Imagine riding your bicycle or driving your car through a fog bank, in the dark, without headlights—not knowing if there was a tree, boulder or cliff in your path, or if you were passing through a desert or a mountain canyon. Under such conditions, your journey would be neither safe nor enjoyable. You certainly wouldn’t learn much about your surroundings.

For much of history, this has been the experience for underwater explorers. Divers wearing masks and helmets have long been able to see beneath the waves, but their view of the ocean realm has always been limited by physics and water clarity.

“Water filters light, so the deeper you go, the less light there is,” says diving expert and author Karen Berger in her book Scuba Diving. “The amount of light that actually penetrates to a certain depth depends on the clarity of the water. In murky water with lots of suspended particles such as silt and organic matter, a depth of only 10 feet can be dark and gloomy.”

Mapping the ocean bottom poses its own challenges. Because water bends light, it is virtually impossible to produce an accurate map of the seafloor by looking down on it from the surface. Early seafloor maps were based on depth measurements, or “soundings,” made with weighted lines that were dropped from the surface, the length of a taut line indicating the distance to the bottom. Seafloor mapping was, therefore, a slow, laborious process.

It wasn’t until sonar (short for SOund, NAvigation and Ranging) was invented in the early 1900s that humans were able to “see” through the dark, murky waters of the deep sea and accurately map the ocean floor. Until then, mariners risked striking or colliding with an unseen submerged object, and fishermen inevitably snagged their nets on rocks, reefs and wrecks.

Sonar devices measure the distance to a given object by bouncing sound waves off the object and measuring the amount of time it takes for the reflected waves to return to the device. Because sonar involves sound instead of light, water clarity is not an issue. Sound waves can penetrate even the murkiest water.

In addition to measuring distances, sonar can be used to calculate a moving object’s speed or determine the shape of a structure on the seafloor, whether natural or man-made. This makes sonar an invaluable tool for ocean explorers.  NOAA researchers routinely use sonar and other sensing systems to get a better picture of the seafloor and submerged resources within national marine sanctuaries.

“We use sensing technologies to look at everything from seamounts to shipwrecks,” said Dr. Steve Gittings, science coordinator for the National Marine Sanctuary Program. “Sidescan and multibeam sonar systems are particularly useful to our research efforts.”

Sidescan sonar uses sound to produce what looks something like a black and white photo of the ocean bottom. Resembling small torpedoes, sidescan sonar devices, or “towfish,” are typically towed on a cable behind a research vessel over an area of interest, usually in a “mowing the lawn” pattern.

As the towfish passes over the seafloor, it transmits data to a computer on board the survey ship. The data is then transformed into an image, which can be seen in “real time” or later put together electronically to generate the overall picture of the area. The images show reefs, rocks, sand waves and other bottom features in remarkable detail.

“Sonar is now so sensitive that we can identify the type and age of a sunken ship,” said Bruce Terrell, senior maritime archaeologist with the sanctuary program.

Multibeam systems also use sound, but do so in a way that gives precise measurements of depth as well as bottom features.

“Multibeam sonar give us millions of soundings from which we can construct excellent maps of seafloor depths, and depict physical features that would never have been shown with older, traditional survey methods such as lead line casts and single beam sonar,” said Sanctuary Program Geographer Christine Taylor.

Multibeam can also generate what are called “backscatter” images, which indicate how hard the bottom is and what the composition is likely to be, whether it’s silt, mud, sand or rock.

“We use bottom imaging to investigate submerged hazards, find out where contaminants are likely to accumulate, determine how sediments move during storms, study how reefs may have grown and died, and see what fish and invertebrates are using as habitat,” said Gittings.

To complete the picture of the undersea environment, sonar systems are often used in conjunction with other devices, such as conductivity and temperature sensors, metal-detecting magnetometers, and remotely operated vehicles, or ROVs.

Usually equipped with lights and video cameras, ROVs give researchers the ability to “fly” down to and around a sunken object and investigate it visually, all from the relative safety of a research vessel. Marine archaeologists with Stellwagen Bank National Marine Sanctuary recently used an ROV to examine the wreck of the ill-fated 19th century steamship Portland, known today as New England’s Titanic. But first, they used sidescan sonar to locate the wreck and identify debris that could potentially ensnare or damage the ship-tethered ROV.

“Together, sensing technologies enable us to explore areas that we just couldn’t get to in person,” said Gittings. “Sidescan, multibeam and ROVs are to us what the Hubble Space Telescope and the Mars Rover are to space scientists,” said Gittings. “They are windows onto worlds that, until recently, we have only dreamed about.”

For more see:
Sound Science: Undersea Exploration Tools