Sonar, sonar, and more sonar: Multibeam and CHIRP

In addition to the buoy deployment and the ground-validation drop camera operations occurring during this cruise, myself and the rest of the science team were busy collecting bathymetry and sub-bottom profiles with two different sonar systems. The first, multibeam, was constantly collecting bathymetry data for the two offshore regions that were the focus of the ground-validation efforts. The scientist team processed the multibeam data after the ship completed individual transit lines across the area of interest.

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Using the CARIS software package, scientists ‘clean’ the individual lines of multibeam data one section at a time. The top screengrab is an uncleaned section of one line, with the entire line pictured on the left portion of the screengrab. The preliminary processing of the lines focuses on the removal of definite errors, like the parabola shape in the center of the screen. The bottom screengrab focuses in on the center of the line section and shows a cleaned version of multibeam data.  Further processing occurs in the the Fletermaus program suite to prepare the bathymetry attributed grid (BAG) file that facilitates transfer of the bathymetry data. Photo credits: Erika Koontz

The errors in depth in multibeam data vary in causation, but can be commonly caused by fish schools swimming through the acoustic pings produced by the ship’s transducer or bubbles produced in the water column from rough seas. The ground-validation work that the Beaufort NOAA lab coordinated directly depended on the collection and processing of the multibeam data because the day to day drop camera site locations were chosen based upon the multibeam collected during the night. Up to fifty sites were chosen from the processed multibeam files, and the small boats together collected drop camera video from over one hundred and sixty sites between the two regions of interest.

Following the multibeam operations was the Compressed High-intensity Radar Pulse technology (CHIRP), a sub-bottom profiling system that is towed behind or alongside the ship. This particular CHIRP system has the capabilities of penetrating up to 200 meters (EdgeTech) below the seafloor to see past geologic formations such as paleochannels, which are ancient river outflows. The sub-bottom profiles collected are being used by Coastal Carolina University to better understand past and present oceanographic conditions to contribute to their ultimate goal of locating offshore regions that could potentially support wind energy development (Coastal Carolina University 2015). 



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The CHIRP is gently lowered to the water by one of the ship’s winch frames (left), where it is sunk about three meters below the surface (right). Photo credits: Erika Koontz

Occasionally, the acoustic signals emitted by CHIRP and the multibeam attract marine wildlife. One evening after the CHIRP was put into the water, a pod of twelve dolphins were swimming alongside the ship and around the CHIRP in an effort to see what it was and if it would also ‘leap’ out of the ocean with them. Preliminary studies seem to indicate that the sound produced by the CHIRP does not negatively affect the wildlife. As for the dolphins, only their curiosity for the CHIRP has been affected.

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Photo Credit: Erika Koontz

References:

Coastal Carolina University. Coastal and Marine System Science. Coastal Carolina Scientists Work on Wind Energy Studies. Coastal News. Coastal Carolina University, 21 July 2015. Web. 26 July 2015. <http://www.coastal.edu/newsarticles/story.php?id=4048&gt;.

“Sub Bottom Profiling.” EdgeTech. EdgeTech, n.d. Web. 26 July 2015. <http://www.edgetech.com/products/sub-bottom-profiling/3200-high-penetration-sub-bottom-profiler/#productTab2&gt;.

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