Abstract:

The topography of geophysical exploration areas is often undulating and complex, and the traditional long-offset transient electromagnetic method (LOTEM) 3D forward modeling generally does not consider the influence of topography. However, the topographic factors seriously affect the reasonable interpretation of LOTEM data. The 3D forward modeling of LOTEM under real topography by the unstructured edge finite element method is developed in this study. Firstly, the 3D modeling software Blender is used to accurately build a complex geoelectric model, and the unstructured tetrahedral mesh is used to finely describe the real topography. Secondly, the dipole discrete field source processing technique is used to separate the long conductor source into several electric dipoles, and the second-order backward-Euler scheme is used to complete the time discretization. In this way, fast and high precision 3D forward modeling of LOTEM can be realized. The transient electromagnetic field response characteristics of the complex geological model under real topography conditions are simulated on the premise of guaranteed computational accuracy. The results demonstrate that due to the influence of topography, the LOTEM electromagnetic field response is severely “distorted”, which can weaken or even completely “cover up” the transient electromagnetic field response of underground targets, and it seriously affects the effective detection of underground targets.

Key words: long-offset transient electromagnetic method, topography, unstructured edge finite element method, Blender, second-order backward-Euler scheme, 3D forward modeling