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Full Fathom Five

September 6, 2017

Full fathom five thy father lies;

Of his bones are coral made;

— From Ariel’s Song by William Shakespeare

The Tempest, Act I, Scene II

Todd and Robert working with the Multibeam.

Todd and Robert working with the Multibeam.

When you look at a nautical chart, you see the depths of the area scattered about. There are also contour lines indicating the general slope of the seafloor. So why then, you might ask, are we spending some of our time mapping?

An example of a nautical chart.

An example of a nautical chart.

We are mapping because there are only limited areas with detailed maps of the seafloor. Every chart lists the dates when the surveys were conducted to gather the printed information. You’ll see specific areas, such as places important for commerce, that have been updated between 1990 and 2013. However, if you look at larger, more remote areas, you’ll find that most of that information is pre-1900. So not only is the data old, it was made using less sophisticated technology. Additionally, these old maps are not very detailed. Imagine if your road map only gave you the interstates and you wanted to explore a little town off the beaten path?

Multi-beam mapping allows us to generate much more detailed maps of the seafloor. Mapping still relies on sonar, but the signal is split into several beams. As the information bounces back, it is compared to the other pieces of information to keep everything within the same parameters. If all the beams except one read the depth as 1000 meters, and the outlier reads ten meters, then the ten meter reading will be averaged out automatically at that point. The ship is able to send out these beams to a width about three times the depth of the sea beneath it. If we are in 100 meters of water the ship can map a track about 300 meters wide. Generally, to map an area, we run back and forth, following lines drawn over the area of interest. On board this is sometimes affectionately called “mowing the lawn” because the motion of the ship is much like that of a lawnmower.

An example of mapping the track, or 'mowing the lawn.'

An example of mapping the track – “mowing the lawn.”

On this mission the beams come off of the ship in a V-shape, and the information gathered by the center beam is more accurate and detailed than the beams on the sides. When the sounding files return, a computer program makes corrections to account for a wide variety of variables, including water temperature, the speed of the ship as it moves along, sound velocity, tidal effect and where we are in the world. When all of these parameters are checked we have a 95% confidence in the accuracy of our depth.

We have access to some extremely accurate maps, but there are places where information is missing. We are attempting to help fill in some of the missing data by targeting specific locations that are near our study sites.

Todd Walsh is the Hydrographic Senior Survey Technician on the NOAA Ship Pisces. With help from Robert Figueroa-Downing, Todd is leading our mapping activities. Todd says that being a hydrographer is a great way to travel all over the world. He has worked from within the Arctic Circle to Midway Atoll, and has enjoyed mapping new places, including newly discovered wreck sites. While he says it is good to have a math and science background, having excellent computer skills is really key to this career.

In Ariel’s song, Shakespeare references “full fathom five,” meaning Ariel’s father is lying in 30 feet of water (1 fathom=6 feet). The desire to know what lies beneath the waves, and how deep it lies, has been a question for hundreds of years. Today’s work is helping answer that question.

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