Water Column Mapping
What is water column mapping? As with most things, it can take on different definitions depending on the user or application. At CCOM/JHC, it is a form of acoustic remote sensing that is used to explore aspects of the marine environment that are found between the ocean surface and the sea floor. This can include gas bubbles, biology, and physical processes within the ocean. When an acoustic wave interacts with one of these water column features, the acoustic field is changed in some observable way. CCOM/JHC scientists use multibeam sonar to make their measurements, and so their observations consist of time series of acoustic backscatter that we can map onto a space representing the ocean. Characteristics of the acoustic backscatter (e.g., how strong, how variable, what the frequency dependence is, where it originated from) are then used to infer properties of the feature causing the backscatter.
A good example of water column mapping using multibeam sonar is shown in the image above. Here, the acoustic backscatter helps describe the spatial distribution of Atlantic herring observed below a research vessel. This data is collected from a Reson SeaBat 7125 multibeam sonar which images a fan-shaped slice of the water column. Several of these images can be combined in order to examine basic school metrics (e.g., volume, area, and other shape parameters) that give the observer information about the fish and/or their behavior. Like most of CCOM/JHC's work related to water column mapping, this type of work is conducted in collaboration with other biologists from NOAA fisheries, private research institutions, and various universities around the world.
Research in water column mapping cuts across several CCOM/JHC research themes. The Center's technological expertise in multibeam sonar has allowed researchers to employ hydrographic multibeam sonars for use in water column mapping, as shown above, but also to utilize fisheries multibeam sonars for seafloor characterization. This is perhaps best illustrated through work with the Simrad ME70 multibeam sonar, where water column modes are used to provide high fidelity information about both the water column and the seafloor.