Summary
Rough topography encountered when conducting controlled-source electromagnetic (CSEM) surveys significantly distorts electromagnetic (EM) fields. However, there have been few studies on the accurate consideration of topography into modeling and inversion algorithms. We have developed a three-dimensional (3D) CSEM inversion algorithm based on the forward modeling algorithm which rigorously incorporates topography based on an edge-based finite element (EFEM) method. We focused on the strict reflection of topographic information to the modeling algorithm and the accuracy of the algorithm was verified through comparing with the semi-analytic solutions. In numerical experiments, we successfully performed the inversion of data obtained from a ridge and valley model. This implies that real data acquired at rough terrain can be handled reasonably by using the inversion algorithm developed in this study.
Introduction
Controlled-source electromagnetic (CSEM) method has been used to image the distribution of physical properties, such as electrical resistivity. In the mineral exploration which is one of the main applications of CSEM method, it is important to consider rough topography because many surveys are conducted in mountainous areas. Katayama and Zhdanov (2004) carried out inversion of real data for exploration of kimberlite using a topographic correction based on the integral equation (IE) method. However, the correction could not be applied to data acquired rough terrain because it considered the effect of topography as the linear part of electromagnetic (EM) fields. These problems could be handled by incorporating topography into the modeling and inversion. Sasaki and Nakazato (2003) rigorously incorporated topography into their algorithms based on a staggered-grid finite difference method (FDM). They showed that strict treatment of topography is essential to avoid the misinterpretation due to the distortion caused by topographic effects. However, their algorithms had the limitation in simulating topography precisely because of intrinsic limitation of FDM. Although this limitation had almost no effect on their study because they used airborne EM method, it can cause significant distortion o
