Painting a Portrait of the Seafloor
This is Will Sautter again. We have been steadily painting the portrait of the Great Reserve boundary with beautiful multibeam and backscatter imagery. Every day, we are discovering new features and unique patterns of sediments that have been shifting with the swift currents flowing off the deep escarpments from the north. In my last post I gave a general summary of multibeam sonars and how they work. Then, Dr. Chris Taylor told us about his fisheries acoustics work where he uses a single-beam sonar to detect plankton and fish. Now I am going to tell you about one of the most important data products that we generate from multibeam sonars. It’s called backscatter and we will use it to classify the different habitats of the Grand Reserve of Puerto Rico.
All about Backscatter
Backscatter imagery is basically the reflective intensity of the sea floor, or the intensity of the sound waves from sonar pings that return to the ship. It is kind of like trying to play tennis on different courts. When you play on a concrete court, the ball bounces harder on the ground as opposed to playing on a clay or grass court which would make the ball bounce softer. The tennis ball would, in this case, be like a ping of sound from a sonar transducer: the harder the sea floor is (such as a muddy bottom to a coral reef), the more intense the acoustic return will be. Also, backscatter can give us a scale of how fine or course the bottom sediments are. When a sound wave hits bottom, it scatters in every direction including directly back to the transducer where it came from. Course sediments or features on the sea floor will scatter the pings more intensely than a flat muddy bottom. The imagery shows up as a black and white model of different habitats on the seafloor. The brighter the values, the harder the bottom and the more intense the acoustic energy returns. The darker values indicate less intense acoustic returns, meaning that the bottom is softer.
We can then analyze the imagery and actually tell if the bottom is sandy or hard reef. We can even see sand ripples, scattered rock fragments and geological features like faults and bedding planes. Information from backscatter can make a seemingly flat and homogenous sea bottom reveal finite changes in sediments which can tell marine ecologists and geologists more about the morphology of the seafloor. It is also of great use to environmental scientists to see how man-made impacts such as sediment runoff or harbor dredging can affect the ecosystem over time.
Combining Backscatter with Other Data Completes the Portrait
We use the backscatter in conjunction with several different bathymetrical products such as depth, slope, and rugosity (the ratio of area to surface area) to classify the seafloor into a habitat map. Using imagery enhancement software and GIS (geographical information systems), we can take a mosaic of our sonar data and generate a map that shows the different bottom types that can be classified by the different features like grain size, coral or algae presence, or by its ecological zone. The final product can tell us where the reefs or sea grass beds are, how deep they are, and how much area they cover. This map can also be combined with Dr. Taylor’s fish acoustics and be used to show how abundant these habitats are with fish or where there might be potential spawning grounds.
Once we have a complete map and all of the data is processed, the Biogeography Branch at the National Coastal Center for Ocean Science will provide all our maps to the public for free. We even have a virtual mapping tool called BIOMapper that will allow any user to view all of the data that we are providing on our website. Thanks for checking out the NOAA Ocean Science Blog, and keep posted for more of our daily life as ocean explorers and discoveries we encounter along the way as we map the Grand Reserve of Puerto Rico.
To see the the Nancy Foster throughout the 2012 mapping mission, visit the NOAA ship tracker site and click on “Enter NOAA’s Ship Tracker link, then scroll down to “NF – Nancy Foster” in the box on the upper right of the screen to see where she is at any given time!
Be sure to visit this blog often for field updates, pictures and videos posted by members of the science team.