大地电磁电场远参考道法适用条件及应用效果

Applicable conditions and effects of remote reference method of magnetotelluric electric field

远参考道法是大地电磁法(MT)时间序列处理中行之有效的方法之一。一般认为磁场作为参考道的效果较好,而电场因易受地下结构影响,不宜作为远参考道。从实际资料处理出发,尝试利用电场远参考道对MT数据进行处理,发现对于常规MT数据,电场远参考对1 Hz以上的高频部分效果良好,低频部分效果相对较差。基于此,提出利用电场远参考信息处理高频数据的新思路。通过对音频大地电磁(AMT)数据的精细处理,对比采用不同远参考道的处理效果,验证了电场远参考数据在高频段(>1 Hz)的有效性。将此思路应用于AMT数据处理中,结果表明采用电场远参考数据与采用磁场远参考道的计算效果相差无几,尤其是对AMT“死频带”(1k~5k Hz)的数据处理结果有很好的改善作用。由于AMT数据采集一般都是多个测点共用一个磁道,电道数量远大于磁道,因此,电场远参考处理能大大提高AMT数据处理效果,可作为常规处理方法推广应用。通过对天然源电磁场的传播规律和MT维性参数的讨论分析,证明了高频电场受地下结构影响较小是电场远参考数据有效的内在原因。同时,该研究也给远参考站的选址指明了方向,即远参考站应该选在地电构造简单的区域,以保证与测区同源的天然电磁场信号受地下电性结构的影响尽量小。

The remote reference method is effective for processing magnetotelluric(MT)time series data.It is generally believed that magnetic fields yield better effects as reference channels,but electric fields are not suitable as remote reference channels due to their susceptibility to underground structures.Based on actual data processing,this paper explores the utilization of electric fields as reference channels for processing MT data and finds that they per-form well in the high-frequency range above 1 Hz but poorly in the low-frequency range.Then,it puts forward a new approach that employs electric fields as reference channels for processing high-frequency data.Fine processing of audio magnetotelluric(AMT)data confirms the validity of electric fields as reference channels in the high-fre-quency range(>1 Hz).Application of this approach to AMT data processing indicates that the results obtained by adopting electric fields as reference channels are comparable to those by utilizing magnetic fields as reference chan-nels,with sound improvement in data processing of AMT“dead band”(1k~5k Hz).AMT data collection typically involves multiple measurement points sharing a set of magnetic channels,and the electric channel number greatly ex-ceeds that of magnetic channels.Thus,processing of remote reference channel electric fields can significantly im-prove AMT data processing results and can be popularized as a conventional processing method.Analysis of natural source electromagnetic fields and MT dimensionality parameters shows that the limited influence of high-frequency electric fields on underground structures is the underlying reason for the remote reference channel data effectiveness of electric fields.This research also provides guidance for the location selection of remote reference stations,suggest-ing that the stations should be situated in areas with simple geoelectric structures to minimize interference of under-ground electrical structures on natural electromagnetic field signals from the same source as the survey area.