DATA PROCESSING – SIDE-SCAN SONAR
Side-scan sonar data were imported to CARIS SIPS version 7.1.1 format. In CARIS, navigation and attitude data were examined, spike filtered, and smoothed. Corrected navigation and attitude were then merged and recomputed to the sonar returns. The towfish altitude (height from seafloor) data was roughly calculated by the data processing engine, based on scan contrast in the water column, and in this survey, the altitude varied considerably during the cruise due to the nature of shallow-water surveying operations. Each trackline was opened in the side-scan editor SIPS processing module and manually corrected for towfish altitude, or “first return.”
Once slant range was corrected, selections of homogeneous scans within a trackline were examined, and a beam pattern which represents the inherent intensity trends across the beam was calculated from the sample. The beam pattern was then de-spiked, smoothed, and applied to the data. This process helped to remove inherent instrument variations in beam intensity across the scan. Typically, a single daily beam sample is used to process the data for beam pattern anomalies for the entire dataset. After the beam pattern correction, an angle varying gain (AVG) was applied, which filters trend differences across the grain (with respect to scan angle from the towfish). Data were then de-speckled using a 5x5 surrounding pixel array to apply a mean pixel intensity deviation correction.
After all corrections were made, the tracklines were converted to CARIS Georeferenced Backscatter Rasters (GeoBars). From the series of GeoBars a full mosaic was created to a common intensity histogram and exported in high resolution (0.5 m) GeoTIFF format. The GeoTIFF backscatter layer histogram has values from 0 to 255 in 8-bit digital number (DN). These values represent the acoustic reflectivity of the seafloor. In general, low backscatter values (dark colors) have weak acoustic reflections and are generally finer grained material such as muds and fine sands, while light colors have strong reflections and suggest coarser material on the seafloor.