Lecture 2 Flashcards
What does the shape of the ocean look like
- it is a terrain that is highly varied
- > includes troughs, ancient volcanoes, submarine canyons and great mountain chains
- there is significant relief(variations in elevation)
- > it isn’t just like dry land and flat
What is bathymetry and what is its goal
- it is the measurement of the ocean depths and the charting of the shape or topography of the ocean floor
- determining bathymetry involves measuring the vertical disance from the ocean surface down to the mountains, valleys and plains of the sea floor
How was bathymetry originally measured compared to now
Originally
- > let out a rope with a weight on the end until it hit the bottom
- > this is referred to as SOUNDING
Today
->this measurement is done with a sonar
Problems using bathymetry with the weights
- determining when the weight has hit the bottom(especially in deep water)
- > several km’s of line can continue to pull more wire from the drum even if the line is on the bottom
- currents can cause the line to drift sideways while lowered
- > so distance measured is not straight
Describe the current way of measuring bathymetry through echo sounders or a fathometer
- it transmits a burst of 10-30 kHz sound(ping)
- > researchers listen for echo from the sea floor
-time between the pulse and return of echo, when multiplied by speed of sound in seawater, is twice the ocean depth
2x Distance= Velocity x Time
- > this equation works because the speed of sound is known in sea water
- > and varies little as the temperature and salinity of water changes
What are the limitations of bathymetry overall
- its tracks are not well distributed
- lacks detail, averages over some region of the bottom
- > sound cone from these instruments spreads with depth and thus can be measuring a large area in deep water
- > also, there is an average over spectrum of return times(does not represent local extrema in depths
- essentially, this system is biased towards returning the highest peak within a broad area
- > so that being the shallow end
Describe Modern Bathymetry Measurements using the Multibeam Echo sounders
- there are multiple simultaneous sound frequencies
- sound waves bounce back with different strengths and timing, allowing the computer to analyze these differences
- > then the computer produces detailed maps of the depth and shape of the ocean floor
- > and if the bottom is made of rock, sound and or mud
Describe the SeaBeam
- the first multibeam echo sounder
- >it maps sea floor strips up to 60 km wide
Describe the side scan sonar
- it is used in modern bathymetry measurements
- >can be towed behind a ship on a cable so that it “flies” just above the ocean floor to provide detailed images
Describe why it was difficult to find the Malaysian Airline Flight 370
- problem was that search crew did not have a good idea of where the plane went down
- > and thus searching for an area that is that big in a remote region of the Indian Ocean which isn’t mapped well is difficult
Can satellites be used to measure bathymetry
- yes
- > sea surface topography can be measured by radar altimetry
- > this measures the height of the sea with an accuracy of 3-6 cm
What are the two reasons for variations in bathymetry measurements done by satellites
- local variations in gravity
- > amplitude of +/- 60 metres
Effects of ocean currents
->amplitude of +/- 1 metre
Why is the sea surface slightly higher over an undersea mountain than a deep area?
- the undersea mountain is pulling water towards it
- > which then piles up over it
- > raising the sea surface
- deep areas in the ocean such as trenches may correspond to a lower gravitational attraction
- > and large undersea objects such as sea mountains can exert an extra gravitational pull and then those differences can affect sea level
- > these ocean mountains can create a bulge on the sea surface, which creates a bulge
- > the satellites detect this bulge
What are the advantages of using satellite bathymetric maps in comparison to non-satellite bathymetric maps
Satellite ones
- > main advantage is the resolution(because of greater spatial coverage)
- > color on the map represents sea floor elevation
- > it is able to pick out the big features, such as the Atlantic ridge
Non-satellite ones(based on bathymetric data from ships)
->it is measured on conventional echosounder with records from the ship
Describe XBT’s water column sampling
- it is an expendable bathymetric thermograph
- measures only temperatures in the 1000-1500 metres
- > as it falls through the water, it measures the temperature
- > small wires transmit temperature data back to ship to be analysed
- because probe is falling through water at its own rate, depth of probe can be inferred
- > which allows us to make a plot of temperature vs depth
- they are cheap and expendable
- > can be launched from merchant ships
- it is so simple
- > you don’t need to be an oceanographer
- > have been able to give us a lot of data about ocean temperatures and how it varies across large blocks of ocean
Why are XBT’s valuable?
- they are deployed along established transects but are valuable
- > they represent the largest fraction of the temperature profile observations since the 1970’s
- > some have been going on for 20 years
- note the transects are in high density and high frequency
- > high density=occupied 4 times a year every 25 km
- > high frequency= occupied 8 times a year with 100 km intervals
What are sampling bottles
- they are sometimes called Nansen, Nisken, or Go-Flo bottles
- > they collect samples to be brought up for future examination
- > care must be taken to avoid contamination of the bottle
-note thermometers can be added to the bottles for additional temperature measurementa
How can sampling bottles be deployed
- they can be deployed off a big ship or little ship
- > 10 L bottle
- > usually sent down in the open
- > space them out to take measurements
- to get the bottles back
- > put weight on the line to be sent down and then this weight hits the top of bottle called trigger and it closes the bottle
- > this hauls the bottle back up and get water from different depths of the ocean
Describe CTD water column sampling
- it is a conductivity temperature device
- > measures temperature electronically
- > also measures the conductivity of water, which gives salinity(knowing the temperature)
-note the CTD allows measurements at all depths as the probe descends
What is the Rosette Sampler
- includes CTD and sampling bottles
- >bottles can be activated at any depth by signal down wire
What does a real time profiling of ctd require
-it requires a winch and a cable that’s able to transmit data
What factors can the ctd pick up?
- conductivity
- temperature
- pressure
- dissolved oxygen
- photosynthetically active radiation
- fluorescence
- turbidity
Describe the planning and execution of ocean research missions
- cruise plans must be generated
- > where do you want to go and what types of scientific questions are you interested in
- identify stations where observations are to be made
- > stationary is a ship that is stationary and where you deploy a rosette CTD or a CTD
- > unlike multibeam scanner or side scanner, ship isn’t moving when you make measurements
- > the ship has to come to a complete stop before deploying the CTD or rosette CTD
- important to make measurements through a region to understand the spatial structure
- > as you’re going down the water column, we’ll see different types of water masses coming in
- > to understand water masses, you can’t just take measurement after transition
- > need to make measurements throughout the first whole first region and the second region
Describe the voluntary observing ship program
- NOAA provides the voluntary observing ships
- > these ships make observations about temperature and weather
- > observations are transmitted to the NOAA and used by oceanographers
Describe the world ocean circulation experiment
- had a field phase
- > ran from 1990 to 1998
- then had a model phase
- basically, these are all one time bottling stations
- > where ships went out and got measurements
Is salinity harder to measure than temperature
- yes
- >seasonal salinity(winter vs summer) is even harder
What are ocean currents
-masses of water that are in the ocean
What are the two things we want to know about ocean currents
-want to know how oceans are moving at one instance and how these movements are changing over time
How do we determine ocean currents
1) Direct methods
- >directly determine speed and direction of ocean motion
- two types: passive and active
- passive means let’s place something in the water and let it flow(simplest ones are unplanned based on objects that end up in water)
- >eg; in 1990, a container vessel lost 30, 000 pairs of Nike sneakers, which drifted with the North Pacific Current East
- >for passive, we need to know where the object entered in the ocean and where it was retrieved
- active
- >deploy an instrument like current meter that determines speed by counting rotations of propeller or impeller
- >actively directly measuring current
- >but a more common method is instead using acoustic current meters
2) Indirect Methods
- infer motion from other types of measurements
- useful for motions over larger areas and longer time scales
- most commonly based upon ocean tracer measurements
- egl helium 3 percentage(most of the helium is in underwater vents. Current flow must be from high helium concentration to low helium concentration).
Describe the process used by an acoustic current meter
- send out sound pulses and measure the return
- > the return comes from particles and other objects travelling with the current
- current speed and direction given by changes in the frequency of sound
- > if the current approaches, the frequency increases
- > if the current is moving away, the frequency is decreasing
-these devices can measure currents at all depths within the range of the beam
What are moorings
- sets of current meters at different depths
- >they are any structure attached to an instrument or ship
