Oceanography Basics Flashcards
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)
3 types of bottom typography
Shelf
Slope
Deep ocean basin
Explain the areas to continental Slope (3)
Slope - sharp decrease from shallow to deep. Steep, jagged Rock
Rise - base of slope, begins to flatten
Sea fans - deposits of sediment at the mouth of submarine canyons
Sub canyon - ancient valleys cut into the rise. Subs hide due to scattering
Explain the deep ocean basin
Deep regions between slopes
Describe the 4 areas of deep ocean basin
Abyssal plain - deep layer of sediment/mud, smooth, BB
Trenches - very deep canyons in ocean floor
Ridge - underwater mountains, barriers to sound (mid-Atlantic/reykjanes)
Seamounts - isolated mountains
Explain the two types of seamount & Atolls
Guyot - flat topped
Pinnacles - jagged
Atoll - ring of reef islands
SPL
Sound pressure level
Name the two types of transmission losses
Spreading - distribution of energy as it moves from source
Attenuation
Name the 3 types of spreading and their dB losses
Spherical - unbounded - 6dB
Cylindrical - sound channel - 3dB
Dipolar - source at SLD break - 12dB
What are the Types of attenuation loss
Absorption
Scattering
Leakage and boundary effects
Describe absorption
Energy converted to heat through molecular action
Salinity up, temp down, frequency up = more loss
What are the 4 types of scattering
Volume - bio
Bottom - sediment
Deep Scattering Layer - bio
Surface - wave action, wind.
What is the DSL
Deep scattering layer - daily migration of plankton and bio causing higher volume scattering
200m night 900m day
Describe leakage and boundary effects
Low frequencies can leak out of channels due to diffraction and scattering.
Deeper and higher frequencies mean more ducting chance
Describe the active equation
SE = SL - 2PL - AN - RD + TS
Describe the passive equation
SE = SL - AN - RD - (DI) - PL
Describe passive FOM equation
FOM = SL - AN - RD - (DI)
Types of AN and frequencies (5)
Shipping - low freq (<500)
Seismic - low
Sea state - 500hz +
Bio - various
Precipitation - 10000hz
Describe the 5 layer ocean model
Mixed layer
Seasonal thermocline - temporary where temp decrease
Transient thermocline - mixing affect, short term (hours)
Perm thermocline
Deep water
Describe upslope transmission
A source in deeper water may send sound rays into shallower water
Causes more reflection loss at boundaries causing less detectable sound
Describe down slope enhancement
Source in shallow water may reflect sound down slope til the water is deep enough for refraction into channels
Larger range, more detection
Describe the Diurnal effect
The heating and cooling of the surface layer by the sun
Means daily or during the day
Heating causes more shadow zones/transient channels
What is the MLD
Depth at greatest temperature
Captures heat and effects bio/plankton depth (AN)
Rarefaction
Pressure decrease point in a medium
Describe half channels
Sound rays refract upwards (if a + BLG exists)
Describe Compression
Pressure increase at a point in a medium caused by sound energy
Factors effecting SOS
Salinity 2 ft/s per 1/1000 increase
Pressure 4ft/s per 100ft depth increase
Temperature 6-8ft/s per F increase
