IAGA 2005 - Toulouse, France 22/07/2005
Inversion of Terrestrial and Martian magnetic anomalies:
geological implications
Y. Quesnel (1), B. Langlais (1), S. Le Mouélic (1), C. Sotin (1) and A. Galdéano (2)
(1) Laboratoire de Planétologie et Géodynamique de Nantes, UMR-CNRS 6112 (2) Géomagnétisme Interne, IPG Paris, UMR-CNRS 7577
Crustal magnetic anomalies ↔ Crustal sources ?Correlation between magnetic anomaly and
geological maps (near-surface investigation…)
Modeling methods
Forward models (geometry,
magnetization)
Inversion (depth, orientation)
Crustal sources
IAGA 2005 - Toulouse, France 22/07/2005
Outline
• Modeling method• Application to Martian magnetic satellite
data• Application to Terrestrial aeromagnetic
data• Geological implications• Future work
IAGA 2005 - Toulouse, France 22/07/2005
MethodForward models: uniformly magnetized spheres (Blakely, 1996) and prisms (Plouff, 1976)
Generalized non-linear inversion (Tarantola and Valette, 1982):
• Data: Bx, By, Bz• Parameters: mx, my, mz, x, y, z (moment and location of one or several dipoles)• Equations: three components of the magnetic field created by a sphere• Iterative process: minimum chi-squared criterion• Final test: data misfits, gaussian distribution
IAGA 2005 - Toulouse, France 22/07/2005
Application to Martian satellite data
IAGA 2005 - Toulouse, France 22/07/2005
A remanent field !
(Acuña et al., 1999)
Type of data: three vector components AB (100-250 km) and MO (400 km) MGS MAG/ER measurements
Area: ancient southern highlands of Terra Sirenum
AB (Br)
MO(Br)
Application to Martian satellite data
IAGA 2005 - Toulouse, France 22/07/2005
AB (100-250 km)
MO (400 km)
Data
Results of three different data set tests:
(input dipole location parameters from forward modeling studies)
Data set λ (°N) dλ (°N) φ (°E) dφ (°E) z (km) dz (km) m (A.m2) dm (A.m2) I (°) dI (°) D (°) dD (°) Chi-
squared AB final fit MO final fitAB -32 0,02 191 0,02 49 0,46 3.1016 0,09 -49 0,03 8 0,04 1,15 good acceptableMO -34 0,02 191 0,02 123 0,28 9.1016 0,04 -33 0,01 29 0,01 0,75 bad good
AB+MO -33 0,02 191 0,02 160 0,30 9.1016 0,03 -45 0,01 26 0,01 1,18 bad good
Application to Martian satellite data
IAGA 2005 - Toulouse, France 22/07/2005
AB (100-250 km)
MO (400 km)
Data
Model(3
dipoles)
12
31
23
Dipole λ (°N) dλ (°N) φ (°E) dφ (°E) z (km) dz (km) m (A.m2) dm (A.m2) I (°) dI (°) D (°) dD (°)1 -32 0,02 191 0,02 49 0,46 3.1016 0,09 -49 0,03 8 0,042 -35 0,04 199 0,02 89 0,69 3.1016 0,04 -66 0,01 -29 0,033 -33 0,02 204 0,02 26 0,70 1.1016 0,07 18 0,04 4 0,03
Nantes
Application to Terrestrial aeromagnetic data
Type of data: total magnetic field airborne measurements at 120 m above topography (summer 1998 ; BRGM)
Area: the Armorican Massif (France)
IAGA 2005 - Toulouse, France 22/07/2005
I
Geological setting
Nantes
Application to Terrestrial aeromagnetic data
IAGA 2005 - Toulouse, France 22/07/2005
λ (°N) φ (°E) z (km) r (km) M (A/m) I (°) D (°)Sphere 1 47,47 -1,90 4 1 11 63 -3Sphere 2 47,40 -1,90 15 4 3 -63 177Sphere 3 47,67 -1,90 15 4 3 -63 177
1
32
Forward modeling (first results):
3 spheres model:
λ1 (°N) λ2 (°N) φ1 (°E) φ2 (°E) z1 (km) z2 (km) M (A/m) I (°) D (°)Prism 1 47,46 47,48 -1,91 -1,89 1,8 4 11 63 -3Prism 2 47,34 47,44 -1,95 -1,85 14 17,5 3 -63 177Prism 3 47,62 47,75 -1,95 -1,85 13,5 17 3 -63 177
1
32
3 prisms model:
N
Geological implications
IAGA 2005 - Toulouse, France 22/07/2005
The Armor2 seismic profile from Bitri et al., C.R. Geosciences, 335, 2003:S N
3 A/m
11 A/m0
5
10
15
20
25
30
35Depth (km)
Geological implicationsResults of the Martian investigation: - no correlation between adjacent sources - no links with craters in the studied area - strongly magnetized materials → magnetite ? how ?
IAGA 2005 - Toulouse, France 22/07/2005
Serpentinization model (early Mars)
Alteration of forsterite (10% Fe) with water:30Fo90 + 41H2O 15Lizardite + 2Fe3O4 + 9Mg(OH)2 + 2H2
Future work…• Inversion of terrestrial aeromagnetic data• Terrestrial satellite measurements investigation• Study of other areas on Mars• Correlation with surface mineralogy : OMEGA Mars Express
IAGA 2005 - Toulouse, France 22/07/2005
Comparison with terrestrial spectra of olivine, pyroxene, serpentine and magnetite (example of the Ronda massif: Launeau et al., 2002)
Acknowledgments
IAGA 2005 - Toulouse, France 22/07/2005
This work benefited from the support of the European Community’s Improving Human Potential Programme under contract RTN2-2001-00414, MAGE.
