Sound Speed Profile Flashcards
What is snell’s law
Property of sound that causes it to bend away from higher velocity and towards lower velocity. HALT. Higher away lower towards
Positive sound speed gradient
sound speed increases with depth. Ray produced is curved upward toward the depth of minimum sound speed
Negative sound speed gradient
sound speed decreases with depth. Ray produced is curved downward toward depth of min sound speed
Isospeed
speed of sound is the same at all points. Sound travels in strait lines
Isothermal
Constant temperature and sound speed increases with pressure/depth. Will have a positive sound speed gradient
Sonic Layer Depth
SLD. Area/point of highest sound speed near the surface (most positive). Located at the elbow near the surface, energy will move away from the SLD toward an area of lower sound speed
Deep sound channel axis
DSCA. Depth of minimum sound speed. Opposite of SLD. Elbow at most negative sound speed. Transitions from a negative to a positive profile. Energy moves towards DSCA (lowest sound speed)
Sound channel
Region where sound speed first decreases with depth to a minimum value, then increases
Conjugate depth
Depth below the DSCA where the sound speed equals that of a source depth between the SLD and DSCA
Critical depth
depth below the DSCA that equals the SLD
Depth Excess
Vertical distance from the critical depth to the bottom. Whatever is beneath the critical depth. Minimum depth excess of 200 fathoms is required for 50% probability of CZ. 300 fathoms depth excess gives 80% chances of CZ
What are the axes of a sound speed profile?
X-axis: Sound Speed…. Y-axis: Depth
What is the unadjusted base sound speed through water?
4800 fps or 1600 yds/sec
What factors affect the sound speed?
Temp, pressure, salinity
How does temp affect sound speed?
Greatest impact above minimum sound speed (DSCA). Sound speed increases 6 ft/s for every 1 deg increase. Temp increases w/depth = positive gradient. Temp decreases w/depth = negative gradient. Below the DSCA, water reaches a depth in which the cold water becomes constant temp (isothermal), and pressure takes over as predominant factor
How does pressure affect sound speed
Pressure in the water column increases with depth at a relatively constant rate which increases sound speed. Increases 1.6 ft/s per 100 ft in depth. Pressure becomes dominant factor in sound speed below DSCA when temp becomes relatively constant
How does salinity affect sound speed
Increases as salinity increases. Most minor factor, especially in deep water. Bigger factor in shallow water. Increases 4.6 ft/s per parts per 1000 increase in salinity
Where do you place sensors to enhance detection of dominant bottom bounce path?
Below the shallow SLD
Where do you place sensors if you have a large surface duct/deeper SLD
75% of SLD to monitor for shallow running diesels (more likely DP)
Where do you place a sensor with a strong negative gradient toward the DSCA?
Deep to Increase Pd (more likely for BB sound path)
What is direct path?
First portion of all propagation paths. Short range propagation where there is approx. a straight-line path between source and receiver. Only change in direction is due to refraction. Deeper SLD allows for greater surface duct propagation (sound refracted back up above SLD) and better ranges from direct path. In order to have a useable duct, the sound speed gradient must be positive
What’s a cutoff frequency?
at low frequencies, sound energy will not be trapped in the surface duct above the SLD. The wavelength is too large for all the energy to fit inside the duct. The frequency at which the sound will no longer be trapped in the surface duct.
What’s a thermocline?
Water below a mixed layer
What’s a mixed layer
Water near the surface that is more isothermal (constant temp); tends to go deeper during winter
