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The migration of sand waves on the Atlantic continental shelf in response to storm-driven
currents has been documented by side-scan sonar. Sediment on most of the continental slope and
rise off the Atlantic coast, if resuspended by seafloor uses, will travel tens of kilometers before
settling to the bottom. Because of high turbulence in continental shelf regimes, any fine-grained
suspended material tends to be well-mixed and dispersed rapidly.
As discussed in greater detail later in this chapter, benthic organisms also play an important
role in sediment dynamics because they may enhance the credibility of sediments by mixing and
resuspending sediment or may undercut a slope by burrowing in steep canyon walls. Although
recent research has shed light on the complex processes of sediment dynamics, the capability to
quantitatively predict sediment erosion and its subsequent fate under a variety of environmental
conditions still does not exist. For some applications, such as disposal of wastes, it will be necessary
to enhance understanding of long-term sediment dynamics in order to reasonably predict their fate.
SEDIMENT PROPERTIES
Knowledge of physical, chemical, and biological properties of ocean sediments is important to
potential uses of the seabed. Engineering behavior depends on all three. For example, the strength
of fine-grained sediments is largely controlled by the geochemistry (mineralogy) of the constituents,
which in turn is often closely linked to biological processes. Also, the properties of a given
sediment deposit are not constant, but may change significantly over time. Some common
properties of sediments are described in this section and elsewhere in this report as they pertain to
particular uses.
Physical Properties
Physical properties are the geological and engineering properties of sediments that must be
understood in order to make calculations related to seabed processes and uses. In general, physical
properties of marine sediments (with the exception of carbonate and siliceous materials) are similar
to those of water-saturated terrestrial soils (Chancy and Fang, 1986~. Thus, with some important
modifications, most geotechnical principles developed for land apply to engineering analysis of the
seabed.
Physical properties of marine sediments are important to geotechnical engineering (Tables 2-1
and 2-2~. Site-specific properties are important because within most regions properties vary
vertically, from the seafloor down through the sediment column; and longitudinally, from the coast
out across the shelf, slope, and rise. Special conditions that affect physical properties and seabed
behavior are dynamic loading by waves, earthquakes, and sediment-structure interactions; high
carbonate content; gas in sediments; high organic content; permafrost and freeze-thaw processes; ice-
seabed interactions; and state of consolidation (compaction).
Compressibility and Permeability
Knowledge of compressibility and permeability (perviousness) of marine sediments is important
in analysis and design of structures for seabed applications. For example, the loading imparted by a
structure placed on the seabed causes compression and settlement of underlying sediments (Figure
2-5~. The state of consolidation within the sediment column is also important in reconstructing the
geological history of an area and assessing the suitability of a site for a given use. Permeability data
are critical for evaluating the potential for pore fluid migration in sediments being considered for
waste disposal.
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TABLE 2-1 Site Data Requirements for Categories of Geotechnical Engineering Applications in
Marine Sediments
Topography Sediment
Macro Micro Index In situ Laboratory Dynamic
Application (>1 m) (<1 Properties strength strength response
Shallow foundations/ highs high high lowb high low
deadweight anchors
Deep foundations/ high low high high high high
pile anchors
Direct-embedment anchors low 0c high low high high
Drag anchors high low high low low low
Penetration 0 0 high high low low
Breakout low low high low high high
Scour high high high low low 0
Slope stability ~high high high high high high
NOTES: a High = mandatory, b low = can design without, c 0 = not needed
SOURCE: After Rocker, 1985.
TABLE 2-2 Sediment Engineering Parameters Normally Required for Categories of Geotechnical
Engineering Applications
Strength Compression
Sediment Alter- properties properties
classi- Grain berg Subbottom
Application fication size limits Clay Sand Clay Sand depth of survey
. . _
Shallow foundations Yes Yes Yes Yes Yes Yes Yes 1.5 to 2x foundation width
Deadweight anchors Yes No No Yes Yes No No 1.5 to 2x anchor width
Deep pile Yes Yes Yes Yes Yes Yes No 1 to 1.5x pile group
foundations width, below tips
Pile anchors Yes Yes Yes Yes Yes No No To depth of pile anchor
Direct- Yes Yes No Yes Yes Yes No To expected penetration of
embedment anchor; max 10 to 15 m
anchors clay; 3 to 10 m sand
Drag anchors Yes Yes No Yes No No No 10 to 15 m clay; 3 to 5 m
sand large anchors
Penetration Yes Yes No Yes Yes No No 10 to 15 m clay; 3 to 10 m sand
Breakout Yes Yes Yes Yes Yes No No lx object width plus
embedment depth
Yes Yes No
Yes No No No 1 to 5 m related to
object size and water
Slope stability Yes Yes Yes Yes Yes Yes No 10 to 30 m; more on rare occasions
SOURCE: Rocker? 1985.
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Accurate determination of compressibility and permeability requires recovering good quality
samples for laboratory testing, although some success has been achieved in measuring permeability
characteristics in situ. In either case, it is usually difficult and costly to obtain detailed data on
these properties.
Sediment strength and response to loading, called "stress-strain behavior," are important criteria
for evaluating slope stability and designing seabed installations. Knowledge of undrained shear
strength, based on short-term static loading, is adequate for some applications, but most engineering
situations require determination of shear strengths for varied loading conditions. Loadings that
result from complex interactions among environmental forces, the structure, and the seabed (Figure
2-5) are dynamic and cyclical. Thus, it is not usually possible to designate a single strength value to
assess seabed stability; rather, it is necessary to determine the full range of stress-strain-time
properties that apply to a given situation. This variability, especially the presence of weak layers, is
important in analyzing slope stability and calculating the stability of bottom-supported structures.
Organic content can also influence the physical properties of sediments. The organic content
may vary vertically in the sediment column due to a change from oxidizing to reducing conditions,
and horizontally due to zones of high productivity or oxygenated areas. Hence, changes in
consistency and strength within fine-grained sediments may be due to variations in organic content
rather than changes in texture, mineralogy, pore water chemistry, or sedimentation (Keller, 1982;
Bennett et al., 1985; Booth and Dahl, 1986~.
Modules ~ i
Deck
Concrete
Shafts
Conductors
Concrete
Base
Cyclical Loading
of Sediments
Wind Environmental Loading:
Waves
Currents
Earthquakes
Winds
Ice
Water
Earthquake
Structural Response:
Displacement
Stresses
Rocking
Shearing
Fatigue Life
Foundation
Sediment Response:
Cyclical: Tension,
Compression, Shearing
Property Degradation:
(Strength)
Stability
Displacements: Vertical,
Horizontal
Creep
Scour
FIGURE 2-S Interactions among environmental forcing effects, structural behavior, and
foundation/sediment responses for a typical oil production gravity platform. The dynamic and static
ocean structure-sediment interactions can lead to complex sediment behaviors, including degradation
or enhancement of strength properties, depending on the nature of the sediment beneath and
around the structure.
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Chemical Properties
Most of the sediment varieties in the world's oceans can be found in the U.S. EEZ. For
example, there are deposits rich in biogenic silica in the Bering Sea; organic-rich hemipelagic
sediment along the Pacific coast; metal-bearing ferromanganese crusts and nodules off Hawaii, other
Pacific islands, and the Blake Plateau; gas hydrate deposits in the Bering Sea and the Gulf of
Mexico; and well-o~ygenated abyssal lutites off Puerto Rico and the U.S. Pacific trust territories.
This diversity makes it difficult to summarize the geochemical properties of EEZ sediments, so only
the more important aspects are discussed here.
Gas Hydrates
Gas hydrates form when dissolved gas concentrations exceed thermodynamic solubility under
local temperature and pressure. Much of the continental margin sediments could contain hydrates,
although hydrates containing methane are less likely in depths less than 500 m and temperatures
warmer than 7°C. Brooks et al. (1984) reported gas hydrates of thermogenic and biogenic origin in
the Gulf of Mexico.
Thawing of hydrates that are near the sediment-water interface will adversely affect geotechnical
properties and installation of facilities. Such effects are inferred, however, since little experience
exists as to the effects of hydrates on engineering properties and behavior of sediments. Formation
of gas hydrates may be associated with the supply of reduced gases (such as methane, CH4, and
hydrogen sulfide, H2S) to sedimentary regimes in which microorganisms chemosynthesize the gases
and form the basis for a food web. Therefore, extraction of hydrates could possibly adversely affect
ecosystems that have evolved in these regions.
Ferromanganese Deposits
In many EEZ seabed regions, complex chemical interactions among the overlying water,
interstitial (pore) fluids, and rocks lead to concentrations of mineral deposits. One important
process involves precipitation of ferromanganese compounds on or in the sediment. Most of these
deposits contain varying amounts of economically important or strategic metals (such as cobalt,
platinum, manganese, and chromium) that could be in short supply during a natural or political
crisis. Deep-sea ferromanganese nodules have 0.24 percent cobalt, equivalent to the cobalt content
of ore from Zaire, which supplies the metal imported by the United States. Shallow-water
ferromanganese crusts have two to three times more cobalt than deepwater nodules. Upper slopes
of seamounts and ridges tend to have 2 to 4 cm of black ferromanganese oxide crusts containing
high contents of cobalt as well as other metals, such as nickel, cerium, molybdenum, and vanadium.
Sediment Oxidation-Reduction Chemistly
The average oxidation state is perhaps the most critical parameter in predicting the chemical
reactions in sediment. In general, shallow-water sediment near the coast is reducing, and deepwater
sediment on the continental slopes is oxidizing, although there are exceptions. The amount of
available oxygen affects the abundance and species composition of benthic organisms, which can
strongly modify the biogeochemical processes that occur (Aller, 1982~. Burrow abundance and
geometry can considerably alter distribution of the principal oxidants within sediments, and the
average size and distribution of burrows can strongly influence fluxes of solutes across the sediment-
water interface.
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Knowledge about redox processes and their possible effects on EEZ sediments is reasonably
good, but on a gross scale, EEZ sediments along the Pacific coast are richer in organic carbon (up
to 2 percent), and therefore more reducing than EEZ sediments along the Atlantic coast (0.25 to
0.5 percent).
Biological Properties
Biological processes can have important effects on the character and behavior of sediments.
Physical and chemical alteration of sediments occur when benthic animals move about and feed, and
in areas such as steep canyon walls burrowing animals can cause sediment instability. In most areas
of the seabed, benthic organisms affect sediment stability and can alter erosion rates and
resuspension.
Under what physical conditions the sediment in a given location will be eroded is important
information for a number of ocean engineering applications. The water velocity needed to initiate
sediment movement and transport is an especially critical engineering parameter, and much
theoretical and experimental work has been directed at predicting this value for different sediments.
Recent work has demonstrated that much of the discrepancy between these predictions and what we
observe in the ocean is due to biological effects on sediment properties (Jumars and Nowell, 1984~.
Bioturbation is the physical reworking and redistribution of sediment particles in the normal
course of movement and feeding of benthic organisms. The rate of bioturbation varies with
temperature, amount and input rate of organic matter in the seabed, type of benthic community, and
abundance of organisms. Bioturbation rates decrease with sediment depth, due to decreased
abundance of organisms, and generally are important only within the upper 1 m of sediment.
Knowledge of bioturbation processes is important to predicting sediment response to activities
such as waste and dredge disposal, since bioturbation can reintroduce materials from sediments into
the overlying water and possibly disperse them over large areas. By itself, bioturbation generally
acts to increase credibility of sediment by maintaining a high water content and physically moving
material toward the sediment surface where it can be moved about by bottom currents. However,
an important counteracting consequence of feeding of sediment-dwelling animals is the packaging of
small sediment particles into large fecal pellets, which have different transport thresholds and
hydrodynamic properties than the ambient sediments.
Adhesion of sediment particles due to mucous secretions of organisms can also alter sediment
erodibility. Mucus is produced by many benthic animals as an aid in locomotion and feeding, as
well as by sediment microalgae and bacteria as an anchorage or protective mechanism. Mucous
secretions increase the shear stress required to erode sediment by promoting particle-to-particle
contact. The resulting effect is strongest near the sediment surface, since most biological activity
concentrated there.
Once adhesive effects or other stabilizing biological factors are overcome by a strong enough
current, the underlying sediments tend to be rapidly eroded since the overlying resistant Recaps is
gone. The effects of biological modification of credibility may vary with time, given the strong
coupling among seasonality, temperature, and biological activity. At continental shelf depths, this
coupling is especially important since seasonal increases of bottom stress from winter storm waves is
likely to coincide with the period of minimal biological activity.
SEABED RESEARCH
Research in the EEZ requires investigation-generally long-term and multidisciplinar~of
complex interactive systems to acquire the quantity and quality of data necessary to test theories,
develop a framework of knowledge, and verify predictive models of seabed processes; and the
application of results to specific problems related to utilization of the seabed. Of the three major
geomorphic subdivisions of the EEZ seabed-the shelf, slope, and rise, collectively called the
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"continental margins-the shelf has been studied most extensively because it is the most accessible.
Many research topics are being actively pursued- within the U.S. EEZ by numerous applied and basic
research organizations. A brief overview of these activities follows.
Surveying
Mapping of the EEZ is being conducted to assess nonliving resource potential under the
auspices of a cooperative mapping agreement between the National Oceanic and Atmospheric
Administration (NOAA) and the U.S. Geological Survey (USGS) carried out by the Joint Office for
Mapping and Research (JOMAR) (McGregor and Lockwood, 1985~. The plan incorporates
NOAA's Sea Beam swath mapping coupled with GLORIA side-scan surveys conducted by the
USGS. The resultant products, high-resolution bathymetric maps and reconnaissance side-scan sonar
maps, provide an excellent geologic framework for future research.
JOMAR's program is carrying out blanket coverage of the EEZ and providing high-quality
seafloor maps to industry and local, state, and federal agencies for mineral exploration extraction,
and resource assessment. Continued and expanded high-resolution surveying will follow, building on
the data base provided by GLORIA (Lockwood and McGregor, 1988~. These detailed surveys will
lead to improved terrain evaluation procedures using survey data for quantitative classification of
areas. Sound scientific questions and problems related to present and anticipated uses of the EEZ
require information on resource potential and assessment of bottom conditions that may inhibit or
constrain development. Water depth, bottom slopes, seafloor topography, sediment properties, and
effects of various geologic processes must be known in order to design, install, and maintain
engineering structures such as platforms, pipelines, and cables. Surveying and mapping of seafloor
characteristics provide an important basis for development decisions. lithe needs Drill evolve as
development proceeds and as knowledge of the EEZ seafloor improves. However, it is clear that
each use of the seabed will require site-specific bathymetry, seafloor imagery, near-surface sediment
profiles, and measurement of sediment properties. By comparison, reconnaissance information may
provide useful background context, but in many cases is not appropriate for development of
particular sites or uses.
Sampling
Near-surface sampling is used to primarily correlate sediment distribution characteristics with
acoustic imagery analysis and process evaluation. Hindcasting of processes based on sedimentologic
results and age dating to determine magnitude and frequency of recent geologic events is typically
done from box and gravity cores.
In situ measurements of shear velocity and attenuation as a function of depth in unconsolidated
shelf sediments have been made in a few isolated locations (Jacobson, 1987; Heacock, 1988~. In
conjunction with these tests, shallow-water, in situ sediment probes were recently developed and
tested. In situ measurements of sediment porosity, permeability, and excess pore pressure
fluctuations have been made, and these data are being compared to acoustic information (Yamamoto
and Torii, 1986~.
Three acoustic systems for use in EEZ shelf waters are being evaluated. The first is the shear
sled receiver system, which can generate and receive shear sound waves to determine the rigidity
modulus of sediments. A deep-tow, high-resolution, shallow-water, subbottom seismic system is also
available that can be used for three-dimensional seismic mapping of shelf and upper slope geologic
structures and sedimentary facies to better understand their geometry and development processes
(Milkman et al., in press). Another high-resolution seismic system in shallow-water use is the Chirp
Sonar, a broad-band system designed for high-resolution profiling and for ascertaining lateral and
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vertical variability in sound attenuation. This system is undergoing extensive field testing and is
considered developmental (Schock et al., 1989~.
Deep coring (e.g., Ocean Drilling Project) can be used to provide sediment data from the
ancient rock record for correlation with acoustics to establish overall geologic development of
sedimentary sequences and provinces. Although sampling techniques using bore holes are well
developed, there is still no reliable and quick method for getting deep (tens of meters) cores in
noncohesive sediments (sand).
Sediment Transport and Organism-Sediment Interaction
Studies of cohesive sediment transport processes influenced by bottom currents in the absence of
waves on the continental rise off the northeastern United States show that most sediment particles
are transported and deposited during short periods (days) by benthic storms that occur about ten
times a year in this region (Nowell and Hollister, 1985~.
The present level of understanding of the effects of benthic organisms on sediment transport is
inadequate for predictive models. Only a few studies have considered the cumulative effects of
organism communities on sediment erosion, deposition, and transport (Grant et al., 1982~; most
research has focused instead on single species maintained in laboratory flumes. Even under
controlled laboratory conditions, the net effects of an organism on erosion and deposition are
difficult to predict (Jumars and Nowell, 1984), particularly the point at which biological effects
become unimportant relative to physical processes.
Outstanding questions on the effects of biological processes on chemical processes in sediments
include how organisms' activities and biogenic structures alter the flow in the boundary layer,
especially the viscous sublayer. Another important research area is the effect of sedimentary
material passing through the guts of benthic organisms on remobilization of particle-bound
materials, such as metals and organic pollutants. Feeding, irrigation, and burrowing activities of
benthic animals are particularly important in shelf environments, where a substantial portion of
organic matter remineralization takes place on the seabed, and nutrients are returned, with only
minor time lags, to the photic zone.
Alterations of the benthic community during disturbances due to environmental or normal
population variations can influence short-term transport of solutes and particles within the
environment as well as long-term storage in the seabed. How such changes influence primary
production, plankton species composition, or cycling of different bioactive elements through the food
web and water column are largely unknown.
Recent advances in the understanding of nonlinear wave-current interaction theory has provided
the breakthrough necessary to initiate research into the predictability of changes in the sediment
regime and microscale topography of the continental shelf resulting from forcing functions active
during major storms (Nowell et al., 1987~.
Gas and Frost in Sediments
Studies of the distribution and geometry of gassy sediments in the Gulf of Mexico, California,
and Alaska are being conducted, and models are being developed to explain their distribution
(Anderson and Bryant, 1987~. The geochemistry, stability, occurrence, and transformation of frozen
gas hydrates, and their relation to deep source gas resources and geohazards (to drilling) are
continuing concerns.
A subsea permafrost study is under way in Prudhoe Bay, Alaska, where physical and
geotechnical measurements of thawed and permafrost layers will be interpreted in terms of heat and
mass transport using existing theories and numerical calculations. Seismic methods have been used
to detect and map discontinuous permafrost in the Beaufort Sea, but results are equivocal. A
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combination of acoustic soundings, measurements, and deep cores would be useful to delineate the
extent of the permafrost.
Slope Stability
There is abundant evidence of deformations and failures on submarine slopes, including
catastrophic movement of large masses of sediment in fairly coherent blocks or slumps, debris flows,
and turbidity currents (Campbell et al., 1986~. There is also evidence of some slope sediments
gradually deforming downslope (creeping) and that accumulated creep strains may eventually lead to
catastrophic failures (Booth et al., 1984; Silva and Booth, 1985; Silva et al., 1989~. Regions
subjected to these processes can encompass enormous masses of seabed over areas exceeding
100 km.
Quantitative analysis of undersea slopes has so far been largely restricted to crude estimates of
slope stability using limit equilibrium procedures. There are no reliable techniques for predicting
initiation of debris flows and turbidity currents or for modeling their behavior. At the other end of
the spectrum, long-term creep deformation of slope sediments is just beginning to be understood.
Understanding of slope sediment dynamics is complicated by the effects of earthquakes and wave
loadings (surface and internal waves), the morphology of subbottom stratification, and the pertinent
stress-strain-time and theological properties of sediments. Improved analytical models to determine
deformations and stability of complex submarine slope situations is needed, including long-term
(creep) deformations of undersea slopes, prediction of creep-rupture mechanisms, stability of slopes
for a- variety of forcing conditions, and post-failure behavior of flows. Significantly more focus is
required to develop a theoretical framework to describe and analyze seabed materials behavior, and
to develop methods to predict seabed stability and dynamics (Nelson and Smith, 1989~.
Physical and Biological Research
Physical oceanographic research will use the seafloor as a base for instrument packages for
long-term monitoring of oceanographic phenomena. Bottom boundary layer studies relevant to
erosion and sediment transport within canyons are examples of such research that can be envisaged
to expand. The seafloor will also be used for deployment of sensors that measure processes and
properties in the water column (Brink, 1987; Allen et al., 1987~: for example, acoustic current
meters and inverted echo sounders look at topographic control of currents, warm core ring
degradation, and internal wave generation.
Long-term biological monitoring of specific sites is anticipated to examine population changes
and ecosystem dynamics in response to different uses, such as oil and gas development, waste
disposal, and mining. Biological research will expand its data base on benthic communities, such as
infaunal and epifaunal donations, structures, and controls. Considerable site-specific work can also
be expected on new exotic communities, particularly at vents.
Remote Sensing
Satellite and aerogeophysical technology provide useful instrumentation for exploring and
monitoring the EEZ. Geophysicists have developed a number of remote sensing techniques for
. . . . . · ~ , · .. . . . - · . · . . . , .. ~
gravity analysis that provide information on sediment loading and basin Development, slrengln and
age of the lithosphere, continental rifting, and location of faults, basins, sediment types, and other
geological features (Hammer, 1983~. Magnetic anomaly analysis of data from remote sensing
technology indicates the distribution of ferromagnetism within the lithosphere and therefore is linked
to the mineralogy of the crust. Magnetic basement techniques, which rely on the wavelength of the
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observed field, can be used to infer thickness of sediment cover. Magnetic analysis can be useful in
determining the age of the oceanic crust, studying continental rifting mechanisms, and modeling the
configuration of the continents and their margins prior to the formation of the bordering ocean
(Webster et al., 1985~.
Satellite and aircraft remote sensing can provide synoptic and repetitive information about
environmental changes associated with development of seabed resources and other uses of the
seabed, such as waste disposal. More intensive experimentation with these techniques is needed to
optimize their use for acquiring information about the seafloor.
A system for airborne seismic surveying is in the experimental stage, based on elements of
systems used by the U.S. Navy and researchers to map the acoustic properties of the oceans
(LaBrecque et al., 1986~. Surface and bottom arrays of expendable hydrophores are deployed by
aircraft and monitored using the Global Positioning System (GPS). Explosive charges are used as
sound sources. The advantage of this proposed technique would be its ability to cover large areas
rapidly. It is unlikely that aeroseismics could ever achieve the resolution of three-dimensional
multichannel surveying, but it would serve as a useful reconnaissance tool.
Research Activities
Many government agencies and private industries support research related to the benthic
boundary layer, the seabed, and the subseabed. Research on seabed processes is conducted by the
National Science Foundation (NSF), the Office of Naval Research (ONR) (Advisory Committee on
Ocean Sciences [NSF], 1987; Jacobsen [ONR], 1987; and Heacock [ONRi, 1988,j, the USGS, and
NOAA An ongoing mapping and surveying program in the EEZ is underway through the
USGSINOAA Joint Office for Mapping and Research (Lockwood and McGregor, 1988; Lockwood,
1989~. NOAA, through its Sea grant Program, supports research that includes offshore mineral
resource evaluation (Sea Grant Abstracts, 1988~. Research on particle flux across the EEZ has been
supported by the Department of Energy (DOE) (McCammon, 1988~. Summaries of engineering
research activities on the EEZ can be found in Seymour and Webster (1987) and Yuen (1987~.
Representative terms from entire chapter:
low low low
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