• Introduction
  
• Methods
  
• Data
  
• Acknowledgements
  
• Figure Captions

Methods

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The seismic-reflection, sidescan-sonar, and bathymetric data were acquired using an Edgetech 424 chirp subbottom profiler, an Edgetech DF-1000 dual-frequency (100/500 kHz) sidescan-sonar, and an SEA Submetrix Series 2000 swath bathymetry sonar (234 kHz) systems, respectively.

Figure 2. Map showing location of seismic tracklines.

Chirp subbottom data (Figure 2) were recorded at a 125 ms sweep rate and a 25K sample rate and logged using Triton-Elics Delph Seismic^TM acquisition software. The chirp data were processed using Colorado School of Mines Seismic UNIX seismic processing software. Processing consisted of converting the Triton-Elics SEG-Y format to Seismic UNIX format, stripping the navigation from the headers, and applying an automatic gain control to the data from 0 - 50 ms and 0 - 80 ms, depending on the amount of penetration. Seismic Unix files were then plotted in postscript and JPEG formats.

Sidescan-sonar data (Figure 3) were acquired at a 0.125 second ping rate, yielding a 400 m swath. These data were logged

Figure 3. Map showing sidescan-sonar tracklines.

digitally at a 2K sample rate using Triton-Elics ISIS^TM acquisition package. The data were then processed using USGS software packages XSonar and ShowImage to correct radiometric and geometric distortions inherent in the sidescan-sonar data. Geomatica ^TM Software Solutions was used to generate a sidescan-sonar mosaic. The mosaic was then used as a base from which sample locations were chosen. Nineteen grab samples were acquired using a Van Veen grab sampler in order to ground-truth the various backscatter regimes present within the sidescan-sonar data (Figure 4).

Bathymetric data were acquired at a 0.133 second ping rate and logged at a 2K sample rate using the SEA RTS2000 acquisition software. A TSS DMS2-05 Motion Reference Unit (MRU) mounted directly above the Submetrix Series 2000 transducers measures all motion (pitch, roll, heave, and yaw) of the vessel.

Figure 4. Map showing location of grab samples and sound velocity profiles.

Precise measurements were taken prior to survey operations to record the offsets between the navigation antenna and the MRU (the MRU is treated as the reference location for all systems). The Wide Area Augmentation System (WAAS) was utilized for navigation, yielding horizontal accuracies of less than 3 meters.

Bathymetric swath width varied as a function of depth, ranging from ~20 times water depth in 10 meter water depth and shallower, to ~4-5 times water depth in 40 meter water depth and deeper (Figure 5). The

Figure 5. Map showing swath bathymetric line coverage.

bathymetric data were processed using the SEA RTS2000 acquisition software. Processing consisted of filtering the raw data to eliminate, or reduce, noise and outliers, while accounting for sensor position and other real world offsets and adjustments. Additionally, eighteen sound velocity profiles were collected in order to model the structure of the water column (i.e. variations in speed of sound throughout the water column) (Figure 5). Corrections were applied to the bathymetric data to account for refraction of the acoustic wavefront due to speed of sound variations. The processed data were then gridded at a 10 meter grid cell size using SEA Submetrix GRID2000 program. The data were exported in ASCII x,y,z format. Interactive Visualization Software (IVS) Fledermaus software package was used to interpolate the data to fill in between-line gaps in bathymetric coverage and to display the bathymetric and sidescan-sonar data in 3-D.

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