According to theoretical considerations and recent physical experiments, the so-called second-ord... more According to theoretical considerations and recent physical experiments, the so-called second-order magnetic phase transition may accompany with a very sharp enhancement of the magnetic susceptibility. On the basis of geomagnetic, geothermic, and magnetotelluric considerations, Kiss et al. (2005) suggested this transition between ferromagnetic/ferrimagnetic and paramagnetic states of the material at the Curie/Néel temperature occurring in the Earth's crust at mid-crustal depths. Since then several pro and contra arguments have emerged. Here we present magnetotelluric modelling results, discuss some new ideas about the thickness and magnetic permeability of the critical layer. We conclude that the possible mid-crustal models due to the second-order magnetic phase transition are elongated: linear or curved bodies (thin or thicker plates) of finite length just at the Curie depth.
Summary We present a new adaptive unstructured triangular grid Finite Element approach for effect... more Summary We present a new adaptive unstructured triangular grid Finite Element approach for effectively simulating electromag- netic fields in two-dimensional anisotropic conductivity structures. This requires the elaborate generation of huge irregular grids and their expensive administration to model nearly arbitrary geometries including surface topography. To characterise the diffusion of the electromagnetic fields, a second order elliptic boundary value problem is solved. The appropriate inhomogeneous Dirichlet boundary conditions are calculated analytically for a 1D stratified ground. The accuracy of the Finite Element solution is mainly influenced by mesh parameters, in particular the size of the trian- gles. However, fine meshes and thus large grids require great effort in computing time and memory. Adaptively refined meshes generate fine discretisations only where necessary and therefore obtain optimum trade-off between computational expense and quality. Comparisons to analyti...
We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach... more We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach for plane wave electromagnetic (EM) methods using an adaptive unstructured grid finite element forward operator. Unstructured triangular grids permit efficient discretization of arbitrary 2-D model geometries and, hence, allow for modeling arbitrary topography. The inversion model is parameterized on a coarse parameter grid which constitutes a
Marine volcanoes are particularly demanding when it comes to applying electric or electromagnetic... more Marine volcanoes are particularly demanding when it comes to applying electric or electromagnetic methods to investigate their interiors. First, the surrounding highly conductive sea water represents a significant difference in conductivity with respect to the volcanic edifice, second, the volcano's topography has great impact on the electromagnetic response, and, third, the surrounding sea bed topography heavily distorts electromagnetic fields in frequency bands that interfere with a certain spatial wavelength and amplitude of the bathymetry. By neglecting these issues severe misinterpretations are the inevitable consequence. We present different approaches to 3D vector finite element simulation on unstructured grids which are able to compute plain-wave magnetotelluric fields for models including arbitrary surface and sea bed topography. As an example, we consider Stromboli volcano. One major issue is the incorporation of the Stromboli topography using a digital terrain model s...
According to theoretical considerations and recent physical experiments, the so-called second-ord... more According to theoretical considerations and recent physical experiments, the so-called second-order magnetic phase transition may accompany with a very sharp enhancement of the magnetic susceptibility. On the basis of geomagnetic, geothermic, and magnetotelluric considerations, Kiss et al. (2005) suggested this transition between ferromagnetic/ferrimagnetic and paramagnetic states of the material at the Curie/Néel temperature occurring in the Earth's crust at mid-crustal depths. Since then several pro and contra arguments have emerged. Here we present magnetotelluric modelling results, discuss some new ideas about the thickness and magnetic permeability of the critical layer. We conclude that the possible mid-crustal models due to the second-order magnetic phase transition are elongated: linear or curved bodies (thin or thicker plates) of finite length just at the Curie depth.
Summary We present a new adaptive unstructured triangular grid Finite Element approach for effect... more Summary We present a new adaptive unstructured triangular grid Finite Element approach for effectively simulating electromag- netic fields in two-dimensional anisotropic conductivity structures. This requires the elaborate generation of huge irregular grids and their expensive administration to model nearly arbitrary geometries including surface topography. To characterise the diffusion of the electromagnetic fields, a second order elliptic boundary value problem is solved. The appropriate inhomogeneous Dirichlet boundary conditions are calculated analytically for a 1D stratified ground. The accuracy of the Finite Element solution is mainly influenced by mesh parameters, in particular the size of the trian- gles. However, fine meshes and thus large grids require great effort in computing time and memory. Adaptively refined meshes generate fine discretisations only where necessary and therefore obtain optimum trade-off between computational expense and quality. Comparisons to analyti...
We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach... more We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach for plane wave electromagnetic (EM) methods using an adaptive unstructured grid finite element forward operator. Unstructured triangular grids permit efficient discretization of arbitrary 2-D model geometries and, hence, allow for modeling arbitrary topography. The inversion model is parameterized on a coarse parameter grid which constitutes a
Marine volcanoes are particularly demanding when it comes to applying electric or electromagnetic... more Marine volcanoes are particularly demanding when it comes to applying electric or electromagnetic methods to investigate their interiors. First, the surrounding highly conductive sea water represents a significant difference in conductivity with respect to the volcanic edifice, second, the volcano's topography has great impact on the electromagnetic response, and, third, the surrounding sea bed topography heavily distorts electromagnetic fields in frequency bands that interfere with a certain spatial wavelength and amplitude of the bathymetry. By neglecting these issues severe misinterpretations are the inevitable consequence. We present different approaches to 3D vector finite element simulation on unstructured grids which are able to compute plain-wave magnetotelluric fields for models including arbitrary surface and sea bed topography. As an example, we consider Stromboli volcano. One major issue is the incorporation of the Stromboli topography using a digital terrain model s...
Uploads
Papers by Antje Franke