and shells or shell parts made of calcium carbonate or silica. The sediment grain size and composition along with its porosity are important factors in sediment wave propagation, such as wave speed and attenuation.

Propagation in sediment consists of pressure waves, shear waves, and interface waves. These waves are sometimes referred to as substrate-borne vibration. Pressure waves (or compressional waves) in the sediment are longitudinal waves where the particle motion is in the same direction as the travel direction of the wave. Shear waves in sediment are transverse waves where the particle motion is perpendicular to the travel direction of the wave. Interface waves are waves that are trapped between layers of varying properties.

Diagram of sound waves generated by pile driving. Sound travelling down the pile creates waves in the water (sound waves), in the sediment (pressure and shear waves) and at the sediment-water interface (interface waves). Waves can transition from sediment to water and vice versa. Image modified from Dr. Anthony D. Hawkins.

The speed of pressure waves in the top few meters of sediment are generally similar to, or faster than, sound speed in water with typical sediment sound speeds from 1,450 m/s to 1,700 m/s (sound travels about 1,500 m/s in water and 340 m/s in air). However, shear wave speeds will be less than pressure wave speeds and are typically 50 to 200 m/s in the top few meters of sediment. Compressional and shear wave speeds in sediments generally increase with larger grain size and with depth in the sediments. Attenuation for pressure waves and shear waves increases with frequency.

The presence of infauna and burrows in the surface sediments have been found to alter wave propagation and attenuation. For example, compressional wave speed can vary by ±5% in sediment with high concentrations of infauna such as burrowing worms. Infauna can also produce vibrations themselves.

Interface waves on the seafloor provide another means for vibrational energy to propagate. Interface waves are a type of wave that is only found at the interface between two dissimilar media. Interface waves on land are called Rayleigh waves. Interface waves in ocean sediment are Scholte waves. Both of these waves are referred to as ground roll.

A Rayleigh wave is a surface wave causing elliptical particle motion. Image Credit USGS Earthquake Hazards Program, public domain.

Interface waves have both a longitudinal motion and a transverse motion which results in a particle near the interface moving in a clockwise direction, this includes in both the sediment and the water near the seafloor. The amplitude of these interface waves decreases exponentially with distance from the water/sediment interface. There is little or no particle motion at a distance beyond a few wavelengths. Interface waves travel at about 90% the speed of shear waves.

Animation showing surface motion caused by a Rayleigh Wave.

Rayleigh Wave animation. The particle motion decreases in amplitude away from the interface. Other interface waves behave in a similar manner. Image Credit University of Rhode Island – DOSITS.

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References

  • Buckingham, M. J. (2017). Sound Propagation. In Applied Underwater Acoustics (pp. 85–184). Elsevier. https://doi.org/10.1016/B978-0-12-811240-3.00002-3
  • Buckingham, M. J. (2020). Wave speed and attenuation profiles in a stratified marine sediment: Geo-acoustic modeling of seabed layering using the viscous grain shearing theory. The Journal of the Acoustical Society of America, 148(2), 962–974. https://doi.org/10.1121/10.0001778
  • Fowler, C. M. R. (1990). The solid earth: An introduction to global geophysics. Cambridge University Press.
  • Lee, K. M., Ballard, M. S., Venegas, G. R., Wilson, P. S., Dorgan, K. M., Reed, A. H., & Roosen, E. (2016). Preliminary characterization of surficial sediment acoustic properties and infauna in the New England Mud Patch. 070003. https://doi.org/10.1121/2.0000486
  • Maguer, A., Bovio, E., Fox, W. L. J., & Schmidt, H. (2000). In situ estimation of sediment sound speed and critical angle. The Journal of the Acoustical Society of America, 108(3), 987. https://doi.org/10.1121/1.1285953