Appearance of the Ocean. Sound Energy in the Sea

The appearance of the ocean depends upon the viewer's vantage point, the roughness of the sea surface, and the degree of cloudiness. Near the sea surface the ocean appears a beautiful azure color where the view angle to the water surface is steep. At a distance the sea appears blue or gray depending upon whether the sky is clear or clouds are reflected from the surface.

If the sea surface is rough, more light is scattered to the viewer by specular reflection than if the sea surface is smooth. On a cloudy day the light is gray and the sea has a molten lead appearance, but on a clear day the sky's blue color (caused by Tyndal scattering of sunlight by molecules of air) adds to the blue light refracted and scattered from the ocean.

The appearance of the ocean from a distance (for example, from an airplane, a satellite, or even the moon) is blue, the color of the light least scattered or absorbed by the ocean or atmosphere. Other frequencies of radiation are present, but in smaller proportion.

Satellites bearing multispectral scanners view the ocean's radiation in narrow-frequency bands (Fig. 6-11) to discriminate specific features in the ocean. The red band (0.5-0.6 μ) rejects blue and green light, thereby enhancing kelp (red brown algae) and suspended sediment. The yellow' green band (0.4-0.5 μ) emphasizes plankton blooms and suspended sediment.

Figure 6-11. A multi-spectual scanner mounted on an earth observation satellite. The scanner sweeps the scan area as it moves over the travel path and produces a continuous image of the scanned strip. The light entering the scanner is broken into 4 components of the visible and solar infrared spectrum and an image is constructed from each. The data are relayed to earth where photographic products are prepared. The scenes shown in Figs. 7-22 and 8-2 were obtained with such a system (NASA LANDSAT)

Reflected solar infrared radiation is useful for delineating the wetlands at the edge of the sea. The thermal infrared radiation (5-20 μ) emitted by the ocean is an indicator of the sea surface temperature. Thermal infrared scanners are used to map the temperature field and detect upwelled (cold) water, industrial and municipal effluent discharges (warm), and currents (either warm or cold) in the ocean (Fig. 7-3). Microwave frequencies (201,000 μ) emitted from the ocean are used for remote sensing of thermal activity because they are not absorbed by water vapor or clouds and can be used in any weather.

Sound Energy in the Sea. The ocean is virtually opaque to most forms of radiant energy. Fortunately, an exception to this is acoustic or sonic energy which can penetrate the deepest parts of the ocean and even travel across the largest ocean basin. If enough sonic energy is provided, sediment and rock layers beneath the sea floor can be penetrated.

Sound travels rather rapidly in water, about 1,450 m per second. This is almost five times faster than the speed of sound in air (about 350 m per sec). The reason for the disparity in sound velocities lies in the different densities and elasticities in the two media. The velocity of propagation of sound is equal to

Seawater is about 1,000 times denser than air but is 150,000 times more elastic. This is the same as saying that air is 150,000 times more compressible than water.

The exact speed of sound in seawater depends upon the temperature, pressure, and salinity of the water. The variation in the speed of sound with temperature and depth is shown in Fig. 6-12A. Variation in water salinity is accommodated by applying the appropriate correction obtained from the curves in Fig. 6-12B.

Figure 6-12. (A) Speed of sound in water of salinity = 35 %; (B) Speed of sound correction, m per sec. (Used with permission of McGraw-Hill, Inc.)

Alternatively, the speed of sound in water can be calculated from the following equation:

The most common use of sonic energy in oceanography is to "see” the ocean bottom and to "see” into the ocean bottom. This is accomplished by taking advantage of the principles of reflection of sonic energy at discontinuities in the medium of travel.

These occur wherever there is an abrupt change in the properties of the medium that causes a corresponding change in the velocity of propagation of sound. Discontinuities are found at the air-sea interface, the water-sediment interface, the contacts between layers of sediments or rocks having distinctly different physical properties, and the thermocline or any other interface between layers of seawater having markedly different densities. Schools of fish, whales, masses of plankton, and gas seeps often also form velocity discontinuities.

The effects of the thermocline upon the speed of sound in the sea are illustrated in Fig. 6-13. The reversal of speed values associated with the thermocline causes two important effects upon the transmission of sound energy in the sea If a source of sonic energy is located at a depth near the sound speed minimum, the energy is channeled laterally at that depth because the seawater bends the travel paths of sound toward the channel much as does an acoustic lens. Also, if a sonic energy source is located below the minimum, the travel paths are bent downward to produce an acoustic shadow zone at shallower elevations. The former effect is used to locate aircraft downed at sea. An explosive charge is detonated at the depth of the sound speed minimum.

Figure 6-13. The variation of the speed of sound with depth in the sea. (Used with permission of McGraw-Hill, Inc.)

Listening stations on-shore record the time of arrival of the sonic pulse and calculate the location of the explosion. The latter effect is used by submarines to avoid detection by ships listening at the surface.