2-D Versus 3-D Magnetotelluric Data Interpretation

Abstract

Abstract. In recent years, the number of publications dealing with the mathematical and physical 3-D aspects of the magnetotelluric method has increased drastically. However, field experiments on a grid are often impractical and surveys are frequently restricted to single or widely separated profiles. So, in many cases we find ourselves with the following question: is the applicability of the 2-D hypothesis valid to extract geoelectric and geological information from real 3-D environments? The aim of this paper is to explore a few instructive but general situations to understand the basics of a 2-D interpretation of 3-D magnetotelluric data and to determine which data subset (TE-mode or TM-mode) is best for obtaining the electrical conductivity distribution of the subsurface using 2-D techniques. A review of the mathematical and physical fundamentals of the electromagnetic fields generated by a simple 3-D structure allows us to prioritise the choice of modes in a 2-D interpretation of responses influenced by 3-D structures. This analysis is corroborated by numerical results from synthetic models and by real data acquired by other authors. One important result of this analysis is that the mode most unaffected by 3-D effects depends on the position of the 3-D structure with respect to the regional 2-D strike direction. When the 3-D body is normal to the regional strike, the TE-mode is affected mainly by galvanic effects, while the TM-mode is affected by galvanic and inductive effects. In this case, a 2-D interpretation of the TM-mode is prone to error. When the 3-D body is parallel to the regional 2-D strike the TE-mode is affected by galvanic and inductive effects and the TM-mode is affected mainly by galvanic effects, making it more suitable for 2-D interpretation. In general, a wise 2-D interpretation of 3-D magnetotelluric data can be a guide to a reasonable geological interpretation.

Electrical structure of the central Cascadia subduction zone: The EMSLAB Lincoln Line revisited

“…The tipper is included where available, which is most of the land stations. Caution over use of the TE mode arises over concerns of the impacts of potential 3D structure on a 2D inversion: TM mode data are less sensitive to finite strike effects (e.g., Wannamaker, 1999;Ledo, 2005). Inversions were run using the 2D inversion of Rodi and Mackie (2001) as implemented within the Winglink software package.…”

Integrating Hydrogeological and Geophysical Methods for the Characterization of a Deltaic Aquifer System

“…The result of this analysis shows that our data are mainly 3D. However, if a preferential direction can be determined, the 2D inversion of the impedance tensor determinant makes a reasonable approximation to 3D structures in many cases (Pedersen and Engels 2005;Ledo 2006), which is the case for this deltaic system. Based on the fluvio-deltaic facies distribution, transitional boundaries and main stratigraphical continuity tendencies, it can be observed that profiles PS1 and PS4 follow the actual river channel and its perpendicular direction, respectively, and therefore we have created 2D resistivity models using the determinant of the impedance tensor mode.…”

“…5). This parameter is invariant under rotation and makes it suitable for qualitative interpretation (Vozoff 1972;Ledo 2006). Responses over 3D structures may show complex behaviour for individual impedancetensor elements, whereas much simpler and less distorted behavior of the determinant impedance usually can be well explained by 2D models (Pedersen and Engels 2005).…”

Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users

“…2-D inversions are then performed to yield 2-D cross-sectional models for profile interpretation. The dangers of 2-D inversions being influenced by 3-D structures are demonstrated with synthetic data in Siripunvaraporn et al (2005b) and Ledo (2006) and with real data in Simpson and Bahr (2005) and Newman et al (2003). All studies indicate that if the data contains 3-D structures, 2-D inversion can mislead an interpretation.…”