相位超象限大地电磁观测数据的模型研究——以河西走廊北侧为例

Model for phase rolling out of quadrant magnetotelluric data——an example from north to the Hexi corridor

在大地电磁实测数据中阻抗张量次对角元素的相位值超出正常象限(一、三或二、四象限)的现象称为相位超象限.这种现象无法利用各向同性介质的一、二维反演来解释,而与特定形态的三维各向同性介质有关的电流槽效应有可能是引起相位超象限现象的原因之一.本文首先重建了四个能引起相位超象限的三维各向同性介质理论模型,对模型结构和响应特征进行了总结.随后结合麦克斯韦方程组介绍了电流槽效应产生的原理,并通过近似解析公式分析了电流槽效应对电、磁场的影响.最后,我们针对最近几年来在青藏高原北缘观测到的相位超象限大地电磁测深数据,重点对河西走廊北侧的三个连续相位异常数据进行了三维反演研究,反演模型能够很好地拟合实测的相位超象限数据.通过对模型中存在于上地壳的不同规模高导异常结构进行三维正演检验,最终确定位于测点北东方向的巴丹吉林沙漠构成了区域高导体,它与紧邻测点东侧的局部高导体相连,在平面上形成了"L"型的高导构造,是引起河西走廊北侧相位超象限大地电磁观测数据的主要原因.除此之外,紧邻测点西侧的孤立高导体也会对相位异常的分布范围产生影响.

In observed magnetotelluric data,phases of off-diagonal impedance tensors may exceed the normal quadrants（ as the first,third or second,forth quadrants）,this phenomenon is called Phase Rolling Out of Quadrant（ PROQ）. The PROQ data cannot be explained with one-dimensional or two-dimensional isotropic methods. The current channeling, which is caused by complex three-dimensional（ 3-D） isotropic media,is one explanation for the PROQ data. In this study,we reconstructed four synthetic 3-D isotropic models according to former studies and analyzed their structure features and response features. Then we introduced the theory of the current channeling combining with the Maxwell equations,and analyzed the influence of current channeling on electric and magnetic fields with approximate analytical formulas.Finally,3-D isotropic inversions were applied to three sequential magnetotelluric sites with PROQ at north to the Hexi Corridor,the responses of the final models fit the observed PROQ data very well.With 3-D forward modeling,we tested the contribution of the highconductivity anomalies in the upper crust to the PROQ data. The regional（ large） conductor northeast to the PROQ MT sites is corresponding to the Badain Jaran Desert, it connects a local（ small） conductor on the east side of the PROQ MT sites; these two conductors,forming a L-shaped high-conductivity structure in plane view,are the main contributors to the PROQ data north to the Hexi Corridor. Beside that,there is another isolated conductor west to the PROQ sites that can influence the distribution of abnormal phases.