Eric Koser: Hydrography 101 – and the Tools to Make it Happen, June 28, 2018


NOAA Teacher at Sea

Eric Koser

Aboard NOAA Ship Rainier

June 22 – July 9, 2018

Mission: Lisianski Strait Survey

Geographic Area: Southeast Alaska

Date: June 28, 2018: 0900 HRS

Weather Data From the Bridge
Lat: 57°52.59′ Long: 133°38.7′
Skies: Broken
Wind 1 kt at variable
Visibility 10+ miles
Seas: calm
Water temp: 5.6°C

Science and Technology Log

Long Line Boat

A typical longline fishing boat. The fishing lines get spread out behind the boat from the large booms on either side.

The ultimate focus of Rainier is to assure accurate navigational charts are available to all mariners. This task is critical to the safety of many industries. About 80% of all the overseas trade in the US (by weight) is moved over water. Here in SE Alaska, it appears the largest industry is commercial fishing. Many boats fish both with nets and long lines to catch halibut, rockfish, cod, and several varieties of salmon.

Another major industry here is certainly tourism. As we conduct our work, we often see very large cruise ships. It’s an interesting juxtaposition to be in a narrow inlet surrounded by mountains, ice, and wildlife and then come across a large ship.  We passed the brand new ship Norwegian Bliss around 11 PM on our transit to Tracy Arm. This ship is 1,082 feet long, carries a crew of 2,100 people and has a guest capacity of 4,004 people! The safe navigation of all of these vessels depends upon the accuracy of charts produced by NOAA.

Norwegian Bliss

The cruise ship Norwegian Bliss as we passed her port to port in the evening.

The freely available charts offered by NOAA are created with three essential steps. First, the bulk of the depth data in this area is measured with MBES (Multi-Beam Echo Sounder). This creates a three-dimensional digital image of the bottom.

Secondly, important features to navigation that are shallow are best identified by our launches which travel along the shorelines and inspect for rocks, ledges, and other potential dangers. The locations of features are identified by GPS location and charted digitally by hydrographers on each launch.

Thirdly, bottom samples are collected by launch crews to confirm the type of material present on the bottom.

The MBES systems aboard Rainier and the launches come from Kongsberg Maritime. Two transducers (devices that transmit and receive) work in tandem. The transducer that is oriented front to back sends out an array of sound signals in a wide beam. The width of the beam on the sea floor depends directly on the depth – deeper water allows the beam to spread farther before reflecting. The transducer that is oriented side to side in the water receives a narrow swath of the ‘pings’ of sound that were transmitted. The time it takes any ping to get to the bottom and reflect back to the ship is recorded. The greater the time, the larger the depth.

MBES on a launch

This shows the position of the MBES on the bottom of one of several launches.

MBES transducers

This is the pair of MBES transducers on a launch, looking from the bow towards the stern.

Hydro Sonar

This image, courtesy of NOAA, depicts an MBSS beam below the ship and the mapped results off the stern.

A couple of issues provide challenges to this technique. One, the speed of sound in water depends on several factors. The salinity (concentration of salt in the water),  the conductivity (how easily electricity passes through the water), and the temperature each fluctuate as the depth changes and affect the speed of the sound waves. As hydrographers receive data, the system has to account for these changes in speed to produce an accurate depth measurement. One way to do this is with a static CTD sensor. This device is lowered from the launches all the way to the bottom as it measures the speed of sound in the water.  It provides a set of three charts as the depth changes which are used to adjust the time data from the MBES accordingly. There is also a version of the CTD, called a MVP (Moving Vehicle Profiler or ‘fish’), that can be pulled behind Rainier as we are moving and take dynamic data.

Here is a NOAA article on hydrographic surveying.  Here is further explanation of MBSS.

Deploying Depth Profiler

Here the crew lowers the profiler “fish” into the water.

Speed Profiler Data

These three plots represent the speed of sound, temperature, and salinity (from left to right) vs. depth (on the vertical axis).

A second issue is GPS signal drift. Over time, the location information can shift slightly. To account for this potential problem, the scientists place a HORCON (Horizontal Control) station onshore in the area where they are mapping. I described this tool in my previous post.

Another interesting technology that is currently being developed is called “backscatter” mapping. Here scientists look not only at the time it takes the sound waves to bounce back to the transducer, but also at the quality of the return signal. Different materials on the seafloor reflect the sound differently – hard surfaces like rocks have a sound signature that is much different than soft surfaces like silt or plants. NOAA is continually improving the tools they use to learn!

Here is an example of the chart that we are updating in Tracy Arm.

Personal Log

I had a chance to take the helm yesterday! It’s interesting how sensitive the steering on this large vessel really is. The rudders are able to turn from “amidships” or their center position, up to about 35° to either side. But while traveling at about 8 knots, we tend to use a maximum of about 5° of rudder to alter the ship’s direction. While at the helm, we keep close track of the heading (compass bearing) of the ship as indicated by the gyro compass and magnetic compass on board. Then we provide steering input to hold the ship to the course ordered by the CONN. I had the chance to help steer around several icebergs as we transited into Tracy Arm. Careful attention to detail – and willingness to promptly follow commands make for success!

Helm

My opportunity to take the helm of Rainier.

I also took an opportunity to head out in a kayak from the ship where we are anchored! Two of my new colleagues and I paddled across this bay and had a great chance to look very closely at pieces of ice. The ice is really beautiful and forms many interesting shapes. The quiet of the bay – hearing only the distant waterfalls, birds, and our paddling was beautiful!

Iceberg

This piece of ice drifted through Tracy Arm from the glacier. It was temporarily ‘grounded’ on the bottom by the receding tide.

It’s crazy to consider the ice we were seeing may have been formed thousands of years ago in the glacier – and it just now melting as it floats away.

Did You Know?

President Thomas Jefferson signed a mandate in 1807 ordering a survey of the nation’s coasts. This fundamental task is always ongoing, with 95,000 miles of US Coastline.

About 90% of any floating piece of ice will be submerged below the salt water.  Because the density of frozen fresh water just slightly less than salt water, the ice floats very low in the water!  Read more here!

Who is Onboard?

I’d like you to meet HST (Hydrographic Survey Technician), Amanda Finn! Ms. Finn has been with NOAA since last September – and started working aboard NOAA Ship Rainier in October of 2017. As an HST, Amanda works with the team of hydrographers to collect MBES data from either the ship or any of the launches. Amanda graduated from the University of Connecticut in 2016 with a bachelor of science degree in GeoSciences and a minor in Oceanography. At the end of her college experience, she knew that seafloor mapping was her passion but wasn’t sure how to make that into a job. But it all came together when she found NOAA through a friend of a friend!

HST Amanda Finn

HST Amanda Finn with recently acquired depth data for Lisinaski Inlet!

Amanda was performing at her first harp concert (another skill!) when she met a relation of a hydrographer who works on a NOAA ship! Based on her experience, her advice to students is: “When things don’t seem to be going the way you want, take time to focus on something else you like instead. In good time, things will work out!”

One positive challenge Amanda shares working here on a hydro ship is developing an understanding of systems integration. Many different pieces must come together to create the finished charts. The people aboard Rainier make the experience very positive!  The passion for seeking the unknown is the drive to continue!

 

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