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Oceanography and Naval Special Warfare As discussed earlier, the nature of Naval Special Warfare (NSW) operations makes understanding the envi- ronmental factors that NSW units may face an important aspect of mission planning and execution. Consequently, the symposium was organized to provide attendees with a good introduction to the capabilities and challenges facing NSW (including the Sea, Air, and Land [SEAL] teams) and the meteorological and oceanographic (METOC) community that supports them. The Naval Special Warfare Mission Planning Guide identifies a number of environmental parameters, includ- ing lunar illumination, water temperature, bathymetry, wave height, water clarity, humidity, current direction and speed, that must be considered throughout mission planning and execution. These factors affect many aspects of mission planning, including transportation and communications. The symposium was organized to maximize interaction between NSW operators and the attending scientists, while focusing attention on important environmental parameters. The first two days emphasized operational require- ments while the third day allowed colleagues of similar backgrounds and interests to discuss lessons learned and identify potential future research ideas related to NSW needs. Attendees were therefore organized into five working groups: Bioluminescence and Marine Toxins, Waves and Surf, Currents and Tides, Electromagnetic and Infrared "Above Surface" Signal Propagation and Winds, and Electrooptical and Acoustic "Below Surface" Signal Propaga- tion. The following chapter includes both a discussion of how environmental factors may affect mission planning and execution and a summary of the discussions that took place during the meeting of the five working groups. NAVAL SPECIAL WARFARE MISSION PLANNING The method used by NSW personnel to develop a mission profile has been evolving since the mid 1980s and is referred to as a phase plan. Each member of the SEAL platoon takes part in designing a set of highly orchestrated actions. These actions are tied to a detailed and rigorous time line that describes the role of each team member as he is transported to and enters the mission area, performs one or more assigned missions, and returns. Options and backup contingency actions are explicitly planned to ensure the highest probability of success and the greatest team safety. The planning cycle typically includes a 96-hour work-up phase to allow sufficient time for personnel and resources to be made available. However, this can be compressed in a national emergency or at the direction of the National Command Authority. Until recently, mission plans were literally hand drawn. Recent efforts have been 25
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26 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES made to develop automated mission planning software for use on a personal computer. Automation promises to allow better and faster plan modification, especially if technological advances allow environmental and intelli- gence data to be updated more efficiently. Although the environment plays an important role in any military operation, missions conducted by special operations forces in general, and NSW forces in particular, are extremely environmentally sensitive. Environmen- tal conditions are an important consideration in mission planning as demonstrated by the importance placed on them in the NSW Mission Planning Guide (prepared by the Naval Special Warfare Center). Accurate and reliable information about a range of environmental conditions can make the difference between NSW personnel obtaining a tactical advantage or having the mission's success severely and negatively impacted and personnel placed in unnecessary danger. Compromise is the best option in certain instances where ambient environmental conditions present some operational advantage as well as a hindrance. For example, a moonless night assists by decreasing the possibility of detection of the SEAL team but also increases the degree of difficulty it will encounter in finding the assigned target. Similarly, low winter air temperatures may be advantageous when they reduce the efficiency of a sentry guarding a targeted harbor facility; however, cold water temperatures can cause a SEAL swimmer to breathe faster and more rapidly consume the oxygen supply in his closed-circuit diving support system. Mission planning and execution involves an ongoing series of choices in response to a constantly changing set of environmental and tactical conditions. Consequently, a series of meteorological and oceanographic (METOC) critical thresholds has been developed, as part of the NSW Mission Planning Guide, to help mission planners and the operators make reasonable decisions about when and how to take environmental factors into consideration. The NSW Mission Planning Process addresses the potential impact of a range of environmental parameters on various mission phases. As discussed in Chapter 3, the Naval Oceanographic Office (NAVOCEANO) provides NSW personnel with environmental information in a variety of forms. Much of the most relevant environmental information is conveyed to NSW personnel from the Warfighter Support Center through a STOIC (Special Tactical Oceanographic Information Chart) similar to the example included as Plate I. The following sections describe the general structure of a hypothetical, yet typical, SEAL mission and discuss the impact of a variety of environmental factors on mission planning and execution. This discussion is intended to provide some context for the remaining sections of the chapter, which deal with the wide range of parameters discussed in the NSW Mission Planning Guide. Environmental Factors and Mission Success Although describing any SEAL mission as typical is probably an oversimplification, the typical mission can generally be divided into five components: (1) insertion, (2) infiltration, (3) action at the objective, (4) exfiltration, and (5) extraction. Due to logistical considerations, the boundaries between insertion and infiltration, or exfiltration and extraction, can be indistinct but significant. NAVSPECWARCOM does not typically possess platforms capable of transporting SEAL units long distances in a maritime environment. Therefore, insertion is defined as that phase when a variety of platforms (typically non-NSW submarines, surface craft, or aircraft) are used to transport NSW personnel and equipment to a location from which smaller, shorter-range NSW platforms can be used to begin infiltration (extraction is similarly defined as the boarding and tactical transport of NSW personnel and equipment away from a location reached during exfiltration). Due to the need to coordinate the use of non- NSW platforms during insertion and extraction, the infiltration and exfiltration phases of a mission must be planned to provide maximum flexibility. The following sections describe the impact that environmental factors can have on each phase of a hypotheti- cal SEAL mission to destroy a number of ships docked in a foreign and hostile harbor. As discussed in Chapter 2, NSW personnel carry out an extremely varied range of missions. The example described here thus represents a fairly specific mission profile and is included simply to demonstrate the reliance NSW planners and operators place on environmental information. Other SEAL missions would, by necessity, require more or less emphasis on certain environmental factors discussed in the NSW Mission Planning Guide. The use of SEALs in this situation would indicate that stealth and minimal collateral damage were important considerations in the decision to undertake this mission.
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE Insertion and Infiltration 27 The mission profile calls for use of a SEAL squad transported to waters well offshore from the harbor by a Navy submarine equipped with a Dry Deck Shelter (DDS) and the SDV. Based on a range of mission consider- ations, infiltration by a SEAL Delivery Vehicle (SDV) was chosen (Table 4-1~. In planning the mission, SEAL swimmers and the SDV team considered the potential impact of various environmental factors during the last stage of insertion lockout (during which the SEALs make the transition from the DDS to the flooded SDV) and the infiltration phase. For this mission, infiltration includes (1) the transportation of the SEALs to the enemy harbor aboard the SDV, and (2) the SDV' s approach to, and preparation to enter, the enemy harbor. During infiltration, the SDV may have to avoid unexpected underwater obstacles and wait while unanticipated harbor traffic subsides. Preserving the clandestine nature of the mission is a high priority, especially once the SDV enters the harbor. Actions at the Objective Once inside the harbor, the SDV will make its way to the pier where the target vessels are docked (Table 4-2~. Navigating within a foreign harbor is complex because the SDV must avoid both unknown obstacles and detection by craft entering, exiting, or patrolling the harbor. Successful navigation can be made more complicated since the SDV is flooded and turbidity can adversely affect the SDV operator's ability to read displays on the console. Once the SDV has reached the appropriate position, SEAL swimmers must exit it successfully, find the correct ships, and attach explosive charges at specific locations to ensure that the targets are crippled or destroyed. Successful completion of this delicate phase of the mission will require that SEALs work quickly and accurately while avoiding detection from both enemy personnel and civilians working on piers or on board various vessels in the harbor. Exfiltration and Extraction Many of the factors that can impact execution of the insertion (lockout), infiltration, and action at the objective phases of the mission are relevant during the exfiltration and extraction (lock in) phases (Table 4-3~. However, due to the length of time elapsed and the complex nature of SEAL activities during the first two phases of the mission, the potential impact of these factors can be magnified by the effects of fatigue. Although detection during this phase may not allow the enemy time to prevent destruction of the target, it can place the SDV and the SEALs at risk and may even jeopardize the safety of the submarine (and its crew) that waits to complete their extraction. The hypothetical SEAL mission discussed above is intended to provide some idea of how a variety of environmental factors may impact mission planning or execution. Because NSW missions can be highly varied and complex, this one hypothetical example cannot fully demonstrate the potential impact of all the environmental factors discussed in the NSW Mission Planning Guide on the breadth of potential NSW missions. Consequently, summaries of working group discussions that took place during the symposium (i.e., Bioluminescence and Toxins Working Group, Waves and Surf Working Group, Currents and Tides Working Group, EM/JR "Above Surface" Signal Propagation and Coastal Winds Working Group, EO/Acoustic "Below Surface" Signal Propagation Work- ing Group) have been edited and augmented by the steering committee (working with naval personnel, symposium working group leaders, and other symposium participants) to provide a detailed discussion of a significant number of the environmental factors that may play a role in NSW mission planning and execution. Each of the following sections attempts to discuss the way these individual factors or processes can play a role in NSW operations, to review the present approaches and capabilities used to address these factors, and to identify some of the salient points raised by attendees during the symposium. As with any gathering in which emphasis is placed on attendee participation, this symposium was designed to allow the participants to take the discussion into areas of mutual interest. Therefore, not all of the topics identified in the NSW Mission Planning Guide received equal treatment. The following sections are intended solely to summarize discussions that took place during the sympo- sium. The steering committee did not attend all working group sessions and was not constituted to review their technical content. The inclusions of proposed solutions or future research agendas suggested by meeting participants should not be interpreted as an endorsement by the steering committee or the National Research Council.
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 3 BIOLUMINESCENCE Bioluminescence is an environmental factor that can significantly impact the vulnerability and efficiency of Navy personnel involved in NSW. The critical threshold of bioluminescence is defined militarily as any condition that allows visible detection of an SDV or swimmer submerged to 10 feet under ambient light conditions. Mission Influence NSW personnel rely on stealth in all aspects of the associated logistics and communications to accomplish their mission objectives. Clandestine insertion of personnel and manned or unmanned vehicles through defended, denied, littoral waters requires that no inherent visible indicator betray the presence of NSW forces at night. Littoral waters are most often nutrient rich due to upwelling and terrestrial discharges (such as storm water runoff) and have been shown to support high concentrations of dinoflagellates (as well as other bioluminescent organisms) on a seasonal basis. Consequently, coastal waters can accumulate large populations of bioluminescent organisms above the thermocline, making transit of personnel and vehicles through this zone susceptible to detection by the unaided eye and light-intensified devices. There is an operational need for monitoring and predicting conditions in which coastal bioluminescence may hamper night operations. Operationally, the occurrence of bioluminescence is readily apparent. The challenge is to recognize the potential at the planning stage or immediately preceding the mission or exercise. Currently, the primary sources of data on bioluminescence are Naval Oceanographic Office (NAVOCEANO) products, such as Special Tactical Oceanographic Information Charts (STOICs; see Plate I), Mine Warfare Pilots, and Submarine Insertion Loitering Area Charts (SULACs). (Many of these products should be examined critically before basing operational deci- sions on them as the abundance and distribution of most marine organisms responsible for bioluminescence vary seasonally.) In addition, there are several university studies that are site specific and limited in scope. Given the extreme variability of bioluminescence in the coastal zone, as well as the very limited number of researchers who are collecting field data in this environment, the attendees recognized that additional field data would be needed to identify sources of variability in levels of bioluminescence. Research Issues Long Term Goals Signature Reduction To adequately address the problem of detection or vulnerability of divers and undersea vehicles, two types of activities were initiated in 1991 at Naval Research and Development (NRaD), San Diego, following a request for supporting research in this area from the Naval Special Warfare Center (NSWC). The first attempted to measure the long-term variability of bioluminescence in settings with similar conditions to sites where NSW missions can be expected to take place. The second attempted to conduct a series of exercises with combat swimmers, MK VIII SDV, and a sea-truthing survey craft to collect oceanographic data on bioluminescence intensity, seawater tem- perature and clarity, and on species abundance. The objective of these activities was to develop threshold levels for visual detection of combat swimmers and SDVs during nighttime operations, because of bioluminescence. The threshold level would be used to determine the probability of visual or light-amplified detection of swimmers and their vehicles by the production of luminous wakes at night. Exercises conducted at Naval Research and Develop- ment (NRaD) and Camp Pendleton, California from 1991 to 1997 have shown that visual detection ratio (VDR) (VDR represents the optical relationship between measured bioluminescent intensity and sea water clarity thresh- old) determination is useful for determining the probability of visual or light-amplified detection of swimmers and their vehicles in coastal waters during any season. Several possibilities exist for reducing military signatures associated with bioluminescence. Near-term op- portunities may exist to minimize the bioluminescence signature of slow-moving platforms as much as 50 percent by shielding "hot spots" such as propellers. This might not be as successful for high-speed craft. Bioluminescent
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32 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES wakes extending one full hull length and more behind a vessel have been reported. Covering emissions by shielding flashes and cloaking points of high turbulence could be made available as field expedients or modifica- tion kits. Signature measurements are needed for the boats and vehicles used by NSW units. Environmental conditions will also affect the signal-to-noise ratio and hence the threat of detection from bioluminescence through the addition of ambient light. For example, moon glitter will reduce the signal to noise ratio by adding ocean surface glitter. Therefore, mission planners should consider the beneficial aspects of moonlight from a signature reduction viewpoint. Another environmental condition that can affect the signal-to- noise ratio is sea state. Rough weather has two effects: surface breaking waves (whitecaps) increase surface glitter. High sea state also promotes mixing of organisms down into the water column and may dilute surface concentrations of organisms and hence the magnitude of the signature. Water turbidity and absorption can attenuate the observable signal caused by submerged divers or vehicles, therefore mission planners concerned about bioluminescence should consider moonlight, atmospheric conditions, prior and current sea state, and water optical properties. The fundamental oceanography training provided in Basic Underwater Demolition/SEAL (BUD/S) training was discussed at the symposium. ~ ' ~ ' ' ' ' '' ' ' ' ' a' changes in depth, course, and/or speed. These techniques should be implemented if a reconnaissance swimmer observes bioluminescence or other reliable reports are available. Current NRaD experiments indicate that surveys of inshore areas can provide an excellent opportunity to identify patterns of bioluminescence with both horizontal location and depth. Natural occurrences such as the turbulence produced by waves breaking over a reef or obstacle could also be exploited. Effective training would address obtaining data from activities of the Naval Meteorology and Oceanography Command (NAVMETOCCOM), as located in San Diego, Norfolk, and other fleet centers or naval air stations. These facilities provide ready access to NAVOCEANO data bases and products. NRaD is currently supplying swimmer detection ratios to NAVMETOCCOM and NAVOCEANO during planned exer- cises. Some attendees suggested that bioluminescence could be evaded through Current Understanding and Technology Bioluminescence intensity varies widely among the more than 500 marine genera that produce light. Wave- lengths cover the spectrum of visible light and in several rare cases even extend into the ultraviolet (UV) and the infrared (IR). However, in the marine environment, bioluminescence is confined primarily to blue and blue-green emissions. Low light-level TV cameras or image intensifiers can be carried by ships or aircraft to detect biolumi- nescence, and potentially, the outline of the physical disturbance at a threshold that is slightly better than the fully ~_ dark-adapted human eye. These and other sensing techniques may otter a means to measure bioluminescence rapidly and to identify otherwise dark and undetectable objects. Bathyphotometers measure light beneath the ocean surface. In general, such instruments are designed to draw water that contains organisms through a light-tight chamber where a light detector measures the bioluminescence stimulated by some turbulence-generating mechanism, such as an impeller, constriction, or grid. Since some bioluminescent organisms produce only a single flash whereas others produce multiple flashes and flash durations vary from less than 100 milliseconds to many seconds, the values measured by different bathyphotometer designs are generally instrument specific. In other words, the photon flux that is reported depends on such factors as the detection chamber volume, the flow rate through the chamber, the method of stimulation, and the amount of pre- stimulation that occurs due to light baffling. Some of this variability is evident in the different units used to report measurements of stimulated bioluminescence (see Table 4-4~. Primarv among these units have been photons ner unit volume and photons ner second ner unit volume. Units ~ ~7 1 1 1 1 1 of photons per unit volume are used when the residence time of the bioluminescent organism in the detection chamber is long enough for a whole flash to occur. Under such circumstances the average photon flux measured by the light detector is a function of the concentration of bioluminescent organisms, the total photons per flash, and the volumetric flow through the detection chamber. In these cases the average photon flux (photons/per second) is divided by the volumetric flow (volume/per second) and the results are reported in photons per liter. Alterna- tively, when the residence time in the detection chamber is short compared to the duration of the flash, then the
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE TABLE 4-4 Bathyphotometer Measurements of Bioluminescence per Liter in Different Oceans Location Deptha (m) Biolum~nescenceb,C Sargasso Sea 0-100 1 x 101° Greenland Sea 0-150 1.1 x 101° Pacific Ocean (Alaska to Hawaii) Surface 1-5 x 101°C North Atlantic Ocean (MLML) 12-80 0.7-2.8 x 101° Caribbean Sea 0-120 4.5 x 101° Beaufort Sea (NRaD) 0-60 5 x 108-1.2 x 1ollc Vestfjord, Norway (NRaD) 0-100 0.9-2.7 x 101lc Arabian Sea (NRaD) 0-70 2-9 x 101lc NOTE: MLML = Mixed Light Manne Layer Project (ONR) aDepth of bathyphotometer profiles over which bioluminescence in next col- umn was averaged. bUnits of Photons 1-1. Measurements were made with HIDEX-BPs CUnits of Photons s-1 1-l for short residence time bathyphotometers 33 average photon flux measured by the light detector is a function of detection chamber volume rather than volumet- ric flow. Under these circumstances the photon flux measured by the detector is divided by the chamber volume, rather than the volumetric flow, and the results are reported as photons per second per unit volume. NAVOCEANO is currently using a recently developed bathyphotometer design that measures the full output of the first stimulated flash from an organism and measurements are reported as photons per unit volume. In this bathyphotometer, bioluminescence is stimulated by hydrodynamically calibrated flow through a turbu- lence generating grid at the entrance to a large cylindrical detection chamber. An array of optical fibers embedded in the walls of the detection chamber collects light and directs it to the light sensor. A variable-speed pump draws water through the detection chamber at pumping rates from 20 is-1 to 44 is-1 (Widder et al., 1993; Case et al., 1993~. This bathyphotometer is known as the HIDEX-BP (High Intake Defined Excitation Bathyphotometer) and was designed to standardize bioluminescence measurements. HIDEX-BP design prin- ciples have also been incorporated into a towed system (TOWDEX) and a moored system (MOORDEX). The basic HIDEX-BP design is such that direct comparisons can be made of measurements from these various systems. Although frequently designed to be lowered or towed, bathyphotometers can also be fixed to a submersible. Some exploratory developments include floating bathyphotometers and expendable sensors. Bioluminescence measurements obtained by automated, unmanned vehicles, both airborne and underwater, were also discussed. It was pointed out that the number of researchers engaged in making bioluminescence measurements varies with available funding, but this is not presently a large field. In the past, research emphasis has been placed on open ocean rather than coastal environments. The examples of bioluminescence measurements made from different oceans (see Table 4-4) clearly indicate a trend of low bioluminescence in open ocean, nutrient-poor waters to higher bioluminescence in more productive, coastal environments. It is possible that predictive models could identify regions of lower bioluminescence potential and yield graphics useful in guiding nearshore operations. Solutions Plankton such as dinoflagellates, radiolaria, ostracods, and copepods living near the surface are the dominant organisms that reveal boat wakes, outline boats or swimmers, and highlight SDVs. Some organisms such as the ostracods and copepods may secrete clouds of luminescence, whereas others such as dinoflagellates and radiolaria
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34 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES produce an intrinsic luminescence (i.e., light-emitting chemicals are retained within the organism). In general, luminescent plankton are not spontaneous emitters but require a mechanical stimulus to initiate light emission. Even though bathyphotometers routinely use turbulent flow to excite bioluminescence, the stimulation threshold actually occurs in laminar flow. The minimum threshold stimulus required to excite bioluminescence in di- noflagellates occurs at a shear stress of 1 dyne/cm2, which is sufficient to deform the cell membrane, and within 0.02 seconds produces a flash of light that lasts for 0.1 to 0.5 seconds. Turbulent flow increases bioluminescence by stimulating more organisms due to a thicker boundary layer, increased mixing, and greater rates of entrainment. The vast majority of bioluminescent organisms are capable of multiple flashes in response to repeated stimuli. In coastal environments, bioluminescence intensity may vary over several orders of magnitude seasonally, as well as on time scales of only hours (e.g., during the exponential growth phase of a dinoflagellate bloom, or through tidal current concentration) and on spatial scales of meters (e.g., across a frontal system produced by nutrient runoff). There is also vertical patchiness in bioluminescence due to nonhomogeneous depth distributions of the organisms. This patchiness renders modeling bioluminescence, at least in the near-term, impractical in coastal zones. To determine the relationships between environmental conditions and bioluminescence, a program for time series measurements at several representative coastal locations could be expanded. The field program would quantify the correlation of bioluminescence with environmental parameters such as wind, waves, nutrients, tem- perature structure (i.e., thermocline), rainfall and runoff, and amount of solar illumination. Because luminescent organisms are present during the daytime, it may be possible to estimate nighttime biolu- minescence from daytime measurements taken hours (as opposed to days) before the mission begins. Research should explore the development of diagnostic tools that allow operational teams to test seawater samples for potential bioluminescence. If feasible, such tools, when used in conjunction with existing technology, would give mission operators the means to verify predictions or reports of bioluminescence conditions on a variety of time scales. Suggestions and Summary Several working group members suggested the use of training items on coastal and littoral processes for both NSW and METOC personnel. The bioluminescence and toxins group recognized the need for greater two-way information flow, specifically the greater need for "brief-back" of bioluminescence observations in NSW exer- cises and operations. Presumably NSW after-action reports are analyzed and archived at an appropriate facility. A standardized environmental report form could separate those METOC conditions to be directed to littoral warfare data bases useful to NSW and other military units. More specifically, discussions centered on conducting field exercises that address detection of combat swimmers and their vehicles, using swimmers to determine detection at various depths. Overall, eight specific suggestions were offered by symposium participants to limit the adverse impacts of bioluminescence on NSW operations: · Characterize optical signatures at night for SEAL vehicles such as Combat Rubber Raiding Craft (CRRC), zodiacs, and ribbed vehicles. Although these vehicles produce a surface signature, in contrast to swimmers and SDVs, a series of similar exercises could be conducted emphasizing on-board instrumentation for a "real-time" threat assessment to visual detection. Shrouding the propeller may also reduce the most obvious source of stimulated bioluminescence and dampen an optical signature. · Provide a reasonably simple model for operational planning, and develop a suite of sensors capable of measuring bioluminescence and transmittance to further the development and use of VDRs. Sensors could be mounted in patrol craft, SDVs, and moorings (short term, long term). For example, miniaturized bioluminescence systems developed at NRaD could be mounted in autonomous unmanned vehicles (AUVs) and SDVs that will access in real time the vulnerability of vehicles and personnel to detection within coastal and near-coastal areas. This kind of instrumentation could be used in newer and larger SDVs now in the planning stages. Miniaturized sensors could be deployed in forward areas days, weeks, and months before a planned operation, giving SEALs real-time data on what to expect in vulnerability from bioluminescence. Subsurface moored detectors can transmit nightly bioluminescence intensities and project VDRs if desired.
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 35 · Conduct time series studies in the littoral zone that examine the relationship between bioluminescence and environmental parameters. · Institute a bioluminescence brief-back mechanism for NSW operations. · Standardize a report form for bioluminescence information. · Develop a Web site with input from leading researchers in bioluminescence that can serve as a reliable source of information on the subject. · Create a NSW training item on bioluminescence. · Develop a seawater test kit that will allow nighttime bioluminescence predictions from daytime measure- ments. HAZARDOUS MARINE ORGANISMS NAVOCEANO and other Navy environmental information products typically contain relevant information on three categories of hazardous marine life: (1) venomous organisms capable of injecting venom, (2) wound- inflicting organisms capable of inflicting non-venomous wounds (includes marine predators), and (3) poison- ous organisms possessing toxic compounds capable of producing various degrees of systematic poisoning when ingested as food (Fig. 4-1~. In addition to these three general categories of hazardous marine life, symposium attendees identified a fourth category of hazardous marine organisms: microorganisms capable of transmitting infectious diseases. The following section includes a summary of discussions that took place during the sympo- sium as well as some additional material intended to provide more background on this topic. Large marine predators will pursue other animals in the water if they are threatened or are seeking prey. These animals are generally not aggressive toward humans, although they can, in some situations, harass and harm swimmers. Many marine organisms produce toxins that are used to protect them against predators. Venomous and stinging animals generally do not actively pursue prey but can inflict venomous stings when disturbed. As discussed, some marine organisms are toxic only when consumed. The toxins produced vary but include some of the most lethal neurotoxins known. Perhaps the most widespread of this group of toxin-producing organisms are the dinoflagellates and diatoms associated with harmful algal blooms. These organisms produce potent toxins that generally have neurological or gastrointestinal effects that in some cases are severe enough to be life threatening (Table 4-5~. These toxic organisms are consumed by other marine animals that then accumulate the toxins (Shumway, 1990~. Human health is impacted when these marine animals are consumed, as the high concentration of toxins they contain can lead to illness and even death. NSW personnel are routinely inoculated against a wide variety of potential diseases; however, it is not possible to inoculate against all potential diseases. Exposure to infectious disease can occur by consuming shell fish from, or swimming in, waters that have been polluted by human waste. This is most likely to occur when swimming near areas where untreated sewage flows directly into coastal waters. In addition, some recent evidence suggests that outbreaks of certain infectious diseases, such as cholera, may be associated with algal blooms. It was not clear, based on the limited discussions at the symposium, in what specific ways additional research on infectious diseases could be made to benefit NSW operations. Mission Influence The Navy attempts to provide information on a variety of biological threats (Fig. 4-1~. Missions can be adversely impacted by the death or illness of mission personnel. In the case of harmful marine algal blooms the impact can occur in two ways. If seafood is consumed that has accumulated toxic algae, illness can occur. This is most likely to take place during long missions when nutrition is gleaned from local sources. There is a lower probability of contracting an illness from harmful algal blooms by consuming some quantity of seawater during routine marine activities. In addition, brevetoxins can become aerosolized because Gymnodinium breve, unlike other harmful algal bloom species, is an unarmored form and may lyse (experience cell disruption) under wave
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52 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES units, these must be modeled to a reasonable accuracy a task that is still beyond current production capabilities. Second, since the bathymetry drives the dynamics of the wave field and the wave field drives changes in the topography, there is substantial feedback in the system. This makes the nearshore a nonlinear dynamic system. In comparison with other such natural systems, a richness of unexpected behavior is expected, and observed. The problem is analogous to the prediction of weather. With research, models will be developed that provide some skill in predicting short-term evolution of the bathymetry, perhaps as long as a week or more. However, like weather forecasting predictions beyond this horizon will remain of climatological value only. For the foreseeable future, in situ or remote measurements of the nearshore bathymetry are the only viable tools. Summary it, Although not considered a critical mission parameter, the importance of determining bathymetry pervades NSW operations. For ingress and egress, bathymetry may represent an obstacle in the form of a sandbar and adjacent deeper trough in the nearshore or perhaps some unknown bathymetric obstruction. However, the second- ary effects of bathymetric shoaling are strong. These include wave shoaling and breaking as surf, the generation and channeling of strong currents, and the potential introduction of wave height anomalies through refractive focusing or defocusing caused by progression over offshore shallow banks. Also, determination of bathymetry in waters shallower than 3.5 fathoms is a designated special reconnaissance mission of the SEALs. In waters deeper than 5-6 m, bathymetry can be assumed to be unchanging, and surveys can be carried out in advance of an anticipated need. In shallower waters of sandy beaches, bathymetry is continually changing and profile data must be recent (within a few days for surf zone work) to be useful. The evolution of a pre-existing profile currently cannot be modeled and therefore must be determined either by direct measurement (e.g., using SEAL teams, airborne lidar, remotely sensed measurements). Both in situ and overhead remote sensing ap- proaches are being explored and appear promising. EM-DUCTING NSW and SEAL operations take place in marine and coastal areas where rapidly changing and complex atmospheric conditions are common. Special reconnaissance and other clandestine operations require the collec- tion of data on nearshore bathymetry, obstacles from within the nearshore, and targets and/or defenses on land in the coastal zone. In many situations, onshore NSW units must maintain radio communications with Naval forces offshore. These radio communications, under certain atmospheric conditions, can jeopardize the clandestine nature of the mission, and in some instances, jeopardize the lives of both the onshore and offshore units. The term "EM-ducting" refers to anomalous refraction of electromagnetic (EM) waves by the atmosphere leading to enhanced (over-the-horizon) radar detection ranges and communications channels. Reduced radar detection and communications may also occur for slightly elevated propagation angles (referred to as a "radar holed. The term "anomalous propagation" is used when refraction effects are significantly greater than the standard atmosphere. If the downward curvature of an EM ray exceeds the curvature of the earth, then a radar "duct" is said to exist. The curvature of the rays is a function of the index of refraction of the atmosphere, which depends on the pressure, temperature, and moisture content in a well-known manner. A duct that results from the strong gradient in the atmosphere near the sea surface is usually referred to as an "evaporation duct" because of the strong vertical gradient of moisture near the sea surface. The height above the surface where the gradient of refractive index is reduced to the critical value (i.e., the ray curvature exactly equals the curvature of the earth) is the duct thickness or duct height. Ducts caused by the presence of a temperature or humidity gradient not associated with the surface are referred to as elevated ducts; these are most commonly associated with the temperature inversion at the top of the marine boundary layer (heights on the order of a kilometer). Mission Influence EM-propagation affects most aspects of SEAL operations on or above the sea surface associated with trans- portation to and from the mission area and execution of the mission. EM propagation does not affect most SEAL
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 53 operations below the sea surface (i.e., most SEAL subsurface activities do not produce a radar-detectable surface signature) but is relevant to communications by submerged minisubs or to possible radar detection of a periscope. EM-propagation effects can be broken down into two parts: (1) radar detection and (2) communications. These issues may be looked at from both directions. Radar detection of enemy threats by SEALs or their supporting units, and radar detection of SEALs or their supporting units are relevant. EM-ducting may provide enhanced ranges to improve communications by SEAL units but also will increase the probability of detection of SEAL units by local enemy forces. Ducting may also affect tactics in support of SEAL units. For example, an electronic countermeasure (ECM) aircraft might approach the coast undetected by flying above an evaporation duct and then descend into the duct to turn on jamming signals to mask a SEAL egress. This jamming might affect enemy radar or communications. Research Issues To deal with EM-propagation effects, SEALs need a specification of the propagation conditions in the operational area as a function of time and location for the duration of the operation. For mission planning purposes, this specification is required well before the actual mission in other words a forecast of propagation conditions is necessary, with continual updates and evaluations through the planning and execution of the mission. During execution of the mission, SEALs may need an evaluation nowcast of propagation conditions for tactical purposes (i.e., use one type of radio if dueling conditions exist and another type if they do not). Forecasting of propagation conditions requires forecasting the profiles of pressure, temperature, and humidity in the operations area (both over sea and over land) and forecasting of the surface wave conditions; this information is used as input for the computation of propagation properties with a mathematical model. The surface wave field affects both the propagation of ducted signals (by multiple scatter from the surface) and the detectability of SEAL surface units and/or low-flying airborne units (i.e., radar detection is a signal-to-noise problem and sea clutter is a source of noise). Because NSW operations are conducted primarily in the coastal zone. strong horizontal variations in meteorological and propagation conditions are expected. To summarize, forecasting of dueling requires an environmental forecast model of local meterological conditions to which we apply a propagation model; nowcasting may involve in situ determination of environmental data (with application of the propagation model) or some other method to infer propagation conditions directly (e. g., observations of radar sea clutter intensity as a function of range from supporting ship or aircraft). The U.S. Navy operational forecast models in the United States operate in the classic mode: a set of data is collected, "ridded at some horizontal and vertical resolution, analyzed, and used to establish an initial state of the atmosphere or ocean. The dynamic equations of the system are integrated in time on this "ridded volume from the initial state to form the forecast and to derive variables that are not actually measured as part of the initial state. For global models, the data grid has about the same resolution as the grid for the model computations (except for some big holes over the oceans). For regional-scale models covering thousands of square kilometers (e.g., the continen- tal United States), the surface data have about the same resolution as the model (with each grid cell typically covering a few hundred square kilometers), but the atmospheric profiles are much sparser perhaps 20 model grid points for each rawinsonde. A high-resolution mesoscale model run on a coastal domain might have no opera- tional data in its entire grid space. Such models must be nested within regional or global models that effectively provide data as evolving boundary conditions. Such high-resolution models cover a few tens of square kilometers and provide a reasonable representation of local conditions only to the extent that these conditions are dominated by the larger-scale context (e.g., nesting in the larger-scale model) and the driving force of local surface properties (surface fluxes, terrain, etch. Synoptically driven fronts and land-sea breeze cycles are examples of phenomena that tend to be well described with this approach. To the extent that local processes are not well captured by synoptic forcing and local surface conditions, local data are needed in the mesoscale model. Local jets, squall lines, gravity wave interactions, small orographic eddies, rainbands, fog banks, isolated thunderstorms, roll vortices, and other intermittent mesoscale phenomena are examples of processes that cannot be forecast without first ingesting local data. Even for phenomena that are well described in general without explicitly local initialization, the exact timing or structure of these events may still be too uncertain unless some local measurements are incorporated. For example, if a sea breeze switches to a land breeze three hours earlier
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54 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES than anticipated, dueling conditions over the ocean may be changed radically. The land-sea breeze cycle is driven by the temperature contrast between the sea and the land. The land surface temperature is critically related to soil moisture, vegetation, and cloud cover. These values represent complicated physical processes that must be accounted for correctly to obtain an accurate forecast. Furthermore, the internal parameterizations of the model must account for the uniquely inhomogeneous conditions encountered in coastal domains. This is a major issue in attempting to use models optimized for much coarser resolution. Data from a variety of sources are required. For example, satellite data could be used but they must, by necessity, be derived from sensors with kilometer-scale horizontal resolution. Examples of potentially useful satellite sensors are IR or microwave sea surface temperature radiometers, synthetic aperture radar (SAR), and scatterometers. These data may be used in atmospheric, oceanic, or coupled ocean-atmosphere models. For example, it may be impossible to forecast a land-sea breeze transition accurately without an accurate forecast of unwilling (which can si~nificantlv alter the sea surface temperature pattern in 12 hours). The fundamental 1 ~7 ~ ~7 ~ 1 1 ~ * ~ row problems laced by researchers can be simply stated: What improvements in present models are necessary to realistically handle the coastal domain at such high resolution, and can such unconventional data be incorporated into an operational mesoscale model? More specific questions include the following: can conventional closure models yield reasonable coastal boundary layers in a variety of conditions; how can the basic data be processed to extract the maximum amount of information; which variables are most useful for data assimilation; how can data fields be processed for optimum incorporation into models; how can the models be modified to utilize these data fields; and what are the minimum temporal and spatial resolutions that still yield significant improvements in model analyses and forecasts? Solutions Planning NSW missions requires forecasts and nowcasts in the highly variable coastal region where local in situ measurements are very difficult to obtain. The solutions to this problem involve (1) improved environmental forecast models of local atmospheric conditions (especially horizontally stratified conditions such as humidity); (2) more sophisticated and more accurate signal propagation models (i.e., models that can handle horizontal inhomogeneities); and (3) innovative measurements of local atmospheric conditions. The U.S. Navy now runs operational global (NOGAPS) and regional (NORAPS) forecast models, and these are known to be of limited value for forecasting dueling conditions in coastal areas. The Navy has an experimental high-resolution coupled ocean-atmosphere model (COAMPS) that is under development but is not operational. This model is expected to include wave forecasts. Although this is the only realistic approach to solving the NSW's dueling forecast problem, it is a long way from meeting NSW requirements. Propagation models must be able to deal with surface sea clutter and coastal terrain. Information is needed on radar scattering signatures of NSW vessels and their wakes. The working group considered the merits of a new field of research using diverse forms of information to nowcast dueling conditions (e.g., GPS [global positioning system] methods, radar or communications statistics, inverse methods). ATMOSPHERIC VISIBILITY Naval Special Warfare operations involve both the water and the land portions of the littoral zone. Atmo- spheric visibility is of crucial importance for the success of ingress or egress operations and reconnaissance. Electro-optical (EO) devices operating in the visible region (including the human eye) and the infrared are affected strongly by the propagation environment. Of special importance is the nearshore region some 10 miles either way from the shoreline, which is influenced strongly by both land and water regimes.
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 55 Mission Influence Visibility is crucial to most aspects of NSW operations. Poor visibility can be both detrimental and beneficial, the latter especially if the enemy does not have the technology to operate under such conditions. The primary atmospheric parameter affecting visibility is aerosols. Commonly, aerosols are water droplets formed around a nucleus, but they also could be dust or smoke particles. Molecular absorption is usually not a problem in the visible wavelength band but must be considered for devices operating in the infrared bands. Turbulence and refraction can affect laser and imaging systems. Depending on the local circulation, offshore conditions should be more horizontally homogeneous and temporally stable than in-shore conditions which might be dominated by nonuniform aerosol sources, terrain-induced circulation, and solar heating. Visibility can be of special concern in the surf zone where breaking waves and wind may generate large near-surface aerosols. Usually, spatial scales of interest for EO devices are less than a few tens of kilometers. Temporal variability can be quite high and may be as short as minutes between visibility extremes. This temporal and spatial variability poses a formidable challenge to mesoscale model development. Research Issues Understanding and modeling of near-surface aerosols is incomplete and needs further attention. Surf aerosols close to the ocean surface are difficult to measure and are a function of both surf and wind. The chemical composition of aerosols in coastal environments is often a complex mixture of marine and continental aerosols, including industrial pollution. In aerosol models, the origin of the aerosol has to be known, which requires better characterization and parameterization of air mass characteristics. Remote sensing techniques for aerosol extinction need to be further developed. Lidar techniques have been used to infer aerosol extinction but have not yet resulted in operationally useful instruments. Various lidar techniques for sensing aerosol extinction have to be investigated further. Satellite remote sensing techniques for inferring marine boundary-layer and extinction characteristics should also be developed. High-resolution (e.g., models using 1 km grid sizes) mesoscale models need to address parameters impor- tant for EO propagation, such as drop size distributions and particle transport. Such mesoscale models should be part of data assimilation systems that can accept conventional (e.g., surface observations, radiosonde data) as well as unconventional (e.g., spectral moments from ground and satellite remote sensors) data. Since NSW operations routinely take place in denied areas, satellite-sensed data are often the only real-time data sources. Over water, multi-spectral upwelling radiance can be used to infer optical depth and boundary-layer aerosol properties. Satellite sensing techniques for similar information over land should be investigated. Sea and land background radiances must be sensed, modeled, and integrated into EO tactical decision aids for performance assessment of infrared imaging devices and night vision goggles. Solutions It is not possible to measure accurately the spatially and temporally variable operational environment. Even if it were, a reliable forecast capability is also needed. A data assimilation system capable of accepting a wide variety of directly and remotely sensed data with improved mesoscale and large eddy simulation (LES) models and techniques capable of integrating measured and modeled information appear to represent the best chances to improve predictions. More emphasis should be placed on predicting EO parameters in the numerical modeling area. The models need higher-resolution data input, increased ability to accept non-traditional inputs, and the ability to work over land, surf, and sea. Development and incorporation of expert systems would assist the data fusion task. Satellite sensors provide data globally and for denied areas. For a variety of reasons, information from geostationary satellites is not fully exploited for shipboard use. With the increasingly convenient access to geostationary satellite data through the net (e.g., SIPRNET) it is possible to utilize cloud type and motion to characterize prevailing conditions and make short-term forecasts. Polar orbiters (Defense Meteorological
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56 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES Satellite Program [DMSP] and, in the future the National Polar Orbiting Environmental Satellite System [NPOESS]) carry a number of profilers. Their present resolution and the impact of higher resolution on improved EO predictions should be examined. Data bases on ground cover, soil type, albedo, snow cover, rain rates, and so forth should be assembled, and techniques should be investigated to use polar orbiter sounder data in data assimilation systems. In addition to the operational military and National Weather Service satellites, the National Aeronautics and Space Administration (NASA) has existing and planned satellites that carry special sensors (one example is aerosol lidars). Data from these NASA satellites should be examined carefully for use and recommendations for future operational environmental satellites. Finally, unmanned airborne vehicles (UAVs) and covert in situ sensors may be used to sense temperature, humidity, and aerosols in denied areas. UNDERWATER ACOUSTICS Underwater acoustics is a pervasive issue for NSW underwater missions, especially those that take place in very shallow water. The littoral regime is a dynamic, acoustically harsh environment characterized by high reverberation, high ambient noise, and volume micro-inhomogeneities. The performance of NSW acoustic sys- tems for detection, localization, imaging, navigation, and communications is severely affected by reverberation, attenuation, and distortion of the acoustic signals by the environment. Mission Influence During infilitration or exfiltration, the environment significantly affects the operation and performance of SDV obstacle avoidance and navigational sonar. Additionally, the performance of diver-held acoustic "tools" (e.g., imaging sonar) used to conduct underwater missions depends greatly on environmental conditions. The primary navigation and detection sensor on the SDV is a high-frequency forward-looking obstacle avoidance sonar. The operation of this sonar can vary greatly, depending on the operating environment, with operating ranges typically less than design specifications. Existing mine countermeasures shallow water sonar performance prediction models could be used to provide SDV teams with a range-of-the-day prediction for mission planning. Although such models provide a rough measure of performance, they are limited by the fidelity of environmental acoustic models describing boundary interactions at high frequencies. Specific METOC prod- ucts required for sonar "range-of-the-day" predictions include wind speed, bottom type, and sound velocity profile. NSW capability to detect, classify, localize, and identify underwater threats is severely limited. These threats are usually either volume or bottom sea mines and obstacles. Bottom mines may become completely or partially buried. Currently SEAL swimmers conduct their missions using either the MK-10 magnetic locator or the AN/PQS-2A diver- portable sonar. The MK-10 has detection capability against ferrous mines, a short detection range, and no stand off localization capability. The diver must swim directly over the mine to ensure its detection. The diver-portable sonar requires the diver to discern the sonar's aural output, and the detection performance is greatly dependent on the diver's training, experience, and level of fatigue. Once a target is detected, the diver must swim to it for visual identification. In areas of low visibility, the diver must physically touch the object to identify major features. The Office of Naval Research (ONR) is currently supporting the development of technology that addresses these acoustic detection deficiencies. A video representation of the AN/PQS-2A aural signals has recently been developed that offers improved detection performance. Although this has provided a quick fix to noted opera- tional deficiencies for exposed or partially buried objects, the sonar operates in the upper end of the frequency spectrum for buried object detection, and performance against completely buried objects may be poor. Additionally, many years of basic research in piezoelectric materials and manufacturing techniques are leading to the development of technologies that could enable fabrication of a portable underwater two-dimensional acoustic array imager. The goal is to develop a hand-held underwater device capable of at least 15 frames per second with 0.5 cm resolution at 5 m stand off ranges for positive identification and detection ranges up to 50 m.
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 57 Research Issues NSW operations are typically conducted in shallow water littoral environments. Due to the proximity of boundaries (sea surface, seafloor) in very shallow water, underwater acoustic research issues most relevant to NSW missions include both small-scale characterization of the boundaries and the development of models for acoustic boundary interactions at high frequency. Sea Surface Scattering Although surface roughness and wind-generated bubbles are relevant to acoustic surface provements to modeling the acoustical effects of bubbles constitute the most important research issue. The upper ocean boundary layer is a distinct acoustical environment in which the active hydrodynamics of the wave zone combines with the surface bubble layer to scatter, attenuate, and refract high frequency acoustic signals. Existing models for forwardscattering and backscattering treat the bubble field as a homogeneous nonrefracting surface layer. The ability to generate useful models for the prediction of high-frequency acoustic systems near this environment is limited by knowledge of the temporal and spatial size distributions of bubbles near the air-sea interface. interactions, im Bottom Scattering Reverberation from the seafloor is often the limiting factor for detecting acoustically small targets on the seafloor. Experiments have shown considerable variability in bottom characteristics ranging from millimeter to kilometer scales and temporal variability ranging from seconds to years. In most operational cases, deterministic modeling of the acoustic scattering based on temporal and spatial variations in sediment characteristics is unreal- istic. It is therefore an important research issue to determine the statistical characterization of seafloor properties and roughness that is required for physics-based modeling of acoustic scattering. Present theory and modeling techniques do not adequately allow for the heterogeneity that has been observed in seafloors. Low Grazing Angle Penetration of Sound into the Seafloor The acoustic detection of buried mines depends critically on the transmission of acoustic energy into the seafloor. Accurate prediction of the intensity level and spatial coherence in sediments is therefore important to the design and prediction of performance of buried mine detection systems. Some measurements have shown anoma- lously high penetration of sound into the seafloor for signal incident at grazing angles below the critical angle. The mechanism responsible for the observed anomalous penetration at low grazing angles is an open research issue. Hypotheses for this include generation of a Blot slow wave, scattering by roughness in the water-sediment interface, and scattering of the evanescent wave by volume inhomogeneities. Solutions Surface Scattering To improve the existing understanding of issues related to acoustic surface interactions, improved descrip- tions of the spatial and temporal nature of the bubble field are needed. Symposium attendees identified a need for a stochastic, space-time description of the bubble field that is sufficiently complete to form the basis for acoustic surface scattering model development. For this purpose, an adequate understanding of the important environmen- tal descriptors, in addition to wind speed, that determine the properties of the bubble field, must be developed.
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58 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES Low Grazing Angle Penetration of Sound into the Sea;floor Field experiments are needed to understand the anomalously high penetration of sound into the seafloor at grazing angles below the critical angle. These experimental investigations will require burial of receivers and sources that allow for the identification and measurement of compressional, Blot slow, and shear waves. It is essential that these acoustic experiments include sedimentologic and oceanographic measurements neces- sary to understand the physics of both propagation and scattering. Sedimentologic and oceanographic measure · 1 r 1 , 1~ , ~ 1 1 · · 1 1 , 1 , 1 r ,1 r measuring scales, and meets required for understanding acoustic behavior include stereophotography of the seafloor for roughness, acoustic profilers for determining layering and variability over a wide range of spatial measurements for determining sediment physical composition and behavior such as porosity, density, permeabil- ity, gas content, and shear and compressional wave velocities and attenuation. Bottom Scattering A promising approach to improving the accuracy of bottom reverberation modeling would be the ability to infer the relevant bottom parameters using acoustic remote sensing. However, before inverse techniques can be used reliably to rapidly determine the seafloor environment, the forward problem has to be investigated further. Contributions to scattering by roughness, volume inhomogeneities, and bubbles have not been adequately isolated and related to measured properties of the environment. UNDERWATER OPTICS Water clarity is a key parameter listed in the NSW Mission Planning Guide and is of concern for underwater visibility. Clear water may allow detection by sentries; highly turbid water may impede the mission through minimizing underwater line of sight or the ability to see navigational aids or dive meters. Scientific study of underwater optics focuses on spectral absorption and scattering coefficients that can be related to visibility via theory. These parameters are called inherent optical properties and are independent of the external illumination field. The inherent optical properties of the water depend on the composition of the material (phytoplankton concentration and type, organic and inorganic particles, soluble material). NSW mission planning needs for thresholds of visibility over a specified distance relate to human underwater visual perception. Visibility is related not only to the inherent optical properties but also the external illumination field; such parameters are called apparent optical properties. Details of underwater optics and its theory have been reviewed by Kirk (1994) and Mobley (1994~. Significant work has been accomplished in characterizing the various optical properties of relevant constitu- ents that govern water clarity; in developing instrumentation for measurement of essential parameters; and in the development of models based on radiative transfer theory that can use inherent optical properties as input to predict apparent optical properties such as water clarity. The operational capability of the METOC community to support NSW operations with accurate, time-varying, local information on nearshore optics is very limited. Gross climatologies are available but cannot be adjusted at local scales or in response to wind, wave, tide, or river flow events. Expendable diffuse attenuation meters have been developed and are available for real-time assessment of local-scale optical properties. Although significant research capability, including instrumentation and modeling, is now available, the local operations of SEAL units depend primarily on reconnaissance by swimmers to assess the water clarity of targets. Mission Influence The mission of NSW units can be influenced strongly by the clarity of the water. In general, water clarity and visibility at the target site are of concern; the offshore visibility at the mother ship or insertion point is not of great concern and will not be discussed in detail. The critical mission thresholds included in the NSW Mission Planning Guide indicate that swimmer or SDV operations should not be conducted when water clarity allows the swimmer
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE 59 or SDV to be observed at depths greater than 10 feet by an individual located on the shore or on an adjacent pier or ship. Although detection in clear water is of great concern, operations conducted underwater in very turbid conditions can be adversely impacted by low visibility. NSW operators mentioned issues of visibility of the SDV control panel where extreme turbidity prevented the pilot from being able to read its speed or direction. Such circumstances could adversely impact the mission. Also, near coastal waters, rivers, and estuaries can have strongly absorbing dissolved material, even if there are few suspended particles. Such circumstances could impair the ability of an operator to discern different colors, such as colored electronic wires. Thus, knowledge of the time-varying changes in underwater optical properties could be an excellent asset in mission planning and execu- tion. Nearshore, Riverine, and Estuarine Optics Water clarity can vary rapidly in the near coastal zone, within rivers, estuaries, or bays, as a consequence of tidal changes, river flow, surf action, and watershed rainfall. Furthermore, the types of optical constituents found in the nearshore region are more diverse than those offshore, because heavier sediments may remain suspended due to the greater turbulence characteristic of nearshore settings. In addition, the water column in nearshore settings typically contains higher concentrations of sediments or dissolved material from nearby terrestrial sources. Knowing the type of sediment in the surf zone, or being carried by rivers or tidal flow, is critical to defining the optical properties of the water column. For example, heavy sands may intensely scatter light but settle out quickly, whereas fine silts often have significant organic components with strong spectrally dependent absorption and can remain suspended for longer periods of time. In addition, the contribution of dissolved material to light absorption is highly variable in coastal waters. Thus, the littoral region in which NSW units operate is the most difficult part of the marine environment for making predictions about water clarity. Research Issues Climatologies of optics can be improved by augmenting actual ship-based optical measurements with existing ocean color satellite imagery from instruments such as the coastal zone color scanner (CZCS; Acker, in press) to get basic background information on coastal optics and their variations over time and space. Ocean color sensors on satellites or aircraft can provide estimates of the spectral attenuation coefficient, which can be related to water clarity. Unfortunately, the CZCS sensor had relatively low spatial resolution (1 km at nadir), poor temporal coverage (10 percent duty cycle over the six-year lifetime), and strong effects from bright target adjacency (e.g., land targets adjacent to water targets). Therefore, the CZCS will prove of limited use in developing refined climatologies of the nearshore, riverine, or estuarine domains of greatest interest to NSW operations. A combined use of ship observations, satellite archives (including the recent 8-month global OCTS [Ocean Color and Tempera- ture Scannerl mission launched by the National Space Development Agency of Japan [NASDA] and the recently launched Sea-viewing Wide Field-of-view sensor [SeaWiFs] mission launched by the National Aeronautics and Space Administration [NASA]; Acker, in press), and selected deployment of aircraft sensors in regions of greatest interest would contribute to a better data base from which optical climatologies can be developed. During actual NSW operations, existing tools developed in recent years could be utilized. Small, portable, spectral absorption, attenuation, and reflectance meters, useful for defining detailed in situ optical properties at the local scale, including vertical profiles, are available. Expendable attenuation meters have been developed by the Navy. Aircraft-mounted sensors have the advantage of higher spatial and spectral resolution and flexibility of deployment at the regional or local scale. They could be appropriate for broader surveys but retrieve only surface information. Laser-based systems can be used to estimate backscatter of aquatic particles, and the fluorescence induced by these systems allows for estimates of chlorophyll and soluble organic matter concentrations. Such systems are limited to surface measurements along the flight line and have very restricted ability to resolve vertical profiles. If deployed via aircraft, they would provide survey lines of important parameters. These systems can be flown in day or night and under clouds, a flexibility that passive ocean color sensors do not have. Combined with
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60 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES optical models, this suite of measurement tools could be used to provide local-scale information on the vertical and horizontal variability of water clarity, and other features of significance to NSW (Plate III). If hydrodynamic models of the region of interest were available, the optical data and models could be incorporated to provide time- dependent predictions of the region. These potential capabilities require collaboration of scientists from different disciplines (e.g., optical, biological, and physical oceanographers and modelers). Solutions Various disciplines have been developing advanced measurement tools and models of relevance to predicting the time and space mission-critical parameters included in the NSW Mission Planning Guide. Furthermore, advances in battery systems, submarine propulsion, and microelectronic control systems have resulted in underwa- ter vehicles that are now capable of executing underwater surveys. The technology exists and the modeling ability is evolving rapidly. It is therefore possible to envision the development of prototypes, within several years, of multi-sensor remote vehicles, which would be capable of providing detailed optical and hydrographic surveys of a region without jeopardizing NSW personnel. Ultimately, networks of such systems might be deployed within a region to provide real-time data that can be used in data assimilation schemes and models. This type of system holds the promise for real-time prediction of the suite of parameters that are of critical concern to NSW operations. WATER TEMPERATURE Except in very rare circumstances, the temperature of the ocean decreases with depth. This decrease is often fairly rapid near the surface, where sea surface temperature (SST) can be influenced strongly by solar radiation (Weller and Taylor, 1993~. A typical depth-versus-temperature plot shows a surface layer on the order of a few meters to a few tens of meters thick. This layer is commonly referred to as the mixed layer because surface winds tend to keep the waters in this interval well mixed and essentially isothermal (Knauss, 1978~. The base of this mixed layer is referred to as the thermocline. Conditions in the mixed layer can vary drastically from those within the water column beneath the thermocline. In waters above the continental shelf (where terrestrial influences are most pronounced), turbidity, salinity, and the number and types of marine organisms can vary drastically across the thermocline. Mission Influence Water temperature can have direct ellects on NSW operations since low water temperatures can degrade diver performance and lead to hypothermia. Consequently, operators are instructed to wear dry suits in water tempera- tures less than 60° (Fahrenheit). Because of the effects of water temperature and the location of the thermocline on a variety of important mission parameters, understanding the thermal structure of a water body in which a mission is to take place is of extreme importance (Fig. 4-5~. Research Issues As indicated in Tables 4-1 through 4-3, SEAL operators need access to reliable information about SST and the depth and nature of the thermocline. In coastal settings where the mixed layer extends to the seafloor, SST measurements are of greatest value. Historic data incorporated into static climatologies can give mission planners and operators some indication of the conditions in which they may have to work (Fig. 4-5~. However, these climatologies may be insufficient to meet all mission planning needs, especially in situations where temperature can influence a number of other parameters, such as bioluminescence, or where turbidity or other values vary greatly across the thermocline. At present, NSW mission planning needs may be met by remote sensing capabili- ties when these assets are available. Water temperature (surface and at depth) can be measured using a combina- tion of satellites, ship-mounted thermistors, surface moorings (in some locations), and drifters (in some settings;
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OCEANOGRAPHY AND NAVAL SPECIAL WARFARE o 5 10 20 25 TEMPEF~TURE(C) 10 12 14 16 18 ~ ~ 24 ' ~ ' ~ ~ . . . ~. , ~ - I I I I ~I 50 55 80 85 70 75 TEMPERATURES 5 10 15 C] 20 SOUND SPEED(I`JUS) 1S00 1S10 1520 lS30 1S40 Trio 1560 ~ _ , . . . . 1'.1 1.1 ~ ll ll ll ll >1 l I ~, 1 1 1 / 1 1 I 1 1 1 1 1 NEW RIVER REGION (SEP-NOV) 30 ~O O 20 40E _ ~ ~ 15 80 ~ al 10 20 80 25 SALINITY(PPT) 32 33 34 36 3B 37 . . . . ~--N _ , 1 1 1 \ I' 1 ~ I 511 l I 32 33 34 35 38 37 8ALINITY(PP~ ALTERNATE ~TYPICAL ---- - - M~ 1 MA - 20 O O 5 10 -40 _ ~ at it 15 C] ~ 80 20 80 25 DENSITY(SIGI`M-. 21 22 23 24 25 2B 27 28 30~ ,.,,. -~ 4950 4980 5010 5040 5070 5100 SOUND SPEED(FT/8) - n _ 40 ~ 80 80 O - 20 ~ an 83.8 83.9 84.0 84.1 DEI`JSllY(LEl/Fr8) -40 ~ - 60 61 FIGURE 4-5 Water temperature, salinity, sound velocity, and density plots for waters off New River Inlet, North Carolina taken from Plate I. STOIC courtesy of the Warfighting Support Center, Naval Oceanographic Office (NAVOCEANO), Stennis Space Center, Mississippi.
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62 OCEANOGRAPHY AND NAVAL SPECIAL WARFARE: OPPORTUNITIES AND CHALLENGES Weller and Taylor, 1993~. However, limitations exist in terms of both the length of time required to collect adequate data and the spatial resolution possible by any subset of the methods listed. Predictive capabilities are also limited by the validity of the initialization parameters (e.g., errors in the input data can be propagated through the model) and the spatial resolution of the model. Local and often unknown variations in bathymetry and other input variability can drastically affect the reliability of any predictive method.
Representative terms from entire chapter: