USGS Monterey Bay National Marine Sanctuary Project Seafloor Image Map of the Monterey Bay Region

IMPORTANT DISCLAIMERS

  The digital mosaic of acoustic images, or sound-photos, of the seafloor covering the Monterey Bay continental shelf region was made by digitally mosaicking together a large number of 200-m to 500-m wide swath images collected along parallel individual ship tracks. In northern Monterey Bay the towed acoustic sidescan-sonar imaging systems used either a 100- or a 59-kHz fish that served as both the sound source and reciever of the sound scattered back from the seafloor (Figure 1). In southern Monterey Bay we used a hull-mounted multibeam swath-imaging system (Simrad EM-1000) that produced both backscatter and bathymetric data. (See Footnote 1). Variations in sediment, rock type or biologic features on the seafloor generate variations in the acoustic backscatter received at the sidescan fish. The lighter, or bright, tones in the mosaics (Figures 2a and 2b) represent areas with high backscatter, while dark tones represent areas with low backscatter; black represents zero backscatter (i.e. sonar shadow). Generally, rough textured areas will have a high backscatter, while smooth areas will have a relatively low backscatter. Some of these variations are easy to interpret while others are ambiguous. For example, around Santa Cruz (Figure 2a) the rock types that occur in the shore cliffs can also be seen on the seafloor as outcrops that produce high backscatter. Sometimes these highly reflective rocks have recognizable sedimentary layering that is folded and cross-cut with numerous offsetting faults (Figure 3). These outcropping seafloor sedimentary rocks have been eroded flat by wave action, and now comprise the modern submerged terrace that will one day, thousands of years from now, become the next and youngest uplifted coastal terrace of the many coastal terraces that form the hillside sloping down to the sea near Santa Cruz.

Monterey Bay Region Seafloor Image Maps Click on thumbnails image below to access the mosaics Important: Imagery Disclaimer
Monterey Bay Geographic Reference Image Map Regional Overview (153 k)	Figure 2a: Digital Sidescan-Sonar Mosaic Updated 5/28/98 (102 k) Back to text
	Figure 2b: Digital Multibeam Sonar Backscatter and Shaded Bathymetry Mosaics New - 6/19/98 (shaded bathymetry) (154 k) Back to text

Anthropogenic (man-made) seafloor features imaged include the wastewater pipeline that trends offshore west of Lighthouse Point in Santa Cruz; it is seen as a bright, straight line with a northeast-southwest orientation (Figure 4). A similar pipeline is seen in the southern Monterey Bay imagery off the town of Marina about 8 miles north of the city of Monterey (Figure 2b). Some biological communities in the water column and on the bottom can be recorded as high-backscatter features. However, since most biological features are in motion, they fail to reproduce on adjacent swaths which are recorded many hours apart, so they can usually be distinguished from seafloor geological features (Figure 5).

Besides the layered sedimentary outcrops, elongated patches of bright seafloor seen crossing at angles to the layering of rock outcrops are mostly coarse sand patches with large wave-generated ripples. These patches, which mostly occur in shallow flat-floored troughs, represent wave-driven sands in motion (Figure 6). Off Santa Cruz, such sands on the inner shelf regularly clog the mouth of the Santa Cruz small boat harbor and necessitate seasonal dredging.

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Figure 3 Sedimentary Layering 700 x 700, 498K Back to text	Figure 4 Wastewater Pipeline 700 x 500, 306K Back to text	Figure 5 Biologic Targets 700 x 650, 394K Back to text	Figure 6 Sand Patches 700 x 750, 303K Back to text

Project Team

Related Pages

• USGS: Monterey Bay Bathymetry and DEM Shaded Relief Images
• NOAA: Monterey Bay National Marine Sanctuary
• USGSMIPS Home page: http://TerraWeb.wr.usgs.gov/software/mips/
• Links to Other Central California Internet Resources
  
  

  	Monterey Bay Region Bathymetry and DEM Shaded Reliefs
  	Up To Coastal and Marine Remote Sensing Research and Applications

For more information contact:
Remote Sensing/Image Processing 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		Project Chief/Geologic Mapping Stephen L. Eittreim Email: eittreim@octopus.wr.usgs.gov 999 US Geological Survey 345 Middlefield Rd. Bldg 2 rm 2239 Menlo Park, CA 94025 Tel: (650) 329-5272
Home	Western Region Coastal and Marine Geology	Coastal and Marine Geology Program	USGS Home Page

The bathymetric data suffers from a noise problem, particularly at the margins between swaths of data. This noise is seen as a pattern of generally north-south, along-ship-track lineations, and generally east-west cross-ship-track lineations that are due to mismatch of information from one swath of data to the next. The ship used was an old, rather flexible vessel, some rough weather was encountered, and the distance between the motion-sensor (giving exact pitch, roll and ship heading) and the sonar transducer was too large, introducing a "twist error" that degraded the data. That is, the exact look-angle and direction information was not known as precisely as it should have been at every millisecond of recording. However, features on the seafloor with relief as small as 1-meter can be observed despite this pervasive noise pattern.

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