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Pacific Coastal & Marine Science Center

Text from Proceedings of Marine Technology Society, S. Kelly (ed.), p. 16-50. The figures have not been included for this publication.

Gulf of the Farallones Geologic Inventory Investigation: A Prototype for Environmental Studies on Continental Margins Off Major Urban Areas

H. A. Karl, W. C. Schwab and J. L. Chin

ABSTRACT

The U.S. Geological Survey began a major geologic and oceanographic study of the Gulf of the Farallones in 1989. This investigation, the first of several now being conducted adjacent to major population centers, was designed to establish a scientific data base on a segment of continental shelf adjacent to the San Francisco Bay area that can be used to evaluate and monitor human impact on the marine environment. The data, acquired with a wide variety of instruments and sampling techniques, shows that the Gulf of the Farallones is geologically complex. The principal surveying tool was a high-resolution side-scan sonar system used to produce a digital sonographic mosaic of a 800 km2 area of the continental shelf. Surficial sediment varies markedly in grain-size over distances on the order of tens to hundreds of meters on some parts of the shelf. Four fields of current-generated bedforms of diverse types and scales occur on the shelf. The largest field, an extensive area (800 km2) of linear depressions arranged in a digitate pattern and floored by large ripples, occupies most of the central shelf between Pt. Reyes and the Golden Gate. The complexity of the surficial sediment distribution and morphologic features on the sea bed probably reflect a spatially and temporally complex local current system; existing current measurements support this hypothesis. The observations suggest that in order to evaluate environmental conditions with a high level of confidence at a specific site and time within the geographic region of the Gulf of the Farallones, it is necessary first to understand the regional spatial and long-term temporal variation in geologic and oceanographic processes. This concept applies to other continental shelf areas and must be considered when designing environmental studies to monitor urban ocean areas.

INTRODUCTION

Important issues of universal concern that are becoming increasingly relevant to the marine environment include pollution, resources, hazards, and climatic/global change. The oceans have been impacted by human activities for many decades. Although concerns about possible environmental damage to the world's oceans were expressed at least thirty years ago, only now is the potential effect of these human activities on the marine ecosystem beginning to be fully comprehended by scientists and the general public. The U.S. Geological Survey (USGS) recognizes that in the decade of the 1990's and beyond that the oceans will be increasingly utilized as a source of living and non-living resources, as a place for recreation, and as a repository for waste products. To assure that these human demands are met in a socially responsible manner, it is necessary that the public and the public's representatives have the best scientific data available to guide them in making choices about the use of the marine environment. The USGS has and continues to provide this information through a variety of mapping and sampling techniques.

The USGS has conducted extensive geologic research investigations in the marine environment for more than two decades. In 1983 President Reagan proclaimed that the ocean area out 200 nautical miles off the coast of the United States, its island territories, and territorial seas was the Exclusive Economic Zone (EEZ) of the Nation. Immediately subsequent to this proclamation in summer 1984, the USGS began a major project (EEZ-SCAN) to survey the EEZ using the GLORIA (Geological LOng Range ASDIC) wide-swath side-scan sonar system (Gardner, 1984). This 6.5 kHz system is capable of being towed at speeds up to 10 knots and imaging swaths of sea floor as wide as 60 km. The GLORIA system is ideal for rapidly mapping large areas of sea floor. The data are computer-processed and digital mosaics produced of the ocean bottom. These mosaics are published in atlases (EEZ-SCAN 84 Scientific Staff, 1986; EEZ-SCAN 85 Scientific Staff, 1987a,b; Bering Sea EEZ-SCAN Scientific Staff, 1991). To date the USGS has mapped most of the EEZ adjacent to the contiguous U.S., Alaska, Hawaii, and Puerto Rico. However, the system cannot be used in water depths shallower than about 200 m and cannot resolve features smaller than about 50-150 m. Consequently, it cannot be used for surveying small scale geological features on the continental shelf.

Systematic environmentally-focused geologic investigations of the continental shelf are being conducted as a series of projects generically called Geologic Inventory projects. Areas that are offshore of major population centers have been and will continue to be most impacted by human activities and, therefore, the USGS has planned the initial Geologic Inventory projects off large urban areas. The first of the Geologic Inventory projects, the Offshore Geology of the Farallones Region project, has been implemented offshore San Francisco Bay.

The purpose of this paper is threefold: (1) to explain the philosophy, structure and goals of the Geologic Inventory projects, (2) to show that a wide variety of technologies is necessary to acquire meaningful data in the urban oceans (highlighting especially swath mapping with side-scan sonar as a invaluable tool for environmental monitoring), and (3) to demonstrate the complexity of geologic and oceanographic processes on the continental shelf and the implications for monitoring of the oceans off urban areas (urban oceans). These three objectives are accomplished principally by describing the Offshore Geology of the Farallones Region project and presenting some of the initial results of that study.

Concepts and Structure of Geologic Inventory Projects

An essential aspect of the Geologic Inventory projects is that each is designed and conducted as a basic research study. The basic research, however, must be relevant to one or more specific social issues, and, moreover, provide baseline information that can be used to design other environmental studies. The Offshore Geology of the Farallones Region project was the first of the Geologic Inventory projects structured and organized according to concepts expressed in the introduction and serves as a case study to demonstrate the applicability of basic marine research to critical social issues. The continental margin offshore of the San Francisco Bay area was chosen as the site of the first Geologic Inventory project for several reasons:
  1. The Gulf of the Farallones is an important commercial and recreational fisheries area.
  2. The Gulf of the Farallones National Marine Sanctuary -- an unique marine ecosystem -- encompasses a large part of the Gulf of the Farallones. Very little is known about the geology and oceanography of this area.
  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 47,000 drums (55 gallon) and other containers of low-level radioactive wastes were dumped on the continental margin between 1946 and 1965. These drums now litter a large area (1400 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 -- the San Andreas Fault crosses the Gulf near the Golden Gate. These faults represent a potential seismic risk.
  6. Study of the open ocean environment complements ongoing USGS investigations of San Francisco Bay and provides an unique opportunity to study an estuarine-shelf-slope system.
The Offshore Geology of the Farallones Region project is a multi-disciplinary project that consists of four basic elements:
  1. Framework geophysics and geology to investigate deep structure in order to assess seismic risk.
  2. High-resolution geophysics 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 side-scan sonar, bottom photography, high-resolution subbottom and bathymetry, and coring. Sediment samples and cores will be used for textural, geotechnical, mineralogical, geochemical, and paleontological studies. The primary objectives of these activities are to construct a sonar mosaic of the sea floor and a high-resolution bathymetric map as surveying 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 began this multi-disciplinary project in 1989 by mapping and sampling on the continental shelf east of the Farallon Islands. In 1990 the project expanded greatly in scope when the USGS conducted a multi-disciplinary investigation sponsored by USGS and four other federal agencies (the U.S. Environmental Protection Agency, the U.S. Army Corps of Engineers, the U.S. Navy, and the National Oceanic and Atmospheric Administration) to survey and sample the continental slope west of the Farallon Islands. The federal agency cooperative study was designed to provide information on the location and distribution of 47,500 containers of low-level radioactive waste and data on areas being considered as sites for disposal of dredge material from San Francisco Bay (Karl and Schwab, in press).

This paper focuses on the USGS investigation of the continental shelf and some of the preliminary results of that study are presented in the following sections.

CONTINENTAL SHELF SURVEYS

Data Collection

The USGS conducted three cruises in 1989 on the continental shelf between Cordell Bank and Half Moon Bay east of the Farallon Islands (Fig. 1). Reconnaissance surveys were conducted in January.using a 100 kHz Klein side-scan sonar system and a Huntec system (a system that combines a subbottom profiler and side-scan transducers) (Chin et al., 1990). The 100 kHz high-resolution side-scan was chosen to be able to resolve features as small as ripples that have wavelengths of tens of centimeters and heights of a few centimeters. Approximately 2,500 line km of side-scan imagery were collected during these surveys (Fig. 1). Regional reconnaissance tracks were spaced nominally 4 km apart in a rectilinear pattern. The side-scan sonar data were collected at a total swath of 200 m at ship speeds of from 2 to 4 knots. Although the high-resolution systems are capable of resolving very small objects, only small areas can be surveyed at these swaths and speeds in a given period of time. These data were acquired in analog form and displayed graphically on paper records using line scanning recorders. In addition to the reconnaissance tracks, a small (300 km2) area was surveyed at a track spacing of 100 m so that adjacent swaths would overlap permitting the construction of a mosaic of the entire area (Fig. 1). High-resolution seismic-reflection (ORE Geopulse, Huntec) and bathymetric (10 kHz) profiles were also collected along many of the reconnaissance tracks to provide data on sediment thickness and stratigraphy. Samples (97) of surficial sediment were collected with a Soutar van Veen sampler at the intersections of the reconnaissance tracks spaced nominally 4 km apart (Fig. 2). A denser grid of 171 samples spaced nominally 1 km apart were collected within the small area at 100 m side-scan trackline spacing.

Data from the January surveys were used to select a site to mosaic with a 120 kHz side-scan sonar system (AMS-120) in July. The chosen survey area was an 800 km2 area on the central shelf east of the Farallon Islands between Pt. Reyes and the Golden Gate and a 200 km2 area on the upper slope west of the Farallon Islands (Fig. 3). Initially the AMS-120 system was set to provide a full swath of 500 m and tracks were spaced 400 m apart to provide 20% overlap for mosaicking. After several lines were completed, it was found that track spacing could be increased to 500 m as the swath width approached 750 m. The side-scan data were collected as analog data and these data were then digitized using onboard computers. The side-scan data were computer processed in pseudo-realtime and an enhanced, geographically correct mosaic constructed onboard ship (see Danforth et al., 1991 for a description of processing techniques). The survey was completed in fifteen days.

So that identical features on adjacent swaths overlap and other data sets can be properly registered to the sonographic mosaic, navigation is a critical component of any offshore survey. The accuracy and precision of navigation required depends upon the goals of the survey. Three systems were used to navigate the ship during the surveys described in this paper: (1) Global Positioning System (GPS), (2) LORAN-C, and (3) shore-based transponder net (both microwave and UHF). The primary system used for real-time positioning was chosen either manually by the navigator or automatically by the computer. Actual tracklines were within 100 m of preplotted tracks when using GPS and LORAN-C and within a few meters of preplotted tracks when using the shore-based transponder net. It is beyond the scope of this paper to describe the various navigation systems in more detail. Suffice it to say that navigation is an essential element to any meaningful marine geologic investigation designed for environmental monitoring.

Results

The description of results and discussion and interpretation will focus principally on the sediment samples and side-scan sonar data. Other types of data collected as part of the multi-disciplinary study are not presented in detail, but are used as necessary to support some of the interpretations and concepts expressed in the paper. The Offshore Geology of the the Farallones Region project is ongoing and additional data will be collected on future cruises.

Surficial Sediment Data

The distribution of surficial sediment textures based on the regional grid of 97 stations suggests that depositional processes in the Gulf of the Farallones are complex. A 20-km wide corridor of sand extends westerly from the Golden Gate to the Farallon Islands (Fig. 4). Silty sand and sandy silt bound the corridor to the northwest and southeast and a tongue of silt from the north extends around Pt. Reyes (Fig. 4). More detailed analysis of the sediment texture reveals a slightly more complex regional distribution as shown, for example, by a plot of mean grain-size (Fig. 5). The increased complexity is well illustrated by examining the cross-shelf corridor of sand defined in Figure 4. Plotting the mean grain-size at 1-phi intervals shows that patches of medium and coarse sand exist within a field of fine sand and that sediment texture becomes coarser closer to the Farallon Islands (Fig. 5). Increased sampling density reveals an even more intricate patterns of sediment texture. Note the area of dense stations (171 stations spaced 1 km apart) on Figure 3. Sediment in this area, based solely on analysis of samples from the regional grid of 97 stations, is uniformly fine sand (Fig. 4). Data from the grid of 171 stations (also plotted at a 1-phi interval) demonstrates that the area actually consists of a pattern of mean grain-sizes that range from fine to very coarse sand (Fig. 6). This level of sampling density provides data important to interpretation of the depositional processes operating in the Gulf of the Farallones. However, it is impractical to sample the entire shelf at this level of density. Moreover, even with such a dense sampling grid, it is still necessary to interpolate boundaries between textural fields. Side-scan sonar, on the other hand, provides continuous information about elements of the sea floor. The most intricate textural patterns are clearly revealed when adjacent side-scan swaths are joined into a mosaic of a large area of the sea floor and the images verified with samples of sediment collected at selected sites.

Side-scan Sonar Data

Side-scan sonographs are acoustic images of the sea floor; acoustic energy transmitted from the side-scan tow vehicle is backscattered from the sea floor. The shades of gray that range from black to white that define the features of the sea floor represent varying energy levels of acoustic backscatter. The darker shades correspond to high backscatter levels. Typically coarser sediment, steeper slopes and rougher sea floor surfaces backscatter more energy. Many complex factors, however, determine how sound is backscattered and reflected from the sea floor (Johnson and Helferty, 1990). Consequently, interpretation of the side-scan sonar data is not as straightforward as interpreting an aerial photograph. Interpretation of sonographic images is an art as well as science. Other data sets must be used to supplement and complement the sonar data so that the sonar images can be interpreted as accurately as possible. For this reason other data such as high-resolution seismic-reflection profiles and sediment samples must be collected in the side-scan sonar survey area.

Data collected along the reconnaissance tracks with the 100 kHz side-scan systems (Klein and Huntec) revealed a variety of features on the sea bed that include outcropping rock and several types of bedforms (Figs. 7, 8, and 9). The various features were plotted on a map to establish their spatial distribution. However, it was impossible to determine absolute boundaries of fields of bedforms and geometric relationships of various features owing to the widely spaced and non-overlapping side-scan sonar records. The regional map of the various features did establish that at least four major discrete fields of bedforms occur on the shelf between Pt. Reyes and Half Moon Bay. These fields were separated by monotonous stretches of flat, featureless sea floor. Of particular interest were a series of depressions floored by ripples with wavelengths of about 1 m. These depressions are common east of the Farallon Islands between Pt. Reyes and the Golden Gate and comprise the largest of the four fields of bedforms. The overlapping swaths in the small 300 km2 area on the shelf provided a sense of the continuity of these depressions, but the shapes were distorted because the analog records. were not corrected for slant-range and variation in the speed of the ship. This mosaic area coincides with the dense grid of sediment samples. The side-scan images indicate an even more complex textural pattern than that defined by the sediment samples. In order to define that pattern and the limits of the field of depressions, the entire area was surveyed with the AMS-120 side-scan system and a computer-processed, geographically correct mosaic constructed onboard ship.

The mosaic, supplemented and complemented by sediment samples, bathymetric profiles, and high-resolution seismic-reflection profiles, provides sufficient information to allow a more thorough description of the morphology and texture of the sea floor than is practical with any other method. The side-scan data that comprise the mosaic have been computer processed so that the mosaic represents a true plan view of the sea floor. That is features on the sea floor as seen on the mosaic are in their true correct spatial position and their true geometric shape. In this area broad shallow depressions (less than 2 m relief) floored by medium and coarse sand are arranged regionally in a digitate pattern (Fig. 10). Where fully developed the "palms" of these features are as wide as 2 km with northerly striking "finger-like" projections as long as 8 km that narrow to 250 m. Small-scale bedforms -- ripples that have a wavelength of 1 m and that strike N-NE -- cover the digitate depressions. In order to process the 120 kHz data rapidly onboard ship it was necessary to desample the records. Consequently, although visible on the raw data, the small-scale bedforms are not resolved on the AMS-120 mosaic. These features were also resolved with the 100 kHz Klein system during the reconnaissance survey (Fig. 7). No ripples were resolved with either the 100 or 120 kHz systems on the intervening areas of fine and very fine sand. The digitate depressions are most distinct in the northwestern part of the field; in the southeastern part of the field they are partly covered with a veneer of finer sediment and the boundaries between depressions and intervening areas become less distinct.

Discussion

In this section, we briefly discuss and interpret some of the preliminary results of the data collected in the Gulf of the Farallones. The Gulf of the Farallones appears to be a particularly complex segment of continental shelf. However, the continental shelf in general is a complex environment. In order to demonstrate the universal application of the investigative techniques and technologies described in this paper, we summarize several extensive studies of San Pedro Bay in the Southern California Bight.

Gulf of the Farallones

The continental shelf of the Gulf of the Farallones is morphologically and sedimentologically very complex. This complexity, undoubtedly, reflects an intricate geologic history and a wide variety of geologic and oceanographic processes that operate on the shelf to transport, erode, and deposit sediment. Only limited current measurements have been collected on the shelf to date (Noble and Gelfenbaum, 1990). Current meters were emplaced at two sites from May through October in 1989. Data from these instruments show that the currents in the Gulf are similar in many respects to other currents over the northern California shelf in that the currents tend to flow toward the northwest except when winds drive them southeastward and tides comprise a large percentage of the current field (Noble and Gelfenbaum, 1990). However, currents in the Gulf of the Farallones differ from those elsewhere in that the diurnal tides are stronger than expected, the semidiurnal tides double in amplitude over short distances along the shelf, and the cross-shelf currents flow consistently toward the coast (Noble and Gelfenbaum, 1990). These data are valid for the observation period of May through October and may not be characteristic of the winter months. Noble and Gelfenbaum (1990) suggest that the tides may be an important control over the distribution and movement of sediment on the shelf. The strong variation in tidal amplitude over short (15 km) spatial distances may be a contributing factor to the complex textural pattern of surficial sediment. Moreover, variation in tidal amplitude may be influenced by topography and the baroclinic structure of the water column. Consequently, tidal current strength will vary both spatially and seasonally (Noble and Gelfenbaum, 1990). Other currents that contribute to the patterns of erosion and deposition on the continental shelf in general include strong currents generated by waves during storms. Measurements will need to be made during the winter months to evaluate the effect of such currents in the Gulf of the Farallones.

Not enough information is available to determine conclusively the processes responsible for the intricate pattern of linear depressions on the central shelf. Similar depressions floored by ripples have been observed on other shelf segments. These depressions are usually attributed to combined tidal currents and storm-generated currents (eg., Kenyon, 1970; Cacchione et al., 1984). No previous studies have mapped an entire field of such depressions and defined the field boundaries, thereby establishing the field geometry and spatial relationships with other morphological elements on the shelf. Bathymetry seems to influence the field boundaries. The northwestern boundary of the field is distinct and arcuate and seems to mimic the 80 m isobath. The southeastern boundary of the field, in contrast, is indistinct as the depressions gradually decrease in number and size. This boundary apparently coincides with the transformation of the isobaths from cross-shelf to along-shelf. The field straddles a topographically high saddle in the central part of the shelf These observations suggest a bathymetric control of the currents responsible for the depressions. Perhaps the water mass is constricted and flow intensified over the topographic high contributing to scour of the linear depressions. Large ripples (1 m wavelength) occur in the depressions floored by medium and coarse sand. The ripples appear to be generated by waves. No ripples have been resolved with the side-scan systems on the intervening areas of fine sand. Small ripples probably occur on these areas, but are too small to be resolved by the side-scan systems. The depressions and ripples manifest a dynamic and complex sediment transport system that certainly varies with space and time. Additional data, including bottom photography and long-term current measurements, need to be collected to understand and predict the pathways of sediment transport in the Gulf of the Farallones.

It is not yet known whether the textural patterns and bedforms on the shelf totally reflect present-day processes or whether some of these features are remnants of processes that operated during lower stands of sea level in the past. Ancient relict features produced during lower sea levels are common on the continental shelf. In the past eustatic sea level was as much as 135 m below present-day sea level. Such low sea levels would have exposed virtually all of the shelf in the Gulf of the Farallones. A regional unconformity detected on high-resolution seismic-reflection profiles proves that a large part of the Farallon shelf was subaerially exposed in the past (Chin and Karl, 1991). These facts need to be considered when using textural patterns and sea bed morphology as an aid to interpret present-day current systems operating on the shelf.

San Pedro Bay, Southern California Bight

The continental shelf off southern California is compartmentalized into a series of littoral transport cells bounded on their downdrift ends by submarine canyons (Emery, 1960; Inman and Frautschy, 1966). San Pedro Bay encompasses the cell that begins just east of Palos Verdes Peninsula and terminates where Newport Submarine Canyon incises the shelf at the east end (Fig. 11). San Pedro shelf varies in width from about 20 km at its widest point to about 3 km at its east and west boundaries. Two submarine canyons, San Pedro Sea Valley and Newport Submarine Canyon, indent the outer area of the flat, gently sloping, wide segments of shelf. A great amount is known about the sedimentology and dynamics of sediment transport on San Pedro shelf. Much of this information is contained and summarized in Karl (1976), Karl et al. (1980), and Drake et al. (1985).

San Pedro Bay is not as complex sedimentologically and, presumably, oceanographically as the Gulf of the Farallones. Nonetheless, numerous observations of the sea bed demonstrate spatial variability in the distribution of grain-sizes and bedforms and long-term current meter and wave observations demonstrate temporal and spatial variability in current energy and patterns. Four hydrodynamic provinces characterize San Pedro shelf. Three of these are aligned parallel with the shoreline and isobaths: (1) an inner zone from shore to approximately the 20 m isobath, grading into (2) the central shelf which continues to about 60-70 m before merging with (3) the outer shelf bounded seaward by the shelf-break at about 75-100m. A fourth transverse province, present where submarine canyons incise the shelf, is superposed on the shelf parallel provinces. Characteristic flow fields, revealed by diagnostic features of the substrate, concentration and distribution of suspended sediment, and direct measurements of currents, dominate in each zone. Herein we discuss briefly only some of the characteristics of the central shelf province.

Surficial sediment on San Pedro shelf consists mainly of a thin blanket of modern very fine sands and coarse silts interrupted by patches of relict medium and coarse sands (Fig. 12). Basically textural isopleths tend to parallel isobaths. In the area near the head of San Gabriel Canyon, however, textural isopleths deviate from this pattern and are perpendicular to isobaths. This observation and other data suggest that San Gabriel Canyon modifies shelf circulation patterns (Karl, 1980a). The regional textural patterns are based on samples collected at 2 km intervals. As shown in the Gulf of the Farallones, a denser sampling interval could modify the regional textural trends. Indeed, samples collected at an interval of 0.2 km in an area of fine sand showed considerable size variation from fine to medium to coarse sand between samples (Karl, 1976; Karl, 1980b).

Intricate and sometimes subtle depositional processes influence physical sedimentation on the central shelf. Tidal currents (measured during one month in spring) are large enough to influence the transport of fine-grained particles suspended in the water column. During most of the year waves generate the only currents that exceed the threshold for movement of sediment that covers the central shelf (Karl, 1976; Drake et al., 1985). Surface waves generate currents of sufficient strength to form symmetric ripples in depths as great as 25 m most of the year. Rippling occurs in water depths as great as 60 m in winter. Episodic storms modify these seasonal limits of active rippling and during periods of quiescence, activities of benthic organisms obliterate ripples altogether. In addition to small-scale wave-generated ripples other larger bedforms exist on San Pedro shelf. Mesoscale bedforms called current lineations that consist of sand ribbons (40-120 m wide) and erosional furrows (15-50 m wide) have been observed on side-scan sonar records. These bedforms are oriented normal to shore and have been attributed to helical flow cells known as Langmuir circulations (Karl, 1980b).The variety of bedforms and their spatial and temporal distribution manifest the intricate currents that comprise sediment transport systems on even the least hydrologically complex continental shelves.

SUMMARY

Numerous processes interact to produce a complex and intricate system of currents that transport, erode, and deposit sediment over the continental shelf. Surface waves, internal waves, and tides contribute to and are superimposed on a mean flow which also responds to meteorological forcing. The relative importance of the various processes varies spatially and temporally. For example, shoaling surface waves affect bottom sediments more in shallow water than in deep water and concentrate more energy at points of convergence than divergence. Moreover, there are times when packets of more energetic waves approach the shelf, thereby intensifying bottom currents and making a proportionately greater contribution to incipient sediment movement than other processes; at other times tides or meteorologically-induced currents might be more important. Sediment textural patterns and bedforms are the products of these various processes. Bedforms and suspended sediment concentrations respond most quickly to regular and sporadic, short-term changes in the strength and position of shelf flow fields, whereas textural patterns change and equilibrate more slowly to shelf hydrodynamic processes. Textural patterns and especially accumulations of sediment represent time-averaged depositional processes and can provide a great deal of information about the history of sediment transport on a segment of shelf.

A wide variety of instruments and technologies are required to sample and measure the many products and processes that constitute the continental shelf environment. Because of the great spatial and temporal complexity of processes and products on the continental shelf, in order to understand fully the interactive processes at a specific site it is necessary to study the entire system on a larger scale. These regional investigations must be multi-disciplinary to sample and survey the full range of important variables and parameters, and, we believe, are best structured as basic research projects. Furthermore, current meter measurements and some other types of sampling (suspended sediment concentrations, for example) must be designed as long-term investigations to sample properly seasonal and shorter term fluctuations. The regional surveys will provide information to design experiments to monitor specific sites.

The Offshore Geology of the Farallones Region project has been described as an example of a multidisciplinary environmental inventory and monitoring investigation. Many technologies and tools were used as an integral part of the project. The high-resolution side-scan sonar systems provided a great deal of information critical to the overall success of the study.

Swath mapping with side-scan sonar is an efficient means to obtain a regional map of the sea floor. New technology allows the construction of sonographic mosaics while at sea. Side-scan sonar in effect removes the water column from the sea floor to reveal the morphology of the sea bed. The side-scan sonar mosaic provides a map that can be used to design other surveys and sampling designs. The mosaic reveals the spatial variation in sediment types, bedforms, and other features on the sea floor. The benefits derived from a side-scan mosaic for planning subsequent investigations and monitoring changes in the sea floor are obvious and invaluable to geologists, biologists, and oceanograhpers.

These studies are particularly important in the urban oceans where human impact is having the most effect on the environment. Research and environmental monitoring in the urban oceans requires a major cooperative effort among government, industry, and academia

ACKNOWLEDGEMENTS

Norman Maher analyzed the grain-size data shown for the Gulf of the Farallones and drafted many of the figures. William Danforth and Thomas O'Brien led the teams that processed the side-scan sonar data.

Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.

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