MERIT – A New Magneto-electrical Resistivity Imaging Technique Part 2: Magnetic Sensors and Instrumentation

E Zimmermann1, A.Verweerd1,2, W Glaas1, A Tillmann2, A Kemna2

1 Central Electronics Laboratory, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany 2 Institute of Chemistry and Dynamics of the Geosphere, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany, e.zimmermann@fz-juelich.de

ABSTRACT

We present a data acquisition system for a new Magneto-Electrical Resistivity Imaging Technique (MERIT) for the study of electrical conductivity distributions in cylindrical probes at different laboratory scales. The system is equipped with electrodes, for a sine wave current injection and electric potential measurements, and Anisotropic Magnetoresistive (AMR) magnetic field sensors. The highly sensitive three-axis magnetic sensors (50 pT/√Hz @ 10 Hz) are located on modules with an integrated digital lock-in amplifier. A field-feedback circuit is used in the modules to negate the crossfield dependency on the Earth’s magnetic field to an accuracy of 0.1 %. For an optimal noise rejection a lock-in frequency of 25 Hz, with a sine wave demodulation, and a measurement cycle of 1-10 s is used. Test measurements of the magnetic field due to a circular wire (20 mA current, 0.1 m radius) in a laboratory without magnetic shielding show an accuracy of less than 200 pT measured at a maximum field of 40 nT.

Keywords Resistivity Imaging, Magnetic Sensing, Measurement System Design, AMR Sensors