Frank Hubacz: ADCP Deployment, May 2, 2013

NOAA Teacher at Sea
Frank Hubacz
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.

Doppler Effect

Doppler Effect

Another view of the Doppler 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.

Typical 4 beam ADCP sensor head. The red circles denote the 4 transducer faces.

Typical 4 beam ADCP sensor head. The red circles denote the 4 transducer faces.

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.

ADCP Construction Diagram

ADCP Construction Diagram

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.

Selecting the anchor

Selecting the anchor

Anchor on the back deck

Anchor on the back deck below the gantry

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.

Acostic Release Mechanism
Acoustic Release Mechanism
Bill attaching the acoustic release mechanism

Bill attaching the acoustic release mechanism

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. 

Hoisting ADCP

Hoisting ADCP

ADCP unit assembly

ADCP unit assembly

Assembling mooring unit

Assembling mooring unit

Ready for launch

Ready for launch

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.

Recovering a mooring with a rope lasso

Recovering a mooring with a rope lasso

Personal Log

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)!

180 longitude

Wes Struble: What in the World Is a CTD Cast? March 2, 2012

NOAA Teacher at Sea
Wes Struble
Aboard NOAA Ship Ronald H. Brown
February 15 – March 5, 2012

Mission: Western Boundary Time Series
Geographical Area: Sub-Tropical Atlantic, off the Coast of the Bahamas
Date: March 2, 2012

Weather Data from the Bridge

Position: 26 degrees 19 minutes North Latitude & 79 degrees 55 minutes West Longitude (8 miles west of Florida’s coast)
Windspeed: 14 knots
Wind Direction: South
Air Temperature: 25.4 deg C / 77.7 deg F
Water Temperature: 26.1 deg C / 79 deg F
Atm Pressure: 1014.7 mb
Water Depth: 242 m / 794 feet
Cloud Cover: none
Cloud Type: NA

Science/Technology Log:

There are four different ship’s stations that are involved in a CTD (Conductivity, Temperature, & Depth) operation: the bridge, the survey team, the winch operator, and the computer room. The bridge is responsible to keep the ship on position and stable over a predetermined latitude and longitude. The survey team is responsible for preparing the CTD platform for deployment and securing it back on deck at the completion of the cast. The winch operator controls the actual motion of the CTD platform by the use of a hoist.  The computer lab relays commands to the winch and survey team in reference to testing and sampling depths, and when to start and stop the ascent and descent of the platform. The CTD platform can carry many types of instruments depending upon the nature of the research being conducted. During this cruise our platform usually contained two each of a temperature gauge, conductivity gauge (from which you can obtain salinity), and oxygen gauge.  In addition there is one pressure gauge and a transmissometer (that measures the turbity of water which is an indicator of the phytoplankton), 23 Niskin water sampling bottles, and two Acoustic Doppler Range finders – one pointing toward the surface and one pointing at the sea floor.

The CTD (Conductivity, Temperature, & Depth) platform on the Ron Brown. The long grey cylinders are the water sampling Niskin bottles, the yellow and blue device at the bottom in the Acoustic Doppler Current Profiler (for measuring distance to the sea floor) for measuring the distance to the sea floor during descent phase of a cast, the grey cylinders are weights, and the green cylinder is the power supply.

A Niskin Bottle with my Nike shoe for scale

The CTD platform being lowered over the side for start of another cast.

The "downlooking" ADCP (Acoustic Doppler Current Profiler mounted on the CTD.

The "up-looking" ADCP (Acoustic Doppler Current Profiler) mounted on the CTD

The Niskin Bottle trigger release. This device is used to remotely close the Niskin bottles at depth

The bridge of the Ron Brown during a CTD cast

     A CTD cast begins when the ship arrives at prearranged coordinates of latitude and longitude. The bridge will announce that we are “on station”.

A photo of the Ron Brown off the coast of Grand Bahama Island

   The survey team acknowledges and then raises the CTD platform and places it is the water at the surface for a minute or two. Then after receiving a signal from the computer operator that all functions are operating within normal parameters the platform is lowered to 10 meters and held there for two minutes to allow the instruments to stabilize.

Here I am starting my midnight to 6 :00 am shift at the CTD computer control station in the computer lab of the NOAA Ship Ronald H Brown

The "brains" of the CTD. This device also contains the pressure sensor.

   After the two minute hold at 10 meters the entire platform is brought back to the surface and the log is started as the package is lowered. The descent begins at about 30 meters/minute and eventually reaches 60 meters/minute. Many of the deep water casts on this cruise were between 4000 m and 5500 meters (about 13000 ft and 18,000 ft) and take over an hour to reach the bottom. While the descent takes place all the instruments are recording data which is stored and plotted in real time at the computer monitor.   When the CTD platform is 10 meters from the bottom the descent is stopped and the first water sample is collected by sending a signal that closes the first Niskin bottle. At this point the CTD slowly begins its climb back to the surface (another hour or more) stopping at designated depths to collect water samples.After the last Niskin bottle is closed at the surface, the CTD platform is brought back on deck, the water samples are removed, and the entire platform is prepared for the next cast.

Here I am on the weather deck in my favorite chair on the ship. I enjoy relaxing here in the sun in the morning after a night shift at the CTD computer station.

Another beautiful western Atlantic pre-sunset. I enjoyed many of these during the cruise.

The early sun rising in the east off the stern of the Ron Brown brings another night of CTD's to an end.