Reverberations: Ocean Acoustics and the Environment Basics of sound Sound travel in the ocean Noise pollution Sound as a measurement tool Michael Vera October 15, 2015

Scattering and Reverberation Level This scattering is caused by the many sources of inhomogeneities in the ocean. These sources may include fish, other biologics, air bubbles, dust or dirt as well as the ocean bottom, and surface. http://www.usna.edu/Users/physics/ejtuchol/documents/SP411/Chapter18.pdf

Sound basics Sound is a traveling variability in pressure, small compared to the background value. Motion is along the direction of travel. D.L. Bradley and R. Stern, Underwater Sound and the Marine Mammal Acoustic Environment.

Sound basics Sound can be characterized with either frequency or wavelength. The product of the two is its speed. The speed is dictated by the medium. Air: 343 m/s. p X p t period (inverse of freq) wavelength

Sound basics An increase by a factor of 10 in intensity is 10 dB Level in decibels (dB) is given by: An increase by a factor of 10 in intensity is 10 dB Different reference intensity/pressure used in air vs. water

Importance of sound in the ocean Electromagnetic waves can’t travel far in seawater. They are strongly damped. The structure of sound speed in the ocean can support long-range propagation.

Rays in the sound channel -Snell’s law: refraction toward minimum in c(z)

“…most common biological sound in winter in the tropical Pacific” Minke Whale “…most common biological sound in winter in the tropical Pacific” Sound, quote: http://www.whaleacoustics.com/audiominke.html Image: http://www.noaanews.noaa.gov/stories2006/s2743.htm

Noise in the ocean Shipping noise is an important contribution to noise from around 10 to 1000 Hz. Nearby ships are louder, but the sound can propagate to long ranges. D.L. Bradley and R. Stern, Underwater Sound and the Marine Mammal Acoustic Environment.

Increases in shipping noise (U.S. West Coast) Andrew et al. (2011), “Long-time trends in ship traffic noise for four sites off the North American West Coast,” J. Acoust. Soc. Am., 129, 642-651.

Geological exploration sources (collab. with Dr. B. Kandula) http://www.dosits.org/tutorials/technology/studytheseafloor/

12-km propagation from airgun array (with Dr. B. Kandula)

Active sonar

Active sonar

Use of sound: Tomography

Use of sound: Tomography Decrease in travel time Amount affected by the sound-speed dependence on temperature

RV Revelle on a HOME deployment

An acoustic tomography network http://atoc.ucsd.edu/Explorations_f98/map.html

https://scripps.ucsd.edu/programs/keelingcurve/

https://scripps.ucsd.edu/programs/keelingcurve/

https://scripps.ucsd.edu/programs/keelingcurve/

Acoustic travel times and temp in the North Pacific 1997-2006 Red line: axial warming of 5mC/yr Dushaw, et al. (2009), J. Geophys. Union, 114. 1.8 mC/yr over top 700 m Levitus et al. (2005), Geophys. Rev. Lett:

Backscatter image of contaminants: Deepwater Horizon Weber et al. (2010), “Acoustic observations in support of the response to the Deepwater Horizon oil spill,” http://acoustics.org/pressroom/httpdocs/160th/weber.htm

Simulation environment approximates Gulf of Mexico cw(z) from local ctd Cp=1700 m/s δ zs=500 m fc=1 kHz Total bw=400 Hz Range=100 km

“Deep blob”

“Deep blob”: peak 10^-3

Conclusion Sound is the dominant means of transmitting a signal in the ocean. Man-made sounds can constitute an environmental threat, primarily to marine mammals. Acoustic methods provide a way to measure the state of the ocean and monitor environmental problems.