Avery Marvin: Beaming With Excitement – Sound Waves and the Sea Floor, July 19, 2013

NOAA Teacher at Sea
Avery Marvin
Aboard NOAA Ship Rainier (Ship Tracker)
July 8-25, 2013 

Mission: Hydrographic Survey
Geographical Area of Cruise: Shumagin Islands, Alaska
Date: July 19, 2013

Current Location: 54° 49.684 N, 159° 46.604 W

Weather Data from the Bridge: Foggy and overcast, wind 21 knots, air temperature: 11.5° C

Science and Technology Log:

As the fog horn sounds every two minutes and we sail solitary through the ocean, we are now in full swing surveying the Shumagin Islands, between and around Nagai, Bird, and Chernabura Islands. Unlike the old-time surveyors who used lead lines (lead weight attached to a long string), we are using a multibeam sonar system, which enables us to gather a large quantity of very accurate data in a more efficient and timely fashion.

Sonar, (SOund Navigation And Ranging) uses the principle of sound wave reflection to detect objects in the water. Just as our eyes see the reflection of visible light off of the objects around us to create a visual image, when a sound wave hits something, it reflects off that “thing” and returns to its starting point (the receiver). We can measure the time it takes for a pulse to travel from the Sonar device below the boat to the ocean floor and then back to the receiver on the boat. Using a simple distance=speed * time equation, we can get the water depth at the spot where each beam is reflected.

The skiff that we use for the shoreline activities discussed in the last post has a single-beam sonar system that directs a pulse straight down beneath the hull to get a rough depth estimate. However, for our hydrographic work on the ship and launches, we use a multibeam system that sends 512 sound pulses simultaneously towards the sea floor over a 120° angle. When many sound waves or “beams” are emitted at the same time (called a pulse) in a fan like pattern (called a swath), the reflected information creates a “sound picture” of the objects or surface within that swath range. The actual width of this swath varies with the depth, but with 512 beams per pulse, and sending out between 5-30 pulses every second, we acquire a lot of data. If you piece together many swaths worth of data you get a continuous topographical or physical map of the ocean floor, and thus the depth of the water. For more information about the specific sonar system used aboard the Rainier and its launches, check out the ship page or the NOAA page about their hydrography work.

In order to understand the complexities of sonar, it is important to understand the properties of sound. Sound is a pressure wave that travels when molecules collide with each other. We know that sound can travel in air, because we experience this every day when we talk to each other, but it can also travel in liquids and solids (which whales rely on to communicate). As a general rule, sound travels much faster in liquids and solids than in air because the molecules in liquids and solids are closer together and therefore collide more often, passing on the vibration at a faster rate. (The average speed of sound in air is about 343 meters every second, whereas the approximate speed of sound in water we have been measuring is around 1475 meters every second). Within a non-uniform liquid, like saltwater, the speed of sound varies depending on the various properties of the saltwater at the survey site. These properties include water temperature, dissolved impurities (i.e. salts, measured by salinity), and pressure. An increase in any of these properties leads to an increase in the speed of sound, and since we’re using the equation distance = speed * time equation, it is crucial to consistently measure them when seeking depth measurements.

To measure these properties, a device called a CTD (Conductivity-Temperature-Depth) is used. Conductivity in this acronym refers to the free flowing ions in salt water (Na and Cl, for example), which are conductive and the concentration of these ions determines the salinity of the water. The CTD measures these three properties (Conductivity, Temperature and Depth) so the speed of sound in the water can be calculated at every point in the water column

To use the CTD, lovely humans like Avery and I will drop it into the water (it is attached to a winch system) at the area where we are surveying and as it travels to the sea floor, it takes a profile of the three saltwater properties mentioned before. Back in the computer lab, software takes this profile data and calculates the sound velocity or speed of sound through the water in that region.  As a crosscheck, we compare our profile data and sound velocity figures obtained at the site to historical measured limits for each property. If our measurements fall significantly outside of these historical values, we might try casting again or switch to a different CTD. However, because we are surveying in such a remote area, in some cases, data outside historical limits is acceptable.

Given that we are trying to determine the water depth to within centimeters, variations in the sound speed profile can cause substantial enough errors that we try to take a “cast” or CTD reading in each small area that we are gathering data. The software the survey team uses is able to correct automatically for the sound velocity variations by using the data from the CTD. This means that the depth profile created by the sonar systems is adjusted based on the actual sound velocities (from the CTD data) rather than the surface sound speed. We are also able to account for speed changes that would cause refraction, or a bending of the beam as it travels, which would otherwise provide inaccurate data about the location of the sea floor.

Personal Log:

You can’t go to Alaska without fishing its waters, rich with a variety of delectable fish species.  So I decided to get my Alaskan recreation fishing license and try my hand at it on the fantail (stern) of the Rainier, while we were anchored in Bird Island cove. Carl VerPlanck, an experienced fisherman with arms like Arnold Schwarzenegger, had coached me on the best jigging techniques for catching a halibut and with my eyes (and mind) on the prize I followed his instructions diligently.  It paid off as I landed several fish my first night on the fantail, with one halibut being a true keeper. John Kidd, NOAA Corps. Officer, gaffed my meaty fish over the steep rail of the Rainier and hauled it aboard.  He was impressed with my catch (and hidden fishing talent), stating “This is the biggest fish caught so far this season.” Woohoo! Most impressive was the amount of meat the fish yielded (4 large filets) which I proudly donated to the kitchen and John. (Three big filets to the kitchen and one filet to John for his camaraderie, the use of his high-tech rod set-up and filleting skills). The following night, we all ate delicious baked Pacific Halibut filets, coated in a creamy Caesar glaze, prepared by chef-extraordinaire, Kathy. It’s pretty cool that I got to feed the ship!!

This was my first time catching a halibut and after close examination (and dissection) of this large, rather bizarre looking flatfish I became very intrigued and had several questions: How and why do the eyes migrate to one side?  How do you tell the age of a halibut? What does the word “halibut” mean?

Like any good scientist, I proceeded to find the answers to these questions, and in doing so, learned many more interesting tidbits about Halibut. (The other species of halibut is the Atlantic Halibut which is very similar to the Pacific Halibut and is named as such for the ocean it occupies.)

So lets start with the name “halibut.” It’s origin is Latin (hali=haly=holy, but=butt=flat fish) and literally translates to “holy flat fish” because it was popular on Catholic holy days. Now what’s with the eye migration and why are both eyes on the same side? Well to understand this question thoroughly we must look at the conditions under which the halibut is born. Female halibut are sexually mature at age 12, spawning from November to March in deep water (300-1500 feet). Depending on their size, females release several thousand to several million eggs which are fertilized externally by the males. After the eggs are fertilized by the males, they become buoyant and start to float up the water column, hatching into free floating larva at about 16 days.  As the larva mature, they continue to rise to the surface. At this larval stage they are upright, like any other “regular” fish, with one eye on each side of their head. This eye placement makes sense, considering they are in the open ocean with water on all sides of them.  When at or near the surface, the larvae drift towards shore by ocean currents. As they get closer to shore and at about 1 inch in length, they undergo a very unique metamorphosis in which the left eye moves over the snout to the right side of the head. At the same time their left side fades in color eventually becoming white and their right side becomes a mottled olive-brown color. By 6 months, they are ready to settle to the bottom in near shore areas, hiding under the silt and sand, with just eyes exposed. Their mottled side will be face up, blending into their surrounds and their white side will face down, creating a “countershading” coloration, which helps keep them hidden from predators.

The Pacific Halibut I caught was by no means a monster or “barn door” as the huge ones are called. But it also was not a “chicken”, slang for a small halibut. Female halibut can reach lengths of 8 feet and a weight of 500+ pounds. Males rarely exceed 100 pounds.  Halibut are generally not picky eaters and will pretty much eat anything that lives in the ocean.  Carl joked that a halibut would even eat an old shoe dangling from a fishing pole.

I was surprised to learn that halibut can live as long as 55 years.  Scientists can accurately age a halibut by counting the rings in their ear bone or “otolith”, similar to dating a tree using its annual growth rings. So next time you catch a halibut and plan on keeping it, try to find the ear bone, grab a microscope and age the fish. If that fails, don’t forget to cut the cheeks out of the halibut (along with the 4 regular meaty fillets), for I am told that is the best part to eat. 🙂

Fun factoid: Sonar works a lot like the echo sounding of a bat, and its development was partially prompted by the Titanic disaster.