Ralph-Uwe Börner

Towards an Integrative Inversion and Interpretation of Airborne and Terrestrial Data

The aim of the joint research project is to generate information from airborne geophysical measurements that are properly transferred from physically quantitative descriptions of the subsurface (electrical conductivities, densities, susceptibilities) into spatial structures and information matching the understanding of end-users: geologists, hydrogeologists, engineers and others. We suggest new types of inversion, which are integrated in the interactive workflow to support typical trial and error approaches of inverse and forward EM and gravity/magnetic field modelling for 1D and 3D cases. Subsequently, we combine resistivity and density models with geological 3D subsurface models. The integrated workflow minimizes uncertainties in the interpretation of geophysical data and allows a significantly improved and fast interpretation and imaging of the 3D subsurface architecture. The results of the AIDA project demonstrate that combined 3D geological and geophysical models enable a much better reconstruction of the subterraneous space. AIDA stands for “From Airborne Data Inversion to In-Depth Analysis” and is part of the R&D program: Tomography of the Earth’s Crust—From Geophysical Sounding to Real-Time Monitoring.

Transient electromagnetic fields: their efficient three‐dimensional simulation and application to borehole‐based observation techniques

GEM Beijing 2011: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Beijing, China. October 10-13, 2011. Transient electromagnetic fields: their efficient three-dimensional simulation and application to borehole-based observation techniques Martin Afanasjew, Jana Borner, Ralph-Uwe Borner, Michael Eiermann, Oliver Ernst, Klaus Spitzer, 1 Institute of Geophysics and Geoscience Informatics, TU Bergakademie Freiberg, Germany 2 Institute of Numerical Analysis and Optimization, TU Bergakademie Freiberg, Germany

3D Inversion of Helicopter-borne Electromagnetic Data - A Cut-&-Paste Strategy

We present a cut-&-paste strategy for the 3D inversion of helicopter-borne frequency-domain electromagnetic data. Standard interpretation procedures often involve laterally constrained stitched 1D inversion techniques to create pseudo-3D models that are largely representative for smoothly varying conductivity distributions in the subsurface. Pronounced lateral conductivity changes may, however, produce significant artefacts that can lead to serious misinterpretation. Still, 3D inversions are numerically very expensive. Our approach therefore restricts the full 3D inversion to those parts of the survey where the 1D inversion actually fails. Using a cut-&-paste technique, those regions are extracted, separately inverted in 3D, and finally re-introduced into the original model. We apply a Gauss-Newton inversion scheme using a staggered-grid finite-difference forward operator that takes into account the dielectric response. An explicit representation of the Jacobian matrix serves for optimum computational performance. We introduce a three-way tensor quantity which facilitates the matrix assembly of the forward operator as well as the efficient calculation of the Jacobian. Finally, we deliver the proof of concept for the inversion using a synthetic and a field data set from the Cuxhaven tunnel valley in Germany.

Three-Dimensional Multi-Scale and Multi-Method Inversion to Determine the Electrical Conductivity Distribution of the Subsurface (Multi-EM)

Combining different electromagnetic (EM) methods in joint inversion approaches can enhance the overall resolution power. Every method is associated with a particular sensitivity pattern. By assembling complementary patterns, subsurface imaging becomes more complete and reliable. We describe different paths to obtain multi-EM inversions. First, a joint inversion approach using finite difference forward operators is outlined that formulates the problem of minimizing the objective function using different weights for each individual method. Then we address a sequential approach using finite element methods on unstructured grids to cycle through the different EM methods iteratively. Both methods are based on a traditional parametrization using piecewise constant model parameters which may be inefficient when describing the usually rather coarse models. Therefore, we investigate wavelet-based model representations as an alternative.

3-D simulation of the bathymetry effect for marine CSEM using adaptive finite elements

We numerically simulate marine CSEM measurements using the finite element method. Unstructured tetrahedral meshes easily allow for the inclusion of arbitrary seafloor bathymetry so that natural environments are mapped into the model in a close-to-reality way. A primary/secondary field approach as well as an adaptive mesh refinement strategy improve the solution accuracy. The seafloor bathymetry is shown to produce significant three-dimensional effects compared to a onedimensional, flat seafloor model.