Experiments on the Stress Sensitivity of Natural Remanent Magnetization

Abstract

Ten rock samples were progressively demagnetized by uniaxial compressions up to 300 bars, and the results were compared with the alternating field demagnetization of the same rocks. Relative change of the NRM intensity during the stress demagnetization ranges from 0.4% to 47.7% at 100 bars. The stress sensitivity correlates with the stability to AF demagnetization, and the directional changes during the two types of demagnetization were similar to each other. These observations indicate that the stress level which affect a remanence relates with the magnetic coercivity of the remanence. For a quantitative comparison, equivalent magnetic fields of the uni-axial stress were calculated as a ratio of the stress sensitivity to the magnetic stability. The equivalent field ranges from 2.3oe/100 bars to 37.5oe/100 bars, and the porous rocks tended to have larger equivalent fields. The porosity effect can be attributed to the stress intensification on the magnetic minerals embedded in rocks. Based on the present observations, it is concluded that the magnetic stability and the porosity of rocks have dominant effect on the stress sensitivity of remanent magnetization.