• Introduction
  
• Methods
  
• Discussion
  
• Acknowledgments
  
• Figure Captions
  
• References

Methods

Click on the figure below to view larger image.

The sidescan-sonar data were acquired using an AMS (Acoustic Marine Systems, Inc.) 120-kHz sidescan-sonar system, owned and maintained by the Woods Hole Oceanographic Institution's Deep Sea Submergence Laboratory durint USGS Cruise F9-89-NC aboard F/V Farnella. Initially, sidescan-sonar data (Figure 4) were collected at a swath width of 500m with 400m trackline spacing, providing 20% overlap. After the collection of several lines of sidescan-sonar data, trackline spacing was increased to 500m, and the remainder of the survey was conducted using a 750m swath width (Karl and others, 1993).

Navigation for the cruise used three systems: (1) Global Positioning System (GPS); (2) LORAN-C; and (3) shore-based, line-of-sight transponder net (Del Norte system) (Karl and others, 1993). For real time positioning, the primary system was chosen either manually by the navigator or selected automatically by computer (Karl and others, 1993). Tracklines (see Figure 4 ) were within 100m of preplotted tracks when using GPS and LORAN-C and within a few meters when navigating under the Del Norte system (Karl and others, 1993). See GIS data also.

The analog signal from the sidescan-sonar tow vehicle was acquired with and stored on a QMIPS data acquisition system, manufactured by Triton Technologies, Inc. The data were then transferred from QMIPS to a Masscomp supermicrocomputer for shipboard processing and data archival (Karl and others, 1992).

Figure 4. Map showing location of sidescan-sonar tracklines. (PDF version, 6.1 mb).

The data were processed on board using a suite of software (XSonar and ShowImage) to remove water column artifacts, correct slant-range distance to true ground distance, and to apply a linear contrast enhancement to maximize the dynamic range of the data (Danforth and others, 1991). Individual sidescan-sonar track lines were then mapped within a Universal Transverse Mercator (UTM) coordinate system, printed with a Raytheon 850 thermal display unit, and pasted to a mylar UTM grid in order to build a hard-copy mosaic while at sea. The hard-copy mosaic was used as a base for further sampling and ground-truth efforts.

In 2003, due to the enhancements in computing technologies and improvements in sidescan-sonar processing software suite, the sidescan-sonar data collected during the 1989 USGS cruise F9-89-NC were revisited in order to create a digital sidescan-sonar mosaic of the Gulf of Farallones sonar data. All raw data were copied from the archival format of 8 mm tapes to a LINUX computing system. The Xsonar and ShowImage software packages were used to process the sonar data as follows: 1) a median filter (2 x 3 boxcar) was applied to the raw data, effectively minimizing speckle and stripping noise present in the data; 2) slant-range distance was converted to true ground distance (using a flat seafloor assumption) and the water column was removed; 3) navigation was edited and re-merged with the sonar files; 4) across-track normalization was applied in order to minimize across-track variation in the data primarily due to distance from the source; and 5) finally, each sonar line was mapped at a 4 meter/pixel resolution within a universal transverse mercator (UTM) coordinate system and saved in raw format (Danforth and others, 1991; Danforth, 1997).

The processed sidescan-sonar data were then imported into Geomatica Software Solutions, PCI software in order to create the digital sonar mosaic. The digital sonar mosaic was mapped at 4 meter/pixel resolution in UTM coordinate system Zone 10N, WGS84 datum. The mosaicking procedure was as follows: 1) ground control points, or common features present in overlapping sonar lines, were chosen to ensure proper geographic positioning of each sonar line; 2) a stencil, or cutline, was drawn around individual sonar lines; 3) the outlined sonar line was then pasted into a ‘master’ mosaic file. This procedure was followed for each sonar line, to build a sidescan-sonar mosaic of the study region (Paskevich, 1992).

After completion of the sidescan-sonar mosaic, the data were exported from PCI in TIFF format with a corresponding ESRI world file in order to correctly display the image within a Geographic Information System (GIS). The image file was imported into Adobe Photoshop 6.0. A linear contrast enhancement was then applied in order to maximize the dynamic range of the sidescan-sonar mosaic.

To provide a background image for the sidescan-sonar mosaic, bathymetric and topographic data were extracted from the National Geophysical Data Center (NGDC) Coastal Relief Model (CRM), Central Pacific Coast, Volume 7 in ESRI grid format. Using the ArcMap Spatial Analysis extension, a hillshade was then created from the ESRI grid file (Azimuth:30; Altitude:45; z-factor:1). After creating a desirable color scheme, the ESRI grid file and hillshade were combined using the ArcView extension grid-to-image converter tool, available on the ESRI Website (www.esri.com), to create a TIFF image file and world file. The sidescan-sonar mosaic and coastal relief image were then imported into ESRI ArcGIS and ArcView GIS software in order to produce a digital, spatially accurate map of the Gulf of the Farallones sidescan-sonar data.

Interactive Visualization Software (IVS) Fledermaus software package was used to display the sidescan-sonar mosaic and coastal relief data within 3-D space. DMagic, a support application to Fledermaus, was used as an interface to prepare the ESRI coastal relief grid for three-dimensional visualization. DMagic was used to create a Digital Terrain Model (DTM) with a desirable color legend, a shaded imagery file and a georeferencing file from the imported ESRI coastal relief grid. These files were then assembled into a Fledermaus object file within DMagic, and imported into the Fledermaus 3-D visualization program. The sidescan-sonar TIFF image was imported into Fledermaus to view the sidescan-sonar mosaic along with a three-dimensional coastal relief background. Using a Polhemus Isotrack II Bat (3-D input device with six degrees of freedom) supplied by IVS, a three-dimensional flight path was recorded within the data space. The flight path was then imported into the IVS Movieclient application in order to produce an MPEG movie file of the 3-D data.