Abstract: Natural ultra-low frequency (ULF) electromagnetic field signal was observed at Beijing 119 geothermic drill at different period of time and orientation by using the BD-6 ULF electromagnetic wave detect instrument developed by Peking University. Mean voltage-depth curves and apparent resistivity-depth curves of different period of time and orientation were obtained by ULF data processing and reversing. Solar radiation can influence the signal intensity of natural ULF electromagnetic field in some way according to the result of analysis and interpretation. However， it has a little impact on the stability of natural ULF electromagnetic field. The maximum of ULF signal is accord with the magnetic north direction， and it becomes weaker in condition that the angle between the direction of instrument detector and the magnetic north increasing. The observation indicates that the interpretations of difference apparent resistivity-depth curves are obviously better than the mean voltage-depth curves.

Key words: natural electromagnetic field, ultra-low frequency, extreme-low frequency, electromagnetic wave, stability

摘要： 利用北京大学研制的BD-6型超低频电磁探测仪，在北京地热119井按不同时段和方位对天然场源超低频电磁信号进行观测，经数据处理得到不同时段和不同方位的电压-深度曲线和差分视电阻率-深度曲线，经分析与解译发现：太阳辐射在一定程度上影响天然场源超低频电磁信号的强度，但太阳辐射对其稳定性影响程度不大；天然源超低频电磁信号强度存在明显的方向性，磁北方向信号最强，随仪器探头方向与磁北方向夹角的增大信号强度降低；实验表明超低频电磁信号的差分视电阻率-深度法明显优于电压-深度法的反演与解译精度。

关键词: 天然电磁场, 超低频, 极低频, 电磁波, 稳定性