David Tourtellot: A Musical Perspective of Sonar, July 24, 2018

NOAA Teacher at Sea

David Tourtellot

Aboard NOAA Ship Thomas Jefferson

July 9-26, 2018

Mission:  Hydrographic Survey – Approaches to Houston

Geographic Area of Cruise: Gulf of Mexico

Date: July 24th, 2018

Weather Data from the Bridge

Latitude: 29°09.1270’N

Longitude: 093°46.5544’W

Visibility: 5 Nautical Miles

Sky Condition: 8/8

Wind: Direction: 70.1°, Speed: 13.3 knots

Temperature:

Seawater: 29.24°C

Air: Dry bulb:26.9°C          Wet bulb: 24.7°C

 

Science and Technology Log

Coming to NOAA Ship Thomas Jefferson, I was eager to learn all I could about sonar. I am amazed that we have the ability to explore the ocean floor using sound.

uncharted wreck

An uncharted wreck discovered by NOAA Ship Thomas Jefferson

Over the course of my previous blog entries, I have described the tools and processes used to survey using sonar. This time, I am going to try to frame the sounds that the sonars are using in a musical context, in the hope that doing so will help students (and myself) better understand the underlying concepts.

Note – many aspects of music are not standardized. For the purpose of this blog post, all musical tuning will be in equal temperament, at A=440. When I reference the range of a piano, I will be referencing a standard 88-key instrument. Many of the sonar frequencies do not correspond exactly to an in-tune pitch, so they have been written to the nearest pitch, with a comment regarding if the true frequency is higher or lower than the one written.

In sonar and in music, when considering soundwaves it is important to know their frequency, a measure of how many waves occur over the course of a set period of time. Frequency is measured in a unit called Hertz (abbreviated as Hz), which measures how many soundwaves occur in one second. One Hertz is equal to one soundwave per second. For example, if you heard a sound with a frequency of 100Hz, your ears would be detecting 100 soundwaves every second. Musicians also are concerned with frequency, but will use another name for it: pitch. These words are synonymous – sounds that are higher in pitch are higher in frequency, and sounds that are lower in pitch are lower in frequency.

Below are the eight octaves of the note A that are found on a piano, each labeled with their frequency. The notes’ frequencies have an exponential relationship – as you move from low to high by octave, each note has a frequency that is double that of its predecessor.

Piano As with frequencies

The frequency of each A on a piano

The highest note on a piano, C, has a frequency of 4186.01Hz

Highest Note on a piano

The frequency of the highest note on a piano

Average, healthy young humans hear sounds ranging from 20Hz to 20,000Hz. All sounds outside of that range are inaudible to people, but otherwise no different from sounds that fall within the human range of hearing. The highest note we would be able to hear would be an E♭, at a frequency of 19,912.16Hz (a frequency of exactly 20,000Hz would fall in between E♭ and E♮, though would be closer to E♭). If put on a musical staff, it would look like this:

High Eb 19kHz

The frequency of the highest note in the human range of hearing

The hull of NOAA Ship Thomas Jefferson is equipped with several sonar transmitters and receivers, which can operate at a wide variety of frequencies.

TJ Sonar

The hull of NOAA Ship Thomas Jefferson, with several sonars. Note that the projectors that transmit lower frequencies are larger than the ones that transmit higher frequencies. This is similar to musical instruments – instruments that make lower sounds, like the tuba or the double bass, are larger than instruments that make higher sounds, like the trumpet or the violin

Higher frequencies provide higher resolution returns for the sonar, but they dissipate more quickly as they travel through water than lower frequencies do. Surveyors assess the depth of the water they are surveying, and choose the frequency that will give them the best return based on their conditions. Most of the sonar frequencies are too high for humans to hear. The ship’s multi-beam echo sounder has a variable frequency range of 200,000Hz-400,000Hz, though as I’ve been on board they’ve been scanning with it at 300,000Hz. Likewise, the multi-beam sonars on the launches have also been running at 300,000Hz. The ship has a sub-bottom profiler, which is a sonar used for surveying beneath the seafloor. It operates at a frequency of 12,000Hz, and has the distinction of being the only sonar on the ship that is audible to humans, however, we have not had a need to use it during my time aboard the Thomas Jefferson.

The ship’s side scan towfish (which I described in my previous blog entry) operates at 455,000Hz.

Here, we can see what those frequencies would look like if they were to be put on a musical staff.

Assorted Sonars and reference pitches

The frequencies of sonar, with reference pitches

Altering the frequency isn’t the only way to affect the quality of the reading which the sonar is getting. Surveyors can also change the pulse of the sonar. The pulse is the duration of the ping. To think about it in musical terms, changing the pulse would be akin to switching from playing quarter notes to playing half notes, while keeping the tempo and pitch the same. Different sonar pulses yield different readings. Shorter pulses provide higher resolution, but like higher frequency pings, dissipate faster in water, whereas longer pulses provide lower resolution, but can reach greater depths.

Personal Log

Mariners have a reputation for being a rather superstitious bunch, so I decided to ask around to see if that held true for the crew of the Thomas Jefferson. Overall, I found that most didn’t strictly adhere to any, but they were happy to share some of their favorites.

Everyone I spoke to told me that it is considered bad luck to leave port on a Friday, though the Commanding Officer, CDR Chris van Westendorp, assured me that you could counteract that bad luck by making three 360° turns to the left as soon as the ship is able. Many on the crew are also avid fishermen, and told me that bringing bananas aboard would lead to a bad catch, and one went so far as to be mistrustful of yellow lighters as well.

Certain tattoos are said to bring good luck – I was told that sailors often have a chicken and a pig tattooed on their feet. According to custom, those animals were often stored in wooden crates that would float if a ship went down, and having them tattooed onto you would afford you the same benefit. When asked if he was superstitious one of our helmsmen Jim proudly showed me a tattoo he has of a dolphin. He explained that having a sea creature tattooed on your body would prevent drowning. “It works!” he said with a grin, “I’ve never drowned!”

Several maritime superstitions deal with foul weather. Umbrellas are said to cause bad weather, as is split pea soup. Whistling while on the bridge is said to “whistle in the winds.” While not a superstition per se, many crew members told me variations of the same meteorological mantra: Red sky at night, sailor’s delight. Red sky in the morning, sailors take warning.

One of the NOAA Corps Officers aboard, ENS Garrison Grant, knew several old superstitions related to shipbuilding. When laying the keel (the first piece of the ship to be put into place), shipbuilders would scatter evergreen boughs and tie red ribbons around it to ward off witches. Historically, having women aboard was considered bad luck, though, conversely it was said that if they showed their bare breasts to a storm, it would subside. This is why several ancient ships had topless women carved into the masthead. Legend has it that in order to assure that a ship would float, when it was ready to be launched for the first time, virgins would be tied to the rails that guided the ship from the ship yard into the water. The weight of the ship would crush them, and their blood would act as a lubricant, allowing the ship to slide into the water for the first time. Yikes! Thankfully, as society became more civilized, this practice evolved into the custom of breaking a bottle of champagne against a ship’s bow!

Did you know? Musical instruments play an important role in ship safety! In accordance with maritime law, ships will use auditory cues to make other vessels aware of their presence in heavy fog. For large ships, this includes the ringing of a gong at regular intervals.

Latest Highlight: During this week’s fire drill, I got to try the fire hose. It was very powerful and a lot of fun!

David Tourtellot during a fire drill

David Tourtellot during a fire drill

Gregory Cook, On Sea Sickness and Good People, August 10, 2014

NOAA Teacher at Sea

Gregory Cook

Aboard NOAA Ship Oscar Dyson

July 26 – August 13, 2014

Mission: Annual Walleye Pollock Survey

Geographical Area: Bering Sea

Date: August 10, 2014


Science and Technology Log:

Last night and afternoon was by far the craziest we’ve seen on the Oscar Dyson. The winds were up to 35 knots (about 40 miles an hour). The waves were averaging 12 feet in height, and sometimes reaching 15-18 feet in height. Right now I’m sitting on the bridge and waves are around 8 feet. With every rise the horizon disappears and I’m looking up at stark grey clouds. With every drop the window fills with views of the sea, with the horizon appearing just below the top of the window frames.

UpDownUpDownUpDown

In the space of three seconds, the view from atop the bridge of the Oscar Dyson goes from looking up to the sky to down at the sea. The above pic is a MILD example.

Ensign Gilman, a member of NOAA Corps, explains to me how the same thing that makes the Bering Sea good for fish makes things rough for fishermen.

“This part of the Bering Sea is shallow compared to the open ocean. That makes the water easier for the wind to pick up and create waves. When strong winds come off Russia and Alaska, it kicks up a lot of wave action,” Ensign Gilman says.

Andrew, Bill and Nate

Lt. Andrew Ostapenko, Survey Tech Bill Potts, and Ens. Nathaniel Gilman on the Bridge

“It’s not so much about the swells (wave height),” he continues. “It’s about the steepness of the wave, and how much time you have to recover from the last wave.” He starts counting between the waves… “one… two… three… three seconds between wave heights… that’s a pretty high frequency. With no time to recover, the ship can get rocked around pretty rough.”

Rough is right! Last night I got shook around like the last jelly bean in the jar. I seriously considered finding some rope to tie myself into my bunk. There were moments when it seemed an angry giraffe was jumping on my bunk. I may or may not have shouted angrily at Sir Isaac Newton that night.

Which brings us to Sea Sickness.

Lt. Paul Hoffman, a Physician’s Assistant with the U.S. Public Health Service, explains how sea sickness works.

“The inner ears are made up of tubes that allow us to sense motion in three ways,” Hoffman explains. “Forward/back, left/right, and up/down. While that’s the main way our brain tells us where we are, we use other senses as well.” He goes on to explain that every point of contact… feet and hands, especially, tell the brain more information about where we are in the world.

“But another, very important piece, are your eyes. Your eyes are a way to confirm where you are in the world. Sea Sickness tends to happen when your ears are experiencing motion that your eyes can’t confirm,” Hoffman says.

For example, when you’re getting bounced around in your cabin (room), but nothing around you APPEARS to be moving (walls, chair, desk, etc) your brain, essentially, freaks out. It’s not connected to anything rational. It’s not enough to say “Duhh, brain, I’m on a boat. Of course this happens.” It happens in a part of the brain that’s not controlled by conscious thought. You can’t, as far as I can tell, think your way out of it.

Hoffman goes on to explain a very simple solution: Go look at the sea.

“When you get out on deck, the motion of the boat doesn’t stop, but your eyes can look at the horizon… they can confirm what your ears have been trying to tell you… that you really are going up and down. And while it won’t stop the boat from bouncing you around, your stomach will probably feel a lot better,” Hoffman says.

The Deck is your Friend.

Everything is easier on deck! Clockwise from left: Winch Operator Pete Stoeckle and myself near Cape Navarin, Russia. Oceanographer Nate Lauffenburger and myself crossing the International Date Line. Survey Tech Alyssa Pourmonir and Chief Scientist Taina Honkalehto near Cape Navarin, Russia.

And he’s right. Being up on the bridge… watching the Oscar Dyson plow into those stout waves… my brain has settled into things. The world is back to normal. Well, as normal as things can get on a ship more than a third of the way around the world, that is.

Personal Log:

Let’s meet a few of the good folks on the Oscar Dyson. 

NOAA Crew Member Alyssa Pourmonir

Job Title: Survey Technician

Alyssa and the Giant Jelly!

Survey Tech Alyssa Pourmonir assesses a giant jelly fish!

Responsibilities on the Dyson: “I’m a liaison between crew and scientists, work with scientists in the wet lab, put sensors onto the trawling nets, focus on safety, maintaining all scientific data and equipment on board.” A liaison is someone who connects two people or groups of people.

Education Level Required: “A Bachelors degree in the sciences.” Alyssa has a BS in Marine and Environmental Science from SUNY Maritime with minors in oceanography and meteorology.

Job or career you’ve had before this: “I was a life guard/swim instructor in high school, then I was in the Coast Guard for three years. Life guarding is the BEST job in high school!”

Goal: “I strive to bring about positive change in the world through science.”

Weirdest thing you ever took out of the Sea: “Lump Sucker: They have big flappy eyebrows… they kinda look like a bowling ball.”

Lump Sucker!

Lump Sucker! When provoked, this fish sucks in so much water that it becomes too big for most other fish to swallow. That’s its defense mechanism! It sort of looks like a cross between a bowling ball and grumpy cat!

Dirtiest job you’ve ever had to do on a ship: “Sexing the fish (by cutting them open and looking at the fish’s gonads… sometimes they explode!) is pretty gross, but cleaning the PCO2 filter is nasty.  There are these marine organisms that get in there and cling to the filter and you have to push them off with your hands… they get all slimy!”

Engineer Rico Speights

Engineer Rico Speights shows off how nasty a filter can be! He and his wife (Chief Steward Ava) sail the Bering Sea together with NOAA!

NOAA Rotating Technician Ricardo Guevara

Job Title: Electronics Technician

Responsibilities on the Dyson: “I maintain and upkeep most of the low voltage electronics on the ship, like computer networking, radio, television systems, sensors, navigation systems. All the equipment that can “talk,” that can communicate with other devices, I take care of that.”

Education level Required: High school diploma and experience. “I have a high school diploma and some college. The majority of my knowledge comes from experience… 23 years in the military.”

Tech Guevara

Technician Ricardo Guevara shows me an ultrasonic anemometer… It can tell the wind speed by the time it takes the wind to get from one fork to the other.

Job or career you’ve had before this: “I was a telecommunications specialist with the United States Air Force… I managed encryption systems and associated keymat for secure communications.” This means he worked with secret codes.

Trickiest problem you’ve solved for NOAA: “There was a science station way out on the outer edge of the Hawaiian Islands that was running their internet off of dial-up via satellite phone when the whole thing shut down on them… ‘Blue Screen of Death’ style. We couldn’t just swap out the computer because of all the sensitive information on it. I figured out how to repair the disk without tearing the machine apart. Folks were extremely happy with the result… it was very important to the scientists’ work.”

What are you working on now? “I’m migrating most of the ship’s computers from windows xp to Windows 7. I’m also troubleshooting the DirecTV system. The problem with DirecTV is that the Multi-Switch for the receivers isn’t communicating directly with the satellite. Our antenna sees the satellite, but the satellite cannot ‘shake hands’ with our receiver system.” And that means no Red Sox games on TV! Having entertainment available for the crew is important when you’re out to sea for two to three weeks at a time!

What’s a challenging part of your job on the Dyson? “I don’t like it, but I do it when I have to… sometimes in this job you have to work pretty high up. Sometimes I have to climb the ship’s mast for antenna and wind sensor maintenance. It’s windy up there… and eagles aren’t afraid of you up there. That’s their place!”

Lt. Paul Hoffman

Job Title: Physician Assistant (or P.A.) with the U.S. Public Health Service

Paul and Peggy

Lt. Paul Huffman and the small boat Peggy D behind him. Lt. Huffman is with the U.S. Public Health Service. But secretly I call him the Bat Man of Health Care. Peggy Dyson is a beloved part of the Alaska Fishing Industry’s history. Before the internet and satellite telephones, her radio service served as a vital link home for fishermen out at sea.  She was married to Oscar Dyson, the man for whom the ship was named.

Responsibilities on the Dyson: He’s effectively the ship’s doctor. “Whenever a NOAA ship travels outside 200 miles of the U.S. coast, they need to be able to provide an increased level of medical care. That’s what I do,” says Hoffman.

Education required for this career: “Usually a Masters degree from a Physician’s Assistant school with certification.”

Job or career you’ve had before this: “Ten and a half years in the U.S. Army, I started off as an EMT. Then I went on to LPN (Licensed Practical Nurse) school, and then blessed with a chance to go on to PA school. I served in Iraq in 2007-2008, then returned for 2010-2011.”

Most satisfying thing you’ve seen or done in your career: “Knowing that you personally had an impact on somebody’s life… keeping somebody alive. We stabilized one of our soldiers and then had a helicopter evac (evacuation) under adverse situations. Situations like that are what make being a PA worthwhile.”

Could you explain what the Public Health Service is for folks that might not be familiar with it?

“The Public Health Service is one of the seven branches of the U.S. Military. It’s a non-weaponized, non-combative, all-officer corps that falls under the Department of Health and Human Services. We’re entirely medical related. Primary deployments (when they get sent into action) are related to national emergency situations… hurricanes, earth quakes… anywhere where state and local resources are overrun… they can request additional resources… that’s where we step in. Hurricane Katrina, the Earthquake in Haiti… a lot of officers saw deployment there. Personally, I’ve been employed in Indian Health Services in California and NOAA’s Aircraft Operations Center (AOC)… they’re the hurricane hunters,” Hoffman concludes.

Kids, when you’ve been around Lt. Hoffman for a while, you realize “adverse conditions” to him are a little tougher than a traffic jam or missing a homework assignment. I’ve decided to call him, and the rest of the Public Health Service, “The Batman of Health Care.” When somebody lights up the Bat Signal, they’re there to help people feel better.

Coming up next: International Teamwork!

 

Marla Crouch: Pitch and Roll, June 24, 2013

NOAA Teacher at Sea
Marla Crouch
Aboard NOAA Ship Oscar Dyson
June 8-26, 2013 

Mission:  Pollock Survey
Geographical area of cruise:  Gulf of Alaska
Date: June 23, 2013

Weather Data from the Bridge: as of 2100
Wind Speed 6.30 kts
Air Temperature 11.7°C
Relative Humidity 73.00%
Barometric Pressure 1,004.20 mb

Latitude:  56.42N   Longitude: 158.20W

Science and Technology Log

Who can tell me the direction longitudinal and transverse waves move?  Think about the electromagnetic spectrum; what is the relationship between wavelength and frequency?  The physics of these wave actions are experienced in fields other than earthquakes (seismic), and light (optics) and sound (audio).

Picture provided by NOAA NWS Prediction Center

Picture provided by NOAA NWS Prediction Center

There are two different types of water waves that mariners regularly encounter, wind waves and swell waves.  An analogy for wind waves and swell is a wind wave is to weather as swell is to climate.  In other words, wind waves are local and swell occurs over a great distance.

Waves are formed when repeated disturbances move through a medium, such as, air, earth and water.  As the wind moves or blows across the open waters, energy is transferred from the friction of the moving air particles to the waters’ surface creating wind waves.  The speed, and fetch (unchanged direction) of the wind, and the distance the wind has traveled unimpeded; influence the amplitude and frequency of the waves.  As wind speed picks up so does the amplitude of the waves.  Wind waves can be identified by their white caps.  Wind waves have short wave lengths.

Graphic courtesy of Tammy Pelletier, WA State Dept. of Ecology http://www.vos.noaa.gov/MWL/apr_06/waves.shtml

Graphic courtesy of Tammy Pelletier, WA State Dept. of Ecology
http://www.vos.noaa.gov/MWL/apr_06/waves.shtml

Swells are a formation of long wavelength surface waves, which travel farther and faster than short wavelength wind waves.  Swells can be formed by storms that occurred somewhere else in the ocean.  For example, Tropical Storm Leepi formed off the China coast south of Japan, and was active June 17 – 19, 2013.  The energy from Leepi’s 40 mph winds and rain radiates outward from the storm, like the ripples that form when you drop a rock in a puddle of water, creating swells.  Swells can travel in a multitude of directions as they bounce off landmasses back into the open waters.

Wind waves and swells transfer energy to ships, such as the Oscar Dyson.  The energy causes ships to pitch, roll and yaw.

Pitch, roll, and yaw are three dynamic ways crafts, such as airplanes and ships move in a fluid.  In my “Surf your Berth” blog I used a teeter totter as an example of pitch.  If you think about the way energy moves in waves, pitch is a longitudinal wave where the energy is moving front to back, so that the bow of the ship goes up and down.  Roll is a transverse wave, the energy is moving side to side, rolling the ship from port to starboard (left to right).  To describe yaw, think about sitting in a chair that swivels.  Yaw is the swiveling action of you in the chair moving in the chair or a ship rotating around a vertical axis.  Watch the horizon in the video to get an idea of what pitch looks like from the vantage point of the bridge of the Oscar Dyson.

If you turn your field of view 90°, so you are looking either port or starboard and see the same motion that is roll.

The officers of the Oscar Dyson work to navigate through both the wind and swell waves to give us the smoothest ride possible.

Personal Log 

Recently we experienced sustained wind speeds between 30 and 40 kts.  Needless to say, we were a pitchin and a rollin.  Chiachi Island afforded us calmer seas, as we reached the lee (wind shadow) side of the island.  I noticed something different in this last encounter with rough seas, instead hearing the water race past the hull, this time the water slammed into the side of the Oscar Dyson.  The crashing transformed some of the wave’s kinetic energy into thunderous claps of sound…BAM!…BAM!  What caused the difference, I’m not sure, maybe we were in a convergent zone, before reaching the lee, where wind and seas raced around the island creating a collision akin to clapping your hands together that buffeted the Dyson from both sides.

What do we do aboard ship to cope with the pitch and roll?  We anchor ourselves.  Chairs at our work stations, do not have casters and are tethered to the desk with a cord.  The dressers in our berths have straps to keep the draws shut and the closet doors lock into their closed position.

On a ship the dining hall is called the Mess.  Here the chairs are tethered to the floor.

Amanda Peretich: Awesome Acoustics, July 13, 2012

NOAA Teacher at Sea
Amanda Peretich
Aboard Oscar Dyson
June 30, 2012 – July 18 2012

Mission: Pollock Survey
Geographical area of cruise:
Bering Sea
Date:
July 13, 2012

Location Data
Latitude: 59ºN
Longitude: 174ºW
Ship speed: 11.7 knots (13.5 mph)

Weather Data from the Bridge
Air temperature: 7.3ºC (45.1ºF)
Surface water temperature: 7.6ºC (45.7ºF)
Wind speed: 4.3 knots (4.9 mph)
Wind direction: 12ºT
Barometric pressure: 1010 millibar (1.0 atm, 757.5 mmHg)

Science and Technology Log

How sonar works: energy (sound) waves are pulsed through the water. When it strikes an object, it bounces back to the receiver. (from http://www.dosits.org/)

How sonar works: energy (sound) waves are pulsed through the water. When it strikes an object, it bounces back to the receiver. (from http://www.dosits.org/)

Before stepping onto the Oscar Dyson, I wasn’t quite sure about much of the science going on. Did they just put the nets in the water every so often and hope to catch some fish? Carefully lean over the side of the ship saying “here fishy fishy” with the hope that the pollock would find their way into the net? Neither of these scenarios is correct (good thing I’m not actually a fisherman!). So today’s lesson is going to be all about what the chief scientist actually uses to find fish: hydroacoustics (hydro meaning water and acoustics meaning sound). This also involves SONAR, which is short for SOund Navigation And Ranging.

Fishfinding Basics

Fishfinding basics.

If you’ve ever been on a smaller boat, yacht, fishing vessel, or the like, you may have seen something called a fishfinder. The basic concepts are the same as what is happening on the Oscar Dyson. An echosounder sends a pulse of energy waves (sound) through the water. When the pulse strikes an object (such as the swim bladder in fish), it is reflected (bounced) back to the transducer. This signal is then processed and sent to some sort of visual display.

Swim Bladder

Swim bladder in a fish.
(from https://www.meted.ucar.edu/)

The Oscar Dyson uses acoustic quieting technology where the scientists can monitor fish populations without altering their behavior. The Scientific Sonar System and various oceanographic hydrophones (underwater microphones) are raised and lowered through the water column beneath the ship on a retractable centerboard. This is important so that the transducers can be lowered away from the flow noise generated by the hull, which in turn will improve the quality of data collected. In addition, there is a multibeam sonar system located on the forward hull. Ultimately the hydroacoustic data is all used as one piece to the puzzle of measuring the biomass of fish in the survey area.

OD acoustics

The different sonar signal transmitter/receivers (transducers) used on this leg of the pollock survey and their location on the ship.

Neal at work

Chief scientist Neal working away in the Acoustics lab. The second screen from the left on the upper row is showing the information from the ME70 multibeam.

So how does this all work when we are looking for fish? The chief scientist (Neal on the 0400-1600 watch) or another scientist (Denise on the 1600-0400 watch) will spend a lot of time analyzing the various computer screens in the acoustics lab, which has been affectionately termed the “cave” (no windows). They are looking at the information being relayed from both the multibeam and the EK60.

What is a multibeam? The Oscar Dyson has the Simrad ME70 scientific multibeam echosounder. It is located on the hull (underside) of the ship on the front half and sends 31 sonar beams per second down to the bottom of the sea floor.

Multibeam

Multibeam echosounder.
(from http://www.simrad.com/)

Aft of the multibeam (on the centerboard) are the five Simrad transducers. It may seem confusing, but hopefully I can walk you through a teensy little bit of how it works when we are looking to trawl for fish.

EK60 Transducer

Information from the EK60 transducer at 18kHz (top) and 38kHz (bottom).

Information from the EK60 echosounder is displayed on the far left screen in the acoustics lab while information for the ME70 multibeam is displayed on the next screen. The darker patches are showing that there are fish in that area. When the scientist first starts to see a good amount of fish, they will “mark” it and keep watching. If the screen fills up with fish (as in the EK60 image), the scientist will call upstairs to the bridge and tell them where to head back to on the transect line to start trawling. Depending on the location of the fish in the water column, it may be a bottom trawl (83-112 net), a midwater trawl (AWT net), or a methot trawl. Side note: the 83-112 midwater comparison trawl that I’ve mentioned before is done almost immediately after an AWT midwater trawl to compare the fish caught in a common area.

ME70 Multibeam

Information from the ME70 multibeam. You can determine the sea floor depth and there are five narrow beam slices from the mid-section of the multibeam (of the 31 different beams that span 120 degrees) displayed on screen.

Neal on bridge

Chief scientist Neal up on the bridge.

Then the scientist will head upstairs as the deck crew is preparing the net. One of the many sensors attached to the net is called the FS70 fishsounder or “the turtle”, and it is only used during trawls (because it is attached to the headrope). The scientist can “watch” the fish swimming under the ship using the EK60 information combined with the information from the fishsounder. The yellow “turtle” on the right in the image shows how the FS70 is flying in the water. You want minimal pitch and roll and for the front of it to be facing the back of the ship. This way, we can “see” the fish as they are going through the net. The officer of the deck and lead fisherman or head boatswain can adjust various things to keep the turtle in the right orientation. The middle image below is constantly changing on the screen in the bridge as the sonar is sweeping back and forth, so you can almost watch the individual fish enter the net. It was interesting to watch the delay between when you would see the fish from the EK60 (on the left) and when you saw them with the FS70 (middle).

Trawl Fishsounder

Display screens on the bridge used during a trawl.

Once the scientist is satisfied that enough fish have been caught for a sufficient sample size, the net will be hauled back and the acoustics work is done for just a little bit (giving Neal some time to grab some well-deserved coffee and the rest of us time to get our rain gear on to process the fish).

So some of the questions I had asked (that don’t really fit nicely in the information above):

Why do we use different frequencies in the acoustic studies?

Frequency Wavelength

Relationship between frequency and wavelength. (from http://emap-int.com)

This ties right back in to chemistry (and other sciences) with an equation and the relationship between frequency and wavelength (yay!). Basically there is an inverse relationship which means that at a high frequency there is a smaller or shorter wavelength (wavelength is the distance for peak to peak of a wave). At a low frequency, there is a higher or longer wavelength.

At a low frequency, you will see only see things that are larger, like pollock, whereas you will see very small things like krill and zooplankton at higher frequencies. Having information from both types of frequencies is necessary to complete the scientific research on the Oscar Dyson.

Single Fish

Traveling at 1 knot, showing single fish from EK60 sonar.

Is it possible to see a single fish?
Yes! From sunset to sunrise, the Oscar Dyson doesn’t actually travel the transect lines. This is because the pollock behave differently during darkness than during the day. So instead of traveling between 11 and 12 knots (which is what happens between trawls), it’s almost like the boat is just sitting around for a couple of hours. But during this time, since the boat isn’t moving along quickly, it’s possibly to see individual fish on the sonar as shown in the image.

Hydroacoustics

Hydroacoustic surveys can involve any number of different types and locations of the transducers. (from http://btechgurus.blogspot.com/2012/06/sonar.html)

Personal Log
Today is Friday the 13th but it was far from unlucky – I finally saw something out in the water other than fog: a boat! Again, all good sightings seem to come from up on the bridge, so I’m thankful for Lieutenant Matt for allowing me to ask a billion questions while I’m up there and teaching me more than I ever thought my brain could hold. He has all of the qualities of a great teacher, which is nice to see.

Ship

The ship we saw up on the bridge this morning from about 5 nautical miles away (left), on the sonar (middle), and through the binoculars (right).

Dancing in the fish lab on the Oscar Dyson

Neal and I dancing while waiting for the fish!

Highlight from the other day? Chief scientist Neal finally dressed out in his Grundens (rain gear) and came to help process a catch in the fish lab! While waiting, he even took a quick second to dance in the doorway (we were “Dougie”-ing) to my music that was playing over the speaker system.

References
NOAA Oscar Dyson flier
NOAA Oscar Dyson Ship Electronics Suite
HTI Sonar
Wikipedia: Sonar
Simrad