Ocean Acoustics Ocean is largely opaque to EM radiation … but sound propagates well Consider: propagation speed refraction, reflection scattering, attenuation range Applications Things you already know: Marine mammals communicate with sound whales, dolphins Ships use echo sounding to determine the depth The same technology is used for fish finders SONAR is used for underwater detection of vessels, submarines Things we learned in class: The ADCP measures water velocity by sensing the Doppler shift of transmitted sound pulses Underwater instruments can communicate with acoustic modems REMUS AUV home using acoustics (I think) – look it up at Hydroid (.com?) Crowd sourcing whale song analysis http://whale.fm Sound in the ocean: http://www.whoi.edu/oceanus/v2/viewTopic.do?tagId=88

In 1826, Jean Daniel Colladon used a church bell under water to prove that sound traveled faster through water than air. In lake Geneva, assisted by Charles-Francois Sturm, a mathematician, the two split apart by 10 miles in boats. One struck the bell underwater and ignited gunpowder above the surface at the same time. The other started a timer at the flash of the gunpowder and stopped the timer when the sound arrived, using a trumpet-like instrument to detect the underwater noise. Despite their simple devices, they calculated the speed of sound underwater at 1435 m/s.

Speed of sound in seawater is about 1500 m s-1 Sound propagates by compression and rarefaction of a compressible medium. Water is not very compressible, so the speed is much higher than in air (340 m s-1) Affected by: Pressure Temperature Salinity Here z is positive increasing downward Speed of sound  Constant sound speed means no refraction

Since sound velocity is not constant, sound rays follow curved paths. They are refracted following the relationship given by Snell’s Law: In the case of a constant sound velocity gradient, dc/dz, a sound ray initially follows the arc of a circle whose radius is:

SOund Frequency and Ranging (SOFAR) channel

SOund Frequency And Ranging (SOFAR) channel SOFAR floats with acoustic transmitters RAFOS floats with receivers

SOFAR float Floats were tracked using a network of hydrophones in the western North Atlantic that was deployed in the 1950s as part of system to determine the splashdown location of missiles from Cape Canaveral. The missiles would release a small explosive charge that detonates once it reaches the depth of the SOFAR channel. From the arrival times of this signal at three or more hydrophones the splashdown point could be located to <1 km accuracy. SOund Frequency And Ranging (SOFAR) channel SOFAR floats with acoustic transmitters RAFOS floats with receivers

Example Three stations A, B, and C-receive a signal. Determine the position P of the shot when the arrival times are 11:00:00 at A, 11:02:30 at B, and 11:05:00 at C. In the figure, the arrival at B was 2 min 30 sec later than at A, and the arrival at C was 5 min 00 sec later than at A. With the sound speed known, circle B can be constructed with radius equaling the velocity times the time lag at B. Similarly, circle C can be constructed with radius equaling the velocity times the time lag at C. Then the center of the circle that is tangent to circles B and C and passes through A must define the location P from which the signal originated.

Task B: Moorings and PIES in Boundary Current Observing Systems (U.Send) The main goal in the reporting period was the development and initial testing of the fixed instrumentation needed needed to implement the proposed boundary current observing system. After researching the available options and choosing an acoustic modem type (Teledyne Benthos ATM-885), several of these modems were purchased, with and without pressure case (OEM version), for attachment to an inverted echosounder with pressure sensor (PIES) and to the moorings, and also for incorporation into a glider (see task A).

Transmission loss From a point source, sound spreads spherically and intensity falls proportional to R2 Some sound reaches the bottom and is reflected, so the transmission loss is decreased Over very long distances, with much of the energy trapped in the sound channel, the spreading is cylindrical and transmission loss is proportional to R

Sound intensity in decibels Sound Level intensity is described in non-dimensional units (relative to a reference) on a log scale in decibels SL = 10 log ( I/Iref ) = 20 log ( p/pref ) Factor of 10 because it is decibels, not bels Factor of 20 because intensity I is proportional to p2 A 10 dB increase means an order of magnitude increase in sound intensity The reference pressure in water is 1 microPascal Factor of 10 because it is deci-bels, not bels Factor of 20 because intensity is proportional to p squared A 10 dB increase means an order of magnitude increase in sound intensity The reference pressure in water is 1 micro-pascal

baleen whales 10-50 Hz human voice 80-3000 Hz

Attenuation loss Attenuation is the combination of absorption and scattering. Absorption: due to molecular properties of water and increases with square of the frequency Scattering: lack of homogeneity of the ocean – temperature microstructure, air bubbles, plankton, particulates … Essentially all attenuation is by absorption at frequencies > 500 Hz For comparison, attenuation of 10 kHz sound in moist air is about 160 dB/km; in the ocean the attenuation loss for the same frequency is about 1 dB/km. 1000 Hz: air 5 db/km, water 0.05 db/km

Sound waves exert pressure as they pass through water Some ocean chemical reactions are influenced by sound, by influencing acidity Sound waves exert pressure as they pass through water Low-frequency sound waves cause borate to lose an OH- group and become boric acid. Pressure “squeezes” borate ions [B(OH)4-] into boric acid [B(OH)3], a more compact molecule. This process absorbs energy. “What that essentially means is that the water is squishy,” Tim Duda says. “When you push on it with a sound wave, it gives. Boric acid quickly changes back to borate, replenishing the borate (but absorbing no sound) More-acidic conditions reduce the amount of borate in seawater, which led some researchers to suggest that ocean acidification will lead to less absorption of sound energy, allowing sound waves to travel farther in the ocean than they do at present. WHOI scientists showed that the effect will be minimal. (Illustration by Jack Cook, Woods Hole Oceanographic Institution) http://www.whoi.edu/oceanus/feature/will-more-acidic-oceans-be-noisier

Air is more compressible than water, and air bubbles have natural resonant frequencies dependent on their size. Fish have air-filled sacs called swim bladders that are roughly ellipsoidal in shape and which can present a resonant target to sonar. Normalized acoustical cross-section for an air bubble in water, and for a rigid sphere of the same radius, R. The peak corresponds to a natural resonant frequency of 52 kHz.

The differences in swim bladders cause differences in the return echo of a sonar signal. Echo signatures for specific species can be determined and used to identify fish. Fish finder display

Strait of Georgia - East Node - 24 Hours - Zooplankton Acoustic Profiler (ZAP) 8 pm 8 am http://venus.uvic.ca/index.php

Strait of Georgia - East Node – 24 hours data from Zooplankton Acoustic Profiler (ZAP) http://venus.uvic.ca/feature-galleries/hydrophone-highlights/other-sounds/ 8 pm 8 am http://venus.uvic.ca/feature-galleries/hydrophone-highlights/other-sounds

http://venus.uvic.ca/2011/11/bubbles-rising-from-the-delta-sediments/ Dynamic sediments and sediments with active biogeochemistry often generate a variety of chemical compounds, some of which will be gaseous. This inverted echo-sounder image from October 30, 2011 at the Delta Dynamics Laboratory in the Strait of Georgia at 108m water depth near the mouth of the Fraser River has captured numerous clouds of rising bubbles. Such bubbles have several distinct characteristics. First, we can see the uniform rise velocity, suggesting nearly constant bubble size. Bubbles have been detected that dissolve with height (common in unsaturated water conditions, as the bubbles shrink they slow down and the traces arc to the right) or bubbles that grow as a result of reduced hydrostatic pressure (the traces curve upwards). This image coincides with low tide, another condition that is known to encourage gas release as the water pressure is at a minimum. We have also detected bubbles rising from schools of fish. One final observation: the ensonified volume of water is an 8 degree cone expanding upwards from the transducer head. The bubbles may appear in the acoustic back-scatter at any height as they enter the beam, and with the strong tidal currents in the Strait of Georgia, they can easily be advected horizontally both into and out of the beam, thus the random appearance of bubble clouds into and out of the ensonified volume.

Acoustic tags are surgically implanted in striped bass. Acoustic transmitters (tags) for this project produce sound at 67 KHz (above audible range for humans) Acoustic tags are surgically implanted in striped bass. Have a wax coating, to prevent immune system from rejecting it. Two-year battery life. Transmitter "pings" an identifying code detectable by hydrophones up to 500 m away. Hydrophones (underwater microphones) are hung from buoys at fixed listening checkpoints, throughout the Mullica River/Great Bay Estuary. Checkpoints are placed mostly at narrow passages, such as Little Egg Inlet, so that fish must pass through them in order to move up or down river, or into or out of the coastal ocean. Environmental data from REMUS AUVs and moored instruments are compared to the striped bass' movement patterns

Acoustic Tomography A network of acoustic sources and receivers has multiple horizontal paths (in addition to multiple rays in sound channel between any given S-R pair). Travel times along the many paths are used to invert for the 3-dimensional sound speed pattern – which can be related to 3-D maps of ocean temperature.

Acoustic Thermometry of Ocean Climate Changes in acoustic travel times over very long distances are used to monitor seasonal and interannual ocean temperature change related to changing climate. http://atoc.ucsd.edu Heard Island feasibility test http://scitation.aip.org/content/asa/journal/jasa/96/4/10.1121/1.410105 Proposed global ATOC system