Density within the Earth?

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Density within the Earth?

• Mike Warner
• Earth Science Engineering
• Imperial College London

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Outline

• Earth structure
• Methods
• Accuracy
• Recommendations

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Earth Structure

• crust vs mantle
• lithosphere vs asthenosphere
• ocean vs continent

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chemical composition
highly heterogeneous,chemically differentiated
thermal boundary layer
crust
10 50 km
(rather) homogeneous chemically uniform
convecting adiabatic interior
mantle
depth
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temperature
thermal boundary layer
lithosphere heterogeneous, strong, complicated
100 km
asthenosphere highly viscous, convecting at
high Raleigh number well-mixed,
homogeneous,simple
convecting adiabatic interior
depth
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Oceans vs Continents

• Oceans
• young
• simple
• thin crust
• mantle density related to cooling
• Continents
• long history
• complicated
• thick crust
• no simple density relationships

• more accurate
• less accurate

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Methods

• gravity
• seismology
• others

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Gravity

• low resolution
• non-uniqueness
• density heterogeneity
• much improved if boundaries are known from
  another technique

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Seismology

• high spatial resolution
• good absolute accuracy
• determines boundaries
• does not measure density directly
• elastic properties correlated with density
• more money gt better results

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Other methods

• Density varies with
• porosity
• composition
• temperature
• pressure
• Measure or estimate each of these

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Accuracy

• absolute densities
• conversion from seismics
• determination of composition
• determination of temperature
• boundaries
• base of sediments
• base of crust
• base of lithosphere

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Profile length

• European
• 2500 km
• Trans-continental
• 9000 km

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European
lithosphere
2500 km
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European - continental
crust
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European - oceanic
crust
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Errors in density
location length a priori best
Continental 2500 km 4.7 2.9
Oceanic 2500 km 2.6 1.7
Continental 9000 km 2.0 1.7
Oceanic 9000 km 1.8 1.5
Errors are 2 sigma(errors not really Gaussian)
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Recommendations

• If at all possible
• source and receiver on oceanic crust
• maximum profile length beneath ocean
• use maximum profile length
• custom seismic experiments

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Recommendations

• Avoid
• Alps
• central Europe
• thick crust (e.g. Fenoscandia)
• Europe to Japan

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Recommendations

• Best profiles
• Western Europe to Eastern US
• Atlantic Islands (Canaries, Maderia, Azores) to
  Portugal, western Spain, NW France, southern
  Ireland, western England

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Errors in density
location length a priori best
Continental 2500 km 4.7 2.9
Oceanic 2500 km 2.6 1.7
Continental 9000 km 2.0 1.7
Oceanic 9000 km 1.8 1.5
Errors are 2 sigma(errors not really Gaussian)