Density within the Earth?

Mike WarnerEarth 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

dept

h

~ 10 – 50 km

highly heterogeneous,chemically differentiatedcrust

mantle(rather) homogeneous& chemically uniform

chemical composition

less accurate

more accurate

thermal boundary

layer

convecting adiabatic interior

temperature

dept

h

~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 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)