Under Sea Topography
Author: Jeannette Bedard, ocean scientist
“Knowledge of the undersea topography lagged considerably behind our acquaintance with the landscape of the near side of the moon”
From The Sea Around Us by Rachel Carson, in 1951
Putting an eyeball to a telescope's lens reveals intricate features of the lunar landscape facing us even though the moon orbits 400,000 km away (the far side of the moon is another matter). The sea floor is much closer, but looking from a ship's deck won't give you any information about the bottom topography. Today, we can view the deep sea world through bottom-mounted ocean networks, but this wasn't always the case.
Once, the open ocean was considered only as a surface. Depths weren't important as long as fish were caught, cargo transported and ports were safely reached. This attitude resulted in the filling of open sea spaces with whimsical drawings of sea creatures or fancy compass roses on the charts of the day. As our desire to know more about the ocean grew, measuring depth became an important first step.
The simplest way to determine depth is by a weight attached to a line marked at regular intervals. From A Young Sea Officer's Sheet Anchor: A Key to Leading, Rigging and Practical Seamanship by Darcy Lever in 1808: “The hand lead is a plummet of 7, 8, or 9 pounds weight” and “the deep-sea lead is shaped like the hand lead; but is more ponderous, weighing from twenty to thirty pounds.” As the weight travelled to the bottom, a sailor would count the knots and tied-on coloured rags as they passed.
While circumnavigating the globe, the Challenger Expedition (1872–76) took frequent depth sounding with a 91 kg weight attached to a hemp line. A hand-operated winch was used – quite a feat considering the average ocean depth is 4 km. In total, 492 deep sea soundings were made. This was a huge accomplishment, however huge gaps in known ocean-topography remained leaving features like mid-ocean ridges undiscovered until the mid twentieth century.
Accuracy is an issue with using a sounding lead because the assumption that the weight travels straight down isn't necessarily the case. Underwater currents can sweep the weight out at an angle. To get around this problem, all sorts of funky adaptations were tried. The first hint of a solution came from the notebooks of Leonardo da Vinci in 1490 – using sound underwater. It took centuries for this idea to take hold with some false starts along the way.
From 1839-1843, James Ross commanded an expedition to the Antarctic with charting as one of the goals. To get accurate soundings, he received a suggestion to time the interval between tossing an explosive shell overboard, to hearing the explosion as it struck the sea floor. Since the speed of sound in water had already been determined in 1827 and the shell was of a known weight, depth could be calculated. James Ross chose to use the traditional weighted method.
The idea of echo location of underwater features was patented in 1912, ironically a couple of months after the sinking of the Titanic. By 1925 echo sounders were commercially available. Now, echo sounders are accurate to 1 m in 3000 m. If a ship passes above the seafloor, depths can be continuously recorded.
Satellite technology goes one step further in viewing the sea floor from a perch up in space. The force of gravity created by rock masses deep underwater can be seen as changes of sea surface elevation. Mounds on the sea surface form over seamounts and depressions form over trenches – allowing further seafloor mapping.
If you’re interested in how our current oceanographic instruments evolved, including some of the false starts along the way keep your eye on this blog space! COMING SOON: deep sea instrumentation specifically designed to be made out of wood.
Photo credit: Old chart 1571 (found on wikipedia)