P. Schmoldt, PhD - MTNet - DIAS

10.2. Inversion for mantle structures
of the TE mode model, may be a result of galvanic distortion by small-scale off-profile
features or crustal structures which remain in the dataset despite decomposition of the
impedance tensor and cannot be adequately modelled with 2D inversion. The TE mode is
usually more affected by small-scale off-profile features than the TM mode (cf. Sec. 6.3);
therefore, the TE mode is often down-weighted in cases where effects of 3D structures
are assumed. In this initial inversion procedure, error floors of apparent resistivity and
impedance phase for both modes are set to 10 % and 5 %, respectively. Modification of
error floors in the combined-mode inversion can later be used to weigh the two modes,
through that controlling their influence on the resulting inversion search for the best-fitting
model. Such weighting of the modes will be carried out in subsequent inversion sequences
of this work.
A remarkable feature of this initial inversion step is the transition from the resistive
uppermost mantle (10 3 – 10 4 Ωm) to the more conductive region below (≈ 10 2 Ωm). For
the northern and central part of the TM-only model and the southern part of the combinedmode
model, the transition is modelled at a depth of approximately 100 – 150 km, which
is consistent with LAB depth estimates for the Tajo Basin region in proximity of the
PICASSO Phase I profile by other investigations (cf. Sec. 7). The upward extension of
the more conductive region in the south-central area of the TM-only model coincides to
some degree with the location of a low velocity region determined in seismic tomography
studies (cf. Fig. 7.24). The vast downward extension of the resistor at the southern edge
of the TM-only model (indicated by the dashed areas), on the other hand, does not seem
plausible and is most likely an inversion artefact.
Owing to the low validity of the isotropic 2D inversion results when using a homogeneous
crust, the investigation is extended by using projected results of the crustal inversion
model (cf. Sec. 10.1.5) as starting model in the inversion for mantle structures. The
starting model is augmented by assigning resistivity values of 100 Ωm to cells below the
crustal model. Inversion is carried out like the previous inversions at the beginning of
this Section, following the Jones Catechism (Sec. A.2.3), using data of periods greater
100 s to minimise contribution of crustal structures, and examining results for each of the
modes individually and in combination (Fig. 10.15). Therein, structures at crustal depths
(above 30 km) are kept fixed. Data for the TE mode can only be fit poorly by the isotropic
2D inversion models; i.e. despite (unduly) high levels for related error floors of 20% (ρa)
and 10% (φ), an unacceptable RMS misfit of 4.01 prevails. Likewise, the misfit for the inversion
model using both modes is exceedingly high (RMS misfit = 3.39). Thus, isotropic
2D inversion models are not adequately representing the Tajo Basin subsurface but can
only be used to examine characteristics of the models.
As for the inversion with a homogeneous crust (Fig. 10.14), models for the three
datasets (only TE mode, only TM mode, both modes) differ significantly. At greater depth
(>100 km) the TE mode inversion model exhibits a highly conductive region, whereas TM
mode inversion yields a highly resistive region. Different characteristics may in parts be
related to the limited depth of induction for the TE mode (indicated by the grey lines in
Figure 10.15, denoting Niblett-Bostick depth (Sec. 6.3.1) estimates for the longest period
249