Oceanography Basics Flashcards
(50 cards)
Describe the three layer ocean model
Mixed layer (isothermal)
Thermocline (transition to cold water)
Deep water (deep and cold)
What is the limiting ray
Sound ray which grazes or is tangential to a boundary layer
What is the critical angle ray
Sound ray which strikes a boundary or surface at steepest gradient
More than this (steeper) causes reflection loses
Shadow zones
Sound rays reflect away from depths at acoustic maxima. Little sound penetrates causing shadow zones
What are the 5 transmission paths
Direct path
Convergence zone
Surface duct
Bottom bounce
Sound channel
What are the different types of channel
DSC/SOFAR channel (deep/bottom)
Shallow sound channel (45-180m)
Secondary sound channel ??
Half channel ??
How does CZ occur
Sounds rays penetrate the SLD and then refract upwards
What are the conditions for CZ
Deep water (5000’ +)
Bottom velocity larger that SLD velocity
Cx of 10MS +
DE of 360m
Large Signal Excess
What is the annulus on CZ
Ring of energy around source at the surface (10% of CZ Range)
What is the CZ depth excess?
Depth at which the SOS matches and supersedes the SLD.
Depth excess needs to be 360m + for a 50% chance.
Give examples of CZ ranges for the North Atlantic & Med
NA: 25-30nm
MED: 12-17nm
Explain the letters in relation to CZ
Explain the letters in relation to CZ
Where does BB prevail
SLD less that 15m
Shallow water - lots of interference
Deep water (1000’+) - BB ranges exist similar to CZ ranges
Name the two types of ocean fronts
Semipermanent
Temporary
Describe semi-permanent fronts
Associated with water mass boundaries
Stay static for long periods of time
Example: East Icelandic Front (Iceland-FAR-NOR)
Temporary Fronts
Temp changes to ocean structure due to abnormal weather or storms
Causes wind induced currents resulting in upwelling of cold water
Example: summer snow run off - Norwegian Sea
Name the Effects of fronts on sound
Abrupt changes is SVP
Higher AN (bio and fishing)
Errors in bearings (refracts to cold)
What are eddies and what effect do they have acoustically
Circular current flows that form on either sides for ocean currents
Up to 100nm in diameter
What are the 2 types of vertical ocean currents
Sinking of surface water
Upwelling
Describe upwelling and it’s Aco properties
In costal areas wind blows surface water seaward allowing cold water to rise in its place
High AN due to rich water - Bio/Fishing
Reduction in surface temp
Describe sinking of surface water and its acoustic properties
Occurs at front boundaries where the denser cold water sinks
Causes currents, rip-tides, trapped debris
Name the three types of bottom and their properties
Rock - smooth rocky bottoms are best reflectors but are rare and often covered in sediment (600m thick in Atlantic)
Sand - exploitable for BB when flat
Mud/Silt - poor reflector due to smaller particles. <300hz can be refracted
Explain the continental shelf
Continental shelf - Smooth sand or mud
Shallow gentle decrease to 600’
High AN, good BB
Contains areas of continental borderland:
Irregular areas of shelf containing peaks and valleys (poor Aco properties)
