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

Jennifer Richards, September 12, 2001

NOAA Teacher at Sea
Jennifer Richards
Onboard NOAA Ship Ronald H. Brown
September 5 – October 6, 2001

Mission: Eastern Pacific Investigation of Climate Processes
Geographical Area: Eastern Pacific
Date: September 12, 2001

Latitude: 9º 56.5 N
Longitude: 95º 2.5 W
Temperature: 31.2º C
Seas: Sea wave height: 2-3 feet
Swell wave height: 4-5 feet
Visibility: 10 miles
Cloud cover: 5/8
Water Temp: 29.3ºC

Research Objective for the day: Begin taking measurements with the Lidar (ETL), the MMP (UW), weather balloons (CSU), and the SPMR (UCSB). Every group on the ship is in full swing, and will continue their operations for the next 18 days.

Science Log

Today I met with part of the group from NOAA’s Environmental Technology Laboratory in Boulder, Colorado. There are three sets of instruments being used by this team, and today I will introduce you to the researchers associated with two of those groups- the lidar group and the kaband group.

Ms. Janet Intrieri, an Atmospheric Scientist, and Dr. Raul Alvarez, a Physicist, have been working long hours each day on the Mini MOPA Lidar. This is the most labor-intensive piece of equipment on the ship, requiring constant watch and intervention to keep it running properly. It is also probably the fanciest piece of equipment on the ship, using CO2 lasers and an intricate network of lenses and mirrors to measure wind velocity and water vapor in the atmosphere. The really cool thing about the lidar is that it can measure these things at various altitudes simultaneously, up to 6-8 kilometers in range. Without the lidar, scientists could measure a specific point in the atmosphere using planes, satellites, or weather balloons, but the lidar allows Ms. Intrieri and Dr. Alvarez to see everything in a horizontal column of the sky at the same time.

How does lidar work? Lidar (which stands for Light Detection and Ranging, similar to the term Radar as used for radio waves) is a remote sensing technique that allows measurements of atmospheric conditions using laser light. The typical lidar system emits a short pulse of laser light that travels through the atmosphere. As this pulse of light goes through the atmosphere, it can interact or scatter off of various components in that atmosphere. These components can include dust, clouds, water vapor, pollutants, and even the air molecules themselves. When the light scatters off of these things, a small part of that scattered light is going back toward the receiver part of the lidar which is usually composed of a telescope (to collect as much of this light as possible) and a detector that converts the light signals into electronic signals that can be input to a computer.

How the signals that are collected are processed depends on what atmospheric properties are being measured. For information on the total amount of light scattering due to dust and clouds, we can simply look at the strength of the return signal as a function of time (which is proportional to the distance that the pulse has traveled). To gather information about the amount of water vapor in the atmosphere, one technique is to transmit two laser pulses that are at different wavelengths. One of the wavelengths is selected so that it is not affected by the water vapor, while the other is selected so that it is partially absorbed by water vapor. (Each different chemical that we might try to measure has a different absorption of light that will determine which wavelengths and types of laser must be used.) Now, as the laser pulses go through the atmosphere and as the scattered light returns to the receiver, one of the signals is attenuated (reduced) more than the other because it is being absorbed by the water vapor. The amount of water vapor that must have been in the atmosphere to cause a particular amount of signal reduction can then be calculated.

Another thing that can be measured with lidar is the wind velocity. To do this, we rely on the Doppler Effect. This effect states that as the light scatters off of the particles in the atmosphere, the frequency of the light may be shifted if the particles are moving. If they are moving towards the lidar, the frequency will be shifted up while the frequency will be shifted down for particles moving away. Since the frequency of light is extremely high and the Doppler frequency shift is very small, we need to bring the signal (light) frequency down to a manageable level. We can do this by a process called mixing. In essence, the light signal is shone onto a detector along with a small sample of laser light that is at the same frequency as the original pulse that was sent into the atmosphere. When these two beams interfere with each other, the result is a signal on the detector that is the difference in the two light frequencies. At this point, this difference signal tells us the speed of the wind, but not the direction of the wind. A shift of a few megahertz (MHz)(depending on the laser wavelength) could be due to a wind either towards or away from the lidar at a meter per second (m/s). To resolve this uncertainty, the transmitted laser pulse is shifted by a fixed amount of 10 megahertz. Now, when the atmospheric light signal and the laser sample are mixed, the shift in frequency will be offset by the 10 MHz signal. (As an example, let’s suppose that the Doppler shift due to the wind is 2 MHz. Then, the first example without a 10 MHz offset will give you simply a resultant 2 MHz signal for either a +1 m/s or -1 m/s wind, while the 10 MHz offset makes the resultant 12 MHz for a wind toward the lidar and 8 MHz for a wind away from the lidar.)

An additional piece of equipment being used by ETL is the Ka-band radar, operated by Ms. Michelle Ryan. Ms. Ryan uses Ka-band radar to study the clouds- water droplet size, condensation, and the changes between liquid, gas, and solid water. She also uses radiometers to study liquid water and vapor in a column from the ship to the sky. Her equipment complements the lidar by providing information about what’s going on above the cloud base (the lidar focuses on everything between the ocean surface and the clouds).

Thank you very much to Dr. Alvarez for translating enormously complex physics into what you just read about how the lidar works. If you read it through a couple times, it really makes sense! And they say laser physics is complex.

Travel Log

People always wonder what the food is like on the ship. Well, there is lots of it, and it’s better than what you would expect. In fact, I’ve heard some of the scientists challenging each other to see who can gain the most weight on the trip- just an excuse to try a little of everything on the buffet line, and dessert twice. There’s always a salad bar, a couple meat entrees, a couple meatless entrees, and several vegetables. One night we even had crab legs and steak! We eat during designated meal times in the mess hall, and since there are more people on the ship than there are seats in the mess, they try to get you to “eat it and beat it.” The most dangerous part of the mess is the freezer stocked with Haagen Daas ice cream, but I am challenging myself to avoid it until the last night on the ship. There are three stewards on the ship that do all the cooking and kitchen stuff. They’re really nice and friendly.

Question of the day: How much money did the U.S. spend last year on scientific research? What percent of the total budget does it represent? (Please cite your source when you send your answer)

Photo Descriptions:Today’s photos – Since today’s science log focused on the Lidar operated by NOAA Environmental Technology Laboratory (ETL), that’s what is highlighted in today’s pictures. You’ll see the ETL lab on the ship- a large container that travelled via tractor-trailor, plane, and barge to get onto the ship. There are two “vans” like this on the ship, which is where this group of ETL scientists spends most of their time. Inside the van, you’ll see Ms. Intieri at the computer controls, Dr. Alvarez tweaking the lenses in the Lidar, and in another picture, Dr. Alvarez pouring liquid nitrogen into the Lidar to keep the optics cool. Finally, you’ll see Ms. Ryan standing next to the kaband radar (looks like a large drum in the photo).

Until tomorrow,
Jennifer