3D Geophysical-petrological modelling of the lithosphere:
how can GOCE data help us assessing the geothermal
potential of Ireland?
J. Fullea(1), Z. Martinec(2), M.R. Muller(2), A.G. Jones(2)
1. Institute of Geosciences (IGEO) CSIC-UCM Madrid, Spain
2. DIAS, Geophys. Section, 5 Merrion Square, Dublin, Ireland
Structure of the presentation
Summary LitMod (geophysical observables+mantle petrology)
IRETHERM: lithospheric structure and regional thermal field in
Ireland
GOCE gravity gradients: new “directional “ constraints for the
lithosphere??
LitMod3D (http://eps.mq.edu.au/~jafonso/Software1.htm)
# LitMod3D is a 3D computational code that allows to model all relevant physical properties
within a robust and thermodynamically self-consistent framework (Afonso et al., 2008, Fullea
et al., 2009, G-cubed).
LitMod3D
LitMod : Thermodynamic modelling and bulk properties
All thermophysical (e.g. density and seismic velocities) properties depend ultimately on T, P,
and Composition.
dG = V dP - S dT + Σi µi dXi + DdE
In the mantle, stable mineral assemblages are computed by Gibbs free energy minimization either within the system
CaO-FeO-MgO-Al2O3-SiO2 (CFMAS) or Na2O-CaO-FeO-MgO-Al2O3-SiO2 (NCFMAS) [Connolly, 2005]. Each mantle
body is therefore characterized by a specific major-element composition (in wt.%), which translates into specific bulkrock properties. All necessary files containing thermodynamic information can be generated either with the freely
available software Perple_X [www.perplex.ethz.ch, Connolly, 2005] or by using a simple interface provided with
LitMod3D.
LitMod3D
LitMod3D : Thermal field
Cartesian finite differences grid
Mantle thermal conductivity dependent on T, P
and C
IRETHERM (http://www.iretherm.ie/index.html)
Academic-government-industry collaborative project
between DIAS, UCD, UCC, GSI, GSNI, and SLR
Consulting targeting Ireland's geothermal energy
potential through integrated modelling of new and
existing geophysical and geological data.
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INSET
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5
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8
9
10
3
5
9
10
TEMP
(°C)
250
100
60
40
20
Left: regional heat-flow density contours, from ~40 mW/m2 (blue) to ~80 mW/m2 (red). Middle and right: potential survey areas to investigate
eight “type” geothermal targets. Middle: overlain on the surface geology of Ireland (courtesy GSNI and GSI). Right: overlain on modelled
temperatures at 2,500 m depth Inset:
Lithospheric structure: P-wave tomography & S-receiver functions
From O’Donnell et al. et al. 2011
From Wawerzinek et al. 2008
Indication of S-N lithospheric
thinning but resolution is poor,
particularly in the North!
LAB depth from S-receiver functions (Landes et al., 2007)
Geophysical observables: elevation and potential fields
Geoid anomaly (n>10) EGM 2008
Bouguer anomaly (land + satellite data)
FA anomaly (Smith & Sandwell 97)
Elevation (ETOPO2 V9.1)
Lithospheric models
3) N-S thinning model-composition
From seismic refraction+1D inv. of geoid &
elevation
1) Flat model
Average mantle compositions,
M&S95 refers to McDonough and Sun
(1995)
2) N-S thinning model
Lithospheric models: predicted SHF and P-wave tomography
Calculated surface heat flow model 1
Calculated surface heat flow model 2
Flat and N-S thinning
lithospheric models predict
quite different SHF and Vp
patterns
1) Flat model
2) N-S thinning model
Synthetic seismic tomography, P-wave anomaly (average Vp model velocity subtracted at each depth)
Lithospheric models: residuals
Lithospheric models
compatible with
“traditional” potential field
+ topography data
1) Flat model
2) N-S thinning model
GOCE data
Model GOCO02S (satellite only, GOCE, GRACE and CHAMP, and SLR data), 8 months GOCE data
(Nov 2009-July 2010) Goiginger H., et al. (2011)
Coordinates
convention:
X East
Y North
(Cartesian reference
frame in the forward
models)
Gravity gradients at z= 250 km computed by spherical harmonic synthesis code (Z. Martinec). Grid resolution: 15’x15’ (30 km X 30 km approx.).
GOCE modelling: gravity gradients
Gravity gradient residuals (synthetic -GOCE) for models 1, 2 and 3 Std. dev. of
30-50 mE in the different components
Gravity gradient
residuals for models 1,
2 and 3 differ in <1mE
!
At the satellite height
(250 km) the
differences in the
gravity gradients
between the
alternative
lithospheric models
are small
Synthetic gravity gradient residuals at z= 250 km. Grid resolution: 9 km X 11 km approx.).
GOCE modelling: gravity gradients
Exploring different datum's : “raw” downward continuation of GOCE data
GOCE data (diagonal
components) z=20 km
Not too close to the
Earth’s surface different frequency
content!
Synthetic data
(diagonal components)
z=20 km
Gravity gradients: ‘flat’ vs ‘N-S thinning’ models
At “intermediate depths” the frequency content of synthetic and GOCE gravity gradients are
comparable.
1) Flat model
gravity gradient residuals at z= 100 km
Statistics of the
residuals (diagonal +
zy component)
Std. dev.
The N-S thinning. model
shows lower residuals
than the flat model. The
differences between the
models are ~ tens of mE.
2) N-S thinning model
Gravity gradients: ‘N-S thinning-compositional’ vs ‘N-S thinning’ models
The N-S thinning model shows lower residuals than the N-S thinning-compositional model . The
differences between the models are ~ tens of mE.
3) N-S thinning-composition model
gravity gradient residuals at z= 100 km
Statistics of the
residuals (diagonal +
zy component)
Std. dev.
All gravity grad
components improve
except Uxx
2) N-S thinning model
Summary: imaging the Irish lithosphere
• IRETHERM →
Regional component in the thermal
field in Ireland?
• N-S thinning vs flat models →
equivalent from « traditional» potential
field + topography data modelling
• GOCE gravity gradients→
at intermediate heights (z=100 km) the N-S thinn.
model shows over all lower residuals than the other
models (Uzz, Uyy, Uzy), the differences in std. dev. are
~tens of mE (resolution of GOCE data ~ 0.4 mE, R. Pail
dixit). Potential for lithospheric modelling (even
“medium” wavelengths ~ few hundreds of km, and
moderate LAB topography variations )?
LitMod3D
LitMod3D : Potential fields
∆g
FTP
Gγ
r =
(ρ ) = G ρ
− xy ln (r + z
x
2
ρ (z) = ρ
∆N
Mass distribution Right
rectangular prisms in a certesian
coordinate system
FTP
(ρ ) =
+ y
0
2
x ln
0
+ z
)−
(y
+ r
)+
y ln ( x + r
)−
z arctan
x
 xy 
arctan

 +
2
 zr 
2
2
z
arctan
2
 xy 


 zr 
y
 yz 

 +
2
 xr 
x2
y2
x1
y1
z2
+
z1
2
arctan
 xz 


 yr 
x2
y2
z2
x1
y1
z1
2
+ γz
Gρ0
g
Gγ
+
3g
xy ln ( z + r ) + yz ln ( x + r ) + xz ln( y + r ) − P
(
)
(
)
x2
y2
z2
x1
y1
z1
y
x 2
 xy 
xyr +
y 2 + 3 z 2 ln( x + r ) +
x + 3 z 2 ln( y + r ) − z 3 arctan 

2
2
 zr 
x2
y2
z2
x1
y1
z1
LitMod3D (http://eps.mq.edu.au/~jafonso/Software1.htm)
LitMod3D : Forward modelling
Objective: 3D interactive modelling of the lithosphere and uppermost
mantle