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:
- The Gulf of the Farallones is an important commercial and recreational
fisheries area.
- 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.
- 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.
- 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.
- 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.
- 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:
- Framework geophysics and geology to investigate deep structure
in order to assess seismic risk.
- 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.
- 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.
- 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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