NOAA Teacher at Sea
Laura Grimm
Aboard NOAA Ship Thomas Jefferson
July 4 – July 22, 2022
Mission: Hydrographic Survey of Lake Erie
Geographic Area of Cruise: Lake Erie
Date: July 7, 2022
Weather Data from the Bridge
Latitude: 42ᵒ 08.7
Longitude: 080ᵒ 16.9
Sky Conditions: few clouds
Wind Speed: 14.9 knots
Wind Direction: 040 NE
Lake Temperature: 22 ᵒC
Wave Height: 1 ft.
Dry Bulb: 20.6 ᵒC
Wet Bulb: 18.6 ᵒC
Relative Humidity: 83% (calculated using the following table)
Science and Technology Log
The mission of a NOAA hydrographic survey is to make bathymetric maps of the floors of bodies of water. Bathymetry is the study of the “beds” or “floors” of water bodies, including the ocean, rivers, streams, and lakes. So, what is the difference between bathymetry and topography? Topographic maps show elevation of landforms above sea level; bathymetric maps show depths of landforms below sea level.
NOAA ships are equipped with lots of different types of equipment to make such maps. One of these is the Multibeam Echo Sounder (MBES). It is used to survey large swaths or bands of the floor of oceans and lakes. This type of technology collects a tremendous amount of bathymetric data.
Multibeam Echo Sounders (MBES) gather information about how deep a body of water is, the physical features of the seafloor, and how close to the surface items like wrecks and obstructions are that might make it dangerous to maritime travel. Obstructions are things sticking up from the floor.
Multibeam Echo Sounders send out sound energy and analyze the return signal (echo) that bounces off the lakebed, seafloor, or other objects. Multibeam sonars emit (send out) sound waves from directly beneath a ship’s hull to produce fan-shaped coverage of the seafloor. These systems measure and record the time for the sound energy to travel from the sonar to the seafloor (or object) and back to the receiver. The longer it takes, the deeper the water. Multibeam sonars produce a “swath” of soundings (i.e., depths) to ensure full coverage of an area. This is sometimes referred to as “mowing the lawn”. Scientists want to be sure that they don’t miss anything!
Multibeam Echo Sounder (MBES) showing bathymetric data, also known as, seafloor topography. Bathymetry is the study of the “beds” or “floors” of water bodies, including the ocean, rivers, streams, and lakes. Topography is a detailed description or representation on a map of the natural and artificial features of an area.
When looking at a hydrographic image, keep in mind that blue = deep water, red = shallow water.
The red rectangle in the foreground is the transmitter. It sends out the sound energy. The other red rectangle is the receiver. It “hears” or receives the echo of the sound. This information is then sent to a computer that analyzes how long the echo took and then calculates the depth.
The small silver latch-looking piece of equipment is the sound speed indicator. It calculates the actual speed of sound in the conditions under which the measurement it taken. A “ping” is sent out from one end and is received at the other end. The speed of sound is then calculated.
I always thought that the speed of sound was a constant number. I guess not! So why is calculating the speed of sound so important? The speed of sound in water is affected by the temperature and salinity of the water. The warmer the water, the faster sound energy travels. Once a molecule starts to vibrate, it passes this energy on to the next molecule, and to the next, and so forth. Water molecules in warmer water are moving quicker so sound energy transmits faster; cold water is more dense and therefore the sound transmits slower. The colder the water, the slower sound energy travels.
Salinity also affects the speed of sound. Salinity is the measure of dissolved salts in water. This accounts for all salts, not just sodium chloride (table salt). The salinity of fresh water is very low compared to that of the salt water in the oceans. Water that has a lot of salts dissolved within will transfer sound energy more quickly. Electroconductivity is a measurement of salinity. (Students – you may remember that we use an electroconductivity probe to help us understand how much fertilizer is in the water used to grow plants hydroponically in the greenhouse.) Knowing the speed of sound in water helps hydrographers interpret the data from the MBES more accurately.
Something to think about . . .
How is a Multibeam Echo Sounder like and unlike echolocation that is used by bats?
For the little Dawgs . . .
Q: Where is Dewey today? Here is a hint. It is also called the “front” of the ship.
A: Dewey is on the bow of the ship. “Bow” rhymes with “cow”.
Dewey is sitting on the bow by the Jackstaff (flagpole). The Jackstaff is a flagpole that flies a maritime flag called the Union Jack of the United States whenever it is at anchor or in port.
Personal Log
We had fun last evening. Patrick, a Seaman Surveyor, told us that he had several flares that had expired. Instead of throwing them away, he decided to have us light them. What a great thing to do around the 4th of July!
Because we were surveying near Lake Erie, we had the opportunity to watch the 4th of July fireworks over Cleveland and surrounding communities. Such a lovely way to spend Independence Day.
Around 2300 (11:00 p.m.) we started to transit (move) toward Erie, PA. It’s been a good day. I look forward to waking up in the waters near Presque Isle.
