DESMEX Sub-project IV - Modelling and Inversion
The main objective of the joint research project DESMEX (Deep Electromagnetic Soundings for Mineral EXploration) is the development of a semi-airborne (ground based transmitters and helicopter based receivers) electromagnetic exploration system for deep mineral deposits up to 1 km depth. There are two tasks for the LIAG within this project:
- Support of the identification of a suitable target with large-scale resistivity surveys and support in the main semi-airborne experiment in 2017 with current source and data loggers.
- Development of suitable finite element (FEM) modeling and inversion algorithms for 3D controlled source electromagnetic (CSEM) data, particularly for the semi-airborne approach.
Resistivity survey Schleiz
In October 2015, we conducted a first 2D dipole-dipole experiment survey across the Bergaer anticline, containing an antimonite deposit, near Schleiz, Germany. Therefore, it is a favored target for the main experiment. The deposit was exploited up to 100 m depth in the past centuries, but the trend to greater depths is unknown.
For our survey, we used 35 electrodes with 125 m electrode spacing over a total profile length of more than 4 km. The injected currents varied between 15 and 22 A depending on subsurface conditions. A preliminary inversion result of the processed DC data is shown in Fig. 1 with significant coverage up to a depth of about 800m. On the one hand, it is possible to identify the trend of the antimonite deposit. On the other hand it delineates the transition from conductive black shales to resistive conglomerates.
The goal is to provide a 3D FEM CSEM modeling code, which supports ground, semi-airborne and airborne CSEM setups, strong resistivity changes and topography. For this purpose, existing codes, developed mostly since 2010, have been reviewed. Unfortunately, these codes do not fit the requirements or are not open available. Thus, a 3D CSEM code will be developed as part of pyGIMLi based on existing open source libraries for mesh generation, finite element modeling and solving large sparse equation systems. Afterwards, first inversion approaches will be implemented and applied to synthetic data and finally to data of the main experiment.
Federal Ministry of Education and Research (BMBF)