Abstract: The upper mantle electrical conductivity structure of the Pacific back-arc beneath northeastern China was investigated by the long-period magnetotelluric (Network-MT) at Changchun city and Sanchahe site. Time series data of the long baseline electric field were collected by using land telephone line cables and three component magneic field variations were recorded at Changchun observatory by digital flux magnetometer. Network-MT responeses have been obtained up to 10⁵～10⁶s, which were inverted to one-dimensional model down to depths of about 1 000km. Resulted models show that: (1) The discontinuity were detected at 100～200km, and in mantle transition zone for around 300 km and 500～600 km depth, respectively. (2) The discontinuity appears at 850km for the lines in Changchun. (3) The conductivity almost monotonically increases with depth in the smooth conductivity models. (4) The conductivity reaches 1 S/m at 600 km depth in Sanchahe sites and at 850 km in Changchun sites, respectively. The above results indicate the discontinuity and the heterogeneity of the mantle and can be compared with the results obtained by petrology studies.

Key words: Network-MT, Upper Mantle, Conductivity

摘要： 使用长周期大地电磁测深网(Network-MT)等技术,在长春和三岔河等地对太平洋板块弧后的上地幔电导率结构进行了探测。用陆地电话线或电缆接收长基线的电场,由设置在长春地磁台的三分量磁通门磁力仪记录磁场变化,得到Network-MT等观测资料的最长周期可达10⁵～10⁶s,反演结果的深度可达约1000km。1)在两个区的100～200,300,500～600km深度附近存在电性间断面;2)长春测区的850km深度存在电性间断面;3)平滑模型的电导率随深度增大而单调增加;4)三岔河测区600km深度、长春测区850km深度的电导率达到1S/m。上述结果表明,地幔内存在不连续界面和不均匀性,有些结果可与岩石学的研究结果对比。

关键词: 大地电磁网, 上地幔, 电导率