电磁观测在地震和火山等自然灾害预测方面起着极其重要的作用。要充分利用电磁信号进行地震监测，就必须弄清电磁场的背景变化规律，在观测到的电磁信号中才能更好地识别与地震有关的异常信息。本文利用我国极低频台站观测到的天然场电磁数据，对极低频（0.1～800Hz）电磁场随季节和纬度的背景变化进行了初步分析，并以云南通海台观测到的震前电磁异常为例，阐述一些震前异常特征。今后还需在电磁场背景变化具体数值分析，电磁异常与震例拟合关系方面进行大量的统计工作。

Electromagnetic observation plays a vital role in natural disasters prediction such as earthquakes and volcanism. In order to take full advantage of electromagnetic (EM) signals in seismic monitoring, the background variation of the EM field should be certified firstly to identify earthquake-related anomalies in the observed EM signals. In this paper we use the natural field EM data observed in extremely low frequency (ELF) monitoring stations to conduct a preliminary analysis of the background that changes with season and latitude in frequency band 0.1-800Hz. And take the possible earthquake-related EM anomaly observed in Yunnan Tonghai station for example to expound some specific characteristics before earthquakes. In the future, researches still need to be done on the specific numerical analysis of EM field background change, and the relations between EM anomaly and the earthquake information.