upper few hundred feet of the ocean. Extinction of light is due to 

 both absorption and scattering, the former being greater at the red 

 end of the spectrum and the latter at the blue. Extinction is largely 

 a function of the amount of suspended material and is considerably 

 greater in coastal waters. The refractive index of seawater depends 

 strongly on salinity and is used to measure salinity (with appropriate 

 temperature corrections) . Electrical conductivity rises rapidly with 

 an increase of salinity, and as a result electrical conductivity 

 measurements are used also for the determination of salinity. (Again ^ 

 there is a strong temperature dependence and a correction must be made.) 

 Conductivity, which is approximately 4 mho/m, limits the transmission 

 of radio waves. Figure 4-4 compares absorption of radio waves and 

 sound waves in seawater. It may be seen that sound is transmitted 

 better below about 100 MHz. As seawater has a relative permittivity 

 of 80 for radio waves, it is a conductor below 10 MHz and a dielectric 

 above 10^ MHz. The skin depth 8 at which radio waves decay to 1/e of 

 their surface field Intensity value is given by 8 = 250/ yf" meters, 

 where f, the frequency, is in Hz. At 1 MHz, 8 = 0.25 m. Ten kW of 

 power in a plane wave at 16 kHz is attenuated to 1/aW at a depth of 23 m. 

 The speed of radio waves is reduced considerably by the conductivity, 

 being equal to 2 Trf 8. Thus, at 1 MHz, speed equals 1.5 x 10° m/s and is 

 5.0 X 10'^ m/s at 1 kHz. 



d . Sound Speed 



Of the many acoustic parameters associated with the ocean and 

 its boundaries, the most important are speed of sound and absorption 

 loss. 



The speed of sound, C, in seawater is given by the formula 

 C^ = (dp/dp)^, where S denotes an adiabatic process. Sound speed is 

 related to the equation of state where p , the density, is given in 

 terms of pressure, temperature, and salinity. An equivalent form for 

 the speed of sound is C^ = l/{p Ka) where Ka is the adiabatic compressi- 

 bility. The adiabatic compressibility is the reciprocal of adiabatic 

 bulk modulus. There are several expressions for the speed of sound in 

 terms of temperature, salinity, and pressure. The formula most widely 

 accepted is that of Wilson shown in figure 4-5. 



At 0°C, the speed of sound is 1449.3 m/s for a salinity of 

 35 °/oo at atmospheric pressure and increases by about 1.4 m/s per 

 1 °/oo increase in salinity, by 4.5 m/s per °C increase in temperature, 

 and by 0.016 m/s increase in depth of 1 m (1 atmosphere ~ 10 m) . These 

 variations with depth can result in marked acoustic refraction in the 

 ocean. 



e. Sound Absorption 



Absorption of sound in seawater increases rapidly with acoustic 

 frequency and is dependent on salinity, temperature, and pressure. 



13 



