Gulf of the Farallones Regional Bathymetry and Sidescan-Sonar Digital Mosaics

The availability and use of remotely sensed digital image data have increased rapidly over the last two decades. One application where remotely sensed image data are becoming a critical component is that of environmental analysis and monitoring. As an example, USGS partnered with the Gulf of the Farallones National Marine Sanctuary to use remotely sensed sidescan-sonar images to detect and map the location of 55-gallon drums that were dumped near and around the Farallon Islands from 1946 until 1970. Approximately 48,000 drums containing low-level radioactive waste were dumped in water depths ranging from about 100 to 2,500 meters covering an area of at least 1,400 sq. km (Karl et al., 1994). Because of the high number of drums littering the seafloor and the large disposal area involved, remotely sensed image data are one of the few promising tools for detecting and mapping barrel locations. One of the newer digital imaging systems is sidescan-sonar; it is an active system using acoustical waves to produce an image called a sonograph. The sonograph is a measure of backscatter reflectance properties of the seafloor and has many similarities to a side-looking airborne radar (SLAR) image. Digital sidescan-sonar images collected by a high-resolution (one- to two-meter pixels) system were used as input for barrel detection analysis (Chavez and Karl, 1995), as well as for mapping the regional backscatter characteristics of the area. The sidescan-sonar digital images were processed using the geometric and radiometric correction procedures, as well as the digital mosaicking package, developed by the USGS (Chavez, 1986). These data were used as input to spatial variability analysis, similar to those used on some lower resolution sidescan-sonar data (Chavez and Gardner, 1994).

The image products shown below were generated for our investigations using both bathymetry and sidescan-sonar image data:

1. Bathymetry Digital Mosaic -
   
   (Left: Bathymetry Mosaic, 230 kilobytes. Right: Shaded Relief Mosaic, 190 kilobytes.) This mosaic was generated using Seabeam bathymetric data collected by the National Oceanic and Atmospheric Administration (NOAA) with an original spatial resolution of 250 meters. The brightness values in the image represent depth -- darker implies shallower and brighter implies deeper. The depth ranges from about 50 meters on the shelf to greater than 3,000 meters off the shelf. The bathymetry image can be used to generate several different image products that are useful for structural enhancement and mapping. One of these products is a shaded relief image that is generated by 'placing' the sun at any angle and direction, then enhancing the topographic/structural information. Notice that the shaded relief image generated from the bathymetry data shows the regional structural patterns; the large seamounts just to the southwest of the image center and Pioneer Canyon in the lower portion of the image can be easily seen.
   
   
   
   
   
3. Sidescan-Sonar Digital Mosaic (Non-Stereo, 250 kilobytes) -
   
   This digital image was generated by processing and mosaicking over a dozen individual sidescan-sonar strips. The data were processed for both geometric and radiometric distortions using the USGSMIPS software package (see MIPS Home Page). The data were collected by the SeaMarc 1A sidescan-sonar imaging system and cover an area approximately 50 km (31.4 mi) by 75 km (47.1 mi). The data were collected as part of a cooperative project between the U.S. Environmental Protection Agency and the USGS to obtain geologic information relevant to the process of designating a deep-ocean site for the disposal of dredge material from San Francisco Bay. The brightness is influenced by several different characteristics, including slope and surface roughness. The slopes facing the imaging system (center track lines) will be bright, while those facing away will be dark/in shadow. Surfaces that are relatively smooth (such as clay and silt) will be darker/have a lower backscatter return than those that are rough (such as gravel), which will be brighter/have a higher backscatter return.
   
   
   
   
   
5. Sidescan-Sonar Digital Mosaic (Stereo, 290 kilobytes; Red and Blue/Green Glasses Required for 3-D Viewing) -
   
   The bathymetry and sidescan-sonar digital mosaics were geometrically registered/matched so that the same pixel in each image represents the same location on the seafloor. The geometric registration allows us to apply various enhancement and analysis techniques and procedures. One of the procedures used introduced parallax into the sidescan-sonar image in the horizontal direction as a function of the bathymetry data to generate a stereo pair. The topographic/3-D component can be viewed by using red and blue or red and green glasses (red on the left). Along with the shaded relief image this is an excellent way to study and analyze the topographic component of an area to detect and map the regional topographic structural patterns.
   
   
   
   
   

More Information On the Western Region Coastal & Marine Geology Team Website: Gulf of the Farallones Disposal Issues:Information and related publications The Farallones Atlasis being developed to provide information on the physical setting and living resources of the Gulf of the Farallones and adjacent ocean waters

References

• Chavez, P.S., Jr., and Karl, H.A., 1995, Detection of barrels and waste disposal sites on the seafloor using spatial variability analysis on sidescan-sonar and bathymetry images: Journal of Marine Geodesy, vol. 18, no. 3, 197-211.
  
  
• Karl, H. A., and Chavez, P. S., Jr., 1993. Acoustic mapping as an environmental management tool, EEZ Conference, November 1993, Reston, VA.
  
  
• Karl, H. A., Chavez, P. S., Jr., Chin, J. L., Maher, N. M., Kooker, L., Penvenne, J., Curl, H., and Ueber, E., 1994, Acoustic mapping as an environmental management tool: II. Computer and image-enhancement techniques for detecting drums of radioactive waste on the ocean floor, in review June 1993.
  
  
• Karl, H.A., Schwab, W.C., Wright, A.St.C., Drake, D.E., Chin, J.L., Danforth, W.W., and Ueber, E., 1994, Acoustic mapping as an environmental management tool: I. Detection of barrels of low-level radioactive waste, Gulf of the Farallones National Marine Sanctuary, California: Ocean and Coastal Management, vol. 22, 201-227.
  
  

Related Publications

• Chavez, P.S., Jr., 1986, Processing techniques for digital sonar images from GLORIA: Photogrammetric Engineering and Remote Sensing, vol. 52, no. 8, 1133-1145.
  
  
• Chavez, P.S., Jr., Gardner, J.V., 1994, Extraction of spatial information from remotely sensed image data -- an example: GLORIA sidescan-sonar images: Canadian Journal of Remote Sensing, vol. 20, no. 4, 443-453.
  
  
• USGSMIPS Home page: http://TerraWeb.wr.usgs.gov/software/mips/
  
  

Related Links

• Central California Internet Resources
  

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For more information about this project, contact:
Pat S. Chavez Jr. Email: pchavez@usgs.gov U.S. Geological Survey 2255 N. Gemini Dr. Flagstaff, AZ 86001 Tel: (520) 556-7221 FAX: (520) 556-7169		Herman A. Karl Email: hkarl@octopus.wr.usgs.gov U.S. Geological Survey 345 Middlefield Road, MS-999 U.S. Geological Survey Menlo Park, California 94025 Tel: (415) 354-3084 FAX: (415) 329-3191
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