深拖式频率域海洋可控源电磁法的三维反演研究

Three-Dimensional Inversion Study of Deep-Towed Frequency-Domain Marine Controlled-Source Electromagnetic Data

摘要: 海洋可控源电磁法是一种有效探测海底电性结构的地球物理方法，在油气、可燃冰及深海矿产勘探中具有重要应用。传统的海洋电磁勘探常采用单船拖源——海底接收器接收的方式进行观测，存在施工组织复杂、作业成本高等问题。本文针对深拖式频率域海洋可控源电磁法的观测方式，开展了三维反演的系统研究，以实现对海底电性分布的精确成像。首先介绍正演理论，包括电磁场的控制方程、单元分析；随后讨论反演理论中的目标函数、正则化策略及求解方法，最后通过数值算例验证所提方法的有效性。反演采用近似高斯牛顿法，并结合正则化约束与步长搜索策略，以提高大规模三维模型参数求解的稳定性与效率。针对布设的收发距分布特点，本文在网格细化进行了设置，保证了反演在有限观测条件下的收敛性和分辨率。数值试验表明，该方法能够有效识别异常体的空间分布，具有较好的稳健性。为低成本、高效率的深拖式海洋可控源电磁勘探以及深海资源探测与海底构造研究提供一定参考。

Abstract: Marine controlled-source electromagnetic (MCSEM) surveying is an effective geophysical method for investigating the electrical structure of the seafloor and has been widely applied in the exploration of hydrocarbons, gas hydrates, and deep-sea mineral resources. Conventional marine electromagnetic surveys are commonly conducted using a single-vessel towed source combined with seafloor receivers, a configuration that is often associated with complex operational logistics and high acquisition costs. In this study, a systematic investigation of three-dimensional inversion is carried out for a deep-towed frequency-domain marine CSEM acquisition configuration, with the aim of achieving accurate imaging of subseafloor electrical resistivity distributions. The forward modeling framework is first introduced, including the governing equations of the electromagnetic field and element-based discretization. Subsequently, the inversion methodology is discussed, covering the formulation of the objective function, regularization strategies, and numerical solution approaches. The effectiveness of the proposed method is finally demonstrated through a series of numerical experiments. The inversion is implemented using an approximate Gauss-Newton scheme in combination with regularization constraints and a line-search strategy, thereby improving the stability and computational efficiency of parameter estimation for large-scale three-dimensional models. In consideration of the characteristics of the deployed source-receiver offset distribution, mesh refinement strategies are employed to ensure inversion convergence and adequate resolution under limited observational conditions. Numerical experiments indicate that the proposed approach can effectively delineate the spatial distribution of subsurface anomalies and exhibits good robustness. This study provides a useful reference for low-cost, high-efficiency deep-towed marine CSEM surveys, as well as for deep-sea resource exploration and investigations of seafloor structures.