Magnetic Barkhausen Noise Measurements on Rocks

Abstract

Context: The magnetic properties of rocks may reflect the modal abundance, composition, and microstructure of the magnetic grains contained within them, usually in a very small proportion. Magnetic Barkhausen Noise (MBN) is a non-destructive technique applied to magnetic materials, and it is very sensitive to microstructure and residual stresses. In this work, measurements of MBN in samples of rocks extracted from nature and containing varying proportions of magnetite were studied and analyzed. Method: Measurements were taken by magnetically exciting the cylindrical samples, using a magnetic yoke and measuring the induced field inside the rock as a consequence of the excitation. For the detection, a sensor coil placed on the excited surface on one of the flat faces of each cylinder was used. The signals obtained from the MBN were digitized, and a digital 5-200 kHz Butterworth filter was applied, calculating the root mean square (RMS) values. Results: The linear fit of the MBN RMS values with the increasing percentage of ferromagnetic minerals showed an increasing trend with a moderate correlation. A correlation between coercive force and the MBN RMS values was observed only for samples with abundant magnetite (> 25 vol%). Conclusions: An increasing variation of the RMS values of the MBN signals was observed in relation with coercive force for abundant magnetite samples. This may be related to the geological processes involved in magnetite genesis. Method: The measurements were made exciting magnetically the samples (cylindrical shapes), using a magnetic yoke and measuring the induced field inside the rock as a result of the excitation. For the detection, a sensor coil placed on the excited surface on one of the flat faces of each cylinder was used. The signals obtained from MBN were digitized. A digital 5-200 kHz Butterworth filter was applied and the RMS (Root Mean Square) values were calculated. Results: The linear fit of the MBN RMS values with the increasing percentage of ferromagnetic minerals showed an increasing trend with moderate correlation. A correlation between coercive force and MBN RMS was observed only for samples with abundant magnetite (> 25 vol%). Conclusions: A variation of the RMS values of the MBN signals was observed in relation with coercive force for massive magnetite samples. This can be related to geological processes involved in magnetite genesis.   Keywords: Rocks, Magnetization, Barkhausen Magnetic Noise, Non-Destructive Test.