复杂地质条件下浅海区电性源瞬变电磁法三维响应特征  

Three-Dimensional Response Characteristics of Transient Electromagnetic Method with Electrical Sources in Shallow Sea Areas Under Complex Geological Conditions

作　　者：雷松达 王显祥 刘遂明 Lei Songda;Wang Xianxiang;Liu Suiming(School of Geophysics and Measurement-Control Technology,East China University of Technology,Nanchang 330013,China)

机构地区：[1]东华理工大学地球物理与测控技术学院,南昌330013

出　　处：《吉林大学学报（地球科学版）》2024年第3期1016-1030,共15页Journal of Jilin University：Earth Science Edition

基　　金：国家自然科学基金项目(42274185,41964006)。

摘　　要：与频域电磁法相比,瞬变电磁法可有效区分空气波和海底电磁响应,在浅海油气藏探测中具有良好的应用前景。近海海域地质条件一般较为复杂,普遍具有强切割地形及复杂构造,使电磁场响应特征变得异常复杂,给数据解释工作带来极大困难。本文基于时域有限元法,采用非结构化网格对复杂地质模型进行剖分,通过构建时域有限元方程并结合偶极子离散的长导线源近似技术以及后退欧拉离散技术,实现了复杂地质条件下浅海区电性源瞬变电磁三维正演模拟。在验证算法精度后,通过复杂地质模型的三维正演,分析了不同海水深度对空气波与海底油气藏目标体的影响,在此基础上,进一步分析了不同围岩电阻率以及海底地形对油气藏目标体分辨率的影响,结果表明:在浅海条件下,脉冲响应受空气波影响较大,阶跃响应受空气波影响较小,随着深度增大空气波影响变小,对油气藏的分辨率也降低;围岩电阻率及海底复杂地形对电性源瞬变电磁影响严重。Compared with the frequency domain electromagnetic method,the transient electromagnetic method can effectively distinguish between air waves and seafloor electromagnetic response,which has a good application prospect in the shallow offshore oil and gas reservoir detection.The geological conditions of offshore area are generally more complicated,and there are strong cut terrain and complex structures,making the electromagnetic field response characteristics become extremely complex.These problems will bring great difficulties to the data interpretation work.In this paper,based on the time-domain finite element method(FEM),the unstructured mesh is used to dissect the complex geological model.By constructing the time-domain finite element equations and combining the long wire source approximation technique with dipole discretization and the backward Euler discretization technique,the three-dimensional forward modeling simulation of electromagnetic transient with electrical source in shallow sea area under complex geological conditions is realized.After verifying the accuracy of the algorithm,the influence of different seawater depths on air waves and the target body of seafloor oil and gas reservoir is analyzed through the three-dimensional forward modeling of the complex geological model.Based on these results,the influence of different surrounding rock resistivity and seafloor topography on the resolution of the target body of oil and gas reservoir is further analyzed.The results show that under shallow sea conditions,the pulse response is greatly affected by air waves,while the step response is less affected by air waves.As the depth increases,the influence of air waves decreases,and the resolution of oil and gas reservoirs also decreases.The electrical resistivity of surrounding rocks and the complex seafloor topopraphy have a serious impact on the transient electromagnetic properties of electrical sources.

关 键 词：瞬变电磁法 浅海 非结构化网格 三维正演 时域有限元 

分 类 号：P631.3[天文地球—地质矿产勘探]