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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 chemical composition less accurate ~ 10 – 50 km depth more accurate thermal crustboundary layer convecting adiabatic mantle interior highly heterogeneous, chemically differentiated (rather) homogeneous & chemically uniform temperature less accurate thermal boundary layer lithosphere heterogeneous, strong, complicated depth ~100 km more accurate convecting adiabatic interior “asthenosphere” highly viscous, convecting at high Raleigh number well-mixed, homogeneous, simple Oceans vs Continents Oceans – more accurate • • • • young simple thin crust mantle density related to cooling Continents – less accurate • • • • long history complicated thick crust no simple density relationships 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 lithosphere 2500 km European - continental crust European - oceanic crust 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) 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 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)
