How can we "see" the seafloor beneath the ocean waves?

How can you see through the ocean?

Imagine that one day your boss asked you a question like ‘how many acres of grass are there in our town?’ There are many ways of approaching this question. You could measure the perimeter of the all the lawns and fields yourself with a tape measure or pedometer or you could look at satellite imagery in Google Earth and use digital tools to estimate the areas of grass.

Now imagine you are tasked with doing a similar estimate underwater – let’s say you need to know how many acres of oyster beds there are. Unfortunately, those same satellite images show only big shimmery blue/green patches anywhere there’s water. This is because the water’s surface reflects light, and any light that is not reflected is absorbed in the top layers of the water column. So how can you make any quantitative measurements of the amount of seafloor covered by oysters?

Questions like this are posed to coastal resource managers all the time, particularly with regard to different types of habitat within a body of water. Luckily there are several tools that are gaining popularity to answers questions like this where imagery will not do.

A shipwreck mapped with multi-beam sonar. Photo source.

A shipwreck mapped with multi-beam sonar. Photo source.

Tool of the trade: SONAR!

Many technologies use sound instead of light to get a better picture of what is on the seafloor. Sound is more effective underwater because it travels further in water than light does. This is why submarines use sonar instead of just turning on headlights and looking out a porthole to see what’s around them.

Sonar is actually an acronym that stands for Sound Navigation and Ranging. The basic operating principle of any sonar system is that it sends a pulse of sound energy into the water and then listens for how long it takes that sound to be reflected back (similar to bat and dolphin echo location). If you know the speed of sound (which can be easily measured), you can convert the time it took for the echo to return into a distance.

Sonar technology has been tweaked for different underwater tasks since its development: submarine detection, depth estimation for nautical charts, fish school detection...the list goes on – underwater acoustics is a huge engineering field! Recently, sonar has been used to estimate what types of habitats exist and where on the seafloor, when combined with highly-accurate GPS systems on research vessels.

Schematic showing how sidescan sonar scans the seafloor and creates imagery. The sonar "looks" out from either side of the instrument, often called a towfish, and this results in a small blind spot directly underneath the sonar. Photo source.

Schematic showing how sidescan sonar scans the seafloor and creates imagery. The sonar "looks" out from either side of the instrument, often called a towfish, and this results in a small blind spot directly underneath the sonar. Photo source.

Three main types of sonar are used for mapping seafloor habitats:

Side-scan sonar:

This type of sonar effectively scans the seafloor and creates high-resolution images of whatever is down there. You can create highly-detailed images or a mosaic of several images over a large area of different habitats and even man-made structures such as shipwrecks. This imagery is created by assigning different shades to each point based on how ‘loudly’ that part of the seafloor reflected the initial pulse of sound. In the example below, the gravel and boulder reflect more sound than the mud and sand on the seafloor, and therefore are represented as a brighter shade.

Single-beam sonar:

This sonar takes a single measurement of the water column and seafloor beneath the boat. One of these measurements is depth, which is an important habitat characteristic for many species. Other characteristics of the received echo, such as the strength of that echo, can also tell scientists about different properties of the water column or seafloor, because different seafloor types or habitats reflect sound slightly differently. Think about hitting a pillow versus a hitting a piece of wood – the wood creates a louder sound mostly due to the fact that it is harder and more rigid than the pillow. Seafloor types react similarly – solid rock will produce a ‘louder’ reflected sound than mud.

A visual history of tools for depth estimation, including single-beam and multi-beam sonar. Photo source.  

A visual history of tools for depth estimation, including single-beam and multi-beam sonar. Photo source.

 

Multi-beam sonar:

Multi-beam sonars send out a fan of sound consisting of many beams of sound below the research vessel. By doing so, you can measure more of the seafloor and water column per ping. This type of sonar is used a lot for depth estimation because it gives highly detailed estimates of depth over wider areas, therefore maximizing the efficiency of these measurements while still providing lots of detail.

Multi-beam sonars also can provide seafloor imagery, in some cases comparable to that of sidescan sonar, with different shades and textures representing different types of seafloor. Another exciting development in multi-beam sonars is that they can also collect imagery within the fan of sound in the water column. This is particularly exciting for resource managers interested in protecting critical habitats because animals like to live in areas with a lot of structure – places to hide from predators, or seek protection from strong currents and waves. Water column imagery can be used to detect structural habitats that protrude off the seafloor like kelp forests, seagrass beds, and coral reefs.

A map of the seafloor made from multi-beam sonar data. Reds are interpreted as shallower depths and blues/purples are deeper depths. You can also see different textures - the "rougher" areas most likely represent bedrock outcroppings, and the "smoother" linear features are likely sand waves. Photo source.

A map of the seafloor made from multi-beam sonar data. Reds are interpreted as shallower depths and blues/purples are deeper depths. You can also see different textures - the "rougher" areas most likely represent bedrock outcroppings, and the "smoother" linear features are likely sand waves. Photo source.

Choosing which type of sonar to use for a particular mapping job depends on the kind of map you are trying to create and your budget. Sidescan sonar can give very high-resolution images of the seafloor, but it does not give you a 3-D model of the seafloor like multi-beam sonar can. Multi-beam sonars can give you data for wide swaths of the seafloor, but multi-beam data requires more accurate positioning and more data processing than both single-beam and sidescan data. There's always the issue of costs, too. Some multi-beam and sidescan sonars can be half a million dollars or more! This is why single-beam sonars are still quite popular despite advances in sonar technology like multi-beam - they are significantly cheaper, easier to install, and require less expensive positioning systems and less data processing.

This sidescan imagery shows seagrass (the fuzzy areas) and sand ripples on the seafloor of Cape Cod Bay. Photo source.

This sidescan imagery shows seagrass (the fuzzy areas) and sand ripples on the seafloor of Cape Cod Bay. Photo source.

When we’re collecting this type of data, we use high-accuracy GPS systems to give an exact position to all the sonar measurements. Underwater cameras and divers are often sent out in a few select locations to check that what the sonar is measuring is in fact the habitat type that we infer to be at that location – this is called ‘ground-truthing’. These types of surveys are done from boats ranging from small launches to big oceangoing ships. Recently, these types of sonar have also been installed on robots as well. In order to systematically collect data over an area, the boat or ship runs in a pattern often called ‘mowing the lawn’: running parallel overlapping lines. So, next time you see a boat running back and forth along the beach like they’ve lost something (maybe their minds), it could be a survey vessel!

More information:

http://video.nationalgeographic.com/video/news/us-ocean-floor-mapping-vin

http://oceanexplorer.noaa.gov/explorations/03fire/background/mapping/mapping.html

http://oceanservice.noaa.gov/education/seafloor-mapping/welcome.html

http://www.noaa.gov/features/monitoring_1008/seafloormapping.html

http://www.personal.psu.edu/users/r/m/rmp5006/p3.html