Most of the planet's surface, approximately 70%, is covered with water. Most of the ocean floor is unseen, unmapped, unknown. While humans have mapped 100% of the surfaces of the moon and Mars, only 15% of the ocean floor has now been mapped within a 100 meter resolution. Dr Ferrini displayed a map of the North Atlantic Ocean floor, and noted that little is known, most of our knowledge being limited to the continental shelf.
Dr Ferrini's specialty is bathymetry, the mapping of the depths. She noted that the shape of the seafloor in a very localized area can be observed at the beach. For mapping the seafloor in deeper waters, sound is used. She gave us an overview of mapping conventions, pointing out the rainbow colors of the maps, with warm colors indicating shallows and cooler colors indicating depths:
She then displayed a picture of the open ocean and joked that, on a good day, the view from her office is good, then contrasted that with a picture of a tempest-tossed sea and quipped that a bad day at work is dangerous. On a really good day, she is able to explore the depths of the ocean in the Alvin submersible, which can accommodate one pilot and 2 scientists- the Alvin is a national research asset, the use of which is open to any university faculty who gain approval. Dr Ferrini travels around the world observing the different processes that shape the seafloor, using a sound-based data acquisition system. Building a global ocean map can be likened to solving a puzzle, a coherent image of the sea floor must be pieced together. Different instruments have different resolutions, resulting in different errors- most of what we know about the ocean depths is estimated- much of bathymetry is predictive. Satellites can measure perturbations and produce a global image, but one with poor spatial resolution and low vertical accuracy. Dr Ferrini contrasted a Google maps image of Manhattan around the UN with an image of the McDonnell Seamount near Wake Atoll. In terms of resolution, satellite cannot compete with sonar. Dr Ferrini displayed a video of Alvin exploring the seafloor, and indicated that it is very expensive to map the depths.
Why do we map the oceans? Nautical charts are important for safe navigation. Maps are crucial for oil and gas exploration. Communications cables are often laid on the seafloor. Maps can also help researchers determine the progress of climate change. In one dramatic case, the search for the wreckage of the lost Malaysian Airlines Flight 370 jetliner was hampered by poor maps- one can't zoom in on an area for which no data have been compiled.
Four hundred and thirty thousand square kilometers of the ocean floor have been mapped, and the data is publicly available. Most bathymetry takes the form of ship-based mapping. Historically, this was done with a deadline, a weight on a chain which was lowered into the water, a technique which Dr Ferrini humorously characterized as 'low res/little detail'. In the 1950s and 60s, the use of sound to map the depths came into use- typically, one sonar device pointed straight down would measure directly underneath a boat to make a profile. One of the pioneers of sonar-based bathymetry was Marie Tharp, who was not allowed to go to sea until 1968, but who compiled sonar images into a map of the ocean floor in collaboration with Bruce Heezen. Their map of the mid-Atlantic ridge is remarkably accurate:
Marie Tharp's work was instrumental in the verification of the theory of continental drift. In a retrospective, she described her career:
The whole world was spread out before me (or at least, the 70 percent of it covered by oceans).I had a blank canvas to fill with extraordinary possibilities, a fascinating jigsaw puzzle to piece together: mapping the world’s vast hidden seafloor. It was a once-in-a-lifetime—a once-in-the-history-of-the-world—opportunity for anyone, but especially for a woman in the 1940s. The nature of the times, the state of the science, and events large and small, logical and illogical, combined to make it all happen.
Modern seafloor mapping uses a multibeam sonar array, a 'fan' of sound beams which create many data points when they are picked up by transducers on the bottom of the boat's hull. Dr Ferrini compared this multibeam mapping to 'mowing the lawn', complete coverage of the boat's vicinity is obtained. The multibeam array not only measures the depth of the ocean floor, but the intensity of the returning sound beams allows researchers to determine if the ocean floor is rocky or muddy. Also, anomalies can indicate the bubbles produced by methane seeps.
Continuous surface detail is obtained with these sonar arrays. Sound velocity depends of water temperature, so the data has to be corrected for. Because the ships are moving, the data has to be corrected (Dr Ferrini joked that she gets motion sickness). Many pieces of data are brought together and cleaned up. It's a slow process, a single ship would take one hundred thousand years to finish mapping the depths. One way to speed up the mapping process would be autonomous mapping platforms, a combination of surface and underwater robots... one of her colleagues developed an unmanned surface vessel which is a 'mothership' for underwater drones.
Dr Ferrini then shifted to the topic of the interdisciplinary approach that will take researchers beyond bathymetry... ship-based mapping is the beginning, and the maps can guide other research. Near-bottom mapping has been done with resolutions up to twenty-five centimeters. Dr Ferrini displayed pictures of spiky hydrothermal vents, which form the mineral-rich base of the deep-sea food web. She posed the question, what would it be like to stand on the sea floor and look around? A sense of scale has to be developed. There are emerging visualization techniques which assist researchers- Dr Ferrini displayed an image of the Hudson Canyon and delivered a groan-inducing pun: "How can we fathom this?" Once a base map is built, the Alvin can be taken down for a new perspective on the data. Mosaics of the photos can be color-corrected and compiled by artists into clean images of underwater features.
Dr Ferrini then told us about Earth's newest land, Hunga Tonga, born of a 2014 volcanic eruption. NASA satellite data measured the processes of erosion and redeposition which shaped the island (this data could be used as an analog for modelling how Martian land masses were formed). Dr Ferrini and her crew broke off from some planned work due to bad weather and pulled up to the island in order to conduct a sonar sweep from the depths to the shallows in order to fill in some blanks for NASA. A flat subaqueous portion helped to keep the landmass intact.
No single country can assemble the bathymetric puzzle alone. NOAA is dependent on a global community, a community not only made up of government and academic personnel, but of industry and the general public. Anyone with a sonar unit can contribute data. The United States National Archives make data available to all (Dr Ferrini did note, though, that certain information, such as the location of shipwrecks, has not been revealed in order to protect future archaeological sites). Intergovernmental organizations such as the International Hydrographic Organization also make data available. The General Bathymetric Chart of the Oceans (GEBCO) 2030 project aims to create a complete high-resolution map of the seafloor by (you guessed it) 2030 in collaboration with the Nippon Foundation.
Dr Ferrini stressed that the oceans are critical to human survival- 50% of atmospheric oxygen is produced by oceanic life. Ocean maps are critical to understanding the oceans. Technical innovations, such as improved sensors, improved data processing, and improved data sharing are needed. The world's oceans belong to all.
The lecture was followed with a Q&A session. The first question involved the effects of sonar on whales- the high frequency sound beams used for bathymetry scatter quickly, and pose little danger to cetaceans. Some anthropocene-obsessed Bastard in the audience asked her if she had observed any negative effects of human activity on seafloor communities, and Dr Ferrini answered that she hadn't yet, but that regulations to protect these habitats might be needed in the future. Manganese nodule mining might pose the biggest danger to seafloor biomes. Regarding changes to the seafloor- most changes occur in shallow areas, necessitating continuous surveying, but the depths are not as stagnant as originally thought. Concerning the location of earthquakes, sonar is insufficient, other resources are needed to pinpoint these occurrences. A question about the Alvin resulted in a funny aside- there is no privacy in the submersible, which operates from 8AM-5PM, and there is a sign advising users: PB4UGO. Dr Ferrini joked that occupants tend to bang their knees in the cramped interior, but everybody is too excited to notice. Regarding satellite data, satellites use optical techniques such as LIDAR, which are thwarted by cloudy or deep waters. A question regarding the extinction of oceanic life had Dr Ferrini stating that she is an ocean optimist- the ocean is the world's greatest museum, things die and become life for other things, and that humanity will be extinct before the oceans are depleted of life. In a hilarious tangent elicited by a question regarding the cost of sonar equipment, Dr Ferrini narrated a tale of shopping for a Boston Whaler with her wife and having the salesman giving her a lecture explaining how the sonar system worked.
Once again, the Secret Science Club delivered a fantastic lecture. Kudos to Dr Ferrini for hitting that 'secret science sweet spot', that combination of raw-information, explanation of research methods, and adventure narrative, leavened with good humor. Kudos also to Dorian and Margaret, and the staff of the beautiful Bell House on a job well done. As an added bonus, SSC co-founder Michael Crewdson was back in the ancestral homeland for his traditional Australian Winter break, accompanied by his oldest child. Welcome back!
For a quick taste of the Secret Science Club experience, here is a lecture by the good doctor about making bathymetric data available to the public.
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