As part of the deep electromagnetic sounding for mineral exploration (DESMEX) joint project, a semi-airborne electromagnetic exploration system is being developed for ore deposits up to 1 km deep. This consists of strong electromagnetic transmitters on the ground, as well as novel magnetic field sensors developed at Leibniz IPHT for airborne applications.

By T. Schönau // V. Zakosarenko // M. Schmelz // H.-G. Meyer // R. Stolz // V. Schultze // R. IJsselsteijn // G. Oelsner // F. Wittkämper

Securing the supply of German and European industry with raw materials and sustainably using these sources are tasks for society as a whole, especially in view of the increasing demand from new, emerging industrial nations in the 21st century. The deep electromagnetic sounding for mineral exploration (DESMEX) joint project is dedicated to these tasks. Within the framework of this project, a semi-airborne exploration method for the exploration of mineral deposits in Germany is to be developed and researched, which will enable exploration depths of more than 1 km and thus take into account the need to detect and describe deeper and deeper sources of raw materials.

The signal amplitudes to be detected are very weak, thus it was necessary to develop a new concept for an electromagnetic measuring method. DESMEX uses very strong excitation sources on the ground, very strong periodically varying currents of which are fed via electrodes deep into the Earth and the electrically conductive layers inside. The secondary magnetic field of the currents flowing through the mineral layers is measured during an overflight with the aid of highly sensitive magnetic field sensors (i.e., so-called magnetometers). A system sketch of the semi-airborne exploration process is shown in Figure 1.

For one, in this semi-airborne process new induction coils from Metronix are used. For another, the magnetometry research group at Leibniz IPHT is researching new magnetometers based on superconducting 3D vector sensors and optically pumped magnetometers (OPMs), which determine the magnitude of the magnetic field vector. The 3D vector magnetometers are based on superconducting quantum interference detectors (SQUIDs) with Josephson junctions with sub-µm dimensions, which are fabricated and researched in the cleanroom at Leibniz IPHT together with the micro and nanotechnologies group [1]. In close cooperation with Supracon AG, new selection principles are being developed with these sensors. The results are highly sensitive, hybrid SQUIDs with a resolution of 4.5 fT/Hz1/2and a huge dynamic range of more than 32 bits.

The second class of magnetic field sensors are the optically pumped magnetometers. Worldwide efforts are being made to continuously improve the resolution of OPMs in order to replace cooled sensors in the future. The research group wants to achieve the high sensitivity of OPMs even in magnetically unshielded applications and is therefore researching new OPM instruments and readout methods. One of these methods, the so-called LSD-Mz mode [2], enables the operation of high-resolution OPMs in Earth’s magnetic field. Two cells filled with cesium vapor are irradiated with circularly polarized laser light of opposite helicity. The magnetic field to be measured is determined by the difference between the two signals of the cells.

Within the framework of DESMEX, the concept was to be tested exemplarily at two ore deposit sites of different genesis. In the first main experiment in October 2017, various flight probes and associated magnetometers developed in this project successfully flew over an area in the vicinity of the former mining regions of Schleiz (Thuringia) and Mühltroff (Saxony). Under this area, antimonite mining was carried out up to a depth of about 200 m until the 1950s. The aim was to follow the electrically conductive structures, mainly graptolite shales, to great depths. The first 3D models of electrical conductivity are currently being interpreted together with the geological and geophysical data available. On this surface, an OPM was operated in LSD-Mz mode in Earth’s magnetic field for the first time. Also for the first time, a noise-limited magnetic field resolution of below 150 fT/Hz1/2with a bandwidth of 2.2 kHz was measured, which is significantly better than commercially available OPMs. Based on this newly gained knowledge, the OPMs are to be optimized for both ground-based and airborne applications in the future [3].

The second main experiment at Kiruna in northern Sweden in October 2018 (see Fig. 2) was also completed successfully with the two flight probes. The aim of this campaign was to map the depth of the largest underground iron ore mine in the world better. This deposit will be mined at a depth of about 1,300 meters and is therefore very suitable as a test object for DESMEX. One example of the raw data to be measured from a superconducting 3D vector magnetometer is shown in Figure 3. The data from the instrument developed by Leibniz IPHT and Supracon AG was fully processed and shows very good quality. Currently, work is underway on 3D inversion (i.e., the back calculation to the model of electrical conductivity in three dimensions, as well as its interpretation).

The DESMEX project and above all the two main experiments posed a very big challenge, which could only be successfully mastered and concluded with excellent results through the close cooperation of all partners. The magnetometry team would like to thank the Institute for Geophysics of the Westphalian Wilhelms University, the Federal Institute for Geosciences and Natural Resources Hanover/Berlin, the Institute for Geophysics and Meteorology of the University of Cologne, the Institute for Mineralogy of the Technical University Bergakademie Freiberg, and the two companies Supracon AG and Metronix GmbH.

The continuation of DESMEX’s work is already being planned. The goals of Leibniz IPHT are airborne and miniaturized OPMs, as well as the perspective integration on unmanned aerial platforms.

Funded by: BMBF/ PTJ