Sound speed in the Ocean
Sound is a form of wave motion.
Sound velocity is defined as
V =
p
elasticity /density
Sound velocity is derived from temperature, salinity and depth.
In practice
r
V =
γ
ρK
where γ is the ratio of specific heats, ρ is density and K is the compressibility.
γ, ρ and K depends on temperature, salinity and pressure.
Simplified formula
V = 1449.2 + 4.6T − 0.055T 2 + 0.00029T 3 + (1.34 − 0.01T )(S − 35) + 0.016Z
Vertical variation of sound velocity
Varies from 1400 to 1570 m/s
Sound speed increases with increasing temperature, salinity and depth.
Surface layer (upto 100m): sound velocity increases due to pressure effect
Mid layer (100-1000m): Sound velocity decreases due to decrease in temperature.
Deep layer (above 1000m): Sound velocity increases due to pressure
Shadow zone
Shadow zone
Due to refraction and vertical variation of sound velocity.
Shadow zone is produced when a positive velocity gradient (due to isothermal
layer) lies above a negative velocity gradient (due to themocline) and the sound
source is in the positive velocity gradient.
Applicable for the surface layer
Sound waves will bend upwards lower velocity (upward in the positive layer and
downward in the negative layer).
This creates a ”shadow zone” where no sound waves enters.
Difficult to detect submarines in shadow zones.
SOFAR channel
Most important feature.
Sound is produced in the sound minimum zone.
Sound rays bend upward or downward towards the lower sound velocity area and
is therefore ”channelled”
Creates a channel of sound called as SOFAR (SOund Fixing And Ranging
channel).
Loss of sound intensity is minimum. Sound can travel thousands of kilometers.
Used to detect ships in danger.