GOCE OBSERVATIONS FOR DETECTING UNKNOWN TECTONIC FEATURES BRAITENBERG C. (1), MARIANI P. (1),...

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 /2 l 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:

sinsincos

),(),(

,nm

mnnmnmn

nn

Pmbmam

mm

–Mass model: sheet mass with average radius R

),(),()2

),(),()1

rm

rm

Anomalous potential and derived quantities

n

n

n

n

n

n

n

n

n

mr

R

Rnn

n

GTzz

mr

Rn

n

Gg

mr

RR

n

GT

3

2

1

121

12

4

112

4

12

4

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

/),(),( 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