Distortion of magnetotelluric data: its identification and removal

Introduction Distortion of regional electric fields by local structures is arguably the greatest bane in the application of the magnetotelluric method (MT), and one that has caused poor interpretations leading directly, within the broad geoscience community, to lower acceptance of MT than should be expected. Approaches and methods developed over the past two decades to recognize, analyze and remove these distortions have enabled significant advances in reliable imaging of the subsurface electrical conductivity structure using magnetotellurics. Without the advanced methods described in this chapter, MT would still yield unreliable models and interpretations. Accordingly, it is fitting that this book contains a significant chapter on distortion. Precise, robust interpretation of magnetotelluric data observed at a local site from electromagnetic fields induced regionally is fraught with difficulties, primarily because of the inability to model structure at the scale of the Earth's resistivity variations, from small-scale (smaller than the experimental design, i.e. frequencies too low or electrode line length too large) to large-scale (far larger than the experimental design). This problem is usually characterized as distortion of the regional fields by local effects (or, perversely, distortion of local fields by regional effects, as in the case of current channeling problems). Distortion of magnetotelluric data has no formal, precise definition, as it is subjective – one person's distortion can be another person's anomaly – leading to a lot of confusion in the literature. Essentially, it means that the observed electromagnetic fields include effects due to features that are outside the scope of the experiment or the interpretation – either too small or too large or of higher dimensionality – that distort the regional electromagnetic fields that one wishes to observe caused by the structure(s) of interest.