SCIENCE AND TECHNOLOGY SUMMARIES

Acoustics, Geology and Geophysics, Sediments, and Sediment Transport

This discussion group focused its activities on answering two questions: How well can these scientific disciplines support operational needs today? What new research on these topics is needed to support coastal warfare in the future?

These questions should be answered in the context of both present and future requirements of the Oceanographer of the Navy. Present requirements include surf forecasting, bathymetry, sediment classification, and loss of acoustic signals in the sea floor. Future requirements include beach trafficability, offshore hazards, data perishability, and acoustic coherence.

To stimulate research in these areas, the Navy should maintain a central file of coastal oceanography data requirements that the research community can access. It would aid education, communication, and the generation of new ideas. The symposium's four warfare areas, were all discussed relative to the above two questions:

All warfare areas

The science and technology that can be applied include geographic information systems (GISs) that provide the capability to manipulate and display environmental and operational data through map-based graphics. This important information system could be used by both the fleet operator/briefer and the environmental scientist. The systems would rely on standardized formats and could employ CD-ROM technology to form the basis of a worldwide shallow-water data base.

Anti-submarine Warfare

The present scientific support for ASW research is inadequate. Because of the Navy's relatively low emphasis on the shallow-water environment, models used for coastal ASW are inadequate, and the physics of the environment are not well-understood, because considerably more data are needed to verify the models. In addition, the signal-to-noise ratio for acoustic detection and the effects of bottom variability and the resulting interactions on our acoustic prediction methodologies are important problems that need to be studied.

Based on the symposium's ASW sessions, new research needed to support future coastal warfare is evolutionary, revolutionary, or systems related.



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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY SCIENCE AND TECHNOLOGY SUMMARIES Acoustics, Geology and Geophysics, Sediments, and Sediment Transport This discussion group focused its activities on answering two questions: How well can these scientific disciplines support operational needs today? What new research on these topics is needed to support coastal warfare in the future? These questions should be answered in the context of both present and future requirements of the Oceanographer of the Navy. Present requirements include surf forecasting, bathymetry, sediment classification, and loss of acoustic signals in the sea floor. Future requirements include beach trafficability, offshore hazards, data perishability, and acoustic coherence. To stimulate research in these areas, the Navy should maintain a central file of coastal oceanography data requirements that the research community can access. It would aid education, communication, and the generation of new ideas. The symposium's four warfare areas, were all discussed relative to the above two questions: All warfare areas The science and technology that can be applied include geographic information systems (GISs) that provide the capability to manipulate and display environmental and operational data through map-based graphics. This important information system could be used by both the fleet operator/briefer and the environmental scientist. The systems would rely on standardized formats and could employ CD-ROM technology to form the basis of a worldwide shallow-water data base. Anti-submarine Warfare The present scientific support for ASW research is inadequate. Because of the Navy's relatively low emphasis on the shallow-water environment, models used for coastal ASW are inadequate, and the physics of the environment are not well-understood, because considerably more data are needed to verify the models. In addition, the signal-to-noise ratio for acoustic detection and the effects of bottom variability and the resulting interactions on our acoustic prediction methodologies are important problems that need to be studied. Based on the symposium's ASW sessions, new research needed to support future coastal warfare is evolutionary, revolutionary, or systems related.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Evolutionary research simply expands on the current S&T direction, adapting deep-water methods to shallow water. Evolutionary research would, for example: Attempt to increase our ability to predict sound transmission in nonparallel seafloor layers as well as in lumpy, nonlayered bottoms, Develop data on bottom loss of sound as a function of location and geoacoustics, Seek to understand better the physics of bottom scattering and reverberation. The possible need for a revolutionary approach to the inherently greater variability of shallow water was discussed. A revolutionary research approach may apply to the entire question of coastal ocean acoustics or perhaps only to new unfamiliar warfare regions where there are few or no prior data with which to perform classical statistical analyses. One recommendation was to develop the capability to gather real-time data, whenever shallow water environmental conditions are favorable to either acoustic or non-acoustic approaches. Amenable conditions may prove more amendable to scientific analysis than the proverbial “black cloud” compilation of conditions surrounding a failed mission. Another recommendation was to provide a centralized collection of coastal oceanography requirements that the research community could access. This would improve education, communication, and product definition. It was suggested that the Navy take advantage of the wealth of oceanic data collected in territorial waters of other nations, which reside in our universities and elsewhere in academia. Overall, a number of research and development ideas were recorded in the areas of observational capability, data collection, and understanding the “science” of the problem. For military operations in coastal areas, future research should aim to: Improve performance of existing (deep-water) active sonars by training and modeling or design entirely new sonars for shallow water, Develop acoustic and nonacoustic data integration systems as well as new rapidly deployable sensor units with a remote processing capability, Expand our knowledge of broad-band acoustics for low doppler (slow) targets, Develop detection and targeting techniques for bottom or near-bottom targets, and

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Expand geoacoustic terrain analysis to improve shallow-water modeling for use as both a tactics and planning aid. Mine countermeasures Both detection of mines and countermeasures are principal problem areas. Fundamental burial mechanisms are well known and documented, but linkage to widely varying and extreme environmental events is not well understood. The use of nonmagnetic (e.g. fiberglass) mines complicates acoustic detection. The present low efficiency of sweeping a mined surf zone needs to be improved by using side-scan sonars or other new techniques. Some specific actions suggested for future research include: Develop a series of shallow-water mine detection system packages, such as magnetometers, gravimeters or acoustic subbottom side-scanning systems, Study the mechanisms of suspended sediment seafloor scour and object burial, Investigate the sources and mechanisms of sand transport to improve the forecast of poor nearshore visibility, Develop ROVs for remote mine demolition with time delay activation, Focus planned coastal data collection on areas identified by the Department of Defense as high-probability conflict areas, search the literature for existing research and data for these areas, and incorporate this data base in a GIS to share these data in map format with Navy laboratories and appropriate civilian organizations, Develop high-frequency imaging sonars with high resolution and wider detection swaths. Amphibious warfare The discussion initially centered on the status of research support from scientists studying acoustics, geology and geophysics, sediments, and sediment transport to amphibious warfare. The techniques to predict beach trafficability are well-known, but few beaches of interest have been surveyed and reported. Surveys are important because trafficability is not measured directly but is inferred from other data.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY The dynamics of beach morphology, sand bars, and slopes are not well-understood or predictable, in part because models of along-shore currents and tides are unreliable. The capability to predict the future shape of beaches does not presently exist. Future S&T efforts in amphibious warfare should be directed in the following areas: Coastal morphology models to address both along-shore currents and sediment transport mechanisms, Real-time assessment of shoreline geology and geophysics from aircraft, predictive models, and soil mechanics, and Shallow-water sensor packages for measurement of tides, atmospheric pressure, currents, and winds. These systems should be rapidly deployable and expendable, and be radio linked to command and control decision makers. Special operations Because the discussion group included no special operations personnel, suggestions were limited. Underwater visibility as a function of sediment type and movement and beach trafficability were the two issues discussed. Research for other warfare areas might also be applicable to this area. To date, special operations has had little support from naval oceanography. One area of necessary S&T development work was identified: Navy SEALs need a covert system for swimmer location both for the swimmers and for their controller. Such a system could utilize the global positioning system (GPS). Nonacoustics (Optics, Chemistry, and Biology) This discussion focused its efforts on identifying the key scientific and technical issues and problem areas involved in attempting to exploit optical, chemical, and biological properties of the coastal environment in support of naval warfare. The session began with a discussion of prior efforts in nonacoustic sensing techniques. The goal of efforts in this area is predicting where and when environmental conditions are favorable for nonacoustic sensing.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Optics The key to employing optical methods in the coastal environment is to improve our quantitative understanding of the processes that affect scattering and absorption of visible and infrared energy. This problem is particularly difficult in the coastal regime because of the presence of strong gradients, multiple scattering layers, and the general inhomogeneity of the waters. Biological activity has a large impact on ocean optics, especially in the coastal region, where suspended sediment complicates the environment. Separating the desired signal from the background light levels requires the capability to predict and quantify the integrated reflectance from the water column and to separate out the contribution from the target (i.e., submarine or mine). An adequate data base of attenuation coefficients (k) does not exist, and it is difficult to employ one value across a wide range of optical techniques (e.g., passive multispectral scanning, laser techniques). Secchi disk measurements are the standard method for determining k. In spite of the many shortcomings of Secchi disk measurements, they are useful. Additional research in the following areas should be considered: The typical coherence scales of coastal processes need to be understood better to exploit optical techniques fully. The highly variable aerosol content of the coastal atmosphere must be characterized more completely. The coastal zone color scanner did not perform well for coastal measurements, in part because of problems in removing the atmospheric component of the back-scattered optical signal. A more quantitative understanding of the strength of bioluminescence sources as a function of wave length, along with the development of a digital data base of measurements, should be a priority activity. The development of pattern and clutter recognition algorithms should be investigated. Time-dependent optical techniques (measurements include the use of phase and travel time information) should be investigated. Use of surface films and surfactants should be investigated as a possible method of moored mine wake detection.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Chemistry Because the discussion group did not include a marine chemist, the discussion was limited. It was noted that, in principle, it may be possible to detect the presence of a submerged object by the trace chemicals left in the water. Some problems with chemical sensing include the lack of baseline information related to chemical signatures in specific locations and the need to develop accurate real-time chemical sensors. Recent work revealed that lobsters can determine the distance of objects of interest by measuring the amount of spatial decorrelation between chemical concentrations emitted by the object as measured simultaneously by each of its antennae. If the chemical signal measured by both antennae is highly correlated, the source is probably far away, because of the interaction of molecular and turbulent diffusion processes that homogenize chemical concentrations proceeding away from an object. Over time, molecular diffusion tends to dampen out sharp gradients. If, on the other hand, the lobster 's antennae measured the presence of strong gradients in the chemical signal (i.e., the signal is decorrelated), the implication is that diffusion had not had time to damp the gradients and the source is probably nearby. This crustacean sensing technique may have application for our chemical detection of objects in the water column. Biology Identified as potential areas of additional research were local phenology, predictive modeling, scales of variability, and bioturbation. Combined Variables Potential areas for additional research are: Co-variability. Understanding the interactions among chemical, biological, and geological processes better as well as the effects of these interrelationships on the transmission of nonacoustic energy, Sea Truth. Developing new, expendable in situ devices to measure a full range of nonacoustic variables in order to produce independent time series of simultaneously measured variables, Data Integration. Development of techniques and computer systems to integrate this diverse new data set into user-friendly, easy-to-interpret detection products,

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Process modeling - The short time and spatial scales of coherence need to be taken into account, not only in the outputs of models, but also in their formulation and testing. A particularly insidious problem is covariation on short scales imposed by longer-period variations (e.g., with distance from shore or along shore). These considerations underlie the above recommendation to focus on short, statistically independent time series of simultaneously measured variables. Eulerian/Lagrangian discussion - A Lagrangian approach might appear most profitable in focusing attention on the biological processes of interest. However, the fact that these particles, by virtue of differing specific gravities and by active behaviors in the case of organisms, can leave streamlines, greatly complicates any bookkeeping scheme in numerical modeling and any scheme for Eulerian-Langrangian conversion. This fact also complicates any argument that either an Eulerian or a Langrangian approach is inherently superior. Physical Oceanography The session began with a brief presentation on the history of naval oceanography, including the evolution of oceanographic instrumentation from old, mechanical meters to new, digital, multipurpose systems. The Navy should take more advantage of this new technology and provide “turnkey” data collection systems to the surface fleet. Greater use of ROVs for survey, mine, and intelligence activities was advocated. It was also recommended that there be additional research into more capable microwave instrumentation as a data collection adjunct. The discussion of S&T issues centered around four subject areas: observational capability, modeling applications and verification, the synthesis and understanding of regional data, and basic science processes. Research topics are outlined below. Observational capability The Navy need better ways to collect oceanographic data. To correct this deficiency, the Navy could develop buoys to collect data for model initialization and analysis, self-propelled autonomous instrument packages to collect data in areas where the Navy does not have access.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Collect oceanographic data whenever and wherever possible although there are times when no personnel are available to do so. This limitation might be overcome by repackaging existing sensors to produce a hands-off, calibrated instrument package that could be installed on a ship while in port. Such a package could collect data while at sea, transferring them to a storage device for periodic shipment to a data-processing facility. Much concern was expressed over NAVOCEANO's reinvigoration as a leading “expert” center in oceanographic information. With the great number of potential conflict areas, NAVOCEANO should rely on academic experts in each region for information and guidance. Support for this approach was enthusiastic because no single organization could possess the full spectrum of data and models for complete coverage of all geographic areas. Further research to improve shallow-water observations is warranted. Surf and sediment make use of existing data collection systems unfeasible. Modeling applications and verification The best way to maintain and verify oceanographic models was discussed at some length. NAVOCEANO does not have the resources to maintain the required number of regional models. Again, the Navy should use academic experts for selected regions. Because it is impossible to have a unique model for each region, some generic approaches to modeling these areas should be examined, with emphasis on similar forcing functions, bathymetry, and open or closed basin boundaries. The ocean modeling community should apply a few selected models in many different locations to understand their limitations more completely, and gain confidence in their capabilities. Sensitivity tests should be performed on verified models. The tests would parametrically vary the forcing functions and observe the results of that action. The tests would more accurately measure the performance of the model in an operational environment. A program should be developed in the fleet to assign a junior military officer to all appropriate combat and support platforms as the science and data collection officer (not an 1800 officer). This officer would receive training in environmental science and would serve as a liaison between the warfare and environmental science communities. One Captain indicated that many fleet commanders tend to have an inherent distrust of environmental models. The science officer approach would provide a mechanism for the operating units to participate actively in data collection, model development, and model verification. If users are made part of the overall effort and they understand the rationale behind data collection efforts, better data collection and subsequent modeling efforts will result.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Synthesis and understanding of regional data With examples from Operation Desert Storm there was considerable debate on the utility of NAVOCEANO's oceanographic models, and some concern about NAVOCEANO's ability to have in place adequate, environmental models for each potential conflict area. NAVOCEANO should plan and initiate the systematic development of shelf circulation models. A plan for developing “global” coverage would be much more effective in the long run than attempting to develop models quickly in response to hostile military actions. NAVOCEANO should take advantage of the regional expertise of the academic community in the development of the models generated systematically and in rapid-response military requirements. Basic science processes: Future research Most of the discussion centered on the understanding of basic science processes needed to perform the Navy's mission. There is value in the judicious use of laboratory experiments to test the processes expected or observed in the field. Areas needing further study include: Shallow-water boundary layer processes, including wave-current interaction and wave breaking phenomena, Near shore river-shelf interactions. Whose buoyancy and density effects are of great interest to the Navy community, particularly in high-latitude areas, The important processes that occur in areas with rough topography, Accurate models of wave dispersion over canyons and coastlines, Processes over the continental shelf, including internal waves, mixing, and wave breaking mechanisms, necessary for successful acoustic and electromagnetic measurements in these areas, Three-dimensional circulation in the surf zone, Hydraulic processes associated with atmospheric frontal regions over the shelf.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY “Shelf processes” (i.e., circulation, waves, mixing) in regions with cohesive sediment bottoms, The response of shelf flow to small-scale variations of wind stress, and Research and new models to identify the processes controlling tides, winds, currents, and waves including differences on broad and narrower shelves. The Navy is expending resources on emerging systems whose support will require new data, new models, and new research. The Navy should involve the scientific community early in the process, to ascertain what new oceanographic information is required, thereby allowing the system to be completely operational when deployed. If oceanographers are not involved early in the process, the requirements will become apparent too late, when the system is in the hands of the users. Several federal agencies, such as the Minerals Management Service, the National Oceanic and Atmospheric Administration, and the Army Corps of Engineers maintain extensive operational and research programs in coastal oceanography. The Navy should take maximum advantage of these existing programs and ensure that unclassified data and technology are made available to other federal agencies and the academic community. Coastal Meteorology Background Coastal weather affects nearly all facets of naval warfare. As the location of potential conflict expands to include islands and tropical and subtropical areas, it is becoming more probable that future wars will be fought in coastal regions. Atmospheric impacts on naval operations can be either direct, such as those of surface air temperature on the range of a Tomahawk cruise missile, or indirect, when changes in the atmospheric forcing of the ocean (e.g., surface wind) modifies the acoustic environment for ASW or builds sea and swell during an amphibious assault. The atmosphere also strongly influences the propagation of electromagnetic and electro-optical energy used in detection (e.g., radar) and targeting systems (e.g., lasers). The effects of clouds and rainfall on naval warfare are important because many advanced weapons rely on lasers for target lock-on -- the presence or absence of a cloud will control the weapons' firing decisions. The warfare specialists cited a need for quantitative descriptions of the basic atmospheric variables of wind, temperature, and moisture and weather elements,

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY including clouds, visibility, and precipitation. The coastal zone features sharp gradients of the air-sea-land interface and abrupt changes in air masses. Consequently, atmospheric data may need to be collected on very small spatial scales depending on the specific region. Based on general characteristics of weapons systems and naval vessels, it was estimated that atmospheric parameters should be resolved on scales of 1 kilometer in the horizontal, tens of meters in the vertical (e.g., to resolve a temperature inversion in the marine layer), and 1 hour in time. No single observational system can measure the atmosphere to the required resolution. Thus it is necessary to resort to theoretical models and the fusion of data from multiple platforms to describe and predict the microscales of coastal regions. A wide range of data products can be used in naval operations, from global climate predictions to nowcasts in a small region and descriptive analyses of coastal weather. Model developments The current generation of operational “mesoscale” numerical weather prediction models have a spatial resolution of 30-40 kilometers and 20 vertical levels in the troposphere (about 10 kilometers deep), which is much coarser than required. More significant is the lack of land surface physics (e.g., evapotranspiration) and of crude parameterizations of turbulent transports (e.g., the assumption of horizontal isotropy) and cloud-radiative interaction. Thus, forecasts from present models are limited in resolving features of coastal weather critical to naval operations. Knowledge of these physical processes is crucial to simulating the atmosphere of the coastal zone. Research in model development should stress a better understanding of physical processes. Coastal meteorology has the distinction of covering all aspects of the modeling problem, in particular: Topographic effects, Thermal forcing of the land and ocean air-sea interaction and the ocean mixed layer, Clouds, Moisture transport through land surfaces, Processes that influence latent and sensible heating,

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Surface drag and radiation, Long-wave and short-wave absorption in multiple-frequency bands. Coastal modeling requires the incorporation of complex processes that are not yet well enough understood to be captured in simulations and forecasting techniques. An important issue to be considered before such “super” models can be used operationally is the degree to which a microscale forecast of a coastal meteorological phenomenon is an initial-value problem (IVP) or a boundary-value problem (BVP). An IVP would require high resolution observations to start-up, whereas the BVP could be driven by coarser data from a larger scale model if interface processes were sufficiently defined. There is evidence that some coastal weather phenomena may be dominated by large-scale atmospheric flows and topographic features that give hope to using “super” models in data-limited areas, but only under some conditions. Observational Developments From observations of coastal meteorological events, two extremes have been found. In the case of the northwest flow off the coast of northern California, when the boundary-layer thermal inversion is below the peaks of the coastal range, the spatial structure of the surface wind over the ocean (including the small scale regions of peak wind speed) can be predicted with simple models. This observation suggests that the phenomenon is dominated by topography and the large-scale flow and that perhaps only a few observations are needed to predict coastal weather. By contrast, an offshore flow from a land surface has been observed to induce internal boundary layers that decouple the main boundary layer (and clouds) from the ocean surface. The resulting boundary layer structure results from a complex balance between cloud-radiative forcing and small-scale changes in the ocean surface. In this situation, extremely detailed measurements may be required for even a limited predictive capability to be achieved. Regardless of the type of coastal meteorological phenomena, our understanding and model verification and development will improve only as our observational capabilities advance. Remote sensing techniques will be important when access to conflict zones is denied. The resolution of satellite measurements is still too coarse and is indirectly related to the primary atmospheric parameters. Nevertheless, these data can provide verification of at least the gross aspects of a simulation model. Higher resolution can be achieved with LIDAR, but low portability and high expense limit

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY widespread use. Another avenue for improving the data base is to continue to expand conventional measurement systems (e.g., radiosondes) by using Navy resources and remote control vehicles. Regardless of the sensing methods, more data and innovative data fusion techniques will be needed to gain a better understanding of coastal meteorology and verify models. Overall, no aspect of naval warfare is immune to the effects of the coastal atmosphere. Better understanding and accurate prediction of coastal zone weather represent perhaps the greatest challenges in meteorology because all physical processes in the land-air, sea, and land play a significant role in the evolution of the air-ocean environment. Electromagnetics During the S&T session on electromagnetics, several key issues were discussed. They affect both the basic research and applied development sides of the Navy's R&D programs. Electric field measurements Ongoing Navy development projects in electromagnetic surveillance tend to concentrate on measurement of the magnetic field as opposed to the electric field. This is a natural consequence of the relatively widespread commercial availability of magnetic field sensors, whereas electric field detection in the ocean remains largely a custom technology developed by academic investigators. Nevertheless, electric field detection offers potential advantages, including greater simplicity and lower cost than magnetic field detection. Electric field sensors are passive voltmeters coupled to the ocean by low impedance electrodes, and they can achieve extremely low instrumental noise levels. They are also insensitive to motion of the detector, unlike vector magnetometers. Further, the electric field from a point source attenuates by the inverse of the distance squared, but the magnetic field decreases by the inverse of the distance cubed. Assuming comparable relative noise levels, the detection range might be greater using the electric field. There are many scientific and engineering issues that must be investigated further, and it is not clear whether electric field approaches are superior in all instances. Nevertheless, more attention should be focused on electric field detection, and a more detailed assessment of its value for use by the Navy should be made.

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY Interaction with academic investigators Development projects in the parts of the Navy responsible for research with a more applied focus, the so-called 6.2 and 6.3 sectors of Navy research, could benefit from more frequent interaction with the academic oceanic electromagnetic community, whose experience is primarily in research with longer term application. The reverse is certainly true as well, and academic electromagnetism specialists would be better able to help Navy meet its needs if they were more aware of the Navy's requirements. Electromagnetic environment in shallow water As interest in electromagnetic surveillance increases and more effort is expended on development of prototype systems, it is clear that a considerable base of knowledge on the electromagnetic environment in shallow water is absent. At the present time, no published data from water shallower than 1,500 meters appear in the open literature. The reason, in large part, is that the Office of Naval Research terminated its electromagnetics efforts about five years ago; thus there is no clear avenue for investigators to fund the necessary exploratory basic research. Although considerable information about the electromagnetic environment in deeper water is available, including identification of sources and understanding of interactions with the seafloor, the understanding is not immediately applicable to shallow water. What is needed is a focused program of basic research to understand the two primary sources, external (ionospheric and magnetospheric) and hydrodynamic types, along with their interactions with the continental shelf. It is not sufficient just to make a few measurements and extrapolate them because the shallow-water oceanographic and geological environment is highly variable and difficult to predict (a priori). Instead, it is important that the origins of the fields be understood; what is required is a multidisciplinary scientific approach that clearly lies in the basic research area. In the absence of basic information on and understanding of the noise environment, it is difficult to devise proper noise cancellation hardware and software, and the obvious tendency will be toward “overdesigning” surveillance systems, resulting in higher than necessary costs and possibly poor performance. It is in the Navy's interests to address the basic research issues in a timely fashion. Sensor development Further development of electromagnetic sensor technology for undersea use is clearly not a primary issue at present. All the available sensors (both electric and magnetic) are limited by environmental noise, and the major goals should be a better understanding of that noise so that it can be canceled effectively. Further,

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SYMPOSIUM ON NAVAL WARFARE AND COASTAL OCEANOGRAPHY numerous magnetic field sensors are available to the Navy with differing performance and cost. Until their utility is better understood, it makes little sense to devise new ones. Investigation of active airborne electromagnetics Active source electromagnetic sensing from aircraft is a primary tool in mineral prospecting, and has recently been adapted to bathymetric charting by the Naval Research Laboratory (formerly NOARL), with impressive results. However, only limited attention has been paid to the use of airborne active electromagnetic methods for direct submarine detection, despite the fact that its performance in shallow water is potentially good enough for it to be a practical tool. Further model investigations and some trial experiments should be carried out. Animal electroreception Animal electroreception, especially by sharks, offers much higher sensitivity to oceanic electric fields than can be devised electronically. The physiology of animal electroreception has been only partially explored, and a better understanding could result in the future development of compact and highly sensitive detectors. Nonlinear effects Although there is little evidence for strong nonlinear constitutive relations in earth materials, if they were present, detection of anthropomorphic electromagnetic signals modulated by the natural background could be simplified. Further investigation of the physics of earth materials might find sufficient nonlinearity for this to be practical.