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USGS Coastal and Marine Geology Program

Geophysical Data from Offshore of the Chandeleur Islands, Eastern Mississippi Delta

Data-Acquisition and -Processing Overview

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Data Acquisition
and Processing

The following sections provide basic descriptions of shipboard acquisition and post-cruise processing of the geophysical and geospatial data contained in this report. Detailed descriptions of acquisition parameters, post-processing steps, and accuracy assessments for each data type are provided within the metadata files for geospatial data layers in the GIS Data Catalog.

Field Program

Approximately 500 km2 of the inner continental shelf offshore of the Chandeleur Islands were surveyed during two cruises aboard the Louisiana Marine Consortium vessel R/V Acadiana in July 2006 and June 2007 (fig. 2). Interferometric-sonar, sidescan-sonar, and chirp seismic-reflection systems were deployed simultaneously during both cruises. The two surveys span approximately 100 km, extending from 1 to 2 km seaward of the island chain to 5 to 8 km offshore. Positions of the ship and geophysical data were determined using Differential Global Positioning System (DGPS) navigation, with an antenna mounted directly above the interferometric-sonar head on the port side of the vessel. During acquisition, the R/V Acadiana maintained speeds between 1.5 and 2.5 m/s.

Data were collected along tracklines spaced approximately 100 to 150 m apart in the shore-parallel direction, and about 1 km apart in the shore-perpendicular direction. Shore-parallel trackline spacing was chosen to ensure overlap of adjacent sidescan-sonar swaths. The sidescan system produces a large swath width, typically between 150 and 200 m. Shore-perpendicular tracklines were occupied to facilitate internal comparison and correlation of bathymetric and chirp seismic-reflection data with alongshore versus cross-shelf orientations.

During the July 2007 cruise, not all of the three data types were acquired along all of the tracklines. Interferometric-sonar bathymetry was acquired along approximately 2,100 km of trackline, whereas sidescan-sonar and chirp seismic-reflection data were acquired only along 2,000 and 1,655 km, respectively. The data gaps resulted from system malfunctions caused by rough sea state (chirp seismic-reflection data), and subsequent survey-time considerations (sidescan-sonar data were not acquired over several shore-perpendicular lines so vessel speed could be increased slightly).

Interferometric-Sonar Bathymetry

Bathymetric data were acquired using a SEA Ltd., SwathPlus interferometric sonar (234 kHz). The instrument was mounted on a rigid pole, along the port side of the vessel, about 1.5 m below the sea surface. SwathPlus acquisition software was used to fire the system at a 0.25-s ping rate and digitally log the data at a 1.5-K sample rate (Systems Engineering and Assessment, Ltd., 2008). Ship motion (heave, pitch, roll, and yaw) was recorded continuously with a TSS DMS 2-05 Motion Reference Unit in July 2006 (ProQuest Information and Learning Company, 2002), and an Octopus F180 Attitude and Positioning system in June 2007 (Coda Octopus Group, Inc., 2008). Sound velocity profiles (SVPs) were acquired approximately every 6 to 8 hours during each survey using an Applied MicroSystems SV Plus Sound Velocimeter (Applied Microsystems, 2008).

The interferometric-sonar system acquired bathymetric soundings over swath widths ranging from 20 to 115 m, in water depths between 3 and 16 m. Data gaps resulted between adjacent interferometric-sonar swaths because tracklines were planned to optimize sea-floor coverage with the towed-sidescan system. Gap widths ranged from 0 to 200 m, and varied as a function of trackline spacing, water depth, and avoidance of nautical obstructions. Accordingly, swath bathymetric data were acquired over approximately 45 to 50 percent of the entire survey area.

SwathEd (Hughes Clark, 1998) and CARIS (2008), two swath-bathymetry-processing software packages, were used to post-processs the raw bathymetric soundings. Navigation data were inspected and edited, sounding data were rectified for ship motion, and spurious soundings were eliminated. Corrections for sound velocity changes within the water column (SVP data) and tidal offsets (utilizing a NOAA discrete tidal zoning model and tidal observations from two Mississippi tide stations) were also applied to the raw soundings. Final soundings were referenced to mean lower low water (MLLW) because real-time kinematic (RTK) GPS base stations were not operational for the duration of the survey. The difference in North American Vertical Datum of 1988 (NAVD 88) geoid height and MLLW at the Gulf Port Harbor, Miss., and Bay Waveland Yacht Club, Miss, tidal stations is 13.0 and 9.9 cm, respectively, and is within the vertical resolution of the interferometric sonar. Processed soundings from approximately 4,000 km of trackline yielded a final bathymetric surface area of about 470 km2, which was gridded at a resolution of 50 m/pixel.

Sidescan Sonar

Sidescan-sonar (acoustic-backscatter) data were acquired with a Klein 3000 dual-frequency sidescan-sonar system (100 and 500 kHz), which was towed approximately 3 m astern from a starboard-side davit (Klein Associates, Inc., 2008). Klein SonarPro acquisition software was used to log the data digitally at a sample rate resulting in raw pixel resolutions of approximately 0.18 and 0.14 m in the across-track and along-track directions, respectively. Horizontal offset values between the sidescan fish and DGPS antenna were provided to SonarPro, which calculated fish position dynamically during acquisition. The Klein system produced useable data over swath widths between 150 and 200 m.

XSonar/ShowImage software was used to correct for geometric and radiometric distortions in the raw sidescan data (Danforth, 1997), and PCI Geomatica software was used to create georeferenced mosaics of the final, processed data. Gray-scale GeoTIFF images of the mosaics were produced at 1- and 5-m resolutions. Approximately 3,900 km of trackline acquired during the 2006 and 2007 cruises yielded a total mosaic area of about 500 km2. Much of the processed mosaic from the June 2007 cruise is degraded by acoustic interference that was generated by ship noise and rough sea state. The acoustic noise consists of high backscatter that dominates the port-side sonar channel along the entire length of many survey lines.

Chirp Seismic Reflection

Approximately 3,550 km of high-resolution chirp seismic-reflection profiles were collected using an EdgeTech Geo-Star FSSB system and an SB-0512i towfish (0.5-12 kHz) (EdgeTech, 2008). During the July 2006 cruise, Triton SB-Logger™ acquisition software (Triton Imaging®, Inc., 2008) was used to control the Geo-Star topside unit and digitally log trace data in the SEG-Y rev. 1 standard format (Norris and Faichney, 2002). Data were acquired using a 0.25-s shot rate, a 20-ms sampling interval, and a 0.7-to-12 kHz swept frequency. During the June 2007 cruise, EdgeTech J-Star acquisition software was used to control the Geo-Star topside unit and digitally log trace data in the EdgeTech JSF format (EdgeTech, 2008). Data were acquired using a 0.25-s shot rate, a 5-ms pulse length, and a 0.5-to-8 kHz swept frequency.

SIOSEIS (SIOSEIS, 2007), Siesmic Unix (Stockwell and Cohen, 2007) and Seisworks® 2-D (Halliburton, 2008) were used to post-process the raw chirp seismic-reflection data. Navigation data were inspected and edited, static corrections were applied to correct for fish depth beneath the sea surface, seafloor reflections were identified by peak amplitude, and sea-surface heave was removed. Final trace data, plotted as JPEG images, and geo-located trackline files are presented in this report.


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