P. Schmoldt, PhD - MTNet - DIAS

9.8. Analysis of vertical magnetic transfer function data
The crustal strike pseudo-section plot (Fig. 9.12) exhibits (a) a region of increased
conductivity for periods in the range of 1 second in both modes along the whole profile,
extending to shorter periods towards the ends of the profile. The region (b) related to
shorter periods in the centre of the profile, between stations pic005 and pic017, appears
to be more resistive. For longer periods, pseudo-sections for the two modes exhibit a
significantly different behaviour: the TE mode indicates two continuous features at longer
periods, whereas the TM mode exhibits four different regions of conductivity at longer
periods. This discrepancy can potentially be related to different induction depth of the
two modes, owing to the different resistivities sensed [Jones, 2006], or the disagreement
of longer period data with the crustal strike decomposition. However, common features
for longer periods, generally supported by apparent resistivity and phase data of both
modes, are (c) a considerably more conductive region in the south of the profile as well
as (d) an conductivity increase at the longest periods greater than 10 4 s.
In principle, mantle strike pseudo-sections (Fig. 9.13) support the crustal strike pseudosection
findings: (b) a more resistive region at shorter periods in the centre of the profile,
underlain by (a) a conductive feature at approximately 1 second, as well as (c) a resistive
feature at longer periods in the south of the profile, and (d) an increase of conductivity
at longest periods of the TE mode and the phase data of the TM mode. In these pseudosection
plots, the northern limitation of feature (c) appears to be located beneath stations
pic007 in the area where seismic tomography indicates a change from low (to the south)
to high (to the north) velocity values (cf. Fig. 7.24); an inversion of the PICASSO Phase
I data should enable a more detailed analysis of this circumstance. Feature (d) might
indicate the location of the asthenosphere; inversion of the MT data may therefore also
permit an evaluation and depth estimate of the LAB in this region.
9.8. Analysis of vertical magnetic transfer function
data
Horizontal and vertical H-field data from the LEMI-417 recording systems are used to aid
MT investigation of the deeper regions. LEMI-417 systems record long-period data of the
magnetic field in two horizontal directions (Hx and Hy) as well as in the vertical direction
(Hz) by use of a fluxgate magnetometer (cf. Sec. 6.1.1). Magnetic field data can be used to
derive induction arrows, which help to interpret subsurface conductivity distributions by
pointing towards or away from a conductive region (dependent on convention, cf. Section
3.2.3). Moreover, vertical magnetic field data can help to identify electric anisotropy in
the subsurface; it is possible to distinguish between cases of an anisotropic-1D subsurface
and an isotropic-2D subsurface since only the latter will exhibit a significant Hz (cf. Sec.
4.1.3).
Magnetic transfer function data are derived with the algorithm by Smirnov [2003] (cf.
Sec. 6.2.3) and Figure 9.14 is generated using the WinGLink [2005] software. Results
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