Public lecture by Dr Roland Martin

11 July 2017

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Image: Gravity Bouguer anomalies in the Pyrenees computed using Spectral Element method

Numerical Modelling and Imaging in Geophysics at Different Scales: applications to the Pyrenees chain and the subsurface/laboratory scale

Dr Roland Martin, senior research scientist, National Centre for Scientific Research (CNRS)

In this lecture Dr Martin presented different high order numerical tools using finite-difference or finite element approaches to propagate seismic waves in a wide variety of Earth structures at different scales in order, in the near future, to couple them through different physics related to different frequency content of the sources involved.

He discussed two applications that could be linked in the future: the Pyrenees chain imaging at moderate source frequencies and wet/dry (non-)linear viscoelastic wave modelling in wet/dry/non-consolidated granular materials in the near surface.

Dr Martin presented a hybrid inversion method that allows us to image density distributions at the regional scale using both seismic and gravity data. One main goal is to obtain densities and seismic wave velocities (P and S) in the lithosphere with a fine resolution to get important constraints on the mineralogic composition and thermal state of the lithosphere. In the context of the Pyrenees (located between Spain and France), accurate Vp and Vs seismic velocity models are computed first on a 3D spectral element grid at the scale of the Pyrenees by inverting teleseismic full waveforms. In a second step, Vp velocities are mapped to densities using empirical relations to build an a priori density model. BGI and BRGM Bouguer gravity anomaly data sets are then inverted on the same 3D spectral element grid as the Vp model at a resolution of 1-2 km by using high-order numerical integration formulae. This procedure opens the possibility to invert both teleseismic and gravity data on the same finite-element grid. It can handle topography of the free surface in the same spectral-element distorted mesh that is used to solve the wave equation, without performing extra interpolations between different grids and models. WGS84 elliptical Earth curvature, SRTM or ETOPO1 topographies are used.

Dr Martin reproduced numerically the response of seismic waves in granular/porous media at the laboratory scale (01.-10kHZ sources) and this will enable us to better understand the signals recorded close to the surface when high frequency content will be used to better image the near surface, in particular by taking into account seasonal water content variations and complex rheologies and steep seismic velocity gradients present in the first hundred meters depths.

Dr Roland Martin is a senior research scientist at the National Centre for Scientific Research, Universit√© Paul Sabatier, and has been working for many years in France where he obtained his PhD in Geophysics (1998). He has been a researcher in Mexico City (1999-2004) before integrating the French CNRS (equivalent to the Australian CSIRO) in 2005 at Pau University and GET laboratory in Toulouse. His main interests are the numerical modelling in geophysics at different scales using different numerical techniques for the forward and inverse problems. He is developing and applying those techniques to the modelling and imaging the Earth at different scales: from the near subsurface or laboratory scale to the Earth crust scale with some specific sites of study like the well monitored Pyrenees chain located between Spain and France. Seismic and gravity dense measurements  are mainly used to obtain more information on both seismic wave velocities and densities in the Earth crust and to couple the structures to the surface using not only high resolution numerical tools but also more complex physics in solid-fluid mechanical systems.

In 2017, Roland was awarded a UWA Institute of Advanced Studies Robert and Maude Gledden Visiting Senior Fellowship.