USGS - science for a changing world

Pacific Coastal & Marine Science Center

Report No. MITSG 92-2, p. 61-65.

Techniques of Basic Marine Geological Research: Application to Environmental Management

Herman A. Karl

The U.S. Geological Survey is conducting systematic environmentally focused geologic investigations called Geologic Inventory projects on the continental shelf and slope within the U.S. Exclusive Economic Zone (EEZ). Areas that are offshore of major population centers have been and will continue to be most affected by human activities, and, therefore, the initial study sites are adjacent to large urban complexes. Eighteen areas in the coastal ocean (8 designated and 10 proposed) have been set aside as national marine sanctuaries. These pristine, unusual, rich and diverse ecosystems, particularly those adjacent to major urban areas, are also impacted and threatened by societal use of the oceans. The first of the Geologic Inventory projects, the Offshore Geology of the Farallones Region, was begun offshore of the San Francisco Bay in 1989 where a national marine sanctuary is situated adjacent to a dense population center.

Each Geologic Inventory project is designed and conducted as a multidisciplinary research study. The basic research, must be relevant to one or more specific social issues, and, must also provide baseline information that can be used to design other environmental studies. Most importantly, the data derived from the project must be communicated in a timely way that is clearly understandable not only by professional scientists but also by the public and those charged with management decisions concerning multiple use of the offshore areas.

The continental margin offshore the San Francisco Bay area was chosen as the site of the first Geologic Inventory project for six reasons: (1) The Gulf of the Farallones National Marine Sanctuary--a unique marine ecosystem--encompasses a large part of the Gulf of the Farallones. The geology and oceanography of this area are poorly understood. (2) The Gulf of the Farallones and adjacent waters are an important commercial and recreational fisheries area; fish products from the Gulf of the Farallones are exported worldwide. (3) Selected areas of the ocean floor have been used and are being considered as disposal sites for material dredged from San Francisco Bay. There is a great need to gather information about the geologic and oceanographic processes on the continental margin to understand the effects of these disposal sites on the environment. (4) More than 47,800 drums (55 gallon) and other containers of low-level radioactive waste were dumped on the continental margin between 1946 and 1970. These drums now litter a large area (1200 km2) of the sea floor within the marine sanctuary. The exact location of the drums and the potential hazard the drums pose to the environment are unknown. (5) Many faults have been mapped in the Gulf of the Farallones; for example, the San Andreas Fault crosses the Gulf near the Golden Gate. These faults are a potential seismic risk for the cities of the San Francisco Bay area. (6) Study of the ocean environment complements ongoing USGS investigations of San Francisco Bay and provides an opportunity to study an estuarine-shelf-slope system.

A wide variety of surveying and sampling techniques and technologies is required to sample and measure the many physical products and processes that characterize the continental shelf and slope environments. The Farallones Region project is multidisciplinary and consists of four basic elements or surveying and sampling techniques. (1) Framework geophysics and geology to investigate deep structure to assess seismic risk. (2) High-resolution reflection profiling techniques are employed to investigate near-surface structure and stratigraphy, sediment body geometries and surface morphologies. These studies will help evaluate seismic and slope stability hazards and areas of excessive sediment erosion and deposition. (3) Characterization of the sea floor with sidescan sonar, bottom photography, high-resolution subbottom profiling systems, acoustic profiling systems, and core samples. Sediment samples and cores are used for textural, geotechnical, mineralogical, geochemical, and paleontological studies. The primary objectives of these activities are to construct a sonographic mosaic of the sea floor and a high resolution bathymetric map as survey bases and to use the natural sediments as tracers for identifying pathways of sediment and pollutant transport. (4) Quantitative investigation of sediment transport and ocean currents especially near the sea bed to measure and predict rates of sediment and pollutant transport.

The USGS mapped and sampled the continental shelf east of the Farallon Islands in 1989. In 1990 the project expanded in scope when the USGS conducted an investigation sponsored by the USGS and the U.S. Environmental Protection Agency, the U.S. Army Corps of Engineers, the U.S. Navy, and the National Oceanic and Atmospheric Administration (NOAA) to survey and sample the continental slope west of the Farallon Islands. This cooperative study by these federal agencies was designed to provide information on the location and distribution of the drums of low-level radioactive waste and geologic data on areas being considered as sites for disposal of dredge material from San Francisco Bay.

Many surveying and sample techniques were used during both the shelf and slope investigations. These include but are not limited to geophysical surveys, sediment sampling, bottom photography, and measurements of ocean currents. One valuable surveying tool is sidescan sonar, which allows scientists to characterize the morphology of the sea floor by swath mapping. Sidescan sonar provides an acoustic image or sonograph of the sea floor that is similar to a satellite image of the Earth's land surface. Mapping with sidescan is commonly done in two modes or styles. In a reconnaissance mode track lines are widely spaced to cover an area quickly and swaths do not overlap. In a mosaicking mode track lines are spaced closely together so that adjacent swaths overlap and a contiguous image is built of a given area of sea floor. Owing to the brevity of this summary, only some of the results obtained using sidescan sonar surveying and bottom photography techniques are described in the following paragraph.

The low-level radioactive waste (LLRW) containers that were dumped in the Farallon Islands Radioactive Waste Dump (FIRWD) may or may not pose a risk to the environment. To evaluate the risk, samples of the sediment, biota, and water must be collected near the concentrations of barrels. However, the exact location of the barrels must be known prior to sampling. The USGS, in cooperation with NOAA, used sidescan sonar to map two areas within the FIRWD. Total sea floor coverage was obtained and computer-processed mosaics were constructed on board ship. Many small non-geologic targets were distributed throughout the survey areas that covered about 70 km2 (20 square nautical miles) on the shelf and 125 km2 (37 square nautical miles) on the slope. Analysis of the side-scan data suggests that the targets are 55 gallon drums. This interpretation was confirmed at one site with an underwater video and 35 mm camera system. Camera surveys are also necessary to assess the condition of the drums to determine whether they are breached and their contents exposed to the environment. Maps of barrel distribution derived from the sonographs are being used to design sampling schemes to evaluate the risk that the levels of radioactivity may have on the biota and environment.

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