P. Schmoldt, PhD - MTNet - DIAS

9. Data collection and processing
Fig. 9.14.: Magnitude of the magnetic transfer function for the Tajo Basin subsurface; see text for details.
reveal a significant distortion of the area around stations pic011, pic013, and pic015, indicated
by a transfer function magnitude greater one. A magnitude greater one is physically
implausible as this implies that Hz > Hx, Hy, meaning that the strength of the secondary
field is greater than the strength of the primary field. The interpolated area of
distortion has an approximately parabolic shape with its vertex beneath station pic013, at
periods of approximately 10 3 s. The area of distortion widens at shorter periods, extending
to the regions in-between stations pic009 and pic011 and stations pic015 and pic017
at periods around 10 s. The centre of the anomalous area coincides with the intersection
of the PICASSO Phase I profile with a DC railway line at the surface, a known source
of EM noise (cf. Fig. 9.1, Sec. 9.4). In contrast to processing results by the EMTESZ
working group [e.g. Kreutzmann et al., 2005] for station in proximity of DC railroad lines
in Poland, the DC railroad noise in the PICASSO Phase I cannot be removed by current
remote robust processing schemes; similar cases of prevailing DC railroad noise has
been reported, for example, by Pádua et al. [2002] for an MT investigation in Brazil.
Accordingly, data from the respective region along the PICASSO Phase I profile are to
be interpreted meticulously or even to be rejected. In contrast to these exceptionally high
magnitudes, some regions of particularly low magnitude of the magnetic transfer function
are observable beneath the profile; namely one region beneath stations pic001 – pic005,
and one region beneath most of the profile at periods greater than 10 3 s. The latter could
be an indication for a more 1D mantle beneath the Tajo Basin. The increase in magnitude
for data of the longest periods beneath station pic009 might suggest a deep-seated 2D
feature, but such a feature is not well constraint due to the low resolution at this period
range.
Analysis of the real induction vectors reveals a preferred direction between N10W and
N40W (Parkinson convection: vectors pointing to conducting anomalies) for most of the
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