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2 SEABED PROCESSES AND RESEARCH ACTIVITIES In order to appreciate the problems and challenges associated with uses of the EEZ seabed, it is necessary to understand its environment, the processes affecting it, and the complexities of its highly variable regions. In this chapter, some important features of the EEZ seabed, the processes that affect its utilization, and research that has been undertaken to understand it are discussed. SEABED CHARACTERISTICS The U.S. EEZ embraces a vast range of seabed morphology, water depths, tectonic and transport processes, sediment types, and environmental conditions. Topography varies from relatively flat on the shelf and lower continental rise to very rugged on steeper slopes and canyons. Depths vary from the shallow continental insular shelves, where surface waves affect the seabed, to regions where depths exceed 4,000 m. The EEZ seabed encompasses passive and active tectonic margins and volcanic regions where a variety of tectonic processes, along with other environmental forces, such as currents, surface waves, tsunamis, earthquakes, and ice scouring affect the seabed and reshape and rework its sediments. Seabed materials include rock outcrops, boulders, coarse sands and gravels, biogenic sediments, carbonate reefs, phosphate deposits, silts, clays, gassy sediments, permafrost, hydrothermal crusts, and manganese nodules. These diverse conditions create a highly variable environment with important implications for expanded seabed utilization. While it is possible to assess most seabed conditions and predict many seabed processes, there are still some processes that are poorly understood. Regional Features ~c;' lithe EEZ includes a relatively flat shelf adjacent to the coast, a steeper (5° to 10°) continental slope often incised by canyons that provide conduits to deeper water, and a gently sloping continental rise (see Figure 1-2~. There are many variations of this simplistic model, depending on tectonic processes and other factors (Figure 2-1~. For example, the Atlantic seacoast region is a passive margin with little tectonic activity, a wide shelf, and thick deposits of sediments, and is similar to the classic model shown in Figure 1-2. The active margin along the Pacific seacoast is quite different. The shelf is generally narrow, and dynamic tectonic processes beneath the Pacific EEZ have resulted in greater physiographic variability than in other areas of the U.S. continental coast. Around the volcanic islands of Hawaii and the Pacific trust territories are narrow shelves consisting of coral reefs and volcanic aprons that extend to abyssal depths. Offshore of large rivers such as the Mississippi are thick deposits of underconsolidated and gassy sediments, canyons, diapirs, and large fan deposits of sediments in deeper water (Figure 2-2~. Other important regional features
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6 ATLANTIC MARGIN: (trailing edge) 1~ 1 Sea Level I ,-,\~_~,,\~,,/-,'.~.\'~", i"~'-\'\V~ NORTH PACIFIC MARGIN: (collision) Sea Level I ~_ ~ - - ~r ~ FIGURE 2-1 Typical idealized cross sections of physiographic provinces of the Atlantic and Pacific continental margins of the U.S. EEZ. The Atlantic coast is characterized as a passive margin with little tectonic activity, a wide shelf, and thick sediment deposits. The Pacific coast is composed of an active margin, or subduction region, with a relatively narrow shelf, thin sediment cover, and significant tectonic activity. SOURCE: After McGregor and Lockwood, 1985. include the area off the western coast of Florida, where there are carbonate sediments, and the arctic regions of Alaska, where permafrost, deepwater gas hydrates, and ice packs affect the seabed. EXTERNAL ENVIRONMENTAL EFFECTS Major environmental forces affecting the ocean floor include seismicity and active faulting at ridge crests and active margins; tsunamis, hurricanes and storm-related waves and currents in shallow waters; bottom currents in deeper water; and ice keel gouging in the high latitudes. In order to use the seabed, it is necessary to be able to predict the frequency, magnitude, and duration of these processes, and to understand their impacts on the seabed and on ocean bottom structures. Earthquakes Earthquakes are a particular concern along the active margins of the west coast, Puerto Rico, Guam, the northern Marianas, and along the Aleutian trench in the North Pacific. The seismic history of arctic seas is poorly documented. The Atlantic coast is mostly inactive, although recent (1984) earthquakes have been documented by the USGS Earthquake Information Center. The Gulf of Mexico historically has had only one earthquake of any magnitude, and is considered fairly safe from earth shock (Teleki et al., 1979), although the existence of growth faults ringing the Gulf may indicate potential for earthquake activity in the future.
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7 DIAPIR FLANK EROSI ON TIC I L, DIAPIR CREST 'WIPEOUTS' (EROSION, CARBONATES DISTURBED SEDIMENT) BURIED / SLIDE DEPOSITS / CRESTAL G RABIES BANKS AND SHELF BIOTHERMS EDGE / \ 1 ANDSLIDES ~/ ~ \.',.;:'7~::x I'm SALT DIAPIR W7.2::, hi\^ _ _~. m.::- : / . ~/ 1 HYDRATE GAS NEAR-SURFACE HILL SEEPS LANDSLIDE DEPOSITS GAS SEEPS SHELF EDGE /NEAR-SURFACE EROSION / DISTURBED I / SEDIMENT ROTATED BLOCKS , . / GROWTH FAULTS FIGURE 2-2 Principal geologic features of the outer continental shelf and upper continental slope offshore Louisiana and Texas. SOURCE: After Campbell et al., 1986. Damage due to shaking by earthquakes can be great, primarily resulting from the mass wasting (downslope movement of a large volumes of sediment) that occurs with slope failure. Normally stable slopes can fail when subjected to accelerations caused by earthquakes, and any structures, pipelines, or cables in the vicinity may be displaced or fail. It is difficult to ascertain how widespread and frequent seismically induced seafloor failures are, because so little is known about the geology of the seafloor in most of the EEZ. Furthermore, knowledge of sediment response to strong ground motion accelerations is incomplete. Data on ground acceleration, velocity, displacement, event duration, and frequency are required for the design of large structures (Bee, 1978~. Tsunamis Tsunamis are long-period, open-ocean waves thought to be caused primarily by submarine landslides triggered by earthquakes or induced by shallow-focus-depth earthquakes or submarine volcanoes. Tsunamis are particularly dangerous because they are unpredictable, travel at great speeds, and build up very high waves when they shoal in shallow waters (a wave less than 1-m high in deep water may become a 30-m wave when reaching shore). Effective warning systems have been implemented, especially on Pacific islands, that reduce the human risks from these events. The
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8 areas most susceptible to these waves are along the west coast, the Pacific islands, and Alaska. Although damage to coastal areas by tsunamis is well documented, their influence on the seabed is not. Hurricanes and Storm-Related Waves Most hurricanes can be predicted well in advance with satellite remote sensing. Adequate data exists on the high winds, waves, currents, and storm surges associated with hurricanes. The data base for storm-related extreme wave conditions, however, contains much more hindcasted data than measured parameters. While data coverage is good for the east and west coasts and the Gulf of Mexico, it is sparse for the Arctic. Many existing wind-wave hindcast models are developed for specific geographic areas and storm conditions. Surface waves affect the seabed in shallow water, where the pressure and water motion impinges directly on the seabed and can lead to mass failures and erosion. Internal waves may play an important role in deeper water, but little data is available on these. Currents In most of the EEZ, near-bottom currents exist at various scales of motion, duration, frequency, and magnitude. Currents result from tidal forcing circulation, major oceanic-scale current systems (such as the Gulf Stream), wind setup, and storm surge. While continental shelf circulation has been the subject of extensive research, (Butman et al., 1979), currents on the slope and rise are just beginning to be deciphered. The topography in the EEZ, especially on continental margins and seamounts, influences the magnitude and direction of currents (Heezen and Hollister, 1984~. Episodic currents confirmed on the east coast continental rise are capable of suspending and transporting sediment by "benthic storms" that can occur several times a year (Hollister et al., 1984~. Ice-Bottom Interaction Ice gouging is an important concern in the Arctic. Enormous forces are imparted to the seabed when large ice masses, pushed by winds and currents, contact the seafloor (Figure 2-3~. The depth of most gouges is of the order of 1 m or less, but scour depths exceeding 7 m have been documented. In the Arctic Ocean north of Alaska virtually no part of the continental shelf has been spared from reworking by ice. These are very active, seasonal processes that have important implications for seafloor structures, pipelines, and instruments. / ~ <~rc~,~ FIGURE 2-3 Primary features of ice gouging. Gouges can be several meters in depth and many kilometers long. Sediment properties are altered by gouging, and engineering of structures and pipelines must take this process into account. SOURCE: After Barnes and Reimitz, 1974. d - Gouge Depth s - Saii Height - Gouge Width k - Keei Depth r- Ridge Height ~ - ice Motion
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9 NATURAL SEABED PROCESSES The complex nature of the seabed is the consequence of an array of processes occurring in these regimes over millions of years. In addition to the forces already discussed, the seabed is shaped by the processes associated with the rise and fall of sea level during glacial and interglacial periods, the consequent sediment influx from continents, and changes in bottom current intensity. These forces create a dynamic and continually changing environment. Therefore, understanding the evolution and active processes of EEZ seabed regimes and being able to reasonably predict their effect on the seabed are essential to seabed development activities. Iwo broad categories of active seabed processes are particularly important to EEZ utilization because of their widespread effects: mass wasting and slope deformation, and sediment dynamics (erosion, transport, and deposition). Sediment Mass Movements Mass wasting is the downslope movement of sediment or rock, such as a submarine landslide. Epically, what occurs is catastrophic slumping or sliding along a well-defined failure surface within the sediment, which results in an upslope scar surface and fairly coherent but deformed sediment deposit on the downslope side. A full range of deformations occurs on submarine slopes; very gradual (creep) deformations may eventually lead to more catastrophic failures, fluid-like debris flows, and turbidity currents (Figure 2-4~. These processes may encompass enormous masses of sediment and influence areas of more than 100 km2 (half the area of Rhode Island) and are therefore important considerations in siting offshore facilities. Because mass wasting can occur even on very low-angle slope inclinations (less than 1°), it constitutes an important geohazard to many submarine installations and operations. In addition to gravity, the primary causes of slope failure are earthquakes and associated faulting; changes in slope geometry resulting from oversteepening; or scour, wave loading, creep, loading by structures and construction, gassy sediments, and rapid sedimentation. Failure does not usually result from just one cause, but rather from a combination of factors. Thus a slope that is safe in normal conditions may fail under unique storm-loading conditions. Areas of potential slope instability are deltas, which continually build by rapid sediment input; areas subject to storm wave influence; and · the continental slope and upper continental rise, where slope inclinations are relatively steep and sediments are weak. Slope failures have been documented in many EEZ environments including the Mississippi River delta, Copper River (Alaska), the continental slopes off the Atlantic coast, in the Beaufort Sea, and off the west coast of the Bering Sea. Much of the mass wasting on the Atlantic margin occurred during low sea level stands (50- to 100-m lower than present) when sediment was rapidly supplied to the shelf edge. The amount of mass wasting still occurring is uncertain; however, it is known that erosive processes are active on the continental slope, and mass wasting is occurring upslope as a result of undercutting. Mass wasting is also occurring in the Mississippi delta, offshore of Fords on the northwest coast, in many submarine canyons, and in localized areas of the Atlantic coast slope. There are obvious implications of mass wasting for engineered installations on slopes. As oil and gas operations move to deeper water, the full range of downslope processes (Figure 2-4) will need to be better understood in order to avoid potentially hazardous areas or to design structures
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Representative terms from entire chapter:
low low low
10 I sLoPE L PROCESSES ~ 1 FORCING FUN'CTION'S Gravity Sedimentation Wave loading Earthquakes Faults, e t c . CREEP ~ SLIDE/ SLL'MP it FLOWS . /n t e r n at OCR for page 11
11 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.
12 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.
13 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.
14 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.
15 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
16 "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
17 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
18 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
19 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~.
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Representative terms from entire chapter:
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