P. Schmoldt, PhD - MTNet - DIAS

Depth (km)
a3b1t6
a5b1t3
a2b1t10
10.1. Inversion for crustal structures
Short period range: 0.001 – 10 s Long period range: 0.001 – 100 s
0
0
10
20
30
40
50
1 2
Log 10 resistivity (Wm)
Depth (km)
10
20
30
40
50
a3b1t6
a5b1t3
a2b1t10
1 2 3
Log 10 resistivity (Wm)
Fig. 10.2.: Resistivity–depth profiles of horizontally averaged models for the Tajo Basin subsurface, obtained through inversion of
PICASSO Phase I response data from two different period ranges and three sets of smoothing parameters; see text for details. Different
sets of smoothing parameters are indicated by colour, with solid and dashes lines denoting average values and variance of electric
resistivity for the different regions, respectively.
Initially, models from different smoothing parameter combinations and both period
ranges are examined to identify dominant and robust characteristics of the different regions.
For the sedimentary layer (depth range 0 – 2.5 km) all models yield an average
resistivity of approximately 30 Ωm decreasing towards the bottom of the layer (cf. Fig.
10.2). Except for the short period range and the model with most horizontal smoothing
(a5b1t3), the conductive region at the bottom of the sedimentary layer exhibits values
of approximately 10 Ωm, most likely originating from saline fluid intrusion into the sedimentary
layer. For the top of the upper crustal layer (depth range 2.5 – 5 km) an increase
of average resistivity to values of approximately 50 Ωm is determined by all models. The
upper region of this layer is not well constrained due to the shielding effect of the conductor
above, thus assuming a resistivity of 50 Ωm for the top of the upper crustal layer can be
considered reasonable. At the bottom of the upper crustal layer (depth range 5 – 10 km),
models are significantly different for the two period ranges, with ‘short period range’ models
exhibiting more resistive structures than the ‘long period range’ models (left-hand and
right-hand side plots in Figure 10.2, respectively). At greater depth (≥ 10 km), the behaviour
is reversed: ‘short period range’ models exhibiting more conductive structures
than the ‘long period range’ models. Since shorter period range models approach average
resistivity values of approximately 100 Ωm at a depth of around 10 km, which is the value
of the starting model, the discrepancies between the two model groups is likely to originate
from a lack of sensitivity to intermediate and lower crust structures for the shorter
period range models. Longer period models, on the other hand, appear to be sensitive
down to greater depth as they vary from the 100 Ωm starting values down to a depth of
42 km.
Due to the notable degree of lateral variation in the models (indicated by the dashed
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