Building a map of the ocean floor
Story and Photograph by Hudson Lofchie
Published in The California Aggie (theaggie.org) on March 30, 2011
When Homo sapiens first gazed to the stars, we began to study, learn and catalogue the universe visible from Earth. That drive to investigate the cosmos left a gap in what we know of our own planet. We have mapped a higher percentage of the moon’s surface than Earth’s. Only seven percent of the ocean floor has been mapped to any useful degree.
“In the Southern Hemisphere, there are gaps about the size of New Jersey where no ship has ever gone before,” said David Sandwell, a professor of geophysics at Scripps Institution of Oceanography at UC San Diego. “Within those areas, there could be all kinds of things.”
Sandwell is part of a team from UC San Diego setting out to fill in the maps of our ocean floors.
Ships equipped with complex sonar systems must carry out the precise mapping. One such vessel is the R/V Melville, operated out of Scripps. The Melville is currently mapping a chain of massive, uncharted under-sea mountains in remote areas of the South Atlantic Ocean.
To map a section of sea floor, boats are equipped with a sonar tool called a Multibeam Echosounder, also known as a Swath Echosounder, which is used for swath bathymetry mapping. Bathymetry is the term for topography that is underwater.
A Swath Echosounder uses around 45 sonar pulses (varies with individual system) to get an accurate map of the ocean floor. Whereas normal sonar is used just to tell if an object is present, this method can use the information from multiple sonars to build a picture of what is beneath.
“On the hull of a ship you have mounted an array of these sonar emitters that emit simultaneously,” said Magali Billen, an associate professor of geophysics here at UC Davis. “Then you have sonar receivers in an array that wait for the sonar beams to go down to the bottom of the ocean, reflect off the surface and then bounce back again.”
When the receivers pick up the sonar beams, a computer program analyzes them and constructs an image based on the return time of the sonar waves, the pitch of the sound waves and other minute differences. Through this process, the researchers have discovered seamounts almost 15,000 feet high.
Most of the ocean floor is mapped at a resolution of 10 km. That means that we can differentiate among objects that are over 10 km across. With a precise system like a Swath Echosounder, scientists can achieve resolutions of about 1 km. Some ships tow tiny submarines that float just above the sea floor. These submarines can map resolutions of about one meter.
The Swath Echosounder maps the ocean floor in a strip that is twice the current depth; if the boat were in 5,000 meters of water, the system would map a strip 10,000 meters wide.
“Mapping the ocean floor is like mowing a lawn,” Sandwell said.
For these kinds of missions, the crew will spend an average of two weeks mapping, and then head back to port.
But creating detailed maps of the ocean floor is not as easy as just using a sonar tool. The greatest barrier to missions such as these is the high cost. Running and operating a ship like the Melville costs about $40,000 a day, or 1.2 million dollars for a 30-day mission, so getting the funding is often difficult.
A few methods have been proposed to increase the efficiency of these mapping missions. Researchers could attach sonar pings to drifting buoys, or even use an automated, solar-powered boat. Each of these methods would operate independently, but if maintenance were ever needed, a team would still need to make the long, expensive trip by ship.
The ocean covers 70 percent of our planet, yet we know so little about it. The ocean contains nearly 50 percent of earth’s species. Once we expand our view of the ocean floor, who knows what else we will find.
It would take hundreds of years of ship time to map the entire ocean floor. However, we already know where the most prominent features of the sea floor are based on satellite gravity. Satellites can detect minor differences in Earth’s gravitational field. For example, Earth’s normal gravitational force is 9.8m/s2, but when the satellite passes over a large, under-sea mountain, gravity is 9.80007m/s2 – a difference imperceptible to us, but detectable by the satellites. By knowing where the major features are, we can target where we send our resources for detailed mapping.