P. Schmoldt, PhD - MTNet - DIAS

9. Data collection and processing
9.6. Compensating for distortion of the impedance
tensor
Removing distortion of the data, as described in Section 4, is a crucial, but difficult, step
during the analysis for most MT datasets. Moreover, since previous geological and geophysical
studies indicate a complex subsurface setting for the region under investigation
by the PICASSO Phase I project (cf. Sec. 7), this step is carried out with great diligence.
9.6.1. Geoelectric strike estimation
Determining whether a regional 2D approximation is justified, and (if that is found true)
subsequent identification of the geoelectric 2D strike direction are major elements of MT
impedance tensor analysis as it affects the decomposition of EM fields into TM and TE
mode contribution. The approach used here for the PICASSO Phase I data utilises a script
developed by Jan Vozar, displaying the RMS misfit calculated by the strike algorithm
for different directions at all stations. The strike algorithm is based on the impedance
tensor decomposition method by McNeice and Jones [2001], using the Groom and Bailey
[1989] technique (Sec. 4.4.4), and is today commonly used my many in the MT
community. The RMS misfit is therein calculated for a chosen depth range derived by
the Niblett-Bostick depth approximation for the rotational invariant arithmetic average of
the off-diagonal elements (also referred to as Berdichevsky average) [Berdichevsky and
Dmitriev, 1976b] (cf. Sec. 6.3.1). A relative impedance error floor of 3.5% is used during
the application of the strike algorithm, resulting in an error floor of 2.0 degrees for the
phase and 7.12% for the apparent resistivity.
The range of directions under investigation is limited to the interval 0 to 90 degrees,
owing to the 90 degrees ambiguity of the geoelectric strike estimation. The ambiguity
originates from the fact that during the calculation no assumption can be made about the
orientation of the two modes, TE and TM, except that they are orthogonal. Hence the
true geoelectric strike direction and its orthogonal fit the data equally well. For example,
a calculated strike direction of N50E (50 degrees clockwise from North) indicates that
the true strike of the geological features has a direction of either N50E or N140E. The
decision about the direction has than to be made by considering additional sources of
information, such as results of geological studies or the vertical field responses. For the
strike analysis of the PICASSO Phase I data an increment of one degree is chosen in order
to provide sufficient resolution.
The geoelectric strike analysis using the above-described RMS misfit calculation is
supplemented by a multisite, multifrequency (transformed into Niblett-Bostick depth)
analysis with the same strike program. Therein, optimal strike direction and average
RMS misfit are calculated for the chosen depth and stations ranges of; results are from
here on referred to as multi-strike. These multi-strike directions are calculated separately
for the whole profile and its northern region only, reasoned by the observed difference
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