Summary Interpretation of magnetotelluric (MT) data for three‐dimensional (3‐D) regional conductivity structures remains uncommon, and two‐dimensional (2‐D) models are often considered an adequate approach. In this paper we examine 2‐D interpretation of 3‐D data by considering the synthetic responses of a 3‐D structure chosen specifically to highlight the advantages and limitations of 2‐D interpretation. 2‐D models were obtained from inversion of the synthetic 3‐D data set with different conditions (noise and distortion) applied to the data. We demonstrate the importance of understanding galvanic distortion of the data and show how 2‐D inversion is improved when the regional data are corrected prior to modelling. When the 3‐D conductive structure is located below the profile, the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one‐half of a skin depth. In this case the use of only TM‐mode data determined better the horizontal extent of the 3‐D anomaly. When the profiles are located away from the 3‐D conductive structure the use of only TM‐mode data can imagine phantom conductive structures below the profile, in this case the use of both polarizations produced a better determination of the subsurface structures. It is important to note that the main structures are identified in all the cases considered here, although in some cases the large data misfit would cause scepticism about features of the models.

The study of electrical anisotropy in the Earth, defined as the electrical conductivity varying with orientation, has experienced important advances in the last years regarding the investigation of its origins, how to identify and model it, and how it can be related to other parameters, such as seismic and mechanical anisotropy. This paper aims to provide a theoretical background and to be a review of the current state of the art of electrical anisotropy using electromagnetic methods in the frequency domain, focusing mainly on magnetotellurics. The aspects that will be considered are the modelling of the electromagnetic fields with anisotropic structures, the analysis of their responses to identify these structures, and how to properly use these responses in inversion and interpretation. Also, an update on the most recent case studies involving anisotropy is provided.

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The definitive version is available at: La versione definitiva è disponibile alla Ortuño et al. AbstractThe remnants of a palaeo-basin sedimentary infill outcropping at the Aran valley (Central Axial Pyrenees) have been mapped and studied combining stratigraphical and biostratigraphycal analysis with an audiomagnetotelluric surveying. The basin was developed on top of a preexisting peneplain and was formed by its offset as a half-graben related to the North Maladeta fault activity. The basin had an elongated E-W lensoidal shape and was infilled by an, at least, ~100 m thick sequence of fluvio-palustrine deposits derived from a palaeo-Garona river. The detritic sequence is composed by three major facies assemblages: a basal member dominated by conglomerates; a medium member mainly composed of sandstones and siltstones; and an upper member characterized by a rhythmic sequence of sandstones-siltstones and lignite layers. The results derived from the palynological and carpological analyses have allowed to constrain the age of the basin infill as Late Miocene, and very likely Vallesian; to characterize the vegetation of the belt surrounding the basin as a mainly temperate to warm-temperate assemblage and to estimate the palaeoaltitude of the site between 900 and 1000 m.a.s.l. Specimens of the taxon Hippuris cf. parvicarpa Nikitin have been identified for the first time in a European palaeoflora. The palaeontological dating of the basin infill allows estimating a minimum Upper Miocene age for the peneplains preserved in the area. The significance of the origin, preservation, and uplift of these landforms in the core of the Pyrenean orogen is discussed in the lights of the results of this work.

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