Density within the Earth? Mike Warner Earth Science & Engineering Imperial College London.

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Presentation transcript:

Density within the Earth? Mike Warner Earth Science & Engineering Imperial College London

Outline Earth structure Methods Accuracy Recommendations

Earth Structure crust vs mantle lithosphere vs asthenosphere ocean vs continent

thermal boundary layer convecting adiabatic interior depth ~ 10 – 50 km highly heterogeneous, chemically differentiated crust mantle (rather) homogeneous & chemically uniform chemical composition less accurate more accurate

thermal boundary layer convecting adiabatic interior temperature depth ~100 km lithosphere heterogeneous, strong, complicated “asthenosphere” highly viscous, convecting at high Raleigh number well-mixed, homogeneous, simple less accurate more accurate

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

Methods gravity seismology others

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

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

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

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

Profile length European ~2500 km Trans-continental ~9000 km

European 2500 km lithosphere

European - continental crust

European - oceanic crust

Errors in density location length“a priori”“best” Continental 2500 km4.7%2.9% Oceanic 2500 km2.6%1.7% Continental 9000 km2.0%1.7% Oceanic 9000 km1.8%1.5% Errors are ~2 sigma (errors not really Gaussian)

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

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

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

Errors in density location length“a priori”“best” Continental 2500 km4.7%2.9% Oceanic 2500 km2.6%1.7% Continental 9000 km2.0%1.7% Oceanic 9000 km1.8%1.5% Errors are ~2 sigma (errors not really Gaussian)