Page 83 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 59

r 3
r r a
r 1 1
r 2
r 2
r 3
Frequency
Figure 2.7 MT-apparent resistivity response for a three-layer model.

Dimensionality analysis of the impedance tensor has been proved to be highly
important before multidimensional modeling process, given that many 3D envi-
ronments have been approached with 2D models, which is not always satisfactory.
In a stratified medium, the 1D case, resistivity changes only with depth and the
impedance tensor is independent of the measurement orientation of the field
components. The two polarization curves xy or yx are the same.
In the 2D case (Figure 2.8), which represents the most commonly assumed
situation for MT data interpretation, geoelectrical changes occur with depth as
well as in a direction perpendicular to the electrical/geological strike direction.
Two different polarization modes can be defined with respect to the geological
strike direction: The TE (transverse electric) mode is defined when the horizontal
component of the electric field E is parallel to the strike direction and the horizontal
magnetic field H is perpendicular. Conversely, the TM (transverse magnetic)
mode is defined when the horizontal magnetic field H is parallel to the strike
direction and E is perpendicular. When measurements are not performed along the
electrical strike direction, the latter can be retrieved by the so-called decomposition
analysis using several methods (e.g., Strike, Phase Tensor, and WALDIM) and by
trigonometric rotation of the TE and TM curves.
The 3D case represents the most general type of geoelectrical structure where
resistivity changes in all directions and the impedance tensor contains all the
horizontal electric and magnetic field components independent of the measure-
ment direction. In this case a strike direction cannot be defined. In practice, due
to computational or budget limitations, many 3D environments are investigated
using 2D profiles making it impossible to compute a 3D model. Since most of
these 2D profiles include 3D effects that can lead to misinterpretation, the use
of the determinant of the impedance tensor was proposed as a useful tool for
computing routine inverse models when it is not possible to determine principal
strike direction, given that the determinant is invariant under rotation.
The determinant mode reduces the distortion effects caused by shallow hetero-
geneities and nonfinite lateral structures, and the phase is not affected by galvanic
distortions. The determinant inversion generally allows a good data fit while at
the same time resolving reasonably well both resistive and conductive structures
along any profile. Estimated resistivity values lie much closer to the true subsurface
resistivity in between the extreme resistivities predicted by individual TE and TM