P. Schmoldt, PhD - MTNet - DIAS

4. Distortion of magnetotelluric data
Fig. 4.11.: Simple models of (isotropic) subsurface dimensionality, representing typical cases observed in MT measurements.
Coloured blocks denote regions of different electric resistivity (the region representing the conductivity of free air on top each model
is implied but not shown here). Models in the top row, left to right: 1D (electric resistivity varies only with depth), 2D (lateral change
of resistivity along a vertical interface), 3D/1D (small-scale body imbedded in an otherwise homogeneous 1D body). Models in the
bottom row, left to right are 3D/2D (small-scale body in area of regional 2D structure), 3D (3D bodies dominate the responses and no
regional structure of lower dimensionality can be found).
4.2. Dimensionality
In MT, dimensionality of a body is usually defined by the number of directions in which
boundaries of the body are sensed. The simplest models, comprising only vertical variations
of electric resistivity, are referred to as 1D. Such models contain at least one horizontal
boundary, located at the air-ground interface for cases where the Earth is modelled
as a homogeneous halfspace, an no lateral changes of electric resistivity. Other 1D model
contain a multitude of parallel layers with different resistivities, in American literature
occasionally referred to as layer cake model. 2D models comprise resistivity changes in
vertical as well as in one horizontal direction, for which the x-axis of the coordinate system
is usually aligned with the lateral interfaces. The simplest 2D model contains two
blocks of different resistivity, with the contact zone parallel to one of the axis of the coordinate
systems (Fig. 3.4). In the 3D case, boundaries are detected vertically and in both
horizontal directions, resulting in a much more complicated structure of the MT response
(Fig. 4.2). Presently the interpretation of 3D MT data is intricate and simplification of
the 3D situation are often strived for in order to ease the interpretation. For certain cases,
measured data can be described by a superposition of bodies with different dimensionality,
e.g. a small-scale 3D body imbedded in a regionally 1D (3D/1D) or 2D (3D/2D) host
medium (see Figure 4.11 for an illustration of models with different dimensionality).
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