OCEANOGRAPHY AND THE MARINER 



benefit to be derived from these is of indirect consequence in view of 

 the fact that the properties themselves are not too well understood. 

 Furthermore, as stated above, they are usually determined by com- 

 plex mathematical calculations or by tedious laboratory experiments 

 which always contain an inherent degree of error. If any of these 

 least commonly known properties can be considered to have the 

 slightest value to the mariner, it would definitely be indirect as 

 seen in application of the values of specific heat or heat fusion in 

 basic experimentation for ice-forecasting purposes. Another indirect 

 application is the use of thermal expansion in determination of 

 general ocean circulation. 



Freezing of sea water takes place at lower temperatures than 

 does freezing of freshwater, owing to the presence of dissolved salts 

 which tend to depress the freezing point. 



An ice-forecasting program has been pursued at the Oceano- 

 graphic Office since 1952. Depending on requirements, either short- 

 er long-range forecasts can be prepared, ranging in period from 5 to 

 30 days. The bases for these forecasts are extensive air reconnais- 

 sance missions and comprehensive reports of shore observers. Nu- 

 merous favorable post-operative comments have been received from 

 shipmasters directly concerned. 



Water color may be caused by a variety of factors: sky cover, 

 water depth, and particle content of water to cite a few. Particle 

 content may consist of sediments, nutrients, or plant and animal 

 matter whether dead or alive. As observed from shore or on board 

 a vessel, water color may range from deep blue to intense green. In 

 some special cases, water color may appear brown or red, especially 

 in coastal waters. Open ocean water with a high degree of trans- 

 parency usually assumes a deep blue color owing to scattering of 

 light on water molecules or fine suspended particles. Greens occur 

 in a variety of hues ranging from yellow-green to blue-green, the 

 degree of tinting or shading being dependent upon the amount of 

 suspended material such as algae or sediment particles. Extraor- 

 dinary populations of minute plants and animals near the surface 

 can be responsible for red or brown water. Color variations occur 

 more frequently in coastal waters owing to addition of sediments 

 and nutrients by river effluent or by disturbance of bottom sediments 

 by wave action in shallow waters. 



Transparency of sea water is closely linked with the distribution 

 of suspended particles and concentration of dissolved colored sub- 

 stances. High values of transparency have afforded many mariners 

 with a large margin of safety while navigating shallow hazardous 

 waters. Rate of progress during salvage work is highly dependent 

 on the amount of light penetrating the water. Little information is 

 available with regard to depths at which animal life in the oceans 

 can see, nevertheless, the distribution and color of various organisms, 

 including fishes, are apparently also related to the amount of light 

 at depths. The depths to which electromagnetic waves of various 

 lengths, especially heat and light rays, will penetrate are directly 

 dependent on this suspended or dissolved matter. Most of the radi- 

 ant energy received at the sea surface is absorbed or reflected in the 

 form of heat, thus becoming the basis for the study of the heat budget 

 of the sea. The temperature and vapor pressure at the interface 

 created by the sea surface and atmosphere play an important role in 

 determining the rate of heat exchange between the two. Extent of 

 cloud cover controls the amount of radiation gained or lost by the 

 sea. The rate at which the downward traveling radiation decreases 

 in water is determined by the "extinction coefficient". This coeffi- 

 cient is of concern to the oceanographer in that it can be used to 

 calculate depths to which visible waves will penetrate. Measure- 

 ments of extinction coefficients are rare. 



SOUND 



Oceanographic research has considerably increased man's under- 

 standing of sound in the sea, although many aspects of underwater 

 sound transmission remain to be investigated before full appreciation 

 of its use can be made. 



It has been shown that sound velocity is dependent on values of 

 density and elasticity. Density, as stated above, can be measured 

 directly or determined by use of tables when values of temperature, 

 salinity and pressure are known. Elasticity is determined mainly 

 by pressure. Sound intensity, or the sound pressure level at any 

 point, is dependent on the sound generating source and properties 

 of the medium. 



Sound waves propagated in a homogeneous medium radiate out- 



ward from the source in all directions. This spreading of the energy 

 is known as divergence and will result in reduction of the sound 

 intensity at any distant point as compared with the initial intensity 

 at the source, whether the distance travelled is from source to target 

 (ping) or from target to source (echo). Thus the greater the range, 

 the smaller the amount of energy imparted to a target surface or 

 returned as an echo. 



Other factors contributing to range limitations are reflection 

 loss, reverberation, refraction, and absorption. Reflection occurs 

 whenever sound waves are obstructed by solid objects or sharp dis- 

 continuities, such as the hulls of ships, the ocean floor, and the sea 

 surface. If these surfaces are rough, sound energy will be scattered 

 in all directions. That part which is returned to the location of the 

 original sound source, thereby interfering with echo recognition, is 

 known as reverberation. The bottom effects are generally important 

 enough to be regarded as the controlling factor in echo ranging 

 whenever the depth of water is less than about 100 fathoms. At 

 short ranges (less than 1,000 yards), reverberation from the surface 

 forms the principal background in echo ranging and is usually the 

 controlling factor in the detection of small objects whenever the sea 

 is not calm. Suspended particles and bubbles also cause reverbera- 

 tion by scattering the sound energy as secondary wavelets and 



ABSORPTION 



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REFRACTION 



Factors affecting sound paths in water 



reflecting part of it back to the sound source. Refraction is the 

 bending of sound rays owing to changes of velocity caused by varia- 

 tions of density in the transmission media. These variations occur 

 between water masses having different physical or chemical properties 

 and between dissimilar mediums. Bending of a sound beam always 

 takes place in the direction where velocity is lower. Absorption fur- 

 ther reduces ping and echo intensities when part of the sound energy 

 is converted to heat energy by friction caused by water viscosity or 

 by vibration of suspended particles such as bubbles. 



Echo intensity can also be attenuated by soft forms of biological 

 growth on a target. Echo interpretation is further complicated by 

 surrounding sources of extraneous noises. These noises are known 

 as ambient noises and are classed in four categories according to their 

 source, viz., thermal, biologic, water, and artificial or man made. 

 Thermal noise is created by increased molecular activity of water 

 with increasing temperature and is characteristically more intense at 

 higher frequencies. Biologic noise emanates from numerous organ- 

 isms, primarily fish and shrimp. Water noise is, of course, produced 

 by wave and current action. Artificial noise originates from several 

 sources such as ship machinery, bow waves, wake, or perhaps shore- 

 based noise. 



Whatever its source, ambient noise will tend to mask the echo 

 signal and, as a result, reduce the maximum range at which an 

 object may be detected or identified. In addition, when a vessel 

 backs down, drifts with the wind, or uses excessive engine or rudder 

 changes, little energy may return to the transducer because of bubble 

 formation around the transducer and may result in quenching. 

 Maintaining constant headway has been considered the best solution 

 for avoiding the problem of quenching. 



Keeping in mind the factors which promote or deteriorate sound 



