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Modeling Acoustic Propagation and Scattering in Littoral Areas

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Modeling Acoustic Propagation and Scattering in Littoral Areas C. ALLAN BOYLES and ALBERT C. BIONDO MODELING ACOUSTIC PROPAGATION AND SCATTERING IN LITTORAL AREAS Littoral (near-coastal or shallow) waters are extremely complex environments to model. This article discusses the physical processes that must be considered to accurately model acoustic propagation and scattering in littoral areas and assesses the ability of generic classes of models to represent those processes. No models to date accurately describe acoustic propagation and scattering in littoral areas where the environment is strongly range-dependent, a situation typical in such regions. A discussion of the limitations of existing models, as well as recommendations for gathering information pertinent to the successful development of accurate acoustic prediction codes, is presented. A viable model would provide a cost-effective means of facilitating the design of future shallow-water active and passive acoustic systems and enable an assessment of the performance of potential tactics to be used in littoral areas. INTRODUCTION Research on shallow-water acoustics has been con­ target is sensed. As in a passive sonar system, the ambient ducted for many years by the Navy, but at a low level of noise background is present, but additional interfering effort since no shallow-water threat of any immediate sounds, termed reverberation, are also present. This un­ consequence existed. For at least the last twenty-two wanted acoustic energy results from scattering of the years, the major research focus has been on deep-water transmitted pulse to the receiver from rough boundaries acoustics to counter the Soviet submarine threat and to (sea surface and bottom) and volume inhomogeneities. protect our own ballistic missile submarine force from Littoral areas are complex environments to model. In detection and neutralization. With the collapse of the the shallow waters of coastal regions and on the conti­ Soviet Union, however, and the increasing belligerence nental shelves, the sound-velocity profile is generally of Third World powers, future submarine warfare is likely irregular and unpredictable and is greatly affected by to be conducted in littoral areas. Hence, there arises an surface heating and cooling, salinity changes, and water urgent need to address shallow-water acoustic propaga­ currents. The shallow-water profile is complicated by the tion and scattering and their relevant physics. influence of salinity changes caused by nearby sources of Given a proper understanding of the nature of acoustic fresh water and contains many gradient layers with little propagation in littoral areas, accurate acoustic models temporal or spatial stability.l The structure of the sedi­ may be developed that will enable us to predict propa­ ment in the ocean bottom is also very complex, further gation and scattering characteristics in any shallow-water complicating the nature of the propagation. region of interest. The utility of such models in, for Because of the complexity of the shallow-water envi­ example, experiment planning and analysis, systems ronment compared with the deep-ocean environment, analysis and research, and scenario simulation and war­ more extensive environmental measurements must be gaming, would be great, as demonstrated repeatedly in made to support the modeling effort. Without that sup­ deep-water acoustic research and investigation. With the porting environmental data input to the models, any expected decline in funding for defense-related endeav­ modeling effort will have limited value. The design of ors, accurate modeling would offer a cost-effective future sonar systems based on unvalidated models would means of helping us design future shallow-water systems clearly be futile. Therefore, good acoustic data, taken and assess the performance and effectiveness of proposed concurrently with accurate environmental data, are re­ littoral systems and tactics. quired to validate any candidate acoustic model. Acoustic systems to be employed in littoral areas can To put the difficulty of assessing acoustic propagation be either passive or active. Passive sonar entails listening in shallow water in proper context, we first examine the to or ensing radiated emissions from a potential target physics of shallow-water propagation,2-5 focusing on the (e.g., a submarine). This signal must be detected and physical conditions that must be considered to describe discriminated against a background of oceanic ambient the phenomenon. We then survey the current models and noise, caused, for example, by wind-wave activity at the assess how well they take these conditions into account. sea surface and by shipping. Finally, we specify the supporting environmental data In contrast, in an active acoustic system, an acoustic needed to enhance both our knowledge of shallow-water pulse is tran mitted, and the returning "echo" from the acoustic propagation and our ability to model it. 162 fohns Hopkins APL Technical Digest, Volume 14, Number 2 (1993) PHYSICS OF SHALLOW-WATER deep-water scenarios, and a constant salinity is used to PROPAGATION calculate sound speed. But ignoring salinity gradients in many shallow-water areas can lead to gross errors in the Let us begin by defining the important concept of sound-speed profile. Large salinity gradients near the transmission loss. Strictly speaking, the instantaneous surface can turn a strongly negative temperature gradient acoustic intensity J is defined as into a strongly positive sound-speed gradient directly J = pv, (1) beneath the surface, forming a surface duct. Because of the positive sound-speed gradient, surface ducts can where p is the acoustic pressure, and v is the acoustic cause sound energy to be trapped within them owing to particle velocity. Acoustic sensors, which are usually hy­ upward refraction or to be channeled out of the ducts by drophones, do not measure the particle velocity. Conse­ leakage if the acoustic frequency is low enough. In either quently, ocean acousticians take as a measure of the instance, transmission loss can be affected considerably. intensity the quantity Such gradients are typical near river outlets. Absorption losses in a medium may be divided into (2) three basic types: viscous losses, heat conduction losses, and losses associated with molecular exchanges of ener­ gy. Viscous losses result when there is relative motion where p is the water density, and c is the speed of sound. between adjacent portions of the medium, as during the Equation 2 follows directly from Equation 1 if the wave­ compressions and expansions associated with the passage front is planar (a usual assumption). The intensity, J, is of a sound wave. Heat conduction losses result from the then averaged over time to obtain conduction of heat between the higher-temperature com­ pressions and the lower-temperature expansions of the 1 medium. Viscous and heat conduction losses do not 1=- ITJ dt . (3) T 0 account for the excess absorption in seawater; this excess is caused by molecular exchanges of energy. Two mech­ The units of intensity are average power per square meter. anisms of such energy exchange dominate in water: struc­ Transmission loss (TL) or propagation loss is then de­ tural relaxation and chemical relaxation processes. Struc­ fined as tural relaxation is due to the conversion of kinetic energy into potential energy in the structural rearrangement of I adjacent molecules in some clusters; it results from vol­ TL = -10 log-r (4) 11m ume changes, not temperature changes, and is measured by the coefficient of volume viscosity. Chemical relax­ where Ir is the intensity at a range r, and 11m is the ation is due to the association and disassociation of dif­ intensity at 1 m from the source. ferent ionic species such as magnesium sulfate (above 1 The conditions influencing shallow-water propagation kHz) and boric acid (below 1 kHz). are as follows: Water depth is also an important parameter for prop­ Seasonal changes in the water-column sound-velocity agation. Although it is easy to measure, small errors in profile depth can cause significant errors in transmission loss, Volume absorption in the water particularly at low frequencies due to modal cutoff. Water Water depth depth will play an important role in determining the Sea-surface roughness optimum frequency of propagation in a shallow-water Bubbles waveguide. Biological scatterers Sea-surface roughness can increase transmission loss Bathymetry (bottom topography) at higher frequencies and contribute to significant levels Bottom composition (affecting, e.g., compressional of reverberation. The effects of sea-surface roughness are sound velocity, compressional absorption, density) accentuated by the presence of a surface duct. Surface Shear sound velocity roughness is determined by the sea state; to model the Shear absorption effects of surface roughness, a directional wave-height Bottom interface roughness and sediment volume spectrum is required. Extensive data on wave-height inhomogeneities spectra for the deep ocean exist but are not appropriate Water only supports a compressional sound wave, a in shallow water. wave in which the particle motion of the medium oscil­ Sound may be scattered away from its original path by lates in the direction of propagation of the sound wave. both bubbles and biological scatterers. The bubble distri­ The speed of sound in the ocean depends on water tem­ bution in shallow water can differ from that in deep water. perature, salinity, and pressure. The variation of sound In addition to bubbles caused by wind-wave
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