GOCE OBSERVATIONS FOR DETECTING UNKNOWN TECTONIC

FEATURESBRAITENBERG C. (1), MARIANI P. (1), REGUZZONI M. (2), USSAMI N. (3)

(1) Department of Geosciences, University of Trieste, Trieste ( ITALY), (2) Geophysics of the Lithosphere Department - OGS, c/o Politecnico di Milano - Polo

Regionale di Como, Como, Italy(3) Departamento de Geofisica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas,

Universidade de São Paulo, São Paulo, Brasil

Home page: http://www2.units.it/~braitenberg/e-mail: berg@units.it

Goal

• Locate density changes in Earth’s crust• Crustal parameters necessary for:

– Exploration purposes– Seismic risk estimation– Volcanic risk estimation

• Remote and unaccessible areas: superficial properties known• gravity study useful geophysical means of investigation

TOPIC

• Sensitivity analysis of GOCE for tectonic structures

• Model: spherical shell of variable density or thickness

• Input: simulated GOCE degree error curve • Rms error of tensor components at satellite

height• Error curves of existing gravity field models

(EGM2008)

DENSITY AND TECTONICS

• GOCE measures gravity and gravity gradient• -> sensitive to tectonic structures with density

changes. • -> structures without density change are

transparent• -> GOCE only: upper limit of degree N=200;

tectonic structures greater than l/2 min= 100 km

PREM Earth model (Anderson, 1989)

Earth Density

Lama & Vutukuri, 1978.

Spherical shell model• Spherical shell model: mass layer expanded in

spherical harmonics• Gravity models in spherical harmonic

expansion

Shell model for sensitivity analysis

– Harmonic expansion of sheet:

ll

ll

sinsincos

),(),(

,nm

mnnmnmn

nn

Pmbmam

mm

–Mass model: sheet mass with average radius R

),(),()2),(),()1

llllrm

rm

Anomalous potential and derived quantities

n

n

n

n

n

n

n

n

n

mrR

Rnn

nGTzz

mrRn

nGg

mrRR

nGT

3

2

1

12112

4

112

4

124

Potential

Gravity

Gravity gradient

R: shell radius r: calculation point

Resolution power for geological structures

• Degree error variance: corresponds to smallest detectable field generated by mass source

• Invert for smallest dectable sheet mass• At density discontinuities : • mass layer interpreted as oscillation of

boundary

ll /),(),( mr Boundary oscillation:

Gravity anomaly cumulative and single degree error

55km200km 100kmλ/2=

GOCE error curve:. Dr. Mirko Reguzzoni, POLIMI & OGS

Invert degree error curves

• Mass-Layer: Crust-Mantle discontinuity • We set: average depth (30 to 70 km) and

density contrast across boundary (500 kg/m3)• We find: minimum decetable oscillation

amplitude of boundary.

Minimum detectable Moho undulation amplitude

Single degree error curves

GOCE improvement

• Up to one order of magnitude improvement for degree range 52 to 200

• Average depth important.• Greater depth with reduced resolution• Depth depends on geodynamic context:

Craton (45 km), High topography (up to 70 km), normal crust: 35 km

Basement resolution

• Mass layer represents basement - sediment transition

• Average depth 0 km to 10 km• Density contrast: greatly variable• Sediments follow exponential density increase

due to compaction

Basement resolution

GOCE resolution

• Single degree error curves give meter level resolution

• Basement depth not important• Density contrast predominant effect

GOCE Gradient measurements

• Use tensor components at satellite height

• Infer crustal density variations• Question: how does sensitivity

compare to sensitivity of airborne gravity?

Observation error levels GOCE

• GOCE root mean square error of data along orbit (after processing)

• Diagonal tensor elements [mE]

Along track Across track Radial Tξξ Tηη

Trr

1 10 4

(Migliaccio et al., 2008)

Rms error airborne gravity

(Van Kann, 2004)

Lower crust density sensitivity

• Model: layer 10 km thick above Moho (35 km depth)

• Trr observed at satellite height– rms: 0.1 mE to 100 mE

• dg observed at 1000 m height– rms: 0.01 mgal to 10 mgal

Sensitivity density lower crust

• rms of 1 mgal at 1000m has comparable sensitivity with 1mE rms at satellite height (at wavelengths of 170 km)

• GOCE sensitivity competes with aerogravity surveys

• Sensitivity for GOCE better at longer wavelengths

Example Tibetan crust

• Terrestrial data are scarce and lacking in Himalaya

• Tibetan plateau and Tarim basin contain spectral components accessible to GOCE

• Further investigation is needed of crustal densities

Tibetan Moho

(Braitenberg et al., 2003; Shin et al., 2009)

Power spectrum Tibetan Moho

(Shin et al., 2009)

Conclusions

• GOCE expected to contribute improvement to:– Crustal density structure for wavelengths between

900 km and 220 km.– In particular: crustal thickness variations and

basement undulations– Crustal densities – 1 mE at satellite height

retrieves as 1 mgal airborne – Advantage: truly global