大地电磁相位超象限现象的各向异性模型研究——以上下结构为例

Anisotropic model study for the phase roll out of quadrant data in magnetotellurics:with examples of upper-lower structure

大地电磁观测数据中的相位超象限现象可以由不同的电性结构产生.本文在已实现大地电磁三维任意各向异性有限差分正演的基础上,以具有上下结构关系的三维各向异性模型为例,分析各向异性体的规模以及参数变化对阻抗相位的影响.在下部各向异性体规模明显大于上部各向异性体或表现为层状特征的情况下,上部各向异性体在两个水平方向上的尺度差异较大,可以看作准二维体时容易发生相位超象限;当上部各向异性体在两个水平方向上尺度相近表现为规则三维体时,要产生相位超象限的现象则需要各向异性体具有更高的各向异性比.在同等条件下,增加各向异性体的各向异性比更容易发生相位超象限;而各向异性走向方位角的变化将直接影响到发生相位超象限的范围.对于准二维模型引起相位超象限的条件,沿用二维模型的近似解析分析方法,进一步构建了基于各向异性体的电导率、背景电导率以及各向异性走向角的相位超象限指标函数,从而更加直观地解释在二维或准二维条件下发生相位超象限现象的模型参数特征.

The PROQ (Phase Roll out of Quadrant) phenomenon in magnetotelluric observed data may be caused by different electrical resistivity structure. In this paper, based on the three-dimensional finite-difference forward modeling of arbitrary anisotropic models, we analyzed how the scales and anisotropic parameters of anomalous bodies influence impedance phases with the examples of upper-lower structure. In all the models, the lower anomalous body is apparently larger than the upper one in scale or in a layer. If the upper anomalous body has a great difference in its horizontal length and can be treated as quasi two-dimensional structure, the model is easier to produce PROQ phenomenon. If the upper anomalous body shows similar length in both horizontal directions and can be considered as regular three-dimensional structure, it needs higher anisotropic ratio in anisotropic bodies to generate PROQ effect. Under the same conditions, increasing anisotropic ratio will be prone to PROQ;variations in anisotropic strike will influence the range of PROQ areas directly. We adopted and extended the approximately analytical analysis of PROQ in two-dimensional conditions, and further built an indicator function including the anisotropic conductivity, anisotropic strike, and host conductivity to interpret PROQ phenomenon in quasi two-dimensional or two-dimensional cases intuitively.