NOAA Teacher at Sea
Susy Ellison
Aboard NOAA Ship Rainier
September 9-26, 2013
Mission: Hydrographic Survey
Geographic Area: South Alaska Peninsula and Shumagin Islands
Date: September 13, 2013
Weather: current conditions from the bridge
You can also go to NOAA’s Shiptracker (http://shiptracker.noaa.gov/) to see where we are and what weather conditions we are experiencing
GPS Reading: 55o 15.037’ N 162o 38.025’ W
Temp: 10.44C
Wind Speed: 9.8 kts
Barometer: 1021.21 mb
Visibility: foggy on shore
Science and Technology Log
Since leaving Kodiak 5 days ago, I have been immersed in a hydrographic wonderland. Here’s what I’ve learned, summed up in two words (three, if you count the contraction); it’s complicated. Think about it. If I asked you to make a map of the surface of your desk you could, with a little bit of work and a meter stick, make a reasonably accurate representational diagram or map of that surface that would include the flat surface, as well as outlines of each item on the surface and their heights relative to that surface, as well as their location relative to each other on a horizontal plane. You might want to get fancy and add notes about the type of surface (is it wood, metal, or some sort of plastic), any small irregularities in that surface (are there some holes or deep scratches—how big and how deep?), and information about the types of objects on the desk top (are they soft and squishy, do they change location?). Now, visualize making this same map if your desktop was underwater and you were unable to actually see it. Not only that, the depth of the water over your desktop can change 2 times each day. If that isn’t complicated enough, visualize that the top of the water column over your desk is in constant motion. OK, not only all those variables, but pretend you are transformed into a very teeny person in a small, floating object on that uncertain water over the top of your desk trying to figure out how to ‘see’ that desktop that you can’t actually see with your own eyes? Welcome to the world of the hydrographer; the challenge of mapping the seafloor without actually touching it. It is, indeed, a complex meld of science, technology, engineering, and math (STEM, in educational parlance), as well as a bit of magic (in my mind).
Challenge number one—how do you measure something you can’t see or touch with your own hands? Long ago, sailors solved that obstacle by using a lead line; literally, a line with a lead weight attached to the end. They would drop the weighted line over the side of their ship to measure the depth. These soundings would be repeated to get enough data to provide a view of the bottom. This information was added to their maps along with estimates of the horizontal aspects (shoreline features and distance from the shoreline) to create reasonably good charts that kept them off most of the underwater obstacles. A simple solution to a complex problem. No electricity required, no advanced degrees in computer science needed, no calculus-based physics necessary. Fast- forward to 2013 and the world of complex calculations made possible by a variety of computer-based algorithmic calculations (i.e. some darn fancy computing power that does the math for you). The NOAA Ship Rainier’s hydrographers use sound as their lead line, traveling in small boats known as launches that are equipped with multibeam sonar that send a series of sound ‘pings’ to the ocean floor and measures the time between sending and receiving the ping back after its trip to the bottom. Sounds simple enough, doesn’t it? If it were all that simple I wouldn’t be typing this in a room on the Rainier filled with 20 computer monitors, 10 hard drives, and all sorts of other humming and whirring electronic devices. Not only that, each launch is equipped with its own impressive array of computer hardware.
So far on our survey days 2 launches have been sent out to cover identified transects. Their onboard crew includes a coxswain (boat driver), as well as 2-3 survey technicians and assistants. Each launch is assigned a polygon to survey for the day.
EVERY PING YOU TAKE…
Once they arrive at their assigned area, it’s time to ‘mow the lawn’—traverse back and forth systematically collecting data from one edge of your assigned polygon to the other until the entire area has been surveyed. Just in case you haven’t realized it yet, although that sounds pretty straightforward, it isn’t. Is the area shallow or deep? Depth affects how much area each traverse can cover; the sonar spreads out as it goes downward sending it’s little pings scampering to the ocean floor. Visualize an inverted ‘V’ of pings racing away from the sonar towards the sea floor. If it’s deep, the pings travel further before being bounced back upwards. This means that the width of each row the sonar cuts as it “mows the lawn” is wider in deeper water, and narrower in shallow. Shallower areas require more passes with the launch, since each pass covers a more limited area than it might if the water were deeper. As the launch motors back and forth ‘mowing the lawn’, the sonar signature is recorded and displayed on monitors in the cabin area and in front of the driver. Ideally, each lap overlaps the previous one by 25-50%, so that good coverage is ensured. This requires a steady hand and expert driving skills as you motor along either over or parallel to ocean swells. All you video gamers out there, take note–add boat driving to the repertoire of skills you might need if you want to find a job that incorporates video gaming with science!
Here’s a small list of some of the variables that need to be considered when using sonar to calculate depth; the chemistry of the water column through which you are measuring, the variability of the water column’s depth at specific times of day, the general depth (is it shallow or deep), and the movement of the measuring device itself. So many variables!!
HOW FAST DOES SOUND TRAVEL?
When you’re basing your charts on how sound travels through the water column, you need to look at the specific characteristics of that water. In a ‘perfect world’, sound travels at 1500m/second through water. In our real world, that speed is affected by salinity (the concentration of salts), temperature, and depth (water pressure). The survey crew uses a CTD meter to measure Conductivity, Temperature, and Depth. The CTD meter is deployed multiple times during the day to obtain data on these parameters. It is attached to a line on the rear of the launch, dropped into the water just below the surface for 2 minutes, and then lowered to near the ocean floor to collect data. After retrieval, it’s hooked to the computer on the launch to download the data that was collected. That data is stored in its own file to use when the data is reviewed in the evening back on board the Rainier. This is one of the variables that will be applied to the sonar data file—how fast was the sound moving through the water? Without this information to provide a baseline the sonar data would not be accurate.
ROCKING AND ROLLING…
When you’re out on the ocean in a boat, the most obvious variable is the instability of the surface, itself. This is called ‘attitude’. Attitude includes changes to the boat’s orientation fore and aft (pitch), side-to-side (roll), and up and down (heave) as it is gently, and not-so-gently rocked by ocean swells and waves. This means that the sonar is not always where you think it is in relation to the seafloor. This is like trying to accurately measure the height of something while you, the measurer, are on a surface that is constantly moving in 3 different directions. Good luck. Luckily for this crew of hydrographers, each boat is equipped with a little yellow box whose technical name is the IMU (inertial measurement unit) that I call the heave-o-meter, as we bob up and down on this might ocean. This little box contains 3 gyroscopic sensors that record all those forward and backward pitches, sideways rolls, as well as the bobbing up and down motions that the boat does while the sonar is pinging away. This information is recorded in the launch’s computer system and is applied to the sonar data during analysis back at the Rainier.
TIME AND TIDE…
Now that you’ve gotten your launch to the correct polygon (using GPS data to pinpoint your location), taken CTD readings to create a sound transmission profile for your transect area, and started up the heave-o-meter to account for rocking and rolling on the high seas, it’s time to start collecting data. Wait—there’s still another variable to think about, one that changes twice daily and affects the height of the water column. You also have to factor in changes in the depth of the water due to tidal changes. (for an in-depth look at how tides work, check out this link: http://oceanservice.noaa.gov/education/kits/tides/tides01_intro.html). At high tide, there’s a greater likelihood that subsurface obstacles will be covered sufficiently. At low tide, however, it’s pretty important to know where the shallow spots and rocks might lurk. NOAA’s hydrographers are charting ocean depths referenced to mean lower low water, so that mariners can avoid those low-water dangers.
You might be asking yourself, who keeps track of all that tide data and, not only that, how do we know what the tide highs and lows will be in an area where there are no other tide gauges? NOAA has tide gauges along many coastal areas. You can go online to http://tidesandcurrents.noaa.gov/and find out predicted tide heights and times for any of these locations. While we are working here in Cold Bay, we are using a tide gauge in nearby King Cove, as well as a tide gauge that the Rainier’s crew installed earlier this summer. More data is better.
What do you do if you’re surveying in an area that doesn’t have existing tide gauges? In that case, you have to make your own gauge that is referenced to a non-moving point of known elevation (like a rock). For a detailed description of how these gauges are set, check out NOAA TAS blogs from some of the teachers who preceded me on the Rainier. On Wednesday, I helped dismantle a tide gauge on Bird Island in the Shumagin Islands that had been set up earlier this season (check out TAS Avery Martin’s July 12th posting), but had ceased to report reliable data. Our mission on Wednesday was to find out if the station had merely stopped reporting data or if it had stopped collecting data entirely.
When we arrived at Bird Island we found out exactly why the gauge had stopped sending data—its battery bank had fallen from one rocky ledge to another, ripping apart the connections and breaking one of the plastic battery boxes in the process. That took a lot of force—perhaps a wave or some crazy gust of wind tore the 3 batteries from their mooring. Since each battery weighs over 25lbs, that means that something moved over 75lbs of batteries. Ideally, the station uses solar panels to keep the batteries charged. The batteries power up the station so that data can be sent to a satellite. Data is also stored on site in a data logger, but without power that data logger won’t work.
We retrieved all the equipment and will be able to download whatever data had been recorded before the system broke. The automated tide gauge is, basically, a narrow diameter air-filled tube that is underwater and set at a fixed depth with a narrow opening pointed downward to the seafloor. The pressure required to balance the air in the tube is equal to the pressure of the water column directly above the opening. The tide gauge measures this pressure and converts it to depth. Pressure/depth changes are recorded every six minutes—or ten times each hour. As it turns out, the damaged battery bank was only one of the problems with this station. Problem number two was discovered by the dive team that retrieved the underwater portion of the gauge; the hose had been severed in two locations. In this case, something had caused the tube to break, so it was no longer connected to the data logger. That must have been some storm!
While there, we set to work checking on benchmarks that had been set earlier in the season. We used a transit and survey rods (oversized rulers) to measure the relative heights of a series of benchmarks to ensure accuracy. There are 5 benchmarks along the beach. Each one was surveyed as a reference to the primary benchmark nearest the gauging station. Multiple measurements help ensure greater accuracy.
We also were tasked with checking the primary benchmark’s horizontal location. While this had been carefully measured when it was set back in July, it’s important to make sure that it hasn’t moved. It might seem a crazy concept to think that a benchmark cemented into a seemingly immovable piece of rock could move, but we are in a region that experiences seismic events on an almost daily basis. (You can check out seismic activity at http://www.aeic.alaska.edu/) NOAA Corps Officer ENS Bill Carrier set up a GPS station at the benchmark to collect 4 hour’s data on its position, a process called HORCON (horizontal control). Unfortunately, the winds were in charge of how much data we were able to collect that day, and blew down the station after only 3 hours! [image of station down] Sometimes the best laid plans …..
DATA, DATA, and MORE DATA
While data collection is important, it’s what you do with the data that really gets complicated. Data management is essential when working with so many files and so many variables. Before each launch returns to the Rainier, the day’s data is saved onto a portable hard drive. Immediately after being hauled back up onto the ship, the data is handed off to the ‘Night Processing Team’ and hustled off to the Plotting Room (computer HQ) to be uploaded into a computer. This is where the magic happens and an advanced degree in computer science or GIS (geographic information systems) can come in handy. I have neither of those qualifications, but I know how to read a screen, click a mouse, and follow directions. So, on Friday evening I was ushered into the ranks of ‘night processor’.
First, data is downloaded into the main computer. Each launch’s files are called raw data files and are recorded in the launch’s acquisition logs. Once the data is on the computer, it is important to set up what I call a ‘file tree’; the series of files that increase in specificity. This is analogous to having an accurate list of what files live within each drawer and section of your file cabinet. These files are color-coded according to the operations manual protocols to minimize the chance of misfiling or the data. They are definitely more organized than the files on my laptop—I might change my lackadaisical filing ways after this trip!
Once the data are placed in their folders, the fun begins. Remember, you have files for multiple variables; sonar, CTD casts, the IMU Heave-o-meter, and tide data. Not only that, you have, with any luck, performed multiple casts of your CTD meter to obtain accurate data about the conditions affecting sound wave transmission within your polygon. Now you get to do something I have never done before (and use a vocabulary word I never knew existed and one that I might try to spell in a future Scrabble game); you concatenate your CTD data. Basically, you put the data from all your CTD casts together into one, neat little file. Luckily, the computer program that is used does this for you. Next, you direct the program to add all the variables to your sonar files; the concatenated CTD data, tide data, and IMU data.
Assuming all goes well and you have merged all your files, it’s time to ‘clean’ your data and review it to make sure there are no obvious holes or holidays in the data that was collected. Holidays can occur if the launch was bouncing too much from side to side during data collection and show up as a blank spot in the data because the sonar was out of the water and not pinging off the bottom. You can identify these holidays during the data collection process [holiday signature], but sometimes there are smaller holidays that show up once the data is merged and on your computer screen. There can also be miscellaneous errant pings caused by debris in the water column. Cleaning involves systematically searching each line of your surveyed polygon to identify and delete those ‘bad’ pings. Kind of like photoshopping away the parts of a digital image that you don’t want in the final image. You work methodically in a grid pattern from left to right and top to bottom to ensure that you are covering the whole file. It sounds easy, but to a non-PC person such as myself all that right click, left click, center click stuff was a bit boggling. The program is amazingly complex and, rumor has it, a little bit ‘buggy’ at times.
After all this, guess what?! You still don’t have a chart. It takes almost 2 years to go from data collection to chart publication. There’s endless amounts of data compilation, reports to be written, and quality control analysis to be completed before the final report and charts are issued.
Personal Log
So far I have spent two nights on the ship ‘in transit’, moving between ports. The other nights have been spent anchored offshore. While the first night at sea was a little bouncy, the second was, in my opinion, the wildest roller coaster ride I have ever taken. Imagine being pulled to the top of a high roller coaster, and released to fly down to the bottom while you are lying flat in your bed. That’s what it felt like as we motored from the Shumagin Islands to an anchorage in Cold Bay. An endless series of up, up, ups, followed by a wild ride down, down, down. Luckily all the drawers and doors have latches that keep them from flying open—although I had a jacket hanging on a hook that seemed to hit the latch on one closet door and actually knock it open—after this happened a couple of times I gave up and put the coat on the floor and firmly shut the door. My bathroom trash can ended up in the shower stall. At one point I heard a loud thump in the dark—and realized my survival suit in its orange bag had fallen from the top bunk to the floor—glad I wasn’t in its way! It was time to just hang on and try not to roll out of bed.
We finally stopped rocking and rolling around 3 in the morning. I thought maybe I was just a bit sensitive to the rocking motion, but was comforted to find out the everyone agreed that it had been a wild night. In fact, one of the potential ‘hazards’ for our work on Thursday was ‘lack of sleep’.
After almost a week aboard the Rainier I have been impressed with the teamwork, precision, and overall efficiency which overlays all operations. This crew can get a launch loaded, lowered, and underway in less time than it sometimes takes me to record my morning attendance at school! This is no simple feat (the boat, not the attendance!). It reminds me of a buzzing beehive filled with activity and focused on a single task; data collection. Each day begins on the fantail (the rear of the boat) at 0800 with the FOO (Field Operations Officer) reviewing the POD (Plan of the Day) and a summary of the day’s goals, work assignments, weather, and potential hazards, prior to sending out the survey crews.
The Boatswain (bo’sun) directs the next part of this tightly choreographed activity, as the launches are lowered by their davits (small cranes), while lines and hooks are handled with an eye to safety and efficiency. Within 5 minutes the two launches have been lowered, loaded with crew and supplies, and are on the water, buzzing away from the hive like bees to perform their daily waggle dance as they move back and forth collecting hydrographic data.
At 1630 they return to the hive, filled with the sweet nectar of hydrographic data. Launches are lifted back onto the ship and the data is whisked off to the computer room for downloading. 5 Minutes later a survey team debrief is held to review work accomplished that day and any problems that may have come up so that plans can be made for the next day’s work. This crew is organized!!
