Monday, November 1, 2010

"Current" events on Georges Bank

Out here on Georges Bank, we are busily mapping krill patches, but we also want to understand the physical environment in which we find the patches. The ocean currents on Georges Bank are constantly changing, and this movement may play an important role in determining the size and location of krill patches. To understand what role the ocean currents play, we have been measuring them as we go.
The tool we use to measure the currents is an Acoustic Doppler Current Profiler, or ADCP. The ADCP measures ocean currents at different depths using the Doppler effect. You hear this effect every time a train goes by blowing its whistle. When the source of a sound is moving toward you, you hear the sound as a higher pitch. When the source is moving away from you, you hear a lower pitch. The change in pitch is known as the Doppler effect.
Using this idea, ADCPs can measure ocean currents. The ADCP sends out a pulse of sound, which bounces off particles moving with the water and back to the instrument. Just like a train, if the particles are moving away from the instrument, we will hear a lower pitch on the returning sound. If the particles are moving toward the instrument, we hear a higher pitch. The change in the pitch of the returning sound tells us how fast the water is moving, and in what direction.
We have three different ADCPs on the ship: Two mounted on the bottom of the ship, and one mounted on the HammarHead towfish. We will use the data from these instruments to see how the currents flow, and then determine how these currents may be affecting the distribution of krill.
An example of data collected using the 75kHz ADCP mounted on the bottom of R/V Endeavor
Gareth preparing to deploy the HammarHead towfish with ADCP mounted on top to measure ocean currents

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