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New Magnetic Field Sensors for Deep Electromagnetic Sounding Methods

Within the framework of deep electromagnetic sounding for mineral exploration (DESMEX) new sensors and instruments for a semi-airborne electromagnetic exploration method of high technology minerals have been developed. As part of this method, currents are applied to the soil and flow through the resource. The induced secondary magnetic field is detected by an airborne instrument with highly-sensitive quantum sensors.

New instrument in action near Schleiz.

Map of the transfer function of the vertical field component at 7.46 Hz.

By: Ronny Stolz // Markus Schiffler // Matthias Schmelz // Andreas Chwala // Rob IJsselsteijn // Gregor Oelsner // Thomas Schönau // Vyacheslav Zakosarenko // Volkmar Schultze

Securing the supply of German and European industries with important mineral and high-tech materials is an important task of the society as a whole and requires the research of new highly-sensitive instruments for the exploration of deeply embedded and/or small-scale mineral resources, as well as the re-evaluation of former deposits.

Within this context, the team set up at the Universities of Munster and Cologne, the Freiberg University of Mining and Technology, the Federal Institute for Geosciences and Natural Resources, the Leibniz Institute for Applied Geophysics, the Leibniz Institute of Photonic Technology (Leibniz IPHT), and the companies Metronix Meßgeräte und Elektronik GmbH and Supracon AG is working jointly on the deep electromagnetic sounding for mineral exploration (DESMEX) research project. The aim of the project is to develop a semi-airborne instrument that can detect raw mineral deposits up to a depth of 1,500 m via their electrical conductivity in a quick, low-cost, and efficient manner. For this purpose, electric currents are fed into the sub-surface with a horizontal electric dipole. The primary magnetic field of these currents is overlain by the induced secondary magnetic field response of the currents flowing through the sub-surface geological structures depending on their geometry and electrical conductivity. The sum magnetic field is detected from the air with new, highly-sensitive magnetic field sensor instruments. The systems are moved on long parallel lines perpendicular to the transmitter dipole. From the ratio of the measured magnetic field components against the recorded transmitter current at frequencies of 10 Hz to about 30 kHz, the 3D model of electrical conductivity can subsequently be calculated by means of new inversion methods currently under development. The big hurdle in this project is to reduce the motion noise in the airborne operation of the sensors in such a way that the extremely weak signals of the flowing currents can be extracted. The signal amplitudes are often many orders of magnitude smaller than the motion noise and cause a very large signal-to-noise ratio (SNR) that exceeds 24 bits for the new sensor instruments.

The focus of Leibniz IPHT in close cooperation with Supracon AG is the development of two quantum sensor technologies for high-resolution and high SNR detection of the magnetic field:

The first instrument uses new optically pumped magnetometers (OPMs), which are initially used on the ground and later from the air. In the first ground-based field tests, a white noise floor of about 100 fT/Hz1/2 was measured in the magnetically unshielded operation of an OPM. The development of these sensors is still ongoing. The aim is to finally achieve a sensitivity of less than 20 fT/Hz1/2.

The second airborne instrument uses superconducting quantum interference devices (SQUIDs) which, as quantum sensors, feature a periodic dependence of the output voltage of the externally applied magnetic field. The period corresponds to the magnetic flux quantum Ф0=h/2e. This feature is used to readout the SQUID by enabling the instrument to have an SNR of more than true 28 bits. Two different readout methods are being pursued: the implementation of the Leibniz IPHT patented SQUID cascade arrangement with staggered sensitivity magnetic field sensors, and second, a flux quantum counting readout of SQUIDs with intrinsic feedback. With this instrument, a system noise of about 30 fT/Hz1/2 was achieved in flight mode. In airborne operation, the instrument is installed in an aerodynamically shaped body which is towed by a helicopter about 30 m above the ground (see Fig. 1). In the data acquisition system developed by Supracon AG aboard the towed body, the data is stored, decimated in real time, and transferred to the helicopter in order to control the operation of the instrument in flight.

In late autumn of 2017, an extensive field campaign was carried out by the DESMEX team in the vicinity of Schleiz and Greiz to investigate the performance of the newly-developed instruments over an abandoned antimonite mine. Fig. 2 shows an example of a map with the transfer function of the vertical magnetic field sensor for one particular frequency. The processing of the data recorded with Leibniz IPHT’s instruments is complete. Work is currently ongoing to develop the associated inversion tools in order to transform the data into a 3D model of the underlying electrical conductivities.

In 2018, another field experiment is being planned over a real mineral deposit in Sweden. The preparations for the campaign have already started.

The sensor technologies developed by Leibniz IPHT have thus overcome the hurdle of the extreme SNR, which appears when using highly-sensitive magnetic field sensors with vectorial character in the Earth’s magnetic field coupled with motion noise in airborne operation. As a consequence, various other applications can be addressed in perspective with these sensors.

Funded by: BMBF, PTJ

Related Publications

Schönau et al. (2017): Flux trapping in multi-loop SQUIDs and its impact on SQUID-based absolute magnetometry, Superconductor Science and Technology 31(3), 035001-1, doi: 10.1088/1361-6668/aaa44f
Schultze et al. (2017): An Optically pumped magnetometer working in the light-shift dispersed Mz Mode, Sensors 17(3), 561, doi: 10.3390/s17030561

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