look through the sea
Sunlight entering the ocean only travels about 200 meters below the surface and then almost nothing, leaving the deepest part of our oceans in perpetual darkness. While these vast areas of our planet may be invisible, at R/V Launces, since the multibeam echo sounders illuminate the ocean floor, they can neither see nor forget.This is one of many other geophysical datasets we have collected at sea that complement the Controlled Source Seismic Section ‘s expedition.
A multibeam echo sounder is a highly sensitive instrument that is mounted on a ship’s hull and emits complex sound waves that travel through the water column, reflect off the seafloor, and are then recorded on board – like when you yell “echo” in a large In the canyon, listen to your own answer. The time it takes for a sound wave to travel through the water column is used to determine the depth of the ocean floor. However, we need to limit the speed at which sound travels through the water column to more accurately determine the depth of the ocean floor, and the speed at which sound travels in water is affected by temperature. On this expedition, we’ve been using a disposable deep-sea thermometer (“XBT” for short) to measure the temperature of the water column to precisely limit the depth of the ocean floor.
Inside look of a disposable deep-sea thermometer, showing the small probe that helps us extract the speed of sound in the ocean to better limit the water depth below the R/V Launces. Photo by Brian Boston
A simple sonic wave would only get us a little under the boat, barely cover any seafloor, and would take much longer to get the same type of dataset we have now. Instead, R/V Launces Multibeam echo sounders emit fan-shaped sound waves at the bottom of the ship and can extract directional information on the returning sound waves to produce a depth range with hundreds of data points per sound wave. Since the take-off angle of the fan is fixed on the boat, the water depth mainly determines how much of the ocean floor we can see in space. In shallower waters, the sound waves don’t have the chance to spread out as far, resulting in a narrower frequency band of data points. This is easy to see below, we have plotted some data across the trench, from about 100 meters deep near shore to over 5,000 meters at the trench.
A day’s work on Langseth’s multibeam echo sounder. A color-coded map of seafloor depths exposes much of the trench, with areas that narrow rapidly as we enter shallower waters. Scientists aboard the Langseth have been looking at every ping of the multibeam data to ensure the data is of high quality and remove any unwanted noise.Photo by Brian Boston
We use this data to not only find the location of the seafloor, but also to help us understand the local geological processes taking place in the area. With this depth data, we can discover canyons that cut through steep forearcs, narrow trench axes, deep-sea hill structures, and seamounts spread across the ocean floor. To better show how incredible some of these seafloor features are, we can take a single profile from the trench to the shore and compare it to land features such as the Andes:
This is the contour extracted from the bathymetry in the previous image. It compares the trench offshore Mexico (blue line) with the elevation of the Andes (red line) and the Aconcagua Mountains, the highest point across the Americas, at the same relative starting point (grey line). Not only does our offshore area nearly rival one of the largest mountains on Earth, it reaches those heights at nearly half the distance.Photo by Brian Boston
Brian Boston is an associate researcher at Columbia University’s Lamont-Doherty Earth Observatory.
