2-D Versus 3-D Magnetotelluric Data Interpretation

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.

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

“…Jones (1983), Wanamaker et al (1984), Berdichevsky et al (1998), Park & Mackie (2000), Ledo et al (2002) and Ledo (2005) studied the limitations of 2-D interpretation of 3-D MT data and made various recommendations, showing some situations where 2-D inversion of 3-D data can properly reproduce the geoelectrical subsurface when the content in the 2-D modes is appropriately chosen for inversion. Siripunvaraporn et al (2005b) suggest that if the data are 3-D in nature but collected along a profile rather than on a grid, the results from the 3-D inversion of the MT profile are superior to the results obtained by performing 2-D inversion of the same data, even with the far coarser mesh employed in 3-D as compared to 2-D inversion.…”

Paraguay‐Araguaia Belt Conductivity Anomaly: A fundamental tectonic boundary in South American Platform imaged by electromagnetic induction surveys

“…Our MT survey line was aligned perpendicular to a subsection of the PACA structure that is supposed to be much longer than it is wide, and relatively distant from its borders. Several numerical modeling studies [e.g., Wannamaker et al, 1984;Ledo, 2005] suggest that the response for a centrally located profile across an elongated 3-D body agrees with the TM response of a 2-D body with an identical cross-section, whereas the observed TE response and the vertical magnetic field are greatly depressed in comparison with the calculated 2-D model responses. Thus, to avoid spurious 3-D features on the 2-D resistivity-depth model, we have used the TM mode in the inversion to determine only the lateral extension and top of the conductive structure.…”

Dimensionally adaptive hp-finite element simulation and inversion of 2D magnetotelluric measurements

“…Moreover, sometimes it is possible to interpret 3D MT data as 2D (see, for instance, [26] and [27]). Traditional inversion techniques usually select one fixed dimension (the full 2D (or 3D) problem) for both, the forward simulations and the inversion.…”

“…Then, we solve a higher dimensional IP by employing the information of lower dimensional IPs. This idea relies upon the results obtained in [26] and [27], where authors show that measurements often exhibit a dimensionality signature that can be properly exploited when solving IPs.…”