P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
8. Recovering a synthetic 3D subsurface model using lower-dimensional inversion schemes Distance from the centre of the mesh (km) 300 200 100 0 −100 −200 C−line 11−line 01−line 02−line 03−line 04−line 05−line 06−line 07−line 08−line 09−line 10−line D−line E−line F−line G−line H−line I−line J−line K−line L−line M−line C−line 11−line 00−line D−line E−line F−line 01−line 02−line 03−line 04−line 05−line 06−line 07−line 08−line 09−line 10−line N−line G−line H−line I−line J−line K−line L−line M−line −300 −300 −200 −100 0 100 200 300 Distance from the centre of the mesh (km) Fig. 8.4.: Location of magnetotelluric (MT) recording sites on top of the synthetic 3D subsurface model; North is located to the top of the figure. Displayed are 144 station arranged in a grid (inverted triangles), as well as 13 additional sites, which are a projection of the PICASSO Phase I stations. Locations of these 13 additional stations (pic001 – pic020, situated along the ’pic-line’ in this figure) are a translation of the PICASSO Phase I stations with the respective indices (cf. Chapter 9) onto the synthetic 3D model with pic001 and pic020 located in the North and South, respectively. Nomenclature for the other stations follows the grid system, i.e the name of a station is the combination of the two lines intersecting at the respective location, e.g. the centre-top station is named ‘C00’. Background colours indicate the relatively conductive (red) and resistive (yellow) regions of the crust; the location of the resistivity interface at mantle depth is indicated by the dashed line (cf. Fig. 8.3). Profiles 3D-crust and 3D-mantle (not displayed) coincide with the conductivity interfaces at mantle and crust depth, respectively. (in respect to Figures 8.4 and 8.5) to accommodate plotting of multiple maps side by side. Using Niblett-Bostick depth estimation (Sec. 6.3.1) yields that on the resistive side periods greater than 18 s (appr. 1.26 in log-scale) penetrate into the mantle, whereas on the conductive side penetration into the mantle is first achieved by periods of 72 s (appr. 1.86 in log-scale). In Figure 8.6 ‘XY data’ refers to the TE mode and ‘YX data’ refers to TM mode for the dataset adjusted to the crustal strike direction (N45W), whereas the ‘XY data’ refers to the TM mode and ‘YX data’ refers to TE mode for the dataset adjusted to the mantle strike direction (N45E). The similarity of crustal TE and mantle TM data (and vice versa) is due to the 90 degrees difference between the two strike directions, resulting in swapping of the two modes. Different colour scales are used to display values in maps of different modes and periods (rather than using uniform colour scale values) to highlight structures at the respective period. Note that, in order to enable plotting of all phase data in the first quadrant (0 - 90 degrees), 180 degrees are added to the respective YX phase 174 pic−line pic−line 00−line N−line
Distance from the centre of the mesh (km) 200 100 0 −100 −200 3D−crust 3D−crust−west 3D−crust 3D−crust−east 8.2. Generating synthetic 3D model data −200 −100 0 100 200 Distance from the centre of the mesh (km) Fig. 8.5.: Location of profiles on top of the synthetic 3D model (cf. Figs. 8.3 8.4); North is located to the top of the figure. Stations are associated with profiles of the same colour; for the profile labelled ‘3D-crust’ three different sets of stations are used: red (3Dcrust, using stations representing the PICASSO Phase I recording sites), blue (3D-crust-NS), and green (3D-crust-EW). Every profile contains at least one station on top of each of the four electric conductivity regions (two at crustal depth, two at mantle depth), to assure that data of every profile is affected by the oblique strike directions. Profiles are constructed orthogonal to the geoelectric strike direction of either crust or mantle and station locations are projected onto the profile. Background colours indicate the conductivity distribution at crustal depth (cf. Fig. 8.4). data. At shorter periods (0.01 s, top-left figures), responses are dominated by characteristics of crustal structures, therefore values for the two modes are alike and ρa = ρ; hence ρa = 50 Ωm (appr. 1.7 in log-scale) in the northeastern half and ρa = 200 Ωm (appr. 2.3 in log-scale) in the southwestern half. For longer periods (100 s, top-right figures) mantle structures start to add observable contributions to the response data. Values of ρa at 100 s are similar to values of ρa at 0.01 s (note the different colour scale), but φ differs significantly. In general, φ(100 s) < φ(0.01 s) ≈ 45 degrees owing to the more resistive nature of mantle regions in respect to the crust. At 100 s periods, skin depth (cf. Sec. 3.3) for stations on the resistive side of the crustal fault is appr. 70 km, whereas on the conductive side it is appr. 35 km. Thus, at 100 s periods all sites are sensitive to electric properties of the mantle. Phase anomalies at 100 s exhibit a point symmetry in regards to the centre of the station array, i.e. where crust and mantle interfaces intersect. Phase anomalies are a superposition of effects from crustal and mantle structures with 175
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Distance from the centre of the mesh (km)<br />
200<br />
100<br />
0<br />
−100<br />
−200<br />
3D−crust<br />
3D−crust−west<br />
3D−crust<br />
3D−crust−east<br />
8.2. Generating synthetic 3D model data<br />
−200 −100 0 100 200<br />
Distance from the centre of the mesh (km)<br />
Fig. 8.5.: Location of profiles on top of the synthetic 3D model (cf. Figs. 8.3 8.4); North is located to the top of the figure. Stations<br />
are associated with profiles of the same colour; for the profile labelled ‘3D-crust’ three different sets of stations are used: red (3Dcrust,<br />
using stations representing the PICASSO Phase I recording sites), blue (3D-crust-NS), and green (3D-crust-EW). Every profile<br />
contains at least one station on top of each of the four electric conductivity regions (two at crustal depth, two at mantle depth), to<br />
assure that data of every profile is affected by the oblique strike directions. Profiles are constructed orthogonal to the geoelectric strike<br />
direction of either crust or mantle and station locations are projected onto the profile. Background colours indicate the conductivity<br />
distribution at crustal depth (cf. Fig. 8.4).<br />
data.<br />
At shorter periods (0.01 s, top-left figures), responses are dominated by characteristics<br />
of crustal structures, therefore values for the two modes are alike and ρa = ρ; hence<br />
ρa = 50 Ωm (appr. 1.7 in log-scale) in the northeastern half and ρa = 200 Ωm (appr.<br />
2.3 in log-scale) in the southwestern half. For longer periods (100 s, top-right figures)<br />
mantle structures start to add observable contributions to the response data. Values of<br />
ρa at 100 s are similar to values of ρa at 0.01 s (note the different colour scale), but φ<br />
differs significantly. In general, φ(100 s) < φ(0.01 s) ≈ 45 degrees owing to the more<br />
resistive nature of mantle regions in respect to the crust. At 100 s periods, skin depth (cf.<br />
Sec. 3.3) for stations on the resistive side of the crustal fault is appr. 70 km, whereas<br />
on the conductive side it is appr. 35 km. Thus, at 100 s periods all sites are sensitive to<br />
electric properties of the mantle. Phase anomalies at 100 s exhibit a point symmetry in<br />
regards to the centre of the station array, i.e. where crust and mantle interfaces intersect.<br />
Phase anomalies are a superposition of effects from crustal and mantle structures with<br />
175
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