Sujet de thèse : Imagerie géophysique des structures volcaniques

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PhD& Thesis&& research& project:" i+64%7+" )$%&)7&"" 1/)7&" /')/$)49" &(%5)$'3()4" %72" $%&7'3)4"
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Thesis&supervisor&:&@?"A+13%739"%72"A?"C+16)&%729"#;E'(('?"
This research work is proposed in the frame of a 10 years geophysical study of La Soufriere de
Guadeloupe volcano, French Indies. The objective of this work is to realize the geophysical imaging
of Basse-Terre (40km x 20km x 10km), the active volcanic part of Guadeloupe Island. To reach this
goal, we propose to develop a new method and invert jointly surface wave dispersion curves and
gravimetric measurements.
Situation: Lesser Antilles volcanoes (Soufrière de Guadeloupe, Montagne Pelée de Martinique,
Soufrière de Montserrat, …) are active volcanoes associated to the Caribbean subduction zone that
were all active during historical times (respectively 1976, 1902, 1995-to-present). Seismic and
volcanic hazards have then a strong influence on the economical and social local environment. Despite
this situation, the geological structure of these volcanoes remains mostly unknown. Most studies have
been concentrated on Montserrat volcano with several marine seismic profiles used to detect the depth
of the magmatic storage depth. An important IODP campaign is currently performed to investigate the
upper crustal structure around the islands. For the Guadeloupe Soufriere volcano itself, a joint seismic
and gravimetric tomography has been performed at the scale of the upper geothermal system and a
seismic tomographic study has been performed at the subduction arc scale. There are however no
geophysical studies yet dedicated to the entire volcano; Basse-Terre Island and La Grande Découverte
volcanic complex. The two hypotheses that were proposed to explain the 1976 phreatic explosion
remain still debated: a stillborn eruption with a magmatic intrusion or a geothermal instability in a
slowly cooling magmatic storage system. This PhD research work aims at producing a geophysical
model for the entire volcano, down to a 5-10km depth.
Method: We propose to combine different datasets: surface wave dispersion curves gravimetric and
aeromagnetic measurements. The joint inversion of these datasets allows combining different spatial
resolutions and different geographical data coverage. Two main questions need to be addressed to
conduct this work, one for the direct problem, one for the inverse problem:
• The surface waves that propagate across Basse-Terre Island travel on rather short distances
(40km x 20km) and may be affected by the strong topography. These two characteristics are
common in most volcanic situations. We propose to investigate these effects and to quantify
their influence on the quality of the dispersion curves by performing several numerical
simulation of the elastic waves propagation in heterogeneous elastic media with topography.
We will use the finite-element Galerkin-discontinuous implementation (V. Etienne 2010)
available at the laboratory.
• For the joint inversion, a necessary step is to invert separately all datasets. Since the gravity
and magnetic inversion relies on the same methodology based on potential field, we will use a
standard transform to invert both gravity and aeromagnetic data. These last data can provide
interesting constraints on the age of the volcanic edifice. In a second stage, we will combine
gravity and surface waves, a problem solved by Maceira et al (2009) to perform a joint
inversion. The main issues are 1) how to perform the data discretization? 2) What is the most
efficient inversion scheme for this mixed linear-nonlinear problem? In Coutant et al (2012) we
used a collocation grid discretization, a Bayesian formulation with an iterative (quasi-Newton)
deterministic approach. Different formulations can be investigated using different
discretization schemes (e.g. layered med) or inversion methods (e.g. Monte Carlo).
The different methods (inversion, numerical modeling, surface wave dispersion analysis, noise
correlation) will be used or develop in collaboration with colleagues at ISTerre. The surface wave
modeling part will be developped in cooperation with E. Maufroy, a postdoctoral researcher working
on site effects. C. Bouligand is teaching potential field theory and gravity measurements.
Data: Seismic data come from the permanent broadband seismic network of La Soufriere Observatory
and from the temporary experiment run since 2010 by the Domoscan ANR project. Gravity data come
the PhD work by H. Gunawan (2005). A complementary gravity field survey will be performed during

August-September 2012. La Soufriere Volcanic rock geophysical properties will be obtained from
analysis performed at UAG Antilles University. Aeromagnetic data come from an IPGP survey by J.L
Le Mouël.
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