据一维正反演分析广域电磁法E-E_x方式的分辨能力

Analysis of resolution ability to E-E_x arrangement wide field electromagnetic method using 1-D modeling and inversion

为了研究广域电磁法E-Ex方式的分辨能力,编制广域电磁法一维正反演程序,对不同地电模型进行模拟和反演计算。正演模拟从层厚变化和电阻率变化2方面讨论。研究结果表明:随着中间薄层层厚的增大及围岩与薄层电阻率差异的增大,视电阻率曲线中异常与假极值越来越明显;当收发距为4 km时,对埋藏100 m深、与围岩电阻率10倍差异的薄层,厚度为4 m的低阻层可有效探测,厚度为33 m的高阻层可有效探测;当薄层埋藏深度为厚度2.5倍时,围岩电阻率为低阻层电阻率的2.0倍时才可以有效探测,高阻层电阻率为围岩电阻率4.0倍时可以有效探测,显然广域电磁法对低阻体的分辨率要高于对高阻体的分辨率;广域电磁法一维视电阻率反演结果与CSAMT采用卡尼亚电阻率的结果相比,广域电磁法的反演收敛过程快速、稳定,反演分辨能力比CSAMT的强。

The caculation results with different models were analyzed using one-dimesional modeling and inversion of wide field electromagnetic method to research the resolution ability,and the results caused by changes of thickness and resistivity to thin layer were studied.The results show that with the increase of the thickness of thin layer and difference between the surrounding rock and it,the curve abnormality and false extreme become more and more obvious,so the area with large lithology difference should be paid more attention to.The resolution of low-resistivity is superior to the high one,and dectecting depth of this method is decided by surface risistivity and frequency.In addition,for the layer buried in the depth of 100 m and with difference between the layer and surrounding rock equals 10 times,the layer with 4 m thick can be detected when its resistivity is lower than that of surrounding rock,and with 33 m when its resistivity is higher than that of surrounding one.For the layer buried in the same depth as above with 40 m thick,when resistivity of layer accounts for less than and equals 50% of the surrounding rock,it can be detected.However,when resistivity of layer is 4.0 times larger than that of the surrounding rock,it can be detected.Additionally,compared with CSAMT which applies the cagniard resistivity,the inversion processing of WFEM is faster and steadier.