Figure 1 is a computer-generated image of the seafloor west of San Francisco, from north of Point Reyes to south of Monterey. This three-dimensional view shows the bathymetry of the seafloor; that is, the variations in depth from place to place. To highlight variations, depths are color-coded from light tan (shallow area near the coast) to blue (deep areas far from the coast). Detailed data are not available for the area west of the black line.
Figure 2 is another computer-generated image of the seafloor, in this case showing the bathymetry of Monterey Bay. As in Figure 1, the bathymetry is shown as a three-dimensional image and depths are color coded. Depths are also shown by contour lines, each of which corresponds to parts of the ocean with equal depths. The depth interval between each contour line is 100 meters.
Figure 3 is a map of the seafloor directly west of San Francisco. The depths on this image are shown with bathymetric contour lines but no three-dimensional perspective. Colors are coded to bathymetric variations: light blue=shallow water and dark blue=deep water). The contour lines are not labelled on the diagram, but the depth interval between each line is 500 meters. The Gulf of the Farallones National Marine Sanctuary is a part of the coastal ocean that is protected from most economic activities by federal law.
The maps shown in Figures 1-3 are generated from actual data collected from the seafloor. Technology has progressed a long way since oceanographers estimated depths to the seafloor using weighted lines (sounding lines) that could only measure one depth at a time.
Instruments on ships, or towed behind a ship, emit sound pulses that bounce off the seafloor and return to be recorded by the instrument. These instruments are called "echo sounders" and are similar to the way a voice echoes off a steep canyon wall.
Early echo sounders emitted
one pulse of sound and could only provide a single depth measurement at a time.
Advanced multibeam sonar emits an array of sound pulses so that a wide swath
of seafloor can be measured at one time (see Figure 4).
Figure 4. This diagram shows one type of multibeam sonar, called side-scan sonar. An instrument, called a "fish", contains two sonar units that emit bursts of sound outward, to either side. The instrument is towed behind a ship and creates a broad-swath image of the seafloor.
Figure 5. This photograph shows one example of a side-scan sonar "fish" that is towed by a ship and emits sound pulses that bounce off the seafloor on either side of the instrument.
Figure 6. The sonar fish receives and records sound pulses that bounce off the seafloor. This image shows data that were collected from the seafloor in the coastal region near Monterey Bay. On this image can be seen the path of the ship as it moves back and forth over the seafloor (like mowing the lawn). Nearly complete coverage of the seafloor can be obtained with this technique, but small areas of no data (thin black lines) are visible on this image.
With 200 ships outfitted with the necessary equipment, charting the entire seafloor in this detailed manner would require hundreds of years. The multibeam sonar technique is practical, however, for areas of the seafloor where very detailed information is required. In the early 1980s, as part of an international Law of the Sea treaty, the water and seafloor within 200 miles of a country's coastline were declared Exclusive Economic Zones (EEZ). Between 1983 and 1993, the U.S. Geological Survey mapped >200,000 square miles of EEZ area in the coastal United States.
Figure 7. The addition of offshore EEZ areas (colored pink) nearly doubled the area of the United States. Islands in the Pacific are Hawaii and territories of the U.S.
PART III: Why care about what the seafloor looks like?
Reasons why detailed mapping of the seafloor is useful include the following:
One area that has been surveyed in detail is the part of the EEZ west of San Francisco in the Gulf of the Farallones. Geologic information was needed at this location for two primary reasons:
(1) Evaluate proposed sites for offshore disposal of dredge materials. Roughly 400 million cubic yards of sediment will be dredged from San Francisco Bay waterways over the next 50 years. The dredged material must be disposed of in a manner that is environmentally safe and acceptable to a wide variety of competing interests. A key set of geological characteristics for a proposed disposal site includes a featureless plain with gentle slopes, no evidence of mass movement of sediments or rock, a lack of strong currents that might redisperse dredge materials, and low-level biota.
(2) Determine locations of barrels of radioactive waste. Between 1946 and 1970, nearly 50,000 drums of hazardous and radioactive wastes were dumped over a 350-square nautical mile area that overlaps the Farallones National Marine Preserve. Although the drums were to be dumped at three specific sites, they are not located at those sites. The exact location of the containers and the potential hazard the containers pose to the environment are largely unknown. Previous attempts to locate the barrels were like trying to find a needle in a haystack. In contrast, the new sonar techniques helped to identify the barrel locations.
Figure 8 shows locations where radioactive waste were to be dumped (red stars) and locations that were proposed for offshore dumping of sediments dredged from San Francisco Bay (yellow shapes).
Click here to find out more about the Pollution and Waste Disposal issues in the Gulf of the Farallones.
Ok, you're from Southern California and want to see what the coastal region looks like there. Click here to view an animated "fly-by" of the region around Palos Verde Peninsula. This Quicktime movie will take a few minutes to load.