Basic magnetic properties of rocks under the effects of mechanical stresses
Basic magnetic properties of rocks under the effects of mechanical stresses
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机械应力作用下岩石的基本磁特性
DOI：
10.1016/0040-1951(70)90015-6
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发表时间：
1970
期刊：
影响因子：
2.9
通讯作者：
T. Nagata
中科院分区：
文献类型：
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作者：
T. Nagata
机械应力对地壳磁化的影响可分为两类：（a）当应力消除时消失的可逆效应;和（B）引起可逆磁化不可逆增强或不可逆退磁的不可逆效应。可逆效应发生在磁化率（κ）和硬磁回复磁化（j hr）中，例如热回复磁化和化学回复磁化。沿着磁化方向的纵向单轴压缩（σ）导致磁化强度降低，而垂直于磁化方向的横向单轴压缩（σ）导致磁化强度增加。理论上，可逆效应是由于压缩引起的单个磁畴内的自发磁化旋转。对于较小的压缩值（σ <$100 kg/cm 2），这些效应可近似表示为κ <$（σ <$）= κ 0（1− β′ σ），κ（σ）= κ 0（1+ 1 2 β′ σ ′），J HR（σ）= J 0 HR（1− β ″ σ）和J HR（σ）= J 0 HR（1+ 1 2 β ″σ）式中β= 0. 5· 10− 4~ 5· 10− 4 cm 2/kg和β= 0. 3· 10− 4~ 1· 10− 4 cm 2/kg，对于火成岩。单轴拉伸引起相反的效应。不可逆效应与软等温剩磁（jsr）有关。在磁场（H）存在下，岩石上的α的施加和释放，在纵向和横向效应的情况下，都会导致剩磁的增加，其中横向效应略小于纵向效应（10~ 25%）。纵向压缩和横向压缩都引起软等温剩磁的不可逆退磁。在这种情况下，横向效应也比纵向效应小一点。理论上，这些效应是由于90°畴壁的不可逆位移。对于较小的压缩值（σ <$100 kg/cm 2）和磁场（tH <$10 Oe）时，其影响可近似表示为：J 0 <$SR（σ <$）= CHσ <$，J 0 <$SR（σ <$）= 34 CHσ <$，J 0 <$SR（σ <$<$）= J 0 <$SR（1-ασ ** H）和J SR（σ perp; ∗∗)= J 0⊥ SR（1− 3ασ <$4H），其中σ <$和σ <$<$4H表示σ的不可逆效应，C= 0.2· 10− 6~ 11· 10− 6 emu/Oe/kg/cm 2，α= 0.02~ 0.1Oe cm 2/kg。单轴张力也会产生同样的效果。在永久地磁场存在的情况下，地壳以类似的方式反复受到应力，与可逆效应相比，不可逆效应可以忽略不计。
The effects of mechanical stresses on the magnetization of the earth's crust can be classified in two categories:(a) the reversible effect which disappears when the stress is removed; and (b) the irreversible effect which causes an irreversible enhancement or an irreversible demagnetization of remanent magnetization. Reversible effects take place in the magnetic susceptibility (κ) and in the magnetically hard remanent magnetization (j hr) such as thermoremanent and chemical remanent magnetizations. The longitudinal uniaxial compression (σ) which is along the direction of magnetization results in a decrease of magnetization whereas the transverse uniaxial compression (σ⊥) which is perpendicular to the direction of magnetization causes an increase of magnetization. Theoretically, the reversible effects are due to the rotation of spontaneous magnetization within individual magnetic domains impressed by the compression. For small values of compression (σ≲ 100 kg/cm 2), these effects can be approximately expressed by κ(σ)= κ 0 (1− β′ σ), κ⊥(σ⊥)= κ 0 (1+ 1 2 β′ σ⊥), J HR (σ)= J 0 HR (1− β ″σ) and J⊥ HR (σ⊥)= J 0 HR (1+ 1 2 β ″σ⊥) where β= 0.5· 10− 4~ 5· 10− 4 cm 2/kg and β= 0.3· 10− 4~ 1· 10− 4 cm 2/kg, for igneous rocks. A uniaxial tension causes the inversed effects. The irreversible effect is related to the soft isothermal remanent magnetization (j sr). An application and a release of a on a rock in the presence of a magnetic field (H) results in an increase of remanent magnetization in the case of both the longitudinal and transverse effect, where the transverse effect is a little smaller (by 10~ 25%) than the longitudinal one. Both the longitudinal and transverse compressions cause an irreversible demagnetization of the soft isothermal remanent magnetization. In this case also the transverse effect is a little smaller than the longitudinal one. Theoretically, these effects are due to the irreversible displacement of the 90° domain walls. For small values of compression (σ≲ 100 kg/cm 2) and magnetic fields (tH≲ 10 Oe), the effects can be approximately expressed by J 0 SR (σ∗)= CHσ∗, J 0⊥ SR (σ⊥∗)= 3 4 CHσ⊥∗, J SR (σ∗∗)= J 0 SR (1− ασ∗∗ H) and J⊥ SR (σ perp;∗∗)= J 0⊥ SR (1− 3ασ⊥∗∗ 4H), where σ∗ and σ∗∗ denote the irreversible effect of σ and C= 0.2· 10− 6~ 11· 10− 6 emu/Oe/kg/cm 2 and α= 0.02~ 0.1 Oe cm 2/kg for igneous rocks. A uniaxial tension causes the same effects. In the case of the earth's crust which has been repeatedly stressed in similar ways in the presence of the permanent geomagnetic field, the irreversible effects can be ignored compared with the reversible effects.