Objective of the BMBF funded compound project DESMEX (Deep Electromagnetic Sounding for Mineral Exploration) is the development of a semi-airborne, i.e. ground transmitters and a combined ground and airborne receivers, electromagnetic exploration method that is able to image ore deposits down to 1 km depth. The subproject IV deals with modelling and inversion algorithms.
The joint research project DESMEX (Deep Electromagnetic Soundings for Mineral EXploration) is founded by the Federal Ministry for Education and Research (BMBF) project r4 to develop a semi-airborne controlled-source electromagnetic (CSEM) exploration system for deep mineral deposits up to 1 km depth. In particular, two ground based electric dipole transmitters are used in parallel to provide strong signals for flight areas with 6x8 km dimensions. For optimum signal quality, two novel receiver systems – one SQUID based magnetometer and an induction coil – are applied and referenced with several ground stations. The measurements scheme is illustrated in Fig. 1.
The main goals for the LIAG within the DESMEX project are:
In the years 2015 & 2016, we conducted a 2D large-scale ERT survey with a total length of 7.5 km across the Berga anticline, near Schleiz, Germany. In this region, an antimonite deposit is located which, in former decades, was exploited within the uppermost 100 m, but the trend to greater depths is unknown. <br/> For our survey, we used a dipole-dipole setup with all in all 60 receiver locations and 125 m electrode spacing. The drive-in currents varied between 10 and 25 A. The final 2D inversion result of the processed DC data is depicted in Fig. 2. A significant resistivity distribution of the surrounding host rocks is obtained up to 500 m depth. The main conductive or resistive structures can be assigned to the main geological layers such as black-shales, greywacke or diabase.
We are developing the 3D FEM CSEM modeling toolbox custEM for about 2 years, which supports marine, ground-based, semi-airborne and airborne CSEM measurements for flat-surfaces or topography. The code is based on existing open source libraries, mainly FEniCS & pyGIMLi. First covers nodal, vector and mixed finite elements, higher order basis functions, source implementation in terms of current density or primary fields, anisotropy, parallelization and others. Second is used for mesh generation and visualization.
So far, we implemented different existing approaches and were able to model and validate various CSEM setups. A view examples are presented in Fig. 3. Furthermore, we automated the modeling procedure: