Seeing the Bottom -- 7 August 2010

NOAA Teacher at Sea: Caroline Singler
Ship: USCGC Healy
Mission: Extended Continental Shelf Survey
Geographical area of cruise: Arctic Ocean 41 miles north of Alaska
Date: 9 August 2010

It’s taken me several days to write and post this entry. I wanted to learn more about the sonar technology that we are using for the bathymetric mapping, then we lost internet early on the morning of 8 August 2010 while heading north in the Beaufort Sea. This happened at about the same time as we started encountering heavy ice, but I do not believe that the two events were related. I am including location and weather data for several days to give you a sense of where we were and where we are heading as well as the physical changes in our environment.

Thankfully, email works even when internet does not – it took my non-IT oriented mind a while to wrap itself around that concept. While I am out of range, my dear sister Rosemary has agreed to post for me as long as I can get emails to her. (Thanks, Ro!) You already have her to thank for the polar bear post. Please keep emailing and/or posting comments. I look forward to reading comments when I come home.

Location and Weather Data from the Bridge

Date: 7 August 2010 Time of Day: 1400 (2:00 p.m.) local time; 22:00 UTC

Latitude: 70º47.6’N Longitude: 142º42.3’W

Ship Speed: 15.1 knots Heading: 111º (southeast)

Air Temperature: 5.1ºC /41.6ºF

Barometric Pressure: 1005.3 millibars Humidity: 87 .9%

Winds: 27.7 Knots NE

Sea Temperature: 2.3ºC Salinity: 20.22 PSU (practical salinity units)

Water Depth: 1270 .8 m

Date: 8 August 2010 Time of Day: 1245 (12:45 local time); 20:45 UTC
Latitude: 72º12.72’N Longitude: 138º28.7’W
Ship Speed: 7.7 knots Heading: 36.2º (NE)
Air Temperature: 0.5ºC /32.9ºF
Barometric Pressure: 1012.7 millibars Humidity: 86.3%
Winds: 19.3 Knots NE Wind Chill: -7.48ºC/18.53ºF
Sea Temperature: -1.2ºC Salinity: 25.5 PSU
Water Depth: 2547.8 m

Date: 9 August 2010 Time of Day: 1530 (3:30 local time); 22:30 UTC
Latitude: 72º 29.8’N Longitude: 139º 40.9’W
Ship Speed: 6.3 knots Heading: 183.5º (SSW)
Air Temperature: -0.03ºC /31.94ºF
Barometric Pressure: 1009.7 millibars Humidity: 92.2%
Winds: 17.7 Knots NE Wind Chill: -6.02ºC /21.17ºF
Sea Temperature: -1.2ºC Salinity: 25.08 PSU
Water Depth: 2969.0 m

Science and Technology Log
The primary objectives of the science mission are to map the seafloor and image the underlying sediments. Bathymetry is the measurement of depth of water bodies, derived from the Greek bathos meaning deep and metria meaning measure. Early bathymetric surveys used the "lead-lining" method, in which depths are manually recorded using a weighted line. This method is slow and labor intensive, and it is not practical for depths greater than about 100 feet. (Ironically, I spent the summer of 2009 doing just such a survey of a small lake on Long Island, NY working with two other teachers as DOE-ACTS interns at Brookhaven National Laboratory.) Modern bathymetric surveys use echo sounding, or SONAR (Sound Navigation and Ranging) to determine depth and shape of the seafloor. These systems make it possible to map large areas in extreme detail, leading NOAA to name the 20th Century advancements in hydrographic surveying techniques to its list of Top Ten Breakthroughs during the agency's first 200 years.

SONAR uses sound signals to locate objects beneath the sea surface. Passive systems use receivers such as hydrophones to detect signals transmitted by other sources, such as animals or submarines. Active systems transmit and receive signals. A transmitter mounted on the ship’s hull emits a signal. The signal travels through the water column and bounces off an object in its path. It returns as an echo to a transmitter on the ship that measures the strength of the return signal. The time between transmission and reception is used to determine range, where range equals (speed of sound in seawater) times (travel time divided by 2). When the object that reflects the signal is the seafloor, the range is the water depth.

There are single beam and multibeam sonar systems. Single beam systems measure along a single line beneath the ship and produce a line of depths. Multibeam systems send signals out along a line perpendicular to the ship and generate a “swath” of data for the area beneath the ship. The advantage of this system is that it creates a map that shows depth and shape of the seafloor. The diagram below shows a schematic comparison of three bottom survey methods.

Source: NOAA Top Ten Breakthroughs

Healy is equipped with a hull-mounted multibeam sonar system. It runs continuously whenever Healy is at sea, collecting bathymetric data to add to our knowledge of the seafloor at high latitudes. I serve as one of the watch standers in the geophysics lab each night from 8 p.m. to 12 a.m. We keep an eye on several computer monitors that display the data from the different geophysics tools and others that display water quality and geographic position data. The photo on the right shows me with my watch partner, USGS scientist Peter Triezenberg sitting at the watch station.

There are many variables that can influence the quality of the multibeam data. The speed of sound in water is influenced by many different variables, including temperature and salinity. Therefore, seawater samples are collected from the ship’s seawater intake system to generate a thermosalinograph (TSG) profile to keep the speed of sound accurately calibrated. Additionally, expendable probes (XBTs) are launched twice a day to update the sound speed profiles. Other instruments monitor the attitude (pitch, roll and heave) of the ship and feed that data to the multibeam system. Finally, the ship keeps extremely precise track of time of day and geographical position so that the data can be used for accurate bathymetric mapping of the seafloor. My job as a watch stander is basically to be sure that everything is running properly, and to notify one of the specialists if something is not right.

Multibeam monitors:

TSG display:

The end result is a detailed map of the seafloor in which different colors represent different depths. The picture below shows an image of the raw multibeam data superimposed on a seafloor map which we can see on the ship’s Map Server display. The red line shows the ship’s track, and the new multibeam data extends perpendicular to that line. Other data on the map are from transects mapped on earlier Healy cruises and other sources.

Sources:
NOAA Ocean Explorer http://oceanexplorer.noaa.gov/technology/technology.html
NOAA 200th Top Tens - The Breakthroughs
http://celebrating200years.noaa.gov/breakthroughs/welcome.html

Personal Log

We experienced a range of sea and ice conditions over the last several days as we traveled east of Barrow Alaska and headed north into the Beaufort Sea. Our earliest ice encounters were a gentle preview of what was to come - mostly bumps and scrapes with small pieces as we headed eastward parallel to the Alaska coastline. By midday on Saturday, we began to cross larger floes, and at times the ship was really rocking. One science team member said it feels like riding the subway, that’s a pretty good analogy. Sitting in the Mess on the main deck of the ship – which is about one floor above water line – I hear the grinding of ice on steel and it feels like I’m sitting in a big tin can that’s being crushed in a trash compactor. Fortunately, the ship is tougher than the ice. At times we move so much that everything in the room shakes. Because we are on a ship, everything is bolted down, but I still look up to be sure there is no danger of anything falling on my head. Some team members from California say the sensation reminds them of an earthquake.

Late Saturday morning, we crossed out of ice and back into open water. As we approached the last pieces of ice before open water, I saw waves hitting the distant edges of the ice; it looked like waves breaking on the shore. At first, I did not grasp the significance of this observation – I thought it was pretty and snapped some pictures and marveled at how we could be in thick ice and then suddenly in open water.

In the next hour, I realized that these were the largest waves we had encountered so far on the trip, and while they looked pretty, they also made the ship roll considerably more than it had before. Over the next few hours, I began to sense the movement more than I had in a few days. By dinner time, I had difficulty walking straight across the mess deck, and I was becoming a little apprehensive. I took a motion sickness pill as a preventative measure, and I took a nap because it was far more pleasant to lie in my rack and be rocked by the ship’s motion than to try to remain vertical. We eventually moved into calmer waters, and soon after that, we were back in heavy ice, which I somehow do not find as unpleasant as the waves. Since then, our movement has been slow and steady along our transects through the ice, with an emphasis on slow.

We don’t get much darkness up here in the Arctic, but we do occasionally get treated to some great sunrises and sunsets, if one is awake to catch them. Here are some photos of the sunset on Saturday 7 August 2010. The first was taken about an hour before sunset from the port side of the ship. I was as captivated by the horsetail clouds as I was by the color of the sky. The second was taken just at sunset, right before my camera battery died!

Caroline