Lecture #5 Week #4

A.K. Morris, Ph.D.

Chapter 4Chapter 4

Seafloor  &  Ocean BasinsSeafloor  &  Ocean Basins

OceanographyLecture #5A.K. Morris, Ph.D.

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We are a species of seafarersWe are a species of seafarers

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We have become very good at We have become very good at ““readingreading””the sea surfacethe sea surface

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Despite all that we knowDespite all that we knowhistorically about the oceanhistorically about the ocean

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Seegurke J. Gutt

Our knowledge of the seafloorOur knowledge of the seafloorpales in comparison.pales in comparison.

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Fig. 4.1

No shortage of ways to sample the ocean bottom these days

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Lecture #5 Week #4

A.K. Morris, Ph.D.

Proto‐bathysphere

••Made out of leather and metalMade out of leather and metal••Gaspard SchottGaspard Schott’’ss““Curiosa sive Mirabilia ArtisCuriosa sive Mirabilia Artis”” 16641664

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Historic seafloor MappingHistoric seafloor Mapping• “Heaving the lead”

• Line is “clearly” marked– 2 fathoms from the lead ‐ 2 strips of 

leather.

– 3 fa ‐ 3 strips of leather.

– 5 fa‐ a piece of white cloth.

– 7 fa‐ a piece of red cloth.

– 10 fa‐ a piece of leather with a hole in it.

– 13 fa‐ a piece of blue cloth.

– 15 fa‐ same as for 5 fathoms.

– 17 fa‐ same as for 7 fathoms.

– 20 fa‐ small line with 2 knots.

– 25 fa‐ small line with 1 knot.

– 30 fa‐ small line with 3 knots.

– 35 fa‐ small line with 1 knot.

– 40 fa‐ small line with 4 knots.

UGH! UGH! –– dondon’’t forget your t forget your ““clearclear”” markersmarkers 8

Historic Seafloor MappingHistoric Seafloor Mapping

•• The deeper you went, the less accurate your The deeper you went, the less accurate your measurement wasmeasurement was……

R.V. Minnow

Wind

Current

R.V. Minnow

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Matthew Fontaine Maury (1806‐1873)•• Father of modern physical oceanographyFather of modern physical oceanography

•• First detailed Atlantic seafloor map (1854)First detailed Atlantic seafloor map (1854)

•• Actually detected the MidActually detected the Mid‐‐Atlantic ridge, Atlantic ridge, but didnbut didn’’t know what to make of itt know what to make of it

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Matthew Fontaine Maury (1806‐1873)•• Aside:  commissioned by the American whaling industry to map Aside:  commissioned by the American whaling industry to map 

whale takes (pink represents sperm whales)whale takes (pink represents sperm whales)

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Historic Scientific CruisesHistoric Scientific Cruises• US Exploring Expedition (1838‐1842)

• H.M.S. Challenger Expedition (1872‐1876)

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Lecture #5 Week #4

A.K. Morris, Ph.D.

US Exploring ExpeditionUS Exploring Expedition(1838(1838‐‐1842) 1842) 

• American endeavor – 6 ships• Lt. Charles Wilkes

• Attempted to map the entire Pacific• Smithsonian collection started with this expedition

• Discovered and named Antarctica!

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H.M.S. Challenger ExpeditionH.M.S. Challenger Expedition(1872(1872‐‐1876) 1876) 

• British endeavor

• Professor Sir Charles Wyville Thomson

1865 14

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Sounding the Bottom

•• Only 300 deep water Only 300 deep water measurementsmeasurements

•• Thomson found the bottom was Thomson found the bottom was ““as varied as on landas varied as on land””..

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1922 – Birth of the “echo sounder”

•• USN inventionUSN invention

•• Uses sound speed in Uses sound speed in seawaterseawater

NOAA

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1922 – Birth of the “echo sounder”

•• Changing the frequency and intensity = new applications!Changing the frequency and intensity = new applications!

LARGE SCALE:LARGE SCALE:

•• Seismic Reflection ProfilingSeismic Reflection Profiling

•• MultiMulti‐‐channel seismic systemschannel seismic systems

MEDIUM SCALE:MEDIUM SCALE:

•• MultiMulti‐‐beam echo sounders (sidebeam echo sounders (side‐‐scan sonar is most common)scan sonar is most common)

FINE SCALE:FINE SCALE:

•• Chirp Profilers Chirp Profilers 

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Lecture #5 Week #4

A.K. Morris, Ph.D.

Choosing frequency & intensity

• The “car stereo” example

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Seismic Reflection Profiling (large scale)Seismic Reflection Profiling (large scale)

•• Sound waves fired by an air gun from the shipSound waves fired by an air gun from the ship•• Sound echoes off different subSound echoes off different sub--layerslayers•• Returns to detector at different timesReturns to detector at different times

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MultiMulti‐‐channel seismic systems (large scale)channel seismic systems (large scale)

•• Same concept using sound waves fired by an air gunSame concept using sound waves fired by an air gun•• Uses many different frequencies, soUses many different frequencies, so•• Much better resolutionMuch better resolution

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Actual multiActual multi‐‐channel seismic data channel seismic data 

USGS Gyre Cruise, 1997. Bryant Canyon, Louisiana Gulf Coast [Nealon, Dillon, & Twichell, 2000]. USGS Report 99-449.22

Echo Sounders (medium scale)Echo Sounders (medium scale)

•• SideSide--scan sonar is very popular todayscan sonar is very popular today•• FanFan--shaped pulsesshaped pulses•• Works well over relatively flat areasWorks well over relatively flat areas

Pluses:Pluses:••Accurate, portableAccurate, portable

MinusesMinuses••Difficult over variable bottoms and rough seasDifficult over variable bottoms and rough seas

Wyse's Ferry Bridge on the bottom of Lake Murray, South Carolina, 160 ft depth.

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SideSide‐‐scan sonar modern applicationsscan sonar modern applications•• Wreck HuntingWreck Hunting

© Frank A. Palmer

© Louise B. Crary

•Both ships collided on Dec 17, 1902 and sank off MA•Re-discovered in 2002•Both are still loaded with coal

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Lecture #5 Week #4

A.K. Morris, Ph.D.

NOAANOAA’’s favorite tool these dayss favorite tool these days……..

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Convenience (a rarity in oceanographic sampling)

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Chirp ProfilersChirp Profilers(fine scale samplers) (fine scale samplers) 

© Sandy Baldwin

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Chirp – scientific application

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Chirp – applied uses

Tacoma, WA

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Fig. 4.3

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Lecture #5 Week #4

A.K. Morris, Ph.D.

USS Alligator video…

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Satellite Altimetryseafloor mapping from space???

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Fig. 4.4

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Smith‐Sandwell Map(based on gravity measurements by satellites and ship‐board instruments) 

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Sound in the Sea

• Passive acoustics

• Active acoustics

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Passive acoustics

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Atlantic common dolphin sound:

Lecture #5 Week #4

A.K. Morris, Ph.D.

Active acoustics

Morris, Macauley and Hamner, in press

• Most things in the sea resonate• Must find their unique frequency• Then you can “see” them

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Speed of Sound in the Seaapproximately 4,921 ft per second

• Temperature• Salinity• Pressure

SOFAR channel (sound fixing and ranging)

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Speed of Sound in the SeaSOFAR channel (sound fixing and ranging)

• 3 main zones of concern

• BOTTOM LINE: sound waves get trapped in the SOFAR channel

• VERY low frequencies

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Low‐Frequency Active Sonar (LFAS)formerly Acoustic Thermometry of Ocean Climate (ATOC)

•US Navy plans to cover 80% of the ocean with this “quiet submarine” detection system.

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Low‐Frequency Active Sonar (LFAS)formerly Acoustic Thermometry of Ocean Climate (ATOC)

Growing evidence that it is dangerous/deadly to marine life…

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Low‐Frequency Active Sonar (LFAS)formerly Acoustic Thermometry of Ocean Climate (ATOC)

XXOVERTURNED !

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Lecture #5 Week #4

A.K. Morris, Ph.D.

Bimodal Crust and Isostasy• Bimodal – 2 states of earth crust

– Ocean basins– Continents

• Isostasy – steady buoyant state

• Archimedes principle – an objects buoyancy determined by weight of “stuff” it displaces

– In this case, balance between weight of crust and weight of fluid mantle it displaces

Add more weight, it will sink moreThink: glaciers, sedimentation, etc.

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Earth’s hypsographic curve

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Seafloor Provinces

•• Continental MarginsContinental Margins

•• Deep Ocean BasinsDeep Ocean Basins

•• Oceanic RidgesOceanic Ridges

•• Hydrothermal VentsHydrothermal Vents

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ContinentalContinental MarginsMargins

•• Passive versus ActivePassive versus Active

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Lecture #5 Week #4

A.K. Morris, Ph.D.

••Active margin: seismically activity, border of platesActive margin: seismically activity, border of plates““PacificPacific--typetype”” marginmargin

••Passive margin: no activity, trailing edge of platePassive margin: no activity, trailing edge of plate““AtlanticAtlantic--typetype”” marginmargin

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Fig. 4.10

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Fig. 4.11

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Lesson in Geo‐anatomy

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Fig. 4.12

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Deep Ocean BasinsDeep Ocean Basins

• Abyssal plains – mostly featureless expanse

• Abyssal hills – jagged basement covered by many millennia‐worth of sediment

• Trenches

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Lecture #5 Week #4

A.K. Morris, Ph.D.

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Fig. 4.16

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Oceanic RidgesOceanic Ridges

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www.marum.de

Hydrothermal ventsHydrothermal vents

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Hydrothermal ventsHydrothermal vents

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