Geophysical Mapping of Oyster Habitats in a Shallow Estuary; Apalachicola Bay, Florida
Acoustic backscatter data were collected using two sidescan-sonar systems. A Klein 3000 dual-frequency (100/400 kHz) sidescan sonar was deployed from R/V Rafael, and towed alongside the vessel ~ 1 m below the surface. Track lines were spaced 100-125 m apart, and the sidescan swath was 75-100 m to each side of the track. ASV IRIS collected backscatter data with an Edgetech 4200FS sidescan sonar. The sidescan transducers were mounted on a metal frame between the two pontoons ~ 0.5 m below the sea surface. The sidescan was set to image 50 m to each side of the track lines.
Backscatter intensity, as recorded with sidescan sonar, is an acoustic measure of variations in the physical properties of the sea floor (fig. 5 and Mapsheet 2). Sidescan-sonar imagery was processed such that high backscatter (relatively strong acoustic returns) is represented by white, and low backscatter (relatively weak acoustic returns) is represented by black. In Apalachicola Bay, backscatter variability is generally caused by sea-floor roughness. Due to the low incidence angles associated with towed systems, topographic highs and lows can be interpreted based on acoustic shadows.
Sonar data acquired aboard R/V Rafael used Klein SonarPro acquisition software, and sonar data collected with ASV IRIS used Edgetech's jstar acquisition software. All sidescan-sonar data were processed using Linux-based Xsonar/Showimage (Danforth, 1997) to sub-sample the data to 8-bit imagery, correct slant-range and beam-angle distortions, and remove striped noise. Sonar data from each survey line were mapped in geographic space with Xsonar, using a 1-m pixel resolution, then imported as raw image files to PCI Geomatics GCPworks (PCI Geomatica, version 8.2), where they were combined to create composite mosaics (Paskevich, 1996). The mosaic was exported as a GeoTIFF raster image for further analysis in ArcGIS (ESRI, Inc.).
Both 100 and 400 kHz data were collected with each sidescan-sonar system, but 100 kHz data were used for the final 1-m mosaic covering the main portion of the bay, surveyed by R/V Rafael, because the lower frequency data contained less acoustic noise. The data collected from R/V Rafael is distributed in 3 images, separated by field season and variations in acquisition parameters. ASV IRIS acquired 15 survey days of backscatter data over the shallowest sections of the bay, including known oyster bars (fig. 3). Mosaics of these data are distributed in 13 images, due to substantial geographic separation. The backscatter data collected with the ASV IRIS were processed using the same methods as the data collected from the R/V Rafael, except only the high frequency (400 kHz) data were used and mosaiced to a range of 20-30 meters to remove the noise in the far range of the data.
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Title | Figures | 1-Introduction | 2-Data Collection and Processing | 3- Geologic Interpretation | 4-References | 5-Maps | Spatial Data | Acknowledgments