We study the influence of the ocean effect on the C-response from geomagnetic sounding in coasts of China with the Guangzhou (GZH) station as an example.Using the staggered-grid finite difference method,the oceanic effect is forward modeled in spherical coordinates.Assuming a ring current source in the magnetosphere,a "surface 3-D conductivity +1-D layer-like background model is adopted for the modeling.The results show that the coastal C-response in China is affected by the ocean effect obviously.In space,the single-period C-response changes from a constant into dense gradients parallel to the coastline.In the direction perpendicular to the coast,the ocean effect reduces slowly toward land interiors,reaching the line Harbin—Guiyang.We correct the coefficients of the ocean effect by the ratio method.Taking the GZH station as an example,we find maximum period of the influence on C-response is about 20 days.The correction result with an average 1-D model of China is more reasonable than the global 1-D mantle conductivity model as background model.The C-response affected by the ocean effect is particularly large at the short period,so direct inversion cannot fit the geomagnetic data.On the contrary,the inversion of corrected C-response can fit the data well and yield a fine conductance model.The model shows that the resistivity of upper mantle beneath the GZH is higher than the China's average model by about one order of magnitude.It permits to infer that the mantle transition below the south of southeastern China may be in a relatively cold environment.This is probably a piece of electrical evidence that the Philippine sea plate is westward subducted and stagnant in the mantle beneath the South China land.%以广州台站为例,研究海洋效应对中国沿海地磁观测C-响应的影响.海洋效应的三维正演模拟采用球坐标系下交错网格有限差分方法,假设磁层环形电流源,正演电阻率结构模型采用“地表3-D电导+1-D层状背景”复合模型.数值模拟结果表明,中国地区沿海C-响应受海洋效应影响明显.在空间上,沿海岸线方向,受海洋效应影响,单周期的C-响应由无海洋效应的常值变形为平行于海岸线的等值线密集梯度带;在垂直海岸方向,海洋效应影响向内陆减小,其影响可达哈尔滨—贵阳一线.海洋效应影响采用比值法进行校正,以广州台站为例,在比值曲线上发现海洋效应对C-响应的影响最大周期可达20天左右,并且就中国沿海而言,相对全球平均一维模型,利用中国地区平均一维电导率模型作为背景模型的海洋效应校正结果更加合理.进一步对广州台站海洋效应校正前后的C-响应进行了1-D反演,由于校正前的C-响应在小周期时受海洋效应特别大,直接反演无法拟合数据;但校正后反演拟合明显变好,得到的1-D导电模型表明广州地区上地幔及地幔转换带的电阻率比中国平均电阻率高约一个量级,推测中国华南地区南部的地幔转换带可能处于相对冷的环境,该模型可能成为菲律宾海板块西向俯冲并滞留到华南大陆下方地幔转换带的电性证据.
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