Is Piezomagnetism Influenced by Microcracks during Cyclic Loading?

Abstract

Before magnetic field variations in tectonically active regions can be used to infer changes in the crustal stress distribution, it must be demonstrated that piezomagnetism is a stress dominated rock property. Changes in many rock properties of geophysical interest such as electrical conductivity and seismic wave velocity during compressional loading are dominated by changes in microcrack porosity. If piezomagnetism were to fall into such a category its applicability as a method for determining in situ stress fluctuations would be limited. In order to assess the influence of microcrack dilatancy on magnetic susceptibility and remanent magnetization a series of cyclic loading experiments were conducted at confining pressures up to 200MPa. The results of these tests indicate that piezomagnetism is a stress related property, that is, it is controlled by differential stress and only marginally, if at all, affected by changes in microcrack porosity. Magnetic susceptibility decreased with increasing differential stress. The change in susceptibility was uniquely related to differential stress and totally independent of confining pressure, the number of stress cycles or the magnitude of the inelastic volumetric strain. Remanent magnetization, on the other hand, showed a pronounced demagnetization on the first cycle which rapidly decreased on subsequent cycles as long as the peak cycle stress remained constant. An increase in peak stress caused a pronounced decrease in magnetization at the termination of the cycle. There is a striking similarity between this observation and the change exhibited in volumetric strain; that is, an increase in crack porosity at the termination of the first cycle that became progressively smaller with increasing cycle number. Both remanent magnetization and volumetric strain became reproducible on the same cycle. In spite of this apparent correlation other test suggest that such results are merely fortuitous and changes remanent magnetization during cyclic loading are dominated by differential stress.