Pre polarizing and spin echo method are often used in NML tool, and the magnetic field used has been changed from primitive earth magnetic field to man made magnetic field.

• 核磁测井仪中多采用预极化方式或自旋回波法，所用的磁场已从最初的地磁场演变到现在所用的各种人工磁场，所用的测量方式要与采用的磁场设计相适应。