Sixty (60) kilometers of acoustic bathymetry/substrate data were collected with a dual-frequency (200 and 430 kHz) Biosonics system along 163 transects in Possession Point between 1/30/07 and 2/1/07. Eighteen(18) kilometers of acoustic bathymetry/substrate data were collected with the same system along 65 transects in Browns Bay on 2/2/2007. The Biosonics sonar generates a 6-degree cone of sound, which translates into a footprint on the seafloor ranging 0.55 to 55 m^2 area for the water depths surveyed (2 - 20 m). Ship speed generally ranged 4.0-4.5 kts and data were merged with GPS positions at 1 Hz. Track lines were spaced 25 - 50 m apart.
B) Sediment Samples for Grain Size Analyses
Sediment samples were collected using a van veen grab sampler or taken directly during low-tide surveys.
In the field, a sub-sample of the uppermost 1-3 cm of each grab was collected and placed into storage bags, labeled, and logged.
Samples were disaggregated and sieved through 2000 and 62 micron screens to separate gravel, sands and mud after the organic component was removed by treatment with hydrogen peroxide. Sediment grain size of sands was determined using the settling methods and principles of Stokes Law on the USGS 3-m long settling tubes. The fine fraction (silts and clays) were analyzed on a Coulter 230 Laser Particle Analyzer. Three samples had less than 3% intermediate fraction and were analyzed on the Coulter. Single sample runs were made on the tubes, while results from the laser particle analyzer are averages of triplicates with standard deviations around the mean ranging 18 to 24 microns due to instrument errors.
The combined bathymetric data were merged to create a digital elevation model representing the surface topography of Possession Point and Browns Bay. This surface is referenced to the NAD83 datum in the WA State Plane North, meters projection with a horizontal accuracy of 2.3 cm. Elevations are referenced to NAVD88 with an estimated vertical accuracy ranging 2.6-9.8 cm. This includes error from the RTK-DGPS and Biosonics sonar, and errors introduced in data processing.
Results from the tubes and laser particle analyzer were merged using standard USGS methods found in the USGS particle analysis program pcSedSize.
Sample of digital surface elevation data XYZ ASCII file, bathy/line95.xyz: X Y Z 487989.879521 5381998.198351 -3.31 487990.116289 5381998.892258 -3.34 487989.943243 5382001.029467 -3.40 Sample of sediment sampling log: Station Local_Time UTC Lon Lat Depth_m Comment 1 6/1/07 20:44 6/2/07 3:44 -123.1430877 48.60642247 2.286 - 2 6/1/07 20:35 6/2/07 3:35 -123.1416559 48.60596625 2.1336 18.6 C 3 6/1/07 20:28 6/2/07 3:28 -123.1396819 48.60476647 2.4384 22.4 C
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