斜阶跃电流激励下多匝小回线瞬变电磁场延时特征

采用Duhamel积分对斜阶跃电流激励的瞬变电磁场进行直接计算，应用叠加原理得到了多匝小回线全域瞬变电磁响应.研究了多匝小回线的早、晚延时临界条件以及等磁矩单匝大回线与多匝小回线的延时和关断时间特征，分析了电阻率、厚度等参数变化时斜阶跃电流对层状模型瞬变响应的影响特征，最后采用实测数据验证了关断效应的校正效果.结果显示，回线边长越小或表层电阻率越大，瞬变响应越早离开早期并进入晚期.多匝小回线的关断时间远大于等磁矩的单匝大回线，但由前者激发的瞬变响应进入晚期的时间却大幅提前，更有利于晚期条件下各种理论公式的使用.斜阶跃影响校正结果显示，增加多匝小回线的匝数可以扩展反演深度的范围，减少匝数则可以提高反演电阻率的精确性；实测数据处理结果显示校正方法产生了一定的效果.本研究补充了多匝小回线瞬变电磁理论内容，研究结果可以为复杂环境中小尺寸多匝线圈的工作参数选择提供参考.

The decay characteristics of transient electromagnetic fields stimulated by ramp step current in multi-turn small coil

Applying Duhamel integral and superposition principle, full-time transient electromagnetic response with a ramp current excitation for small multi-turn coils was calculated on a homogeneous half space and a layered medium. The critical conditions of the early and late stages and the decay characteristics during turn-off time for multi-turn small coils were studied in this paper. By varying the resistivity and thickness of the layered medium, the transient response stimulated by ramp current was also analyzed. At the same time, the measured data was adopted to testify the effect on turn-off time correction. The results indicate that, much shorter the loop size is or more higher the near-surface resistivity is, more earlier the transient response leaves the early stage and enters the late stage. Long turn-off time for multi-turn coils compared with single loop, but much earlier critical time entering the late stage, which could be considered as an advantage for using the late-stage formulas. The inversion result after turn-off time correction demonstrates that it has a wide depth range for more turns, while the inverse resistivity is accorded with the model well for less turns. The correction on measured data shows a good effect. The study supplements the theoretical research contents for multi-turn coils TEM, and also provides one option to use small multi-turn coils device at complicated earth surface.