Sound Speed Profile

Question 1

What is snell’s law

Answer 1

Property of sound that causes it to bend away from higher velocity and towards lower velocity. HALT. Higher away lower towards

Question 2

Positive sound speed gradient

Answer 2

sound speed increases with depth. Ray produced is curved upward toward the depth of minimum sound speed

Question 3

Negative sound speed gradient

Answer 3

sound speed decreases with depth. Ray produced is curved downward toward depth of min sound speed

Question 4

Isospeed

Answer 4

speed of sound is the same at all points. Sound travels in strait lines

Question 5

Isothermal

Answer 5

Constant temperature and sound speed increases with pressure/depth. Will have a positive sound speed gradient

Question 6

Sonic Layer Depth

Answer 6

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

Question 7

Deep sound channel axis

Answer 7

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)

Question 8

Sound channel

Answer 8

Region where sound speed first decreases with depth to a minimum value, then increases

Question 9

Conjugate depth

Answer 9

Depth below the DSCA where the sound speed equals that of a source depth between the SLD and DSCA

Question 10

Critical depth

Answer 10

depth below the DSCA that equals the SLD

Question 11

Depth Excess

Answer 11

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

Question 12

What are the axes of a sound speed profile?

Answer 12

X-axis: Sound Speed…. Y-axis: Depth

Question 13

What is the unadjusted base sound speed through water?

Answer 13

4800 fps or 1600 yds/sec

Question 14

What factors affect the sound speed?

Answer 14

Temp, pressure, salinity

Question 15

How does temp affect sound speed?

Answer 15

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

Question 16

How does pressure affect sound speed

Answer 16

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

Question 17

How does salinity affect sound speed

Answer 17

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

Question 18

Where do you place sensors to enhance detection of dominant bottom bounce path?

Answer 18

Below the shallow SLD

Question 19

Where do you place sensors if you have a large surface duct/deeper SLD

Answer 19

75% of SLD to monitor for shallow running diesels (more likely DP)

Question 20

Where do you place a sensor with a strong negative gradient toward the DSCA?

Answer 20

Deep to Increase Pd (more likely for BB sound path)

Question 21

What is direct path?

Answer 21

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

Question 22

What’s a cutoff frequency?

Answer 22

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.

Question 23

What’s a thermocline?

Answer 23

Water below a mixed layer

Question 24

What’s a mixed layer

Answer 24

Water near the surface that is more isothermal (constant temp); tends to go deeper during winter

Question 25

What is a convergence zone?

Answer 25

Region near the ocean surface in which focusing of sound rays occurs. Higher sound levels within CZs. Requires depth excess for any probability of CZ propagation. Depends on sound speed at source depth, critical depth, and depth excess. Negative sound gradient over positive sound gradient with enough depth excess to allow sound to refract and focus.

Question 26

What is bottom bounce?

Answer 26

When a sound wave strikes the bottom of the ocean, attenuation causes loss of energy, reduces the amount of energy returning back to the water column, sound-pressure level of wave is decreased. It can also produce extended ranges via reflection off the bottom.