text and images by Linda Welzenbach
Since leaving the Chilean coastline, the THOR team has been exploring the sea floor… with sound. Similar to the way that sea mammals communicate with each other over long distances and submarines avoid obstacles, scientists use sound to “see” the bed surface and ultimately map the ocean bottom. Sound waves are generated from beneath the ship from an instrument called a multibeam echo sounder which, in effect, is a giant loudspeaker pointing downwards and away from the vessel on either side.
Here’s how it works: the multibeam sends a “ping” (like you hear in the movies with submarines), a pulse of high-pitched sound that moves down and away from the bottom of the ship. It’s called multibeam because each ping contains 432 individual depth soundings that go out in the shape of a fan. The angle and trajectory of each sounding, the time traveled and distance away from the source of sound, combined with the properties of the ocean, (temperature and salinity) are used to calculate the velocity of sound through water. The result is a swath “image” of sound velocity, where the width of the swath is the maximum angle the outermost depth soundings have achieved as they travel some distance to the seafloor and back.
For deeper water, the swath will be wider; for shallower water, the swath will narrow. Each of the reflected soundings are then processed to show a continuous colorful ribbon, or snail trail, of the seafloor below the ship. (example snail trail image) The colors are equivalent to the distance the sound wave has travelled through the depth of the water, where the longest distances, which are blue, represent the deepest parts and the red colors are shallower features. Collectively they “paint” a picture of the seafloor’s surface in 3-dimensions.
Most oceanographic research vessels employ the multibeam echo sounder whenever they can, the combined results of which can paint a detailed picture of processes that we could never witness ourselves through the depths of the ocean. Marine geologists, oceanographers, and glaciologists studying Thwaites Glacier will use this information to unravel the processes operating between the ocean and the ice shelves. Features such as furrows and plough marks indicate past movement of the ice shelf and icebergs dragging along the sea floor, channels show where water flowed beneath past ice sheets, and high ridges or peaks may have been places where ice was once “grounded” (or resting on) the sea bed, a situation which may act to stabilize an ice shelf or glacier, and allay or even stop its’ backwards retreat.
The science team also use these echo sounders to make decisions about where to gather samples from the sea floor by producing an image of the layers of muds, sands, and rocks just below the surface. The samples help to decipher a longer history of how ice interacted with the ocean, and ultimately how the ice will behave in the future. The accuracy of the data is critical to ensuring that the location where those samples are taken will not be in a place that might damage the instruments (coring devices) used to collect the samples.
An important part of collecting accurate multibeam data is accounting for a variety of physical parameters. On the ship, we are effectively a bobbing cork in the ocean. Combined with the echo sounder, a set of reference points are made with each “ping” to account for this unpredictable motion. Similar to how a gyroscope works, the reference point data accounts for real time conditions such as roll (side to side) pitch (forward and backward motion), heave (the up and down motion) and yaw (the horizontal or side to side motion). The data processing software then applies clever corrections to make sure that the data accurately represent the true position and orientation of each sea floor sounding.
Two days after the ship’s passage through rough seas, THOR team members Kelly Hogan and Ali Graham noticed that the seafloor bathymetry data output suggested that the seafloor had a tilt when they knew it should have been flat. In order to understand if the tilt was real, they needed to do an experiment. They would have to go back over an area that had already been scanned, but in the opposite direction. The difference in the two data sets would show an equal and opposite tilt (if they were right that the tilt was in error) and it would help them calculate the degree of that error. In order to accomplish this, the seafloor would have to be flat and they would need about an hour’s worth of backtrack scanning of the same flat seafloor.
Enter the icebergs. The first significant iceberg that appeared was close enough to provide a terrific photo opportunity for the passengers, and the crew of Nathaniel B. Palmer made a spontaneous viewing loop around the iceberg. (pic of the first berg) The ship had also entered the Bellingshausen Abyssal Plain in the Southeast Pacific Basin, the flattest area of the transit before reaching the Antarctic continental shelf, so the sea floor conditions were optimum.
Another iceberg on the horizon looked to provide the appropriate distance and time it would take to conduct the necessary scan. Using that iceberg as a goal post, they made the first multibeam swath, rounded the second target iceberg (which was as photogenic as the first), and retraced the initial snail trail long enough to determine the degree of error and apply the correction.
Two icebergs later, the problem was solved and the colorful ribbon of seafloor bathymetry was returned to its horizontal self. These kinds of corrections are not uncommon and ensure that the data is good for when the THOR team reaches Thwaites itself, where the accuracy of the sea floor measurements will be important for selecting the best sea floor to sample and interpreting its history.
Here are some of the other icebergs we have seen on our traverse through the Amundsen Sea. They have been categorized by shape, age, and how they formed.