That’s a pretty intriguing image isn’t it. What you can see is a map of the seafloor of the Piscataqua River inlet, which lies on the New Hampshire/Maine border in America. Mapping the sea floor has a whole host of benefits. It can tell us not only the topography of the sea floor, but what it is composed of. For science and conservation, mapping can help us understand things like water movement, or figure out where different habitats are – important information if we want to make sure we use our marine environment with minimal impact. We can even see items of historical and archaeological interest – like wrecks. For industry, mapping is useful for aggregate extraction and fisheries. Knowing the layout of the sea floor is also important for things like laying undersea cables for telephones or power. There is a safety aspect too. Accurate mapping of underwater hazards can help reduce the risk of collision.
There are a whole host of different technologies that are now used to map the sea floor. Here is a quick overview of just one of them. Airborne laser scanning ALS – essentially mounting LiDAR (Light Detection and Ranging) equipment onto an aeroplane with a GPS and flying it over the area you want to map - has been used for some time to map the terrestrial environment, but it has also – with some slight modifications – proved useful for mapping the seabed.
Unlike for the land, mapping the seabed requires two laser – an infrared one which reflects back from the surface from the water, and a green one which can penetrate through the water and reflects back from the seabed. Calculating the difference in time it takes for these two lasers to reflect back to the sensors reveals how far down the seabed at any given point is. What type of surface (say hard rocks, soft sediment, or even a kelp forest) the green laser reflects off can affect the intensity of the signal. LiDAR operators are able to use these differences as well as the depth information to work out what sort of habitat is under the waves.
Sticking this sort of system on an aeroplane as some huge advantages. You can cover huge areas in a relatively short period of time, the data it produces is seamless, and crucially it can map areas that may be too dangerous or difficult for a boat-based system to access. This sort of system does have its limitations though – how far down the green laser can go. LiDAR is only useful for shallow waters - if conditions are excellent down to around 50 meters depth. If the waters are unclear – there is a lot of sediment, rough sea state, or even a substantial amount of plankton then the lasers ability to penetrate the water is reduced. For this reason, LiDAR manufacturers will often relate the depth their equipment can reach to the amount of turbidity in an area - something called a Secchi depth... but more on that in a later post.
LiDAR has been used to improve our understanding of the marine environment and the critters that like to live there. Here are just three open access pieces of science that have used this technology:
Mapping Coral Reefs
Coral reefs are highly complex structures found in shallow waters – perfect candidates for LiDAR mapping. David Zawada and John Brock from the US Geological Survey used LiDAR to map an area of reef along the northern Florida Keys. They managed to get a really good level of detail enabling them to make a number of suppositions about how different parts of the reef have formed – and continue to change.
Research into undersea waves
James Churnside at NOAA used LiDAR technology to measure undersea waves (yes – under the sea!) in the West Sound, Orcas Island in Washington. James has done a fantastic short (4 ½ minute) film explaining his work . How much fish in the Bight? Another one from James Churnside. This time, LiDAR was used to work out the density of sardines in the Southern California Bight. How? Well the sardines are reflective too.
Image: Created by Larry Mayer of the University of New Hampshire. Acquired from the WHOI + Oceaus magazine (http://www.whoi.edu/oceanus/)