Marine geology by ship and satellite

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Marine geology by ship and satellite
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Before I forget.

• April 23 last day of classes. So syllabus is
  O.K.
• Next week sidescan sonar instead of terrestrial
  topography? Can do terrestrial topography when
  we do GPS.
• 1960 Trieste (bathyscaphe, deep boat) reached
  10,915 m. Alvin maximum operating depth 4000
  m. Sea Cliff II 6000 m. Shinkai 6500 (6500 m)
  can cover 97 of ocean floor.

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first, ships .
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UNOLS ships

• UNOLS University-National Oceanographic
  Laboratory System
• 61 academic institutions (e.g., WHOI, Scripps,
  Lamont-Doherty, University of Hawaii, Texas AM,
  University of Washington)
• Schedules scientific cruises aboard 28 research
  vessels

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UNOLS ships -- WHOI

• Atlantis
• Built 1997
• 274 feet long
• Endurance 60 days
• Maximum speed 15 knots
• Complement of 60 people
• Submersible Alvin,
• hydrothermal vents

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UNOLS ships -- WHOI

• Knorr
• Built 1969
• 279 feet long
• Endurance 60 days
• Maximum speed 14.5 knots
• Complement of 56 people

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UNOLS ships -- WHOI

• Oceanus
• Built 1975
• 177 feet long
• Endurance 30 days
• Maximum speed 14 knots
• Complement of 27 people
• Physical oceanography

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Drilling into the ocean floor

• Ocean Drilling Program (ODP).
• International partnership of universities and
  research institutions.
• Drill ship is Joides Resolution. 6 cruises per
  year, each 2 months in length and with 30
  shipboard scientists.
• Climate history, creation of oceanic crust.

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ODP drill ship -- JOIDES Resolution

• 469 feet long, 69 feet wide
• Drilling derrick 200 feet tall
• During drilling, ship is kept stationary by 12
  computer-controlled thrusters
• Rig can suspend over 9 km of drill pipe
• Operates 24 hours per day. Lowering drill bit
  takes 12 hours in 5.5 km of water
• Deepest hole penetrated 2.1 km (Hole 504B in
  East Pacific)
• So far, drilled a total of 1555 holes

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ODP drill ship -- JOIDES Resolution
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Whats it like on a ship?1998 LAtalante cruise
Summer of 1998 . Where? 30-day cruise to the
south of the Azores Islands. Why the
Azores? Azores Islands surface expression of
hotspot. Near the Mid-Atlantic Ridge. Wanted
to investigate how the ridge and the hotspot
interact to create oceanic crust. Which
ship? N/O LAtalante. About the size of the
Knorr. French.
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Whats it like on a ship?1998 LAtalante cruise

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Whats it like on a ship?1998 LAtalante cruise
Who? Sixteen scientists French, Chinese,
American, South African, Russian, Portuguese.

• Sidebar The Journey to the Ship
• Visa problems at Logan
• Airplane problems at Lisbon
• Luggage problems at Ponta Delgada

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Whats it like on a ship?1998 LAtalante cruise
What? Used geophysical surveying and rock
dredging to explore volcanic eruptions and huge
submarine volcanic plateau to the south of the
Azores.

• Sidebar French ships
• Three-course meals, waiter service
• Scotch-tasting parties
• Daily cabin cleaning
• Watchstanding and the 3 am hunger

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Whats it like on a ship?1998 LAtalante cruise
Geophysical surveying . 3.5 kHz Acoustic.
Thickness of the sediment layer atop the
seafloor. Seabeam An array of sonar beams to
measure the depth of the seafloor. Magnetometer
Measures the magnetization of ocean floor
rocks. Earths magnetic field reversals are
recorded during seafloor spreading. Can tell
how fast the plates diverge. Gravimeter
Measures variations in the Earths gravity field.
High gravity near mid-ocean ridge.
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Whats it like on a ship?1998 LAtalante cruise
What did we find?
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Bathymetry data
Sonar SOund Navigation And
Ranging Basic Idea Transducers send pulses of
sound to the ocean floor. Receivers measure the
time it takes for the sound pulse to reflect off
the ocean bottom and return to the ship. Distance
(or depth) Rate x Time
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Single beam Multibeam

• Arrays of 12 kHz transducers (as many as
  120), arranged in precise geometric pattern on
  ships hull.
• Signal fans out to cover a swath of seafloor.
• Length of swath about 2 times water depth.
• Accuracy 10 m.
• Ship speed 10 knots.

Hull-mounted instrument sends a single acoustic
pulse toward the seafloor.
More beams are better than one.
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Bathymetry data
For each point in the ships track, ship receives
an array of as many as 120 travel-times extending
to either side of the ship. Travel times are
converted to depths. Ship moves forward at 10
knots. Another acoustic pulse is
transmitted. Swaths are stacked to produce a
continuous profile (onboard computer systems).
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HMR1
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Global multibeam database
Were mapping, baby!
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Multibeam survey examples
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Multibeam survey examples
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Multibeam survey examples
Juan de Fuca Ridge
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Multibeam survey examples
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Multibeam survey examples
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Multibeam is good because
it provides high-resolution (100 m) information
about the topography of the sea floor.
Problem with multibeam bathymetry
Research vessel speed 12 knots Would take 125
YEARS to map the seafloor using swath mapping
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next, satellites .
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Measuring the sea surface

• Satellites (Seasat, Geosat) can measure sea
  surface height with a precision of lt5 cm.
• How? Send a radar pulse from the satellite to
  the sea surface (and back). Satellite orbits at
  a known height of 800 km above the Earth.
• Satellites give information on currents and waves
  -- as well as the topography of the seafloor.

Geosat
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Seafloor topography from satellites
Satellite, orbiting at a fixed height of h,
measures distance to sea surface (h). h is
affected by factors like waves, currents, storms,
and seafloor topography. Over submarine hills,
water piles up. Effects of waves, currents, etc.
can be measured and subtracted from h -- whats
left is a map of the seafloor.
Sandwell and Smith (1997)
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Sea surface topography

• Sea surface height contains information about
• Earth tides (20 cm)
• Ocean tides (1 m in the deep ocean)
• Ocean currents and winds (strong currents change
  sea level as much as 1 m over distances of 100
  km. Influence of strong winds is weaker in
  deeper water).
• Changes in atmospheric pressure (few cm)
• SEAFLOOR TOPOGRAPHY

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Predicting seafloor topography
Geosat orbit Over 1.5 years, mapped the
topography of the oceans surface with a ground
track spacing of 6 km and vertical resolution of
3 cm. Can combine data with other
Earth-orbiting satellite altimeters (e.g., ERS-1)
to produce a grid of topography measurements.
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Predicting seafloor topography
Step 1 Compile all available shiptrack
bathymetry data. Sparse. Step 2 Compile all
available satellite altimeter data.
Abundant. Step 3 Combine shiptrack and
satellite data to produce a global map of
seafloor topography.
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Question Why do we need shiptrack bathymetry at
all?

• Calculating bathymetry from satellite data is a
  mathematical procedure. Have to calibrate the
  satellite altimetry against ground-truthed
  shipboard bathymetry measurements. PREDICTED
  BATHYMETRY
• Footprint of satellite altimeter is relatively
  large (1 to 5 km). Satellite-altimetry-derived
  bathymetry is good only over certain wavelengths
  
• Less than 15-30 km must use shiptrack
  bathymetry.
• 30-160 km Satellite-derived calculations
  compare well to actual shipboard measurements.
• gt160 km Have to use shipboard data.

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Predicting seafloor topography
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Predicting seafloor topography
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Predicting seafloor topography
Petroleum exploration. Major petroleum companies
(Exxon, Mobil, Texaco, UNOCAL) use satellite
altimeter data to find offshore sedimentary
basins. Select targets for seismic data.
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Seafloor topography fromsatellite altimetry
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Seafloor topography fromsatellite altimetry

• New map of the seafloor from satellites gives us
  a global picture of submarine topography.
• Allows us to explore marine geology in areas
  where ships cant go because of politics or bad
  weather -- e.g. the extreme south Atlantic.