NOAA Teacher at Sea
Aboard NOAA ship Oscar Dyson
April 29 – May 10, 2013
Mission: Pacific Marine Environmental Laboratory Mooring Deployment and Recovery
Geographical Area of Cruise: Gulf of Alaska and the Bering Sea
Date: May 2, 2013
Weather Data from the Bridge:
Partly sunny, WindsN 5-10 knots
Air Temperature 1.3C
Relative Humidity 60%
Barometer 1008.2 mb
Surface Water Temperature 2.8C
Surface Water Salinity 31.37 PSU
Science and Technology Log
As I described previously, one of the instruments being deployed on this cruise is an Acoustic Doppler Current Profiler (ADCP), which measures speed and direction of ocean currents across an entire water column using the principle of Doppler shift (effect). The Doppler Effect is best illustrated when you stop and listen to the whistle of an oncoming train. When the train is traveling towards you, the whistle’s pitch is higher. When it is moving away from you, the pitch is lower. The change in pitch is proportional to the speed of the train. The diagrams below illustrates the effect.
- Another view of the Doppler Effect
The ADCP exploits the Doppler Effect by emitting a sequence of high frequency pulses of sound (“pings”) that scatter off of moving particles in the water. Depending on whether the particles are moving toward or away from the sound source, the frequency of the return signal bounced back to the ADCP is either higher or lower. Since the particles move at the same speed as the water that carries them, the frequency shift is proportional to the speed of the water, or current.
The ADCP has 4 acoustic transducers that emit and receive acoustical pulses from 4 different directions. Current direction is computed by using trigonometric relations to convert the return signal from the 4 transducers to ‘earth’ coordinates (north-south, east-west and up-down. (http://oceanexplorer.noaa.gov/technology/tools/acoust_doppler/acoust_doppler.html). The most common frequencies used on these units are 600 KHz, 300 KHz, and 75 KHz. The lower the frequency the greater the distance that the wave can propagate through the ocean waters.
Determining current flow helps scientist to understand how nutrients and other chemical species are transported throughout the ocean.
Prior to sailing, ADCP mooring locations are selected by various research scientists from within NOAA. Next, engineers develop a construction plan to secure the unit onto the ocean floor. Once designed, the hardware needed to construct the mooring is sent to the ship that will be sailing in the selected mooring locations. Prior to arriving at the designated location it is the responsibility of the science team to construct the mooring setup following the engineering diagram shipped with each ADCP unit. ADCP moorings can be constructed to hold a wide variety of measuring instruments depending upon the ocean parameters under study by the research scientist.
The moorings are built on the ship’s deck starting with an anchor. The anchor weight is determined based upon known current strength in the area where the mooring will be located. Anchors are simply scrap iron railroad train car wheels which bury themselves into the sediment and eventually rust away after use. The first mooring unit that we assembled had an anchor composed of two train wheels with a total weight of 1,600lbs. Although this mooring was built from the anchor up this is not always the case. When setting very deep moorings the build is in the reverse order.
Next, an acoustic release mechanism is attached to the anchor by way of heavy chains. This mechanism allows for recovery of the ADCP unit as well as the release mechanism itself when it is time to recover the ADCP. The units that we are deploying will remain submerged and collect data for approximately 6 months.
Finally, an orange closed-cell foam and stainless steel frame containing the actual instrumentation is connected to the assembly and then craned over the back deck. The stainless steel frame has a block of zinc attached to it which acts as a sacrificial anode. Sacrificial anodes are highly active metals (such as zinc) that are used to prevent a less active metal surface from rusting or corroding away. In fact, our ship has many such anodes located on its hull. Once the entire unit is in position, a pin connected to a long chord is pulled from a release mechanism and the unit is dropped to the ocean floor. Date, time, and location for each unit are then recorded.
To recover the unit, an acoustic signal (9-12 Khz) is sent to the ship from the sunken mooring unit to aid in its location. Once located, a signal is used to activate a remote sensor which powers the release mechanism to open. The float unit then rises to the surface bringing all of its attached instruments along with it. The stored data within the units are then secured and eventually sent along to the research scientist requesting that specific mooring location for ocean current analysis.
On my first day of “work” I was able to watch the science teams deploy three different ADCP moorings as well as conduct several CTD runs. I will discuss CTD’s in more detail in future blogs. I was impressed by the camaraderie among all of the science team members regardless of the institution that they represented as well as with members of the deck crew. They all work as a very cohesive and efficient group and certainly understand the importance of teamwork!
Adjusting to my new work schedule is a bit of a challenge. After my work day ended today at 1200 hours, I fell asleep around 1500 hours for about 4 hours. After trying to fall back asleep again, but to no avail, I decided to have a “midnight” snack at 2000 hours (8pm). I finally fell asleep for about 2 more hours before showering for my next shift. I think I now have more empathy for students who come to my 8am chemistry class and occasionally “nap”!
A wide selection of food is always available in the ship’s galley. I have discovered that I am not the only one taking advantage of this “benefit”! I will definitely need to reestablish an exercise routine when I return home. We are currently heading for Unimak Pass which is a wide strait between the Bering Sea and the North Pacific Ocean southwest of Unimak Island in the Aleutian Islands of Alaska.
Did you know that since the island chain crosses longitude 180°, the Aleutian Islands contain both the westernmost and easternmost points in the United States. (172° E and 163° W)!