On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method
On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method
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DOI：
10.1029/jb086ib12p11609
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发表时间：
1981-12
影响因子：
--
通讯作者：
R. N. Edwards;L. Law;J. Delaurier
中科院分区：
文献类型：
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作者：
R. N. Edwards;L. Law;J. Delaurier
大地电磁测深显示，在太平洋下面的深度超过60公里的导电带，但不能解决这个区域以上的岩石圈的电阻率。可控源电法可进一步提高分辨率。其中最简单的是电流技术。偶极-偶极电阻率测深法并不适用，因为要获得电阻率值，需要偶极间距达数千公里。一个可行的替代办法是在洋底测量从海面延伸到海底的垂直双极源的磁场。磁力计发射机之间的距离仅为几公里，就足以确定海洋下半个空间的电阻率。可以构造类似于电阻率法的测深曲线来求解层状岩石圈的电阻率。所构造的曲线在交变频率下是有效的，该交变频率与不是在海洋中而是在岩石圈中的趋肤频率相比是小的。穿透深度大约是发射器-接收器间隔的一半。对于合理的岩石圈折射率和分离度，磁场振幅在皮特斯拉的范围内，达到双极长度的10倍。现代仪器经过改装，可用于海底，可在20公里范围内探测到这种信号，其频率约为0.02赫兹，平均数小时。
Magnetotelluric sounding reveals a conductive zone beneath the Pacific Ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone. Further resolution can be obtained by controlled source electrical methods. The simplest of these are the galvanic techniques. Dipole-dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity. A viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor. Magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean. Sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere. The curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere. The depth of penetration is of the order of half the transmitter-receiver separation. Magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole. Modern instrumentation, modified for the ocean floor, can detect such signals at a range of 20 km at a frequency of about 0.02 Hz averaged over several hours.