地面瞬变电磁法对低阻薄层的分辨能力

Resolution capability of ground-based transient electromagnetic method for low-resistivity thin layer

瞬变电磁法现阶段的资料处理解释仍以一维反演为主,因此分析瞬变电磁法对薄层的分辨能力对于更好地发挥瞬变电磁法优势仍具有重要意义。采用基于Occam算法的一维反演,以100 m深度内10 m层厚的低阻薄层为例,开展了地面回线源瞬变电磁法对低阻薄层分辨能力的定性和定量研究。数值结果表明:低阻薄层电阻率与背景电阻率差异越大,薄层反演电阻率和层厚越接近于真实值;反演电阻率极小值位置始终对应于薄层位置。低阻薄层电阻率与背景电阻率比值为1∶5,1∶10,1∶20,背景电阻率小于100Ω·m时,瞬变电磁法能够良好探测10 m薄层的最大埋深依次为40,50,80 m;背景电阻率为200Ω·m时,上述3个最大埋深依次为30,30,60 m。上述研究成果有助于优化瞬变电磁法野外工作参数设计和提高资料处理解释精度。

At present 1D inversion is commonly used for data processing and interpretation of transient electromagnetic(TEM)method.It is important to study the resolution-capability for the thin layer to take better advantage of ground-based loop-source TEM method.In this study,we employ 1D Occam′s inversion method to qualitatively and quantitatively study the resolution capability for a thin layer.The thin layer has low resistivity,and its thickness is 10 m.The buried depth ranges from 10 to 100 m with 10 m intervals.The numerical modeling results show that the inverted resistivity and thickness will more coincide with the true values of the thin layer if the differences of resistivity between the thin layer and background medium increase.The depth of the inverted minimum resistivity agrees with the position of the low-resistivity thin layer.For background resistivity less than 100Ω·m,the maximum depths are 40,50 and 80 m for ratios of 1∶5,1∶10 and 1∶20 between the resistivities of the thin layer and the background medium,respectively.The maximum depth indicates the one within which the thin layer can be well detected by TEM methods.For the background resistivity of 200Ω·m,the corresponding maximum depths are 30,30 and 60 m.This study will help to optimize the field-work parameter designing and to improve the accuracy of data processing and interpretation.