P. Schmoldt, PhD - MTNet - DIAS

Period (s)
Period (s)
Period (s)
r a XY
0.01
0.1
1
10
100
1000
10000
0.01
0.1
1
10
100
1000
10000
0.01
0.1
1
10
100
1000
10000
Minimum misfit model
LM77 model
Minimum misfit model +ea LM110 model
‘sidelobes’ model LM160 model
pic020
pic019
pic017
pic015
pic013
pic011
pic009
pic007
pic005
pic004
pic003
pic002
pic001
pic020
pic019
pic017
pic015
pic013
pic011
pic009
pic007
pic005
pic004
pic003
pic002
pic001
10.2. Inversion for mantle structures
0.01
0.1
1
10
100
1000
10000
0.01
0.1
1
10
100
1000
10000
0.01
0.1
1
10
100
1000
10000
Period (s)
Period (s)
Period (s)
norm_misf
2.7
Fig. 10.32.: Misfit distribution for XY apparent resistivity data (ρXY a ) of the PICASSO Phase I stations and periods for a selection
of Tajo Basin subsurface models; therein, the x-axis is positive towards North and y-axis positive towards East. Station responses
are derived through
forward modelling with the wsinv3d [Siripunvaraporn et al., 2005a] and the normalised misfit is calculated via
ρ O
a −ρ
misfit = log10 C
a
Err(ρO
; with ρ
a )
O a and ρC a observed and calculated apparent resistivity, respectively. The particularly low misfit for
longer periods of stations pic013 – pic017 originates from rejection of data due to their low signal-to-noise ratio (cf. Sec. 9). The
variation in misfit between models is most significant for longer period data (>10 s) of stations in the centre of the profile (pic007 –
pic011) related to the lithospheric-mantle beneath the central region of the Tajo Basin.
ity and an increase of electric conductivity (cf. Secs. 5.1 and 5.2.2), and may be the cause
of the lithospheric-mantle anomaly beneath the Tajo Basin.
Answering the question regarding the origin of the low velocity – high conductivity
anomaly in the Tajo Basin mantle is impeded by limitations of MT and seismic tomography
approaches: whereas MT investigation is challenging due to the limited penetration
of EM waves for regions of increased conductivity and the resulting lower sensitivity to
deeper regions (cf. Sec. 3.3) as well as the increased uncertainty levels of long periods
in the PICASSO Phase I dataset; seismic tomography suffers from, particularly vertically,
smearing of features [e.g. Nolet, 2008, Section 15.3]. Thus, the extent of subsurface features
is not particularly well defined. In addition, the resolution of MT and seismic models
is limited by the, downward increasing, cell height of the underlying meshes, resulting in
a vertical extent of approximately 20 km and 13 km for cells at a depth of 100 km in the
MT and seismic tomography mesh, respectively. In case of the MT model, increasing cell
size is required to yield a model with a manageable number of cells that can be handled by
the 3D inversion algorithm (cf. Sec. 6.3). A model with a non-linear variation of cell size,
which would facilitate smaller cells in an area of interest, is not feasible as it introduces
inversion artefacts to the model owing to insufficient resolution at this depth range.
In addition to information about the Tajo Basin mantle, the 3D inversion model pro-
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0.0
271