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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: [email protected]
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
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.
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
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)
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