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    Underwater Acoustics - Searching For The MH370 Flight Recorder
    By Patrick Lockerby | April 6th 2014 02:08 PM | 7 comments | Print | E-mail | Track Comments
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    Underwater Acoustics - Searching For The MH370 Flight Recorder

    Australian and Chinese vessels have both picked up acoustic "pings" that could be from the black box of missing Malaysia Airlines flight MH370, search officials have announced.
    Guardian Sunday 6 April 2014 15.44 BST

    A video posted on Sky News shows Chinese searchers in a RIB using a Benthos DPL-275 Locator



    The disadvantage of using a portable locator is that it will not be as directionally sensitive or accurate as a ship-board sensor.  However, a great advantage of using that sensor from a RIB is that the RIB provides a silent environment.  Another advantage is that if a ping is detected by the RIB and mothership together then that gives a more precise search direction through triangulation.


    After the hydrophone was in place on the seafloor, its exact location was determined using a transponder. Knowing the ship's location, the relative position of the hydrophone could be plotted. The ship's location is indicated by circles as it steams around the site of the deployed hydrophone. Image courtesy of Sounds in the Sea 2001, NOAA/OER.

    Locating a black box in water is a very complex problem.  It is not at all like searching for a flashing distress beacon where line-of-sight observation is normal.  In underwater acoustic searches, line-of-sight detection with a hydrophone or ultrasonic sensor is probably the exception rather than the norm.

    Sound in water is reflected at the surface and the bottom, as well as at boundary layers.  The bottom material affects the strength of both reflection and absorption.  Water salinity and temperature affect boundary layer conditions and angles of refraction.  The frequency of the sound affects refraction, in similar fashion to the refraction of light of different wavelengths. Sound can be channeled underwater in the same way that images can be 'channeled' in air to produce mirages.  The problem is further complicated by sea-bed topography - peaks and canyons can reflect or blank a signal.


    This "channeling" of sound occurs because of the properties of sound and the temperature and pressure differences at different depths in the ocean. The ocean is divided into horizontal layers in which the speed of sound is greatly influenced by temperature in the upper layers and by pressure in the deeper layers. As temperature decreases, the speed of sound decreases, and as pressure (depth) increases, the speed of sound increases. Sound waves bend, or refract, towards the area of minimum sound speed. Therefore, a sound wave traveling through a thermocline (a region of rapid change in temperature with depth) tends to bend downward as the speed of sound decreases with decreasing water temperature, but then is refracted back upward as the speed of sound increases with increasing depth and pressure. This up-down-up-down bending of low-frequency sound waves allows the sound to travel many thousands of meters without the signal losing significant energy. The depth of this “channel” varies in different oceans depending on the salinity, the temperature, and depth of the water. At low and middle latitudes, the SOFAR channel axis lies between 600-1200 m below the sea surface. It is deepest in the subtropics and comes to the surface in high latitudes, where the sound propagates in the surface layer. Scientists often take advantage of the properties of the SOFAR channel. We have learned that by placing hydrophones at just the right depth (that is, at the axis of the sound channel) we are able to record sounds such as whale calls, earthquakes and man-made noise that occur many kilometers from the hydrophone. As a matter of fact, sometimes we can hear low-frequency sounds across entire ocean basins!. Image courtesy of Sounds in the Sea 2001, NOAA/OER.
    Image and text source: NOAA

    Finding MH370 will take a lot of patient and unstinting work.  It is imperative that we find out what happened.  This will bring closure to the grieving relatives and will assure us that every effort is being made to make air travel as safe as possible.

    Further reading:
    Understanding Ocean Acoustics
    Sharon Nieukirk, Research Assistant
    Acoustic Monitoring Project
    NOAA Pacific Marine Environmental Laboratory

    Underwater Acoustics Concepts
    SI units and decibels in underwater acoustics (with some airborne parallels)
    NPL

    Comments

    logicman
    CNN have posted some good articles on MH370, but perhaps they should not have published this particular bit of muddled political groupthink.
    A senior Malaysian government source told CNN that Flight 370 flew around Indonesian airspace after it dropped off Malaysian military radar. The plane may have been intentionally taken along a route designed to avoid radar detection, the source said.
    http://edition.cnn.com/2014/04/06/world/asia/malaysia-airlines-plane/
    Avoid radar?  How about 'avoid catastrophe'?

    Absent any evidence of malice, maybe Captain Zaharie Ahmad Shah and First Officer Fariq Abdul Hamid, during a severe but ultimately self-extinguishing electrical fire which left them incommunicado and radar-blind decided to fly a route which would avoid the possibility of crashing a plane which they believed to be on fire into a populated area or of colliding with another plane.
    "ultimately self-extinguishing electrical fire which left them incommunicado" That is not the accepted narrative, that would imply that there was something wrong with the plane. Facts are much less important than the accepted narrative.

    logicman
    Facts are much less important than the accepted narrative.

    I never could sing from the same song sheet.  Maybe I'm tone deaf.  :-)
    Bonny Bonobo alias Brat
    There are some nice graphics on this www.news.com.au website illustrating the complexities of searching for missing flight MH370 with underwater acoustics, here are just a few of them :-










    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    logicman
    Thanks for helping to illustrate my article, Helen.

    One small point: in my view the search area they depict needs to be further north and east.  Maybe the graphic designers were told to put the box away from the JORN coverage area.  It's not safe to assume that JORN must have detected the plane if it was in that area.  The coverage is HF ionosphere reflection which only works 100% in full daylight.  It gets switched off at night because it is blind.  That explains why the radar is also unmanned at night: no point.  When MH370 was running out of fuel Radar 2 would still have been relatively blind to the West.

    Based on last known data and fuel remaining plus magnetic variation I think the plane went down somewhere outside the IRSU coverage but inside the Radar 2 coverage.  I've marked the spot in the image below.  Magnetic var is west in the area, which makes a southerly course deviate ever more east.

    JORN - Jindalee Operational Radar Network
    logicman
    For my latest article please see -
    Could A Fire Have Caused The Loss Of MH370?
    logicman
    MH370 search area drastically reduced after two more pings heard

    The search area for the missing Malaysia airlines flight MH370 has been drastically reduced after two further detections of pings consistent with those of a black box from a plane late on Tuesday.

    The head of Australia’s Joint Agency Co-ordination Centre, Angus Houston, said on Wednesday the Australian defence vessel Ocean Shield had detected two further pings on Tuesday – one in the afternoon and one in the late evening – that had allowed the search area to be further refined to 75,000 sq km. He said he was “optimistic” the plane would be found soon.

    “I can now tell you that Ocean Shield has been able to reacquire the signals on two further occasions; late yesterday afternoon and late last night,” Houston said.

    Guardian Wednesday 9 April 2014 04.46 BST