Oceanography and Mine Warfare (2000)

A number of environmental parameters can have strong impacts on mine warfare operations. These factors, discussed below, can affect operations directly, as with the influence of currents on diver operations and sediment transport, or they can influence sensor performance, as is the case for water column and seafloor acoustic properties. The surf zone, which has few similarities with other offshore regions in either its dynamics or in operational strategies, will be discussed separately.

Naval operations in the coastal zone are divided into the following categories: surf zone (0–10 ft.), very shallow water (10–40 ft.), shallow water (40–200 ft.), and deep water (>200 ft.; Figure 1–2). These depth ranges are somewhat equivalent to regimes defined by oceanographers (Table 4-1). Oceanographic depth ranges are primarily based on physical oceanographic principles, although acoustic, biologic, and geologic characteristics are similarly distinct.

The surf zone is defined by oceanographers as the region where surface waves actively dissipate energy due to depth-limited wave breaking. The 10-ft. depth cutoff selected by the Navy corresponds to a typical global mean value, although oceanographers allow the boundary to vary with wave height and breaker type (an undesirable level of complexity for operational planning). The surf zone water column is always well mixed. Breaking waves make this region noisy and nearly opaque to both acoustics and optics. In the surf zone, sediment transport rates are high, and significantly affect mine burial.

The naval very shallow water regime corresponds to the oceanographically defined inner shelf (Table 4-1), a complicated region where the surface and bottom Ekman boundary layers merge, stratification can be transient, buoyancy fluxes from rivers are commonly important, and fluid motions can be dominated variously by wave, tides, or low-frequency currents. Very shallow water is a complicated transition zone for both the Navy and the oceanographer.

The naval regimes of deep and shallow water (depths greater than 40 ft.) are grouped by oceanographers into a shelf and slope regime. Dynamically, this is the region in which surface and bottom Ekman boundary layers can form and are usually present. Motions are predominantly wind driven, with some buoyancy effects close to shore. In this region, surface waves provide only slight bottom stirring, and stratification significantly influences both circulation dynamics and acoustics. Sediment characteristics within this depth range will vary according to antecedent geology and fluid energy near the seafloor.

The influence of meteorological variability on nearshore mine warfare can be either direct or secondary. Direct influences are primarily related to the effects of atmospheric conditions on sensor capabilities. For instance, broken cloud cover can yield confusing shadows for optical sensing, while solid cloud and rain can degrade both optical and acoustic-sensing performance. Little can be done about these effects, although their influence on sensors can be forecast.

Secondary influences are primarily related to the atmospheric driving of fluid motions that affect mine burial and countermeasures. At smaller scales, wave forcing by local winds will complicate diver operations and influence the rate of mine burial or scour. On larger scales, winds can drive shelf circulation and dramatically change local optical and acoustic properties of the water column. In the coastal zone, surface wind patterns can be strongly influenced by local coastal topography and can exhibit large diurnal changes. Similarly, the presence or absence of turbid outflow waters from nearby rivers and estuaries is usually strongly dependent on local atmospheric conditions.

Presently, naval operations rely on several atmospheric models for meteorological predictions. The Navy Operational Global Atmospheric Prediction System (NOGAPS) is the only global meteorological model operated by the Department of Defense. This model is specifically designed to deliver medium-range weather forecasts, but it also provides supporting information for nearly every application run at the Fleet Numerical Meteorology and Oceanography Center. Regional atmospheric prediction models include the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) and the Third Generation Wave Model (3GWAM). COAMPS consists of a complete three-dimensional atmospheric data assimilation system that can be run simultaneously with one of two ocean models and can also be integrated with 3GWAM.

Although the models discussed above have advanced predictive capabilities, as is the case with all numerical models, continued enhancement and calibration is necessary to improve the quality of predictions. This is accomplished through inter-comparisons of model results between the Fleet Numerical Meteorology and Oceanography Center and the National Centers for Environmental Prediction. In open ocean areas, model predictions are robust, whereas in coastal regions landmass influences can result in the breakdown of model predictions.

Further research and development of models for coastal meteorology will aid the prediction of meteorological factors affecting mine countermeasures. Finer scales than are common for synoptic meteorology have to be resolved in space and time. Proximity to the coast has a number of direct and indirect consequences. Coastal topography (for example, the coastal ranges of the U.S. west coast) can cause atmospheric edge waves with associated progressive

cloud fronts. Longshore-directed winds will drive upwelling or downwelling, and abrupt changes in water column characteristics. Local thermal gradients, especially at the land-sea boundary, often push fog bands over the coast, greatly reducing visibility. In addition, local strong wind fields can generate complicated local seas that augment ocean swell.

Spatial and temporal variations in water depth and seafloor profile can influence the location and height of breaking waves, the position and strength of surface currents, and the propagation of the tide into very shallow waters. In the surf zone, temporal changes in bathymetry can influence local dynamics in times as short as one day. In deeper waters, the fractional changes are smaller and slower, but can still easily be sufficient to cause mine burial. Bathymetry forms the boundary condition for all fluid motions. This influence can extend beyond the local region. For example, an offshore bank can cause focusing of waves onto a short section of beach that sits adjacent to quiet zones. Prior knowledge of these conditions is an important aid to operational planning.

Bathymetry enters the minehunting problem as a boundary condition for wave and current motions through scour or burial (burial related to beach profile ''activity'') and through bottom clutter characteristics. The role of bathymetry as a boundary condition for nearshore waves and currents has received considerable attention, particularly with regard to planning and safety of special operations forces. Bathymetry measurements become more complicated in shallow water because a) the fluid dynamics are increasingly sensitive to small scale features (shelf scale motions average over larger regions and are partially isolated from bathymetry details by the boundary layer and stratification); b) temporal changes are increasingly important; and c) traditional survey methods are unusable.

Profile activity, and hence burial potential, varies strongly in the fluid environment with activity generally increasing with decreasing depth. In the deep and shallow water zone, burial occurs either on seeding or slowly thereafter. On the other hand, bathymetry in very shallow waters changes rapidly due to a wide spectrum of bedforms, and mines can quickly be scoured down and covered over.

The sensitivity of wave and flow fields to the details of regional and local bathymetry can be studied with models, although this approach has not received much attention. Thus, recommendations of bathymetry resolution for operational requirements are often ad hoc rather than model-based. Similarly, there is only limited knowledge on the "shelf life" of a set of bathymetric measurements (the period of time for which depth measurements at a location can still be considered valid). Clearly this is depth-and wave-climate dependent, but predictive capabilities are lacking.

Little is known about the climatology, variability, and importance of small-scale bedforms in shallow waters. These likely provide an important mechanism for mine burial. Bedforms also affect flows through bottom dissipation and act as a clutter against which mines must be detected. Knowledge of the presence and persistence of low-clutter regimes (no bedforms) would be useful. In addition, as bathymetry may be related to antecedent geology, information gathered from onshore sequences may be of use in predicting changes in seafloor character and bathymetry.

Existing data that describe the natural variability of bathymetry at different depths, over different time scales, and in various oceanographic environments should be exploited. Little is known about smaller-scale bedforms, particularly meter-scale mega-ripples that greatly complicate the clutter problem. The dynamics of moveable (sand, for example) but low-clutter (flat bed) bottoms should be studied. Such studies should keep the mine burial problem in mind.

Automatic clutter characterization is a significant issue. Hunting of bottom mines, while usually carried out visually, is technically a feature extraction problem for which bottom clutter worsens the signal-to-noise ratio. Improvement of this ratio by optimum site selection (mentioned above) is one approach; however, considerable success has been achieved in automated clutter classification and anomaly identification (as an aid to visual analysis) through a variety of signal-processing approaches, including fractal description. These approaches are worth pursuing.

Currents are long-time-scale fluid flows arising from a wide variety of processes, such as seawater density gradients, direct wind forcing, and tides. Meteorological and oceanographic analysis, prior to mine warfare activities in the coastal zone, must be sophisticated enough to understand the complexities of forces driving current flow, determine how these forces are interacting with each other, understand the time scale of variability driving current flow, and understand how this may affect the mission. Because of their importance, current and tidal data have long been a major component of the Naval Oceanographic Office (NAVOCEANO) products for mine warfare support.

Tidally driven changes of sea surface elevation vary globally from negligible to tens of feet in very shallow waters. In the shallow and deep water zones, these elevation changes have little effect on minehunting operations or effectiveness. Tidal effects primarily influence mine warfare operations in very shallow water and the surf zone, although in the surf zone tidal currents are usually negligible compared to wave-driven flows. Near-surface mines may begin to broach at extremely low tides, providing a substantial increase in their remote sensing signature. Tidal currents can cause a dip that keeps a moored mine below the surface, can increase scour of bottom mines, and can cause significant transport of drifting mines. Outside the surf zone, tidal currents can often exceed 1 kt and thus affect diver and marine mammal operations. In addition, the onset of wave breaking, with its increased complexity for minehunting and swimmer operations, depends on depth. Thus, timing of swimmer operations in proper phase with the tide can substantially improve both safety and effectiveness.

Currents affect operations, safety, and potentially scour. There is a general decrease in dominant length and time scale of current motions with decreasing depth. Deep and shallow water flows are geostrophic and low-frequency (and are likely to be predictable), while very shallow water and surf zone currents are more likely to be directly forced through wind, wave-driving forces, or buoyancy fluxes due to runoff or river outflow. Naval skill in current prediction decreases with decreasing depth (shallowest flows must be handled statistically or must be measured).

Tidal models are very accurate in deeper waters, however in very shallow water and the surf zone large errors may arise due to nonlinearities (finite height to depth); bottom friction; boundary effects, such as reflection; and local forcing (storm surge). These issues are most important in very shallow water, where the tides are largest and the minehunting problems most difficult.

Some aspects of current modeling over continental shelves are relatively well understood. For example, the alongshelf current can be well predicted for the case of steady wind forcing over an idealized continental shelf. Cross-shelf flows are much less well understood (and have formed the focus of the Coastal Ocean Processes, or CoOP, research program). Known weaknesses in understanding include the details by which wind blowing over the ocean transfers momentum through the surface boundary layer to force shelf flows and the details of the bottom boundary layer that retards those currents. While the upwelling processes have received considerable study, the corresponding downwelling dynamics are relatively unknown (not simply the opposite). Both processes can have important effects on optical and acoustic properties, aside from the importance of the currents themselves.

With the advent of satellite imaging, oceanographers became aware of the rich array of smaller-scale eddies on continental shelves. With currents that often exceed 50 cm/s and spatial scales on the order of 20 km, these features can have large impacts on operations, but are difficult to predict. On the U.S. West Coast, eddies are commonly produced by the interaction of alongshelf currents with capes and other coastal topography. Eddies can also be generated through shear instabilities and can propagate into coastal waters, as is the case with the Gulf Stream. Adequate prediction of a coastal eddy field requires improved numerical modeling coupled with assimilation of either in situ or remotely sensed data.

At even smaller scales, it has become increasingly apparent that a variety of internal waves are generated at or propagate onto the continental shelf. These can yield substantial anomalies to expected water properties and often have surface signatures that are easily detectable by remote-sensing. Best known are the soliton signatures seen in radar images. The possible utility of these signals, and the possibility that alteration of expected signatures by submerged vehicles could be detected, are not well explored.

In very shallow water, local buoyancy fluxes become dominant; depend on proximity to local rivers and estuaries, as well as local weather; and require substantial improvements in small-scale modeling capability. In the surf zone, currents are primarily wave driven and depend strongly on details of the bathymetry, particularly the presence of sand bars and of topographic channels through bars that can cause strong fixed-rip currents. Presently, models of the current field over a barred beach are primitive.

Tidal models are becoming increasingly capable, however model success critically depends on the quality of bathymetric data available. As was found for special operations, there is a strong need for high-quality bathymetry data and increased capabilities to collect these data on denied coasts. Extension of present capabilities into shallower water using nested grids, nonlinear physics, and better ground-truth data (perhaps from remote-sensing) for assimilation purposes should provide good tidal predictions for minehunting and special operations.

Improvements in current predictions will require progress on three fronts. At the most basic level, there are aspects of the fundamental physics (for example, the dynamics of bottom dissipation through a bottom boundary layer) that are poorly understood. Second, due to the expected complexity of coastal flows there is a need for improvements in modeling capability coupled with data assimilation. Third, there is need for increased availability of data for assimilation, either in the form of clandestine in situ sensors or through improvements in the types, accuracy, and availability of remote-sensing data.

Water clarity has always been important for diver missions, in terms of safety and ability to see well enough to detect, classify, and neutralize mines. With the development of modern optical sensors and appropriate platforms for deployment, optical methods of detection and classification have been added to the MCM toolbox. Water clarity is primarily determined by absorption and scattering by phytoplankton, detrital particles, suspended sediments, and dissolved organic material, as well as by the water itself. Scattering by particles blurs images of mines and other dangers, whereas absorption dampens the power of natural or artificial light sources for both divers and optical sensors. Transmission properties of coastal waters vary spectrally as functions of the relative concentrations of absorbers and scatterers. Transmission can be predicted if the spectral absorption and scattering coefficients are measured through radiative transfer models. For divers and sensors using the sun as a light source, underwater visibility is also a function of sun angle and atmospheric conditions (Mobley and Mobley, 1994).

The value of MCM optics has been increased by recent development of in-water sensors for direct measurement of inherent optical seawater properties (i.e., absorption and scattering coefficients), generation of new information on sources of variability in major absorbers and scatterers, and the evolution of radiative transfer models. Progress in understanding in-water optics directly benefits the interpretation of hyperspectral remotely sensed data collected from satellite and aircraft.

With the exception of the surf zone, where breaking waves, bubbles, and suspended sediments create acute problems, optical properties are important to MCM in the very shallow, shallow, and deep water environments. In-water optics play a role in diver missions, in detection and classification of mines through video imagery and other optics-based techniques, and in interpretation of remotely sensed hyperspectral imagery. The latter has relevance to the determination of bathymetry through satellite remote-sensing.

Visibility for divers is a major issue for both safety and ability to detect mines and neutralize them. Divers need to know their visual depth limitations both vertically and horizontally in natural sunlight, their visibility range with an artificial light source either at night or when they are below the euphotic zone, and their vulnerability to visible detection from above the sea surface. The ability to obtain the detailed vertical structure of attenuation coefficients, rather than a water column average, provides divers with valuable information for mission planning. For example, a thin layer with radically different optical properties may serve as an optical haven or shield for avoiding detection by the enemy. Conversely, a thin layer may obscure the diver's ability to see the bottom in an otherwise clear water column. In environments where bioluminescence is present, an additional safety issue for divers is whether bioluminescent light will be observed from the surface or will be rapidly attenuated.

Light Detection and Ranging (LIDAR) systems are operated from helicopters, and cameras for underwater video imaging are incorporated into towed packages or autonomous platforms for minehunting and classification. Some of the issues relevant to divers also apply to cameras and laser detection systems. If the waters are turbid or highly absorbing, the optical signal will be rapidly degraded, limiting the detection distance between the optical sensor and target. Turbidity results in a decrease in the depth that can be effectively probed by helicopter systems and a decrease in the area that can be effectively hunted by towed systems. Knowledge of local inherent optical properties could provide reliability estimates on optical measurements, including bounds for different sizes and shapes of mines under different turbidity conditions and decision criteria for whether camera or LIDAR measurements are worth collecting under low-visibility conditions. Information on vertical structure of optical properties could provide guidance for deployment strategies of in-water sensors.

In coastal regions, the optical environment tends to be vertically and horizontally variable. Causes of spatial variability include phytoplankton patchiness, tidal and estuarine circulation patterns, and topographically forced mixing that redistributes absorbers and scatterers in the water column and resuspends sediments. Some of the mechanisms that produce spatial variability in optical properties of the littoral zone are also responsible for generating temporal variability. Optical remote-sensing with sufficiently high-spatial resolution can provide a context for extending local measurements of inherent optical properties to larger scales in the coastal zone.

Inherent optical properties vary in time and space. In coastal environments, it is often difficult to separate temporal and spatial variability, because both are greatly influenced by high-frequency forcings, such as tides, local winds, and inherently unsteady turbulence. Advection from local circulation patterns plays a major role in optical variability. An optical profile at one time and location may be useless for predicting the range of the day for a broader region, or even for that location later in the day.

The three major contributors to optical variability in the coastal zone are phytoplankton, chromophoric dissolved organic material (CDOM), and suspended sediments. One part of the problem is to understand the factors responsible for changing the specific optical properties of phytoplankton and CDOM, such as photoadaptation of pigment concentration and type for phytoplankton and bleaching for CDOM. Another part is to determine how concentrations of the three contributors change by processes such as phytoplankton growth and death; CDOM in situ production, leaching from the sediments, input with freshwater runoff, or seepage; and sediment resuspension, floculation, and settling. A third part of the problem is to understand how these time-dependent processes are coupled with local circulation and mixing. Both measurement and modeling approaches are needed to better understand how optics change in the compressed nearshore space and time continuum.

Remote-sensing provides a comprehensive spatial picture of the surface optical properties, but also presents three major challenges. One is to develop algorithms for maximal extraction of information from hyperspectral

ocean color remote-sensing, such as sediment concentration. The second challenge is to couple surface imagery with information on vertical structure, collected from a limited subset of locations, to produce a three-dimensional picture of water clarity. The third challenge is to provide spatial continuity during cloudy periods and between repeat satellite passes or aircraft overflights. Autonomous vehicles equipped to carry optical sensors can augment above-water remote-sensing measurements by providing vertical information and by providing coverage when above-water sensors cannot.

Mine-hunters need to be able to turn data rapidly into knowledge. Massive matrices of data on inherent optical properties of the water column can be collected in a very short time, but processing and interpretation can be a challenge. What the diver wants to know is what can be seen vertically and horizontally with the naked eye. By knowing the spectral nature of attenuation in advance, the diver will be prepared for color aberrations. What the operator of a towed package for optical hunting and classification wants to know is the optimal deployment mode and an estimate of reliability of the data to be collected. User-friendly robust algorithms are needed to provide the divers and system operators with information in real time or near real time.

Absorption and scattering degrade the quality of images collected by video and digital cameras. Improved pattern-recognition algorithms could increase the reliability of the information extracted from degraded signals. By coupling multiple sensors, such as optical and acoustical, the reliability of mine detection or classification could be increased. High-frequency acoustics and optics might be combined to classify and size the particles causing scattering. Analysis of underwater images is time consuming and requires an experienced operator. Improved automated pattern-recognition programs could decrease the time between image collection and decision making.

A high-degree of spatial and temporal variability of optical properties is characteristic of many nearshore areas. In a rapidly changing environment, the concept of determining the range of the day from one or a few profiles is futile. An integrated program of measurement and data synthesis incorporating optical data from profilers on surface vessels or easily deployed mini-moorings, pods of autonomous vehicles that can be launched simultaneously, and satellites is necessary to provide a three-dimensional view of the environment. Much progress has been made in the last five years in developing sensors that measure inherent optical properties. In the near future, these sensors need to be incorporated into autonomous vehicles and easily deployable profiling moorings. Process models for predicting changes in optically active constituents need to be coupled with physical circulation models to provide a temporally updated three-dimensional view.

Archived databases of satellite remotely sensed data on ocean color can be mined to provide climatological estimates of scales of spatial and temporal variability for specific locales. Time scales of interest need to span the tidal cycle (mixing and advection), diel cycle (phytoplankton growth, diel winds), and event scales (wind and storms) to the seasonal and interannual scales. With the recent and planned launches of a variety of ocean color satellites by the United States and other nations, unprecedented temporal and spatial coverage of many coastal regions will become available. New methods of integrating data from different sensors should be developed and diverse methods of statistical analysis applied to extract meaningful information for MCM applications.

The nearshore environment, because it is so dynamic, is acoustically complex, being characterized by high-reverberation, high-ambient noise, and volume and boundary micro-and macro-heterogeneities. High-frequency sonars, used for mine detection and localization, as well as for classification, are the primary tools of mine warfare. The frequency range of sonar systems in common use is reasonably broad, but is generally higher than for antisubmarine warfare. Sonar systems are expected to operate in the very shallow, shallow, and deep water zones, and they must perform through the entire water column, including near surface, seafloor, and sub-seafloor. A general problem faced is the presence of reverberative and low signal-to-noise and highly cluttered acoustic backgrounds. Furthermore, incorrect interpretations of mine or mine-like objects can occur when using sound velocity profiles that are not representative of in situ conditions. Another difficulty with high-frequency sonar is the classic flash-

light-in-space problem: a highly directive spatial sensor with insufficient time to completely survey the required volume comprising the minefield.

Mines can be triggered by broadcasting acoustic signals, in both a broadband noise and a narrowband line structure, with frequencies similar to those emitted by propellers and machinery on-board the vessel. The target location can also be simulated by varying radiated signal strengths and by activating directional transmitters that appear to be at a specific position in the water column. This process can be further refined by acoustically simulating specific high-value targets (e.g., troop transports and carriers) or vessels with mine countermeasure (MCM) capabilities.

A mine can be as small as a sphere 3 ft. in diameter or as large as a cylinder 21 in. in diameter and 10 to 20 ft. in length (Table 2-2 ). These dimensions define the detection resolution required for effective minehunting. It is a simple calculation to divide the water mass of interest into appropriately sized volume cells to estimate the time required to properly search the area for mines. Once this is accomplished, the next step involves separation of mine-like objects from objects that are mines. The latter need to be investigated more closely using divers or other sensors. As indicated above, the large amount of time involved in identifying and separating true mines from mine-like objects greatly impacts naval missions. This creates a dilemma for MCM forces, as they are forced to survey the potential minefield at a resolution that slows the rate of search so much that the larger naval mission must be delayed or cancelled.

Acoustic visibility is not the same as optical visibility. Acoustic images must be interpreted with considerably less information than a video image. With the coarser resolution of a sonar image, many natural objects may be identified initially as potential mines, thus requiring additional image processing or other means of eliminating objects as potential hazards.

Countering mines is normally a serial process. That is, objects must first be detected, classified, and visited with a remotely operated/autonomous vehicle (ROV/AUV) carrying a warhead. The warhead is then attached to (or near) the mine, the AUV moves away from the area, and the warhead is acoustically signaled to detonate. This process demands accurate navigational positioning during search and localization to reduce the time required for revisit and neutralization.

For convenience, research issues are grouped into three sections: (1) environmental parameters that impact acoustic performance, (2) acoustic capability, and (3) ocean acoustic models that are the basis for performance prediction.

Water column temperature and salinity datasets typically are not collected in sufficient detail for sound speed calculations at the resolution required for mine localization. In addition, these physical parameters are not predictable over the necessary operational time scales. Generally, knowledge of seafloor physical properties (such as sound speed, velocity attenuation, bulk density, and sediment roughness) are insufficient for acoustic modeling. Data are

often not collected at an adequate resolution to be useful for minehunting sonars. Also lacking is an understanding of nearshore geophysical processes needed for development of interpolation and extrapolation methods to be used in data-sparse regions.

High-frequency scattering of acoustic energy from randomly rough surfaces cannot be quantitatively modeled with sufficient accuracy for sonar system performance prediction. Similarly, acoustic properties of the near-surface water column, which may have a significant bubble content due to wind and wave interaction, are not well enough understood to allow predictions of sonar performance.

Search algorithms and classification tools for intricate shapes in complex backgrounds are currently limited. The ability to "see" mine-like objects at varying degrees of resolution and at multiple "look" aspects needs to be exploited with new signal-processing techniques.

Measurements of water column and seafloor acoustic properties must be made at scales of less than 1 m. The resulting datasets will enable predictive models to be developed that can be used to interpolate and extrapolate acoustic properties to regions where data are sparse. Acoustic measurements from a variety of nearshore regions are required. Data must be collected from the entire water column to a depth of at least 2 m into the sediments. These data will provide important information for the development of robust acoustic models.

Temporal aspects of mine warfare are complicated by mission planning times that are typically on the order of days and weeks and dynamic nearshore environmental processes that occur on time scales of hours and days. Prediction of environmental acoustic properties for planning purposes must be consistent with these time lines. Thus, the acoustic environment must be forecast for time scales of less than hours to weeks or greater.

Signal-processing techniques that can be employed in complex (multiparameter) data fields should be developed. Also, signal-processing algorithms capable of exploiting low signal-to-noise situations are required. The exploitation of detailed seafloor data currently collected (phase and amplitude from sidescan sonar) can be applied to mine-like target recognition methods.

In the coastal zone, knowledge of seafloor bottom type is vitally important for successful mine warfare operations. The seafloor and its physical, chemical, and magnetic properties can be important in all aspects of the mine warfare problem, for example,

1. mine burial probability, a function of sediment properties, drives sweep or hunt tactical decisions;

2. seafloor conductivity and water depth are key factors for determining magnetic sweep paths;

3. bottom reflectivity is a factor in airborne LIDAR performance;

4. bottom sediment characteristics are a key factor in sediment transport, which affects water clarity and mine burial;

5. minehunting sonar performance is normally bottom reverberation limited;

6. sediment properties determine shock wave propagation, a method for mine neutralization in the surf zone.

Fortunately, naval science and technology efforts are relatively strong with respect to mine warfare-specific seafloor studies. The Office of Naval Research (ONR) has a basic and applied research effort (6.1¹; see Chapter 3, "Support for Mine Warfare: Environmental Science Programs Within the Office of Naval Research") with the objective of improving the performance and prediction of mine warfare systems used to detect, classify, and neutralize mines located within or on the seafloor. Furthermore, an exploratory development (6.2) mine warfare program is underway that is oriented toward addressing bottom backscattering issues at high-frequencies.

In the MCM research area, ONR's 6.1 High-Frequency Scattering program is investigating high-frequency scattering issues for various types of sediments in shallow water. ONR is also funding the 6.2 Real Time Swathmapper for rapidly mapping shallow areas using an unmanned undersea vehicle (UUV) platform and the Hyperspectral Classification of Coastal Environments, a remote-sensing effort designed to use multi-frequency techniques to classify seafloor sediments.

To better understand and quantify the significance of the mine warfare problem, the U.S. Navy has embarked on a course that includes coordination and analysis of worldwide seafloor data and development of mission planning systems, such as the Mine Warfare Environmental Decision Aid Library (MEDAL). MEDAL uses descriptive parameters to define bottom composition for a wide range of depositional environments. Information can be obtained in situ from diver reports, extracted from acoustical data, or viewed from a video camera on a mine neutralization vehicle. Bottom sediment databases containing 70 categories of sediment descriptions are automatically input into MEDAL. In MEDAL, there is a direct mapping of these descriptions into 15 categories for the sonar performance model and into mud, sand, and rock for the mine warfare (MIW) doctrine worksheets. Bottom roughness and clutter density databases are input into MEDAL as defined by MIW doctrine. MEDAL also provides manual entry for the above data types based on diver observations, hydrographic charts, etc. Even though detailed sedimentological data are often available from bottom grabs and dive reports, these data are decimated to mud, sand, or rock based on doctrine as given in NWP 3–15 (Chapter 2, "Mine Warfare Doctrine").

Operationally, a better understanding of seafloor bottom types and properties can lead to increased detection range and search rate, improved classification performance, and increased capabilities for buried mine detection. Thus, improved understanding of seafloor properties will greatly improve mine warfare mission efficiency.

Significant naval research efforts have been directed toward a better understanding of sediments and seafloor processes. These studies have demonstrated the importance of seafloor characteristics for littoral mine warfare, however significant deficiencies still exist, for example:

1. A significant amount of acoustic energy has been observed entering the seafloor at angles below the critical angle. Since many acoustic mine warfare systems are operating near the seafloor, low-angle acoustic propagation is critically important to system performance. Work is required to fully understand the physics of this phenomenon.

2. At this time, the use of distributed explosive arrays (e.g., explosive nets) appears to be the most effective way of neutralizing mines in the surf and landing craft zones. Although, it must be acknowledged that waves and wave-generated currents can cause significant deployment problems for explosive nets. Since most surf zone mines will be buried in the sediments, sedimentary explosive shock wave propagation is critical for determining operational performance. Presently, we do not understand the physics of sedimentary stress-strain relationships; therefore, we cannot adequately model or predict the effectiveness of the explosions for mine neutralization.

3. Mine burial is an essential factor in our ability to conduct effective mining operations. If mines are buried in the objective area, we have no recourse but to conduct influence-sweeping operations. Therefore, to make this extremely important tactical decision, it is necessary to have the capability to predict the likelihood

of mine burial. Presently, there are four mechanisms by which mines will bury: scour (current induced and wave induced), migrating sand ridges, burial by deposition, and impact burial. Existing models to predict mine burial are rudimentary at best. Of these four models, impact burial prediction is the most mature, but even this model needs improvement. We do not understand the forces causing mine burial (currents, sediment transport), in particular how these forces interact with various mine shapes. An understanding of these physical processes would allow an improvement of burial prediction models. A significant deficiency in this regard exists for the surf zone.

4. Sediment transport in very shallow water and surf zones greatly affect mining operations. Sediment transport is critical for mine burial, electro-optic transmissivity, acoustic scattering, and bathymetry. Significant work has been done in sediment transport research, and a strong understanding of the physics involved has been developed, but these efforts have been generic rather than mine warfare-specific.

Research efforts should focus on advancing ongoing sediment acoustic property research in the MCM Tactical Environmental Data System (MTEDS) Program, which includes development of techniques enabling the AN/SQQ-32 minehunting sonar to "calibrate itself" for better sonar performance by determining seafloor reverberation statistics. In addition, MTEDS researchers have developed techniques using MCM vessel echo-sounders, in conjunction with the Naval Research Laboratory's Acoustic Seafloor Classification System, to determine sediment physical properties necessary for mine burial prediction. Research efforts should also focus on the problem of below-critical-angle acoustic partitioning. This effort should be cooperative between the ocean acoustics and geology and geophysics communities.

As seafloor type and onshore geology may be related, predictions of seafloor character in the Surf Zone and Very Shallow Water environments can be accomplished with some success by extrapolating known stratigraphy and geologic structure from the adjacent landmass. There is an abundance of information on nearshore terrestrial geology but, comparatively, there is little known about the geology of continental shelves. Thus, the ability to predict seafloor character would benefit from expansion of seafloor databases to include onshore stratigraphic and structural information.

Development of mine burial prediction models should be initiated as part of the exploratory development phase (6.2) of the Environmental Physics for Mine Warfare Program. This research will improve existing impact burial prediction models and develop effective models for other mine burial mechanisms. Mine burial in the surf zone and empirical testing of quantitative models should also be addressed.

Development of an explosive shock wave propagation model should be initiated as part of the Environmental Physics for MCM Program. This research initiative should encourage multidisciplinary interactions among explosive physicists, geophysicists, and mathematical modelers.

It is also recommended that the excellent basic and applied research (6.1) from the various sediment transport programs be advanced to the exploratory development phase (6.2). Sediment transport research should also investigate the effects of this phenomenon on specific mine warfare areas of interest, such as mine burial, E-O system performance, acoustic scattering, and bathymetry.

By doctrine, the surf zone encompasses depths less than 10 ft. (~ 3 m), however the oceanographic definition is the presence of breaking waves. The physical extent of the surf zone, the nature of the dynamics, and the complexity of operations in this area all depend somewhat on the regional framework geology. Coral or mud beaches present different operational constraints than sandy beaches (the primary focus of this discussion). Similarly, the horizontal extent of the surf zone depends on the slope of the beach and on tidal range varying from tens of meters (~30 ft.) for steep slopes to a kilometer (0.6 mi.) for flat dissipative beaches.

Several classification schemes exist to organize surf zone characteristics according to geological setting and dynamic state (Inman and Nordstrom, 1971; Wright et al., 1986). The following discussion deals with a generic intermediate sandy beach and should be extrapolated for other environments.

The fluid environment of the surf zone is energetic and dominated by waves or wave-driven currents. Wave heights are commonly 1 m or more (> 3 ft.). Waves are turbulent, and inject bubbles into the water column through spilling and especially plunging breakers. Currents can exceed 1 m/s (1.8 knots) and can be directed alongshore for obliquely approaching waves or offshore in strong rip currents. Tidal currents are not usually important, but tidal changes in sea surface elevation can shift the surf zone on-or offshore and can substantially change the location and intensity of breaking and the strength of both rip currents and longshore currents.

Bathymetry and the sediment environment are commonly complex and highly changeable. Sand bars introduce meter-high anomalies in the beach profile that trigger wave breaking and can induce strong circulation patterns. Gaps in bars funnel rip channels that reinforce the gaps to form a rich suite of morphologies. Moreover, in most environments, bathymetry changes rapidly with a nearshore survey usually having a useful "shelf life" of a week or so, but as short as a day under storm conditions. Shorter-scale bedforms form or change very quickly in this environment, although perhaps conveniently the seabed is usually flat in the highest energy regions.

Mine warfare in the surf zone is unlike any other region because (a) the environment is hostile; (b) acoustic and optical search tools are generally unusable; (c) burial or scour can be rapid and extensive; and (d) the mines can be cheap, numerous, and mobile. As a consequence, brute force minesweeping methods are most often used. The effect of the environment on mine warfare operations can best be described in terms of the planning and in-stride execution phase and then a follow-up phase.

Because surf zone conditions are so difficult, explosive nets and line charges are the primary method of clearing safe lanes. Thus, planning requirements are primarily concerned with explosive effectiveness (related to pore water gas content) and selection of preferred (safest) routes. For the purposes of this report, the latter is addressed with reference to the mine problem versus other operational and safety issues.

Both of the above concerns are related to sediment characteristics and mobility. Gas content depends on depositional sedimentary organic matter content and the degree of exposure and recent turn over of sediments, the latter being somewhat related to bathymetric variability. Similarity, it is likely that in the time between seeding a minefield in the surf zone and an amphibious operation, the field may undergo substantial change. Heavier mines may become either partially or completely buried through profile variability (sinking during erosion, then covering during subsequent accretion) or through scour. This process is facilitated through all modes of profile variability from the movement of bars to bedforms and can easily result in burials up to 1 m in depth (potentially rendering a mine ineffective or inert).

Similarly, smaller anti-personnel mines are known to be mobile under wave and current motion. Thus, a minefield evenly spread initially may slowly become non-uniform as mines concentrate in, for example, rip channels. Knowledge of this process and of the recent morphologic changes in a beach (from remote-sensing) could allow selection of the safest routes with low-expected mine density and/or deep burial (alternatively, rip channels may be preferred if buried mines are a major concern).

Optimum use of these effects will require study of mine burial and redistribution processes on ocean beaches. It is reasonable to expect that the safest mine routes may also turn out to be the safest routes for other operational purposes.

Minehunting and sweeping in the surf zone is exceedingly difficult. The high levels of turbulence and bubble injection usually render the surf zone opaque to both optical and acoustic search techniques. Water clarity on sandy or mixed grain size environments is often less than 1 m (3 ft.), and decreases rapidly in the presence of fine-grain sediment and biologic components. While diver search is often difficult to impossible, conditions can improve depending on the influence of local river or estuary outflow (for example, the direction and extent of the Chesapeake Bay outflow on the regional environment) or on local winds driving upwelling or downwelling flows that might move turbid waters offshore. These conditions are usually predictable with models and can be incorporated in later clearance planning.

The large bubble density in wave breaking acts both as a source of acoustic noise and as an effective absorber of acoustic energy from active sources. The break point is effectively a blanket that allows no shoreward sonic penetration from offshore active sources. The poor acoustic environment is nearly always a problem, but it is particularly so under plunging breakers. At sites with an appreciable tide range, advantage may be taken in search planning of tidal shifts of the break point and locations of strongest wave breaking.

Environmental parameters in the surf zone have significant influences on diver and marine mammal operations, which are the same as those affecting Special Operations Forces, detailed in a separate Ocean Studies Board (OSB) study (NRC, 1997). Prediction and understanding of waves and currents are very important to diver safety and effectiveness. However, in follow-up overt operations, issues that affected diver observability, such as bioluminescence and aerosols, are less important.