sound waves – NOAA Teacher at Sea Blog

June 14, 2018June 21, 2018

Heather O’Connell: Sound in Seawater and Sleeping at Sea, June 8, 2018

NOAA Teacher at Sea

Heather O’Connell

NOAA Ship Rainier

June 7 – 21

Mission: Hydrographic Survey

Geographic Area of Cruise: Seattle, Washington to Southeast, Alaska

Date: 6/8/18

Weather Data from the Bridge: Latitude: 48.15° N, Longitude: 122 ° South 58.0’ West, Visibility: 8 nautical miles, Wind: 24 knots, Temperature: 14.2° C

Science and Technology Log

I was fortunate enough to sit in on a survey orientation for new survey technicians and junior officers with Lieutenant Steven Loy. He was on Rainier as the Field Operations Officer, F.O.O., in the past and is currently here as an augmenter filling the role of Senior Watch Officer since he has navigated through the Inside Passage several times. In his two hour orientation, he shared a wealth of knowledge and discussed how multibeam sonar and ultrasounds are two opposite ends to the ultrasonic pulse spectrum.

Multibeam sonar sends out sound and measures the time it takes to return to calculate the depth of the ocean floor. The accuracy of the depth data generated from the multibeam sonar relies on the sound speed profile of the water. The combined effects of temperature, salinity and pressure generate a sound speed profile. Because of the inherent importance of this profile, there are several different ways to measure it. The sound velocity profiler measures this right at the interface of the multibeam sonar. C.T.D.s., or conductivity temperature and depth machines, measure water profile while the ship is stopped. M.V.P.s, or moving vessel profilers, take the water profile as the vessel is moving. Lastly, XBTs are expendable bathythermographs that measure temperature while the ship is in motion.

Sound is affected by different variables as it is energy that travels through a medium as a wave. Lieutenant Loy shared an informative website, The Discovery of Sound in the Sea, where I was able to enhance my understanding. Sound can travel through a liquid, such as water, a gas like air, or a solid like the sea floor. On average, sound travels about 1500 meters per second in sea water. However, the rate changes at different times of day, various locations, changing seasons and varying depths of the water. By looking at sound speed at one particular place in the ocean, you can determine how the different variables affect this sound. Usually, as depth increases, temperature decreases, while salinity and pressure increase.

A multi-beam sensor has a metal plate receiver and a transmitter perpendicular to one another. This array geometry enhances sound. The sound velocity profiler is next to the receiver and measures right at the interface. To determine the speed of sound right where the beam is generated, sonar is used to measure speed sound across a known distance. This information is then utilized in the overall determination of the depth of the ocean floor. Once this cast is taken, the Seafloor Information System (SIS), can adjust sonar measurements accordingly.

Another way to measure the sound profile of water includes a C.T. D. This device measures the conductivity, temperature and depth of the water. Conductivity measures the electrical current of the water. The more dissolved salt, or ions in solution, the greater the conductivity and salinity of the water. The depth of the water is directly related to the pressure of the water. Salinity, temperature and pressure affect the sound speed profile of water. This machine has a high data rate that goes up and down the water column. The titanium C.T.D. operates at a high pressure and costs about forty thousand dollars. This accurate technology can only be utilized when the boat is stopped and is used on the smaller survey launches.

C.T.D. used for sound speed profile of water

A third method of measuring sound profile is the M.V.P., moving vessel profiler, which takes the data when the ship is moving. These are calibrated before a survey begins and are an efficient way to collect data. An expansive crane lowers the metal torpedo with the sensor off the fantail, the overhanging back part of the ship, into the water to collect the data. The fish is programmed to stop twenty meters above the ocean floor, at which point it returns to its docked position. On ship Rainier, the deck department deploys the fish with a cable wire and the plot room with the survey technicians controls the sensor.

Boatswain Kinyon and Survey Technicians Finn and Stedman releasing the torpedo of the M.V.P. into the water

Another way to collect the sound profile of water with a moving vessel is to use an expendable probe. As temperature decreases, the sound speed decreases. Since temperature is the most important factor affecting the speed of sound, an X.B.T., Expendable Bathythermograph, or expendable probe created by the military. With bathy relating to depth and thermo meaning heat, this measures the temperature of the water at a cost of about one hundred dollars. These probes descend at a known rate, so, depth is a function of time.

Sources – Discovery of Sound in the Sea

Personal Log

We left port yesterday at 16:30, which has been a highlight of my NOAA Teacher at Sea Experience thus far. Before leaving port, all hands were assigned a different assignment to help with the launch. I watched the crew bring in the gangway that connects the ship to the port then disassemble it. The crew with hard hats and orange work vests took down poles and neatly tied up different sections by knotting ropes. We slowly progressed out of the port after a cargo ship passed us.

Once the ship picked up speed and the ocean breeze was in my hair, I felt a new kind of freedom. With the Seattle skyline behind us and the beautiful green peninsulas in front of us, I was content to be moving forward. Everyone seemed to feel relieved once we were underway. I felt gratitude as I enjoyed watching the sunset from the flying bridge, the area of the ship above the bridge at the front of the ship.

After sunset, I returned to my berth, or sleeping quarters, located in the bow of the ship on the C-deck. I heard the constant white noise of the propellers that got much louder when the pitch, or angle, of them changed. This sound of seawater combined with the rocking motion of the ship lulled me to sleep on our first night at sea.

Did You Know?

Juneau, the American capital of Alaska, can only be entered by plane or boat. It is inaccessible by roads due to large mountain ranges on either side.

June 21, 2017June 21, 2017

Helen Haskell: From Raw Data to Processed Data, June 16, 2017

NOAA Teacher at Sea

Helen Haskell

Aboard NOAA Ship Fairweather

June 5 – 26, 2017

Mission: Hydrographic Survey

Geographic Area of Cruise: Southeast Alaska – West Prince of Wales Island

Date: June 16, 2017

Weather Data

Wind: 3 knots from the east (272° true)

Visibility: 6 nautical miles

Barometer: 997.6 hPa

Air temperature: 9 °C

Cloud: 100% cover, 1000’

Location

54°54.4’N 132°52.3’W

Science and Technology Log

It would be easy to assume that once the small boat surveys are conducted and data from the larger sonar equipment on Fairweather is also acquired, that the hydrographers’ work is done and the data can be used to create navigational charts. As I have learned, pretty quickly, there are many parameters that affect the raw data, and many checks and balances that need to be conducted before the data can be used to create a chart. There are also a significant amount of hurdles that the crew of Fairweather deals with in order to get to their end goal of having valid, accurate data. Some of the parameters that affect the data include tides, salinity of the water, temperature of the water, and the density of the data.

Tides:

Tides play a huge role in data accuracy. But how do tides work and how do they influence navigational chart making? Tides on our planet are the effect on water due to forces exerted by the moon and the sun. The mass and the distance from the Earth to these celestial bodies play significant roles in tidal forces. While the sun has a much greater mass than the moon, the moon is much closer to the Earth and it is distance that plays a more critical role. Gravity is the major force responsible for creating tides. The gravitational pull of the moon moves the water towards the moon and creates a ‘bulge’. There is a corresponding bulge on the other side of the Earth at the same time from inertia, the counterbalance to gravity. The moon travels in an elliptical orbit around the planet and the Earth travels in an elliptical orbit around the sun. As a result, the positions of the moon to the Earth and the Earth to the sun change and as a result, tide height changes. The tides also work on a lunar day, the time it takes the moon to orbit the Earth, which is 24 hours and 50 minutes. So high tide is not at the same time in one area each solar day (Earth’s 24 hour day). There are three basic tidal patterns on our planet. Here is southeast Alaska, the tides generally are what is called ‘semi-diurnal’, meaning that there are two high tides a day and two low tides a day of about the same height. Other areas of the world may have ‘mixed semi-diurnal’ tides, where there are differences in height between the two high and two low tides, or ‘diurnal’ tides, meaning there is only one high and one low tide in a lunar day. The shape of shorelines, local wind and weather patterns and the distance of an area from the equator also affect the tide levels. How does this affect the hydrographers’ data? If data is being collected about water depth, obviously tide levels need to be factored in. Hydrographers factor this in when collecting the raw data, using predicted tide tables. However, later on they receive verified tide tables from NOAA and the new tables will be applied to the data.

Sound Speed Profiles:

Traveling down through the water column from the surface to the seafloor, several factors can change, sometimes significantly. These factors include temperature, pressure and salinity. These variables affect the accuracy of the sonar readings of the MBES (Multibeam Echo Sounders), so have to be factored in to account with the raw data analysis. What complicates matters further is that these factors can vary from location to location, and so one set of readings of salinity, for example, is not be valid for the whole dataset. Many fresh water streams end up in the waters off the islands of southeast Alaska. While this introduction of freshwater has effects on the community of organisms that live there, it also has impacts on the hydrographers’ data. To support accurate data collection the hydrographers conduct sound speed casts in each polygon they visit before they use the MBES. The data is downloaded on to computers on the boat and factored in to the data acquisition. The casts are also re-applied in post processing, typically on a nearest distance basis so that multiple casts in an area can be used. In the picture below, the CTD cast is the device that measures conductivity (for salinity), temperature and depth. It is suspended in the water for several minutes to calibrate and then lowered down through the water column to collect data. It is then retrieved and the data is downloaded in to the computers on board.

CTD Cast

Hydrographers Bekah Gossett and Sam Candio getting ready to deploy the cast.

Data Density:

Hydrographers also need to make sure that they are collecting enough sonar data, something referred to as data density. There are minimum amounts of data that need to be collected per square meter, dependent on the depth of the sea floor in any given area. Having a minimum requirement of sonar data allows any submerged features to be identified and not missed. For example, at 0-20 meters, there need to be a minimum of five ‘pings’ per square meter. The deeper the sea floor, the more the beam will scatter and the ‘pings’ will be further apart, so the minimum of five pings occupy a greater surface area. Hydrographers need to make sure that the majority of their data meets the data density requirements.

Crossline Acquisition:

After much of the initial raw data has been collected, and many of the polygons ‘filled in’, the hydrographers will also conduct crossline surveys. In these surveys they will drive the small boat at an angle across the tracklines of the original polygon surveys. The goal here is basically quality control. The new crossline data will be checked against the original MBES data to make sure that consistent results are be acquired. CTD casts have to be re-done for the crossline surveys and different boats may be used so that a different MBES is used, to again, assure quality control. At least 4% of the original data needs to be covered by these crossline surveys.

Shoreline verification:

Low tides are taken advantage of by the hydrographers. If the research is being conducted in an area where the low tide times correlate with the small boat survey times, then a vessel mounted LIDAR system will be used to acquire measurements of the shoreline. Accurate height readings can be extracted from this data of different rocks that could prove hazardous to navigation. Notes are made about particular hazards and photos are taken of them. Data on man-made objects are also often acquired. Below are pictures produced by the laser technology, and the object in real life. (for more on LIDAT: http://oceanservice.noaa.gov/facts/lidar.html)

Polygons on the sheet

Areas to be lazered

Notes after lazering

Laser

Old abandoned

Laser image of boat with trees behind

Old pier

Night Processing:

Each evening once the launches (the small boats) return, the data from that day has to be ‘cleaned’. This involves a hydrographer taking an initial look at the raw data and seeing if there were any places in the data acquisition that are erroneous. None of the data collected is deleted but places where the sonar did not register properly will become more apparent. This process is called night processing as it happens after the survey day. After night processing, the sheet managers will take a look at remaining areas that need to be surveyed and make a plan for the following day. By 6 a.m. the next day, the Chief Scientist will review the priorities made by the managers and let the HIC (Hydrographer In Charge) know what the plan in for their survey boat that day.

Personal Log

Throughout the Science and Technology log in this blog post, I keep referring to technology and computer programs. What stands out to me more and more each day is the role that technology plays in acquiring accurate data. It is an essential component of this project in so many ways, and is a constant challenge for all of the crew of Fairweather. Daily on Fairweather, at mealtimes, in the post survey meetings, or on the survey boats themselves, there is discussion about the technology. Many different programs are required to collect and verify the data and ‘hiccups’ (or headaches) with making this technology work seamlessly in this aquatic environment are a regular occurrence. I am in awe of the hydrographers’ abilities, not only in knowing how to use all the different programs, but also to problem solve significant issues that come up, seemingly on a regular basis. Staff turnover and annual updates in software and new equipment on the ship also factor significantly in to technology being constantly in the foreground. It often eats in to a large amount of an individual’s day as they figure out how to make programs work in less than forgiving circumstances. Tied to all of this is the fact that there is a colossal amount of data being collected, stored and analyzed each field season. This data needs to be ‘filed’ in ways that allow it to be found, and so the tremendous ‘filing system’ also needs to be learned and used by everyone.

Hydrographer Steve Eykelhoff and ET Sean checking the computers on the small boats

Taking a look to see if anything is loose

Word of the day: Fathom

Fathom is a nautical unit of measurement, and is the equivalent of 6 feet. It is used in measuring depth.

Fact of the day:

Prince of Wales Island, west of which this research leg is being conducted is the fourth largest island in the United States. 4,000 people live on the island, that is 2,577sq mi.

What is this?

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(Previous post: a zoomed in photo of ‘otter trash’ (Clam shell)

Acronym of the day:

LIDAR: Light Detecting and Ranging

July 6, 2011April 28, 2021

Becky Moylan: Acoustics and Trawling, July 5, 2011

NOAA Teacher at Sea
Becky Moylan
Onboard NOAA Ship Oscar Elton Sette
July 1 — 14, 2011

Mission: IEA (Integrated Ecosystem Assessment)
Geographical Area: Kona Region of Hawaii
Captain: Kurt Dreflak
Science Director: Samuel G. Pooley, Ph.D.
Chief Scientist: Evan A. Howell
Date: July 5, 2011

Ship Data

Latitude	1940.29N
Longitude	15602.84W
Speed	5 knots
Course	228.2
Wind Speed	9.5 knots
Wind Dir.	180.30
Surf. Water Temp.	25.5C
Surf. Water Sal.	34.85
Air Temperature	24.8 C
Relative Humidity	76.00 %
Barometric Pres.	1013.73 mb
Water Depth	791.50 Meters

July 5, 2011

Science and Technology Log

Work is going on 24 hours here on the ship. The crew have different shifts, so nothing ever stops. It may be 3:00 in the morning, and you’ll see people sorting fish, filtering water, or working the acoustics table.

To improve the accuracy of identifying what organisms are seen on the acoustic system, Sette researchers collect samples from the scattering layers at night using a large trawl net towed from the ship.One important part of the research here is using the acoustic system to find where groups of fish and other organisms are located. This is done with a “ping”, or noise, sent down in the ocean. The sound waves bounce back when they find something, letting scientists know where, and sometimes what, is swimming underneath. Computers keep data on all the different sound waves showing patterns of fish movement. They have found that some groups move upward during the nighttime, and then move back down during the day.

Every night on the ship, there is at least one trawl. The method of trawling started back in the 1400’s. Some people use these nets to catch large amounts of fish to sell, and that has been an environmental concern. NOAA is using this method as a scientific sampling, or survey, method to try and help the environment. They are trawling in the Epipalagic Zone (mid to shallow) which is around 200 meters deep, depending on the total depth at location and where the acoustics pick up signals.

Scientists want to find out the status of the smaller life in order to try and predict the outcome of the larger life. Only a small amount is caught for sampling. They weigh, sort, count, and study them. The goal is to be aware of what is happening in this area of the ocean. Some of the species they have found are different types of shrimp, squid, Myctopids, small crabs, and jellies. Last night they wound up with two Cookie Cutter Sharks. These results will then be combined with the measured acoustic data in order to improve the accuracy and effectiveness of acoustic monitoring.

One scientist from New York, Johnathan, is looking for specific species of Myctopids. He studies them under the microscope and records detailed data found. The Myctopids are sometimes called Lantern Fish. This is because they have organs that produce light. The lights are thought to be a way of communicating with other fish and also as a camouflage. As mentioned earlier, some fish rise to shallower waters at night and the Myctopid is one of these. The reason might be to avoid predators, yet also to follow zooplankton which they feed upon.

Personal Log

Last night some of us went out on deck to watch the Kona fireworks. I didn’t realize how far out we were until I saw how tiny the little ball of colors appeared. You could see three different areas along the coast where they were shooting off fireworks. As a fourth of July treat, the cooks barbecued on deck and made special deserts. I especially liked the sweet potato pie.

This morning I was out at 6am preparing the CTD for deployment. It is getting easier each time. There are many precautions and steps to make sure the procedure is done correctly and safely. We could only drop it to 200 meters today because this area is shallower here. I watched and learned how to control the computer from the inside. Very impressive!

I’m wondering when the ship is going to have another “abandon ship drill”. That’s when we all carry our floatation suits to the upstairs deck and put them on, and it is not easy to do. You lay the suit down, sit on it, and put your legs in first. Then you stand up, pull the suit hood on, then lastly the arms. This is because the hands don’t have fingers. It is quite a funny sight.

I found out today that the 3am trawl ended up with only one fish because a Cookie Cutter Shark had eaten a round hole in the net. This is where they get their name. They always bite a round hole. Some have even eaten a hole out of humans!

June 28, 2011April 28, 2021

Jason Moeller: June 25-27, 2011

NOAA TEACHER AT SEA
JASON MOELLER
ONBOARD NOAA SHIP OSCAR DYSON
JUNE 11 – JUNE 30, 2011

NOAA Teacher at Sea: Jason Moeller
Ship: Oscar Dyson
Mission: Walleye Pollock Survey
Geographic Location: Gulf of Alaska
Dates: June 25-27, 2011

Ship Data
Latitude: 55.58 N
Longitude: -159.16 W
Wind: 14.11
Surface Water Temperature: 7.2 degrees C
Air Temperature: 9.0 degrees C
Relative Humidity: 90%
Depth: 85.61

Personal Log
Anyone who has seen the show Deadliest Catchknows how dangerous crab fishing can be. Fishing for pollock, however, also has its dangers. Unfortunately, we found out the hard way. One of our deck hands caught his hand between a cable and the roller used to pull up the trawl net and hurt himself badly.

Fortunately, the injuries are not life threatening and he will be fine. The injuries did require a hospital visit, and so we stopped at Sand Point to treat him.

Clouds hang over the hills at Sand Point. The airstrip is in the left edge of the photo.

We stayed at Sand Point for nearly 48 hours. What did we do? We fished, of course! We used long lines and hooks, and had a great time!

Alex with a flounder that he caught! He also caught several cod and a 32-lb Pacific halibut!

As with every fishing trip, we also managed to catch things that we didn't mean too! Tammy (the other NOAA Teacher at Sea) especially liked the kelp!

A few visitors always hitched a ride on the kelp we caught. Here is a tiny sea urchin.

This crab was another hitchhiker on the kelp.

We were bottom fishing for Halibut, and a starfish, the largest one I've ever seen, went after the bait!

A one-day fishing license in Alaska costs $20.00. We had internet, so five of us went online and bought the fishing passes. Was it worth it?

We filleted it and had the cooks make it for dinner. With the halibut, we also cut out the fleshy “cheeks” and ate them as sushi right on the spot! It doesn’t get any fresher (or tastier!) than that!

Science and Technology Log
Today we will look at the acoustic system of the Oscar Dyson! Acoustics is the science that studies how waves (including vibrations & sound waves) move through solids, liquids, and gases. The Oscar Dyson uses its acoustic system to find the pollock that we process.

The process begins when a piece of equipment called a transducer converts an electrical pulse into a sound wave. The transducers are located on the underside of the ship (in the water). The sound travels away from the vessel at roughly 1500 feet per minute, and continues to do so until the sound wave hits another object such as a bubble, plankton, a fish, or the bottom. When the sound wave hits an object, it reflects the sound wave, sending the sound wave back to the Oscar Dyson as an echo. Equipment onboard listens to the echo.

The computers look at two critical pieces of information from the returning sound wave. First, it measures the time that it took the echo to travel back to the ship. This piece of information gives the scientists onboard the distance the sound wave traveled. Remember that sound travels at roughly 1500 feet per minute. If the sound came back in one minute, then the object that the sound wave hit is 750 feet away (the sound traveled 750 feet to the object, hit the object, and then traveled 750 feet back to the boat).

The second critical piece of information is the intensity of the echo. The intensity of the echo tells the scientists how small or how large an object is, and this gives us an idea of what the sound wave hit. Tiny echos near the surface are almost certainly plankton, but larger objects in the midwater might be a school of fish.

good fishing — An image of the computer screen that shows a great number of fish. This was taken underneath the boat as we were line fishing in Sand Point.

poor fishing — The same spot as above, but with practically no fish.

fishing — An image of the screen during a trawl. You can actually see the net--it is the two brown lines that are running from left to right towards the top of the screen.

One of the things that surprised me the most was that fish and bubbles often look similar enough under water that it can fool the acoustics team into thinking that the bubbles are actually fish. This is because many species of fish have gas pockets inside of them, and so the readout looks very similar. The gas pockets are technically called “swim bladders” and they are used to help the fish control buoyancy in the water.

Species Seen
Northern Fulmar
Gulls
Cod
Pacific Halibut
Flounder
Sea Urchin
Crab
Kelp

Reader Question(s) of the Day
Today’s questions come from Kevin Hils, the Director of Chehaw Wild Animal Park in Chehaw, Georgia!

Q. Where does the ship name come from?
A. Oscar Dyson was an Alaska fisheries industry leader from Kodiak, Alaska. He is best known for pioneering research and development of Alaska’s groundfish, shrimp, and crab industry. Dyson was a founding partner of All Alaskan Seafoods, which was the first company actually controlled by the fishermen who owned the vessel. He also served on the North Pacific Fisheries Management council for nine years. He is in the United Fishermen of Alaska’s hall of fame for his work. The ship was christened by his wife, Mrs. Peggy Dyson-Malson, and launched on October 17, 2003.

Q. How do you see this helping you teach at Knoxville Zoo, not an aquarium?
A. This will be a long answer. This experience will improve environmental education at the zoo in a variety of different ways.

First, this will better allow me to teach the Oceanography portion of my homeschool class that comes to the zoo every Tuesday. For example, I am in the process of creating a hands on fishing trip that will teach students about the research I have done aboard the Oscar Dyson and why that research is important. Homeschool students will not just benefit from this experience in Oceanography, but also in physics (when we look at sound and sonar) and other subjects as well from the technical aspects that I have learned during the course of the trip.

Scouts are another group that will greatly benefit from this experience as well. The Girl Scout council wishes to see a greater emphasis in the future on having the girls do science and getting real world experiences. While the girls are still going to desire the animal knowledge that the zoo can bring, they will also expect to do the science as well as learn about it. My experience aboard the Dyson will allow me to create workshops that can mimic a real world animal research experience, as I can now explain and show how research is done in the field.

The same can be said of the boy scouts.

In addition, one of the most common badges that is taught to boy scout groups that come in is the fish and wildlife merit badge. In the past, the badge has primarily focused on the wildlife aspect of this topic. However, I now have the knowledge to write and teach a fisheries portion for that merit badge, as opposed to quickly covering it and moving on. This will enrich future scouts who visit the zoo for this program.

A major focus for all scouts is the concept of Leave No Trace, where scouts are supposed to leave an area the way they found it. The fisheries research being done aboard the Dyson is focused toward that same goal in the ocean, where we are attempting to keep the pollock population as we found it, creating a sustainable fishery. The goal aboard the Dyson is similar to the goal in scouting. We need to be sustainable, we need to be environmentally friendly, and we need to leave no trace behind.

School children on field trips will greatly benefit, especially students in the adaptations section. There are some bizarre adaptations that I never knew about! For example, sleeper sharks slow, deliberate movement coupled with their fin and body shape basically make them the stealth fighter of the fish world. They can catch fish twice as fast as they are! Lumpsuckers are neat critters too! This knowledge will enhance their experience at the zoo during field trip programs.

Finally, I can pass the knowledge from this experience on to my coworkers. This will not only better the experience of my students, but it will also improve the outreach programs, the bedtime programs, the camps, and other programming done at the zoo.

Q. Are you old enough to be on a ship? You look like you’re 13???!!!!
A. SHHHHHHH!!!! You weren’t supposed to tell them my real age! They think I’m 24!

July 16, 2009April 5, 2021

Dan Steelquist, July 16, 2009

NOAA Teacher at Sea
Dan Steelquist
Onboard NOAA Ship Rainier
July 6 – 24, 2009

Mission: Hydrographic Survey
Geographical Area: Pavlov Islands, Gulf of Alaska
Date: July 16, 2009

Weather Data from the Bridge

Latitude: 55°13.522’ N Longitude: 161°22.795’ W Visibility: 10 Nautical Miles Wind Direction: 174° true Wind Speed: 15 knots Sea Wave Height: 0-1ft. Swell Waves: N/A Water Temperature: 8.3° C Dry Bulb: 10.6° C Wet Bulb: 10.6° C Sea Level Pressure: 1021.0 mb

Science and Technology Log

The primary mission of the Rainier is to gather hydrographic sounding data. For this leg of the summer field session, that data collection is done by a number of small launches that go out to work each day from Rainier. On a typical day four twenty-nine foot survey launches are deployed from the ship, each with an assigned area to gather data. Each launch is equipped with a multibeam sonar device that sends sound signals to the bottom and then times how long it takes for the signal to return to the receiver. Knowing how fast the signal will travel through the water, the length of time the signal takes to leave and return to the sounder determines the depth of the water at that point.

Here I am preparing the CTD to take a cast.

For many years sonar devices have only been able to measure the water depth directly below a survey vessel. Now, with multibeam sonar, survey vessels can cover a larger swath of seafloor with hundreds of depth measurements being taken at a time. Once the data is processed, a “painted” picture of the bottom surface can be generated. Once a launch is in its assigned work area, the sonar is turned on and the boat goes back and forth in a prescribed pattern to gather data on water depth, essentially providing total coverage of what the seafloor looks like in that area. The coxswain (person driving the launch) has a computer screen with a chart of the coverage area and steers the launch over the planned area. As the launch moves along the path of sonar coverage its path shows up on the screen as a different color, letting the driver know where the boat has been.

In order for data to be interpreted accurately, there are many steps in the process from data acquisition to actual placement on a nautical chart. There is one very important piece of data that needs to be gathered in the field as the launches do there work with the sonar. Sound waves can vary in speed as they travel through water, depending on certain conditions. In order for accurate depth readings to be acquired, those conditions must be known. Therefore throughout the data gathering session, hydrographers must acquire data on the condition of the water. That is where a CTD cast comes in. CTD stands for conductivity, temperature, depth. Every few hours a CTD cast must be done in order to accurately interpret the data gathered by the sonar. The device is lowered over this side of the launch and allowed to sink to the bottom. As it descends, the CTD gathers data at various depths. When recovered the CTD is connected to a computer and its data is integrated with the sonar data to acquire more accurate depth readings.

Personal Log

I’ve been on the Rainier now for twelve days. While there are certain routines on board the ship, there isn’t much routine about the work these people do. I continue to be impressed with how everyone applies their skills to their work in order for data to be gathered. Much of the area where we are working has never been charted before and much of what has been charted was done before World War II with lead lines (dropping a piece of lead attached to a line, and counting the measured marks on the line until it hits bottom). The details acquired by multibeam sonar are truly amazing. We will be here in the Pavlof Islands for a few more days and then head back to Kodiak, where I will get off the ship. Not long to go, but there is still much for me to learn!

Something to Think About
How long would it take you to paint an entire house with dots from a very small paintbrush? That would be like using a lead line to gather depth information. How long would it take you to paint an entire house with a very small, narrow paint brush? That would be single beam sonar. How much time could you save by using a wide paintbrush? That would be multibeam sonar.

NOAA Teacher at Sea Becky Moylan Onboard NOAA Ship Oscar Elton Sette July 1 — 14, 2011

July 5, 2011

NOAA Teacher at Sea
Becky Moylan
Onboard NOAA Ship Oscar Elton Sette
July 1 — 14, 2011