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
