Rosalind Echols: Sound Off! From Noise to Nautical Charts, July 22, 2013

NOAA Teacher at Sea
Rosalind Echols
Aboard NOAA Ship Rainier (NOAA Ship Tracker)
July 8 — 25, 2013 

Mission: Hydrographic Survey
Geographical Area of Cruise: Shumagin Islands, Alaska
Date: July 22, 2013

Current Location: 54° 55.6’ N, 160° 10.2’ W

Weather on board: Broken skies with a visibility of 14 nautical miles, variable wind at 22 knots, Air temperature: 14.65°C, Sea temperature: 6.7°C, 2 foot swell, sea level pressure: 1022.72 mb

Science and Technology log:

“Teamwork, Safety First”, is inscribed boldly on the Rainier stern rafter and after being aboard for more than 2 weeks, it is evident this motto is the first priority of the crew and the complex survey operation at hand.

It’s a rainy overcast morning here in SW Alaska and we are circled around the officers on the fantail for the daily safety meeting. Weather conditions, possible hazards, and the daily assignment for each launch are discussed. Per the instructions on the POD (Plan of the Day), handed out the previous evening, the crew then disperse to their assigned launches. The launches are then one-at-a-time lowered into the water by the fancy davit machinery and driven away by the coxswain to their specific “polygon” or survey area for the day. A polygon surveyed by a launch on average takes 2-3 hours at 6-8 knots to survey and usually is an area that is inaccessible by the ship. Many polygons make up one large area called a “sheet” which is under the direction of the “sheet manager”. Several sheets make up an entire survey project. Our hydrographic project in the Shumagins has 8 sheets and makes up a total of 314 square nautical miles.

On board each launch we have a complex suite of computer systems: one manages the sonar, another manages the acquisition software, and the third records the inertial motion of the launch as it rocks around on the water (pitch, heave, roll). The acquisition system superimposes an image of the path of the launch and the swath of the sonar beam on top of a navigational chart within the polygon. Starting at one edge of the polygon, the coxswain drives in a straight a line (in a direction determined by the sheet manager), to the other end of the polygon, making sure there is some overlap at the boundaries of the swaths. He/she then works back in the other direction, once again making sure there is some overlap with the adjacent swath. We call this “mowing the lawn,” or “painting the floor” as these are visually analogous activities. Throughout the day, we pause to take CTD casts so that we have a sound velocity profile in each area that we are working.

You might be wondering, why the swath overlap? This is to correct for the outer sonar beams of the swath, which can scatter because of the increased distance between the sea floor and the sonar receiver below the hull of the boat. The swath overlap is just one of the many quality control checks built into the launch surveying process. Depending on the “ping rate”, or the number of signals we are able to send to the bottom each second, the speed of the boat can be adjusted.  The frequency of the sound wave can also be changed in accordance with the depth. Lower frequencies (200 khz) are used for deeper areas and higher frequencies (400 khz) are used for shallower areas.

Despite what might seem like mundane tasks, a day on board the launch is exhausting, given the extreme attention to detail by all crew members, troubleshooting various equipment malfunctions, and the often harsh weather conditions (i.e. fog, swells, cool temperatures) that are typical of southwest Alaska. The success of the ship’s mission depends on excellent communication and teamwork between the surveyors and the coxswain, who work closely together to maximize quality and efficiency of data collection. Rain or shine, work must get done.  But it doesn’t end there. When the launches arrive back at the ship, (usually around 4:30 pm), the crew will have a debrief of the day’s work with the FOO (field operations officer) and XO (executive officer). After dinner, the survey techs plunge head first (with a safety helmet of course) into the biggest mountain of data I have EVER witnessed in my life, otherwise known as “night processing”. We are talking gigabytes of data from each launch just for a days work.  It begins with the transferring of launch data from a portable hard drive to the computers in the plot room. This data is meticulously organized into various folders and files, all which adhere to a specific naming format. Once the transferring of data has finished, the “correction” process begins. That’s right, the data is not yet perfect and that’s because like any good science experiment, we must control for extraneous factors that could skew the depth data. These factors include tides, GPS location error, motion of the launch itself, and the sound velocity in the water column.

In previous posts, I discussed how we correct for tides and the sound velocity. We also correct for the GPS location of the launch during a survey day, so that any specific data point is as precisely located as possible. Although GPS is fairly accurate, usually to within a few meters, we can get even more precise by accounting for small satellite errors throughout the day. The Coast Guard provided Differential GPS allows us to position ourselves to within a meter. To get even more precise, within a few centimeters, we determination location of a nearby object (our Horizontal Control, HorCon, Station) very precisely, and then track the reported position of this object throughout the day. Any error that is recorded for this station is likely also relevant for our launch locations, so we use this as the corrector. For example, if on July 21, 2013, at 3pm, the GPS location of our Bird Island HorCon station was reported 3cm north of its actual location, then our launches are also probably getting GPS locations 3cm too far north, so we will adjust all of our data accordingly. This is one of the many times we are thankful for our software. We also account for pitch, heave, and roll of the launch using the data from the inertial motion unit. That way, if the launch rolled sideways, and the center beam records a depth of 30 meters, we know to adjust this for the sideways tilt of the launch.

After all correctors have been applied (and a few software crashes weathered), the survey technicians then sort through all the data and clean out any “noise.” This noise represents sound reflections on sea life, air bubbles, or other items that are not part of the seafloor. Refraction of sound waves, as mentioned in the last post, is caused by density changes in the water due to changes in the temperature, pressure, or salinity.

Many of the above correctors are applied the same day the data is collected, so the sheet manager can have an up-to-date record of the project’s progress before doing final planning for data collection the next day. After a sheet has been fully surveyed and ALL correctors applied, the sheet manager will complete a “descriptive report”, which accompanies the data and explains any gaps in the sonar data (“holidays”) and/or other errors present. This report, along with the data, is sent to the Pacific Hydrographic Branch for post-processing and Quality Control. After that the data is forwarded to the Marine Charting Division where the data undergoes a final set of Quality Assurance checks and is put in a format that can be printed on a paper nautical chart. From acquisition on the launches to publication on a chart, the process can take up to two years! The length of the process is designed to ensure maximum accuracy as many mariners rely on accurate charts for safe navigation.

Personal Log

As the saying goes, “When in Rome, do as the Romans.” One of the attractions of life in Alaska is access to excellent fishing, and a wide variety of tasty fish species. Although I normally consider myself to be a fairly outdoorsy person, thus far in my life this had not extended to the activity of fishing. However, inspired by Avery’s enthusiasm, and her first successful halibut catch, I decided at least give it a try, obtaining an Alaska fishing license and preparing myself for yet another adventure. I am, after all, always encouraging other people to try new things, especially things that make them a bit nervous, so it only felt right to follow some of my own advice. Honestly speaking, though, the thought of catching the fish and then having to deal with the consequences made me a little anxious.

Fortunately, I had excellent guidance in this activity, setting out with Avery and two very patient crew members, who put up with my initial lack of skill and muscle, and intense enthusiasm about even the smallest jellyfish in the water. I had realized after my shoreline rock verification expedition that pointing at everything in the water and shouting “Look!” was probably not that helpful if we were trying to identify rocks, but here it seemed more appropriate. At least if you think jellyfish are cool. After several lackluster hours, we finally found a spot where a group of Black Rock Fish were schooling, and caught quite a few very quickly. Not surprisingly, the fish aren’t that happy about being caught and flail around a fair amount. Considering that they have pointy, venomous spines in their dorsal fin, it takes great care to get the fish in the bucket without injury, but we successfully managed it.

Somehow, in all my years of school, I never actually dissected anything, and have always felt a little squeamish around dead animals. However, after helping catch the fish, I couldn’t very well leave my colleagues alone to deal with the arduous task of filleting and cleaning the fish, so I decided to do my best to participate. It actually went much better than I expected, and I learned quite a bit about fish anatomy along the way. For example, fish have an air bladder that allows them to float. They look much less impressively large when this is deflated.

All in all, it was a very satisfying experience. It is nice to be able to say that I have developed a somewhat useful life skill (fishing as well as avoiding my fingers with large knives). Our wonderful cook, Kathy, even used some of the fish for a delicious lunch of fish tacos, which I hope to try to replicate myself some time in the near future.

Fun Fact: a fathom, a maritime measurement for depth equal to six feet, was originally based on the distance between a man’s outstretched finger tips. The word itself derives from an Old English word meaning outstretched or embracing arms. Given that we use it to measure depth, it is also interesting to note that it is related to the word to fathom something, or the adjective unfathomable, meaning immeasurable. The word is also related to the phrases “six feet under” and to “deep six” something.