Using 2-D techniques interpret 3-D magnetotelluric data
Thong, Duy Kieu
Abstract
In recent years, 3-D magnetotelluric techniques are being used. However, there are sometimes impractical in using 3-D techniques, both field experiments and computational time. In such cases, the important question is how can we valid apply the 2-D hypothesis valid to extract geoelectric and geological information from real 3-D environments? The aim of this presentation 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.
The fundamentals of the mathematics and physics of the electromagnetic fields generated by a simple 3-D structure are reviewed, and allows us to priorities the choice of modes in a 2-D interpretation of responses influenced by 3-D structures. The numerical models analysis shows us how the sensitivity of the TM mode and TE mode, as well as their robustness to the 3-D structures. Moreover, one important result of this analysis is that the most unaffected by 3-D effect depend on the position of 3-D structure with respect to the regional 2-D strike direction.
According to the ?above fundamentals, using 2-D inversion for 3-D magnetotelluric data, we know that the effects of finite strike are not significant when the 3-D conductive structure is located below the profile and the structure has a strike extent greater than about one-half of a skin depth. Joint 2-D inversion of both the TE- and TM-mode responses resolved acceptably the top and location of the conductive structure. Inversion of TM-mode data only gave superior determination of the horizontal extent of the 3-D anomaly, although the deeper 2-D structures were not well imaged. However, when the profiles are located off the conductive 3-D body, TM-only 2-D inversion can image phantom conductive structures that are laterally off the profile. It is important to note that all the inversion models obtained resemble the main characteristics of the true model. Thus, although the data are 3-D, we obtained the first-order structures with 2-D techniques.
References:
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