Through the Porthole: What instruments do oceanographers use in the field to collect data?
Through the Porthole. Artwork by Springs School Student. June 2012. Photo by Lisa Seff.
Part I: The acoustic Doppler current profiler
The acoustic Doppler current profiler (ADCP) is an instrument that measures the ocean current speed and direction throughout the water column using the Doppler effect. On the R/V Ukpik the ADCP is mounted on a BioSonic towing body (like a very heavy sled) and towed alongside the vessel at approximately 5 knots.
accoustic Doppler current profiler (ADCP). Photo from NOAA Ocean Explorer Website August 2012.
Towing the ADCP mounted on the Biosonic Towing Body alongside the Research Vessel Ukpik. August 29, 2012. Photo by Lisa Seff.
Not sure of what the Doppler effect is? You experience the Doppler effect anytime you go driving and pass another vehicle, however the effect is especially evident when an ambulance approaches, and passes you with its siren on. As it zooms past and continues down the street you’ve probably noticed that the siren sounds changes to a lower pitch. This is a classic example of the Doppler effect. For a quick audio example, click on the video:
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You only hear the true sound, or the sound the ambulance driver actually hears, for the exact moment that the ambulance is right next to you. Think about it. As an ambulance approaches, the sound waves get pushed together and hit our eardrums more frequently than if it were standing still. This increase in frequency results in a higher pitch. As the ambulance heads away from you the sound waves stretch apart and hit our eardrum less frequently, which results in a lower pitch. I like to think of the Doppler effect as a magic trick! Magic is all about illusion and perception of what you see and hear, not what’s really happening. In the case of the Doppler effect the source’s sound never actually changes. The change perceived is due to the relative motion between the source and the receiving object, which in the case of the ambulance is you!
Specifically the ADCP transmits continuous high frequency sound waves, called chirps or pings. As the chirps from the instrument travel they bounce off of particles suspended in the water column and are reflected back to the instrument. Particles moving toward the instrument result in higher frequency waves reflecting back to the ADCP. The difference between the frequency of the outgoing and incoming waves the ADCP receives is called the Doppler Shift. Researchers can use data from the ADCP to measure the velocity (speed and direction) of particles as well as the relative type of organisms that are in the water column. Organism identification is, in part, based on the different frequency signature of each organism and the location in, and movement patterns through the water column.
If you’d like to know more about the ADCP go to: http://oceanexplorer.noaa.gov/technology/tools/acoust_doppler/acoust_doppler.html
If you’re interested in additional information about the Doppler Shift and how scientists measure distant stars and galaxies using light wave frequencies, just click on the following video. It starts out a little strange, but then ends with a great example and explanation of the Doppler Shift! http://
http://ed.ted.com/on/q8FeukzZ