Abstract: Laochang Ag-Pb-Zn polymetallic deposit in Lanchang,Yunnan Province is a typical deposit in the "Sanjiang" area of southwest China,which is facing a resource crisis recently. The audio-frequency magnetotellurics(AMT) has been carried out in the margin exploration of this deposit,but the frequency distortion of some measurement points is caused by the interference of high voltage lines. To deal with this problem,a method is proposed for suppressing industrial frequency noise based on isolated frequency point extraction and magnetic field data reconstruction technology. By setting a limit to distinguish all measurement points,it can quickly distinguish the measurement points with different degrees of industrial frequency interference; then the data extraction is carried out by avoiding the interfered 50 Hz and multiples frequency band data;For the measurement points with strong industrial frequency interference that cannot be avoided,the magnetic field data reconstruction is carried out by using the inverse distance weighted for the strong interfered measurement points using the data of their nearby measurement points. The AMT data of the Laochang Ag-Pb-Zn polymetallic deposit in Lancang was processed by the above method,which effectively avoided the 50 Hz and odd times harmonic interference,achieved a relatively good noise suppression effect,and circled two low-resistance anomalous areas with ore finding potential,which provided a direction for ore finding exploration at the side of the mine. The method also provides a new solution to the industrial frequency interference in geophysical exploration work.

Key words: audio-frequency magnetotellurics,power interference,isolated frequency extraction,magnetic field reconstruction, Ag-Pb-Zn polymetallic deposit