Molasse Basin

This example is devoted to demonstration on how IGMAS+ can be used to perform a lithosphere-scale modelling based on a European Molasse basin (MOLA) model using "flying carpets" (horizons) as input data.

Description

Goal

The goal of this modelling example is to show the typical modelling workflow using horizon files from a data publication: load the structural model, convert the horizons to a suitable format, import and use the horizons for construction of an IGMAS+ model, import stations, run anomaly calculation and apply the border effect minimization.

Model

The MOLA model is described in the following publication¹:

Przybycin, A. M., Scheck-Wenderoth, M., & Schneider, M. (2015). Assessment of the isostatic state and the load distribution of the European Molasse basin by means of lithospheric-scale 3D structural and 3D gravity modelling. International Journal of Earth Sciences, 104(5), 1405-1424. https://doi.org/10.1007/s00531-014-1132-4

The structural model itself is published on Zenodo²:

Szymanski (Przybycin), A., Scheck-Wenderoth, M., Schneider, M., & Anikiev, D. (2024). MOLA: 3D lithospheric-scale structural model of the European Molasse basin (Version 1). Zenodo. https://doi.org/10.5281/zenodo.10869954

Representative horizons of the MOLA model: Top of the Folded Molasse sediments (light green color), Top of the Upper Crust (blue color), Top of the Lower Crust (turquoise color), Top of the Lithospheric Mantle (Moho, lavender color).

The MOLA model consists of the following units with assigned densities (Table 2 in Przybycin et al. (2015)¹):

Number	Unit name	File name	Density
1	Nördlinger Ries impact structure	2024-MOLA_01_Ries.txt	2000 kg/m$^3$
2	Alpine Body	2024-MOLA_02_AlpineBody.txt	2730 kg/m$^3$
3	Folded Molasse	2024-MOLA_03_FoldedMolasse.txt	2400 kg/m$^3$
4	Foreland Molasse	2024-MOLA_04_ForelandMolasse.txt	2350 kg/m$^3$
5	Cretaceous	2024-MOLA_05_Cretaceous.txt	2640 kg/m$^3$
6	Upper Jurassic Malm	2024-MOLA_06_Malm.txt	2650 kg/m$^3$
7	PreMalm Sediments (Jurassic and Triassic)	2024-MOLA_07_PreMalm.txt	2680 kg/m$^3$
8	Tauern Body	2024-MOLA_08_TauernBody.txt	2800 kg/m$^3$
9	Upper crystalline crust	2024-MOLA_09_UpperCrust.txt	2850 kg/m$^3$
10	Lower crystalline crust	2024-MOLA_10_LowerCrust.txt	3150 kg/m$^3$
11	Lithospheric Mantle	2024-MOLA_11_LithosphericMantle.txt	3180 kg/m$^3$

Gravity data

The evaluation of the resulting density model in Przybycin et al. (2015)¹ was done by comparing the gravity calculations with the Earth Gravitational Model 2008 (EGM2008) provided by the International Gravimetric Bureau (BGI 2012³; Pavlis et al. 2012⁴).

Download

Load the structural model archive from Zenodo and unpack to folder MOLA_3D_model_files.

The scripts necessary for the conversion of the data, and the gravity data are available here.

Scripts

At least one script (convert_horizons.sh) is required to process the original publication data files, to be able to involve them in IGMAS+ modelling. The second one (cut_horizons.sh) is required only when working with earlier versions of IGMAS+ (prior to v1.5).

Note

Originally the bash scripts are designed for Unix, but one can run them on Windows. The best way to run these scripts is to use Git Bash:

• load Git-SCM from the official site
• install
• run Git Bash (right click menu in the folder with scripts and select "Open Git Bash here")
• run scripts as explained below

Convert horizons

convert_horizons.sh is a bash script that converts ASCII (TXT) files in the format of data publications to CSV horizon files readable by IGMAS+.

Usage:

./convert_horizons.sh

takes files from folder MOLA_3D_model_files (must be in the same folder as the script) from the original data publication data archive, converts them and saves to folder MOLA_3D_model_horizons.

Cut horizons

Note

Using this script is optional.
Cutting of horizons can be done inside IGMAS+ (for v1.5 and higher).

cut_horizons.sh is a bash script that processes the CSV horizon files by adjusting Z-coordinate values such that the units are cut at zero level.
It is necessary to convert the files in this way in order to be able to model Complete Bouguer Anomaly in IGMAS+ correctly (versions below v1.5).

Usage:

./cut_horizons.sh

processes files from folder MOLA_3D_model_horizons (created by convert_horizons.sh), converts them (cuts the unit surfaces) and saves to folder MOLA_3D_model_horizons_cut.

Modelling

Importing horizons

To load the horizons first start a new project with File → New Project:

Select Irregular/Regular Horizon(XY-Plane) Import and click Finish

Then navigate to the MOLA_3D_model_horizons folder created by the convert_horizons.sh script:

Ensure all relevant files are selected and proceed with Next:

The window shows the information about the loaded horizons.
You can interpolate them by adjusting the number of points or spacing in $x$ and $y$ dimensions, if necessary.
Details can be found in this chapter.

Or simply proceed with Next:

On the next step you can define model borders extension, minimum vertical distance, vertical model limits (Z-Top, Z-Bottom) and model units:

Brief explanaion on the selected parameters:

• Extend model borders: Extending model borders and adjusting the Range can be applied to remove the edge effect.
  In the case of MOLA model we will use automatic Border effect reduction instead, so simply uncheck Extend model borders.
• Minimum vertical distance: here we defined the minimum thickness of the bodies to avoid identical positions of the vertices.
  MOLA model has been constructed in a way that there is a minimum 10 cm thickness, so it makes sense to set 0.1 m here.
• Z-Top: in order to perform Complete Bouguer Anomaly modelling it necessary to have all masses in the model to be below the zero level.
  To cut the model horizons at zero level, set the Z-Top parameter to 0.

Warning

Cutting of horizons in this way can not be done in IGMAS+ version earlier than v1.5.
Therefore, when using earlier IGMAS+ versions you should use the pre-cut horizons from folder MOLA_3D_model_horizons_cut prepared by the cut_horizons.sh script and Z-Top will be automatically determined as 0.

• Z-Bottom: we don't need to change this parameter as it is automatically defined and the depth of the model is sufficient¹.
• Units: original coordinates in the horizon files are in meters (UTM32N), so we must keep "m".
• Project Points (Mundry): we don't need to use interpolation algorithms of Mundry⁵ because the horizon grids are regularly spaced². Keep it unchecked.

After adjusting parameters, proceed with Next:

In this window one can setup the orientation and number of working sections.
Due to the nature of the subsurface structures¹, it makes sense to span the sections from North to East, therefore we use the → 90 button to zet Azimuth to 90°.
Since the original horizontal grid spacing in MOLA is 2.5 km², to keep the original resolution, we change the spacing to 2500 m.
After changing parameters, click on Preview and then Finish.

IGMAS+ will import the working sections and create the corresponding model domain.

Triangulation

After that, click on Triangulate Sections icon :

Simply proceed with Finish. IGMAS+ will show the triangulated model domain:

Visualizing horizons

To visualize the horizons, in the Object Tree switch off the Sections, and in Interfaces switch off interfaces both related to reference and Top

The 3D View will show the model horizons (or interfaces) as surfaces:

Sometimes it is more convenient to visualize the models with vertical exaggeration.
In the Object Tree select Model, then go to the Property Editor Tab and change vertical exaggeration to 4:

Horizons of the MOLA model after triangulation and applying vertical exaggeration factor of 4.

Setting up model parameters

Now we should set up model parameters.
We are modelling gravity, so it is necessary to set up densities according to the Table of densities.

In the Body Manager Tab use Add parameter button and add density:

You can choose any units, but keep the entered values consistent with the units. Here we select the default t/m$^3$ and click OK.
After that in the same Body Manager Tab we fill in the values:

The reference density is set to 3.1 t/m$^3$ to minimize the edge effect when modelling Bouguer anomaly.

Importing stations

Import stations with File → Import → Stations:

Switch file type to [csv | xyz] - Comma Separated Values, select the provided gravity data file Molasse_Basin_Measured_Gravity.csv, and proceed with Open:

Here you can see the columns imported from the CSV file. If all looks good, finalize the importing with Finish.
The measured gravity field will show up on top of the model as a color-coded surface, and stations are visualized as red dots:

Calculating anomalies

Once the project has triangulated model, stations are imported (or created) and parameters are assigned, it is possible to calculate the anomalies.
Just use the Calculate Anomalies icon or use Tools → Calculate Anomalies:

Here we are interested in the default vertical ($G_z$) component. Click Finish to start calculations.

Visualizing misfit

To visualize the gravity misfit it is better to use the 2D View. Click on the 2D View icon or use View → View → 2D View.

in the 2D section view one can see the measured (solid red (for the vertical component) line), calculated (dashed red line) and residual (dotted red line) anomalies.
One can also see the edge effect on the sides of the model (too much mass due to the reference body).

Minimizing border effect

To reduce the border (edge) effect we can use the automatic border effect minimization algorithm.
In the Object Tree select Model, then go to the Property Editor Tab and find ... button on the right from the border-algorithm in the Border effect section.

First uncheck User border anomaly to set it to false and then click ... button or, alternatively, use Research → Wizard [Border effect]:

Warning

With User border anomaly checkmark set to true it is not possible to modify the voxel dimensions and resolution required for the border effect algorithm.

Select Automatic and proceed with Next:

Leave the defaults here and proceed with Next:

The minimization of the border effect is done using a special voxel cube. The initial resolution of 250 m is too fine, it is enough to reduce it to 2500 m (resolution of the original horizon grid²):

Click with Finish and check User border anomaly back to true.

Re-calculating anomalies

In order to recalculate the anomalies, click Re-Calculate Anomaly icon or use Tools → Re-Calculate Anomaly.

After recalculation is finished, the border effect minimization is applied automatically.
To visualize the updated gravity misfit switch to the the 2D View:

The border effect has been successfully minimized.

Visualizing misfit on the map

To visualize the anomaly misfit on the 2D map, use 2D Maps View: click 2D Maps View icon or use View → View → 2D Maps View:

It is possible to adjust the visualization options using Map rendering preferences (brush) icon in the top toolbar panel of the 2D Maps View window:

Select z component and adjust font size for both measured/calculated and residual anomaly maps:

Click +"OK"++ and you can see changes of the fonts directly on the maps:

In the same way you can change the contours, contour labelling, color maps, as well as the color limits.

Exporting the calculated anomalies

To export the calculated fields as well as the misfits, use File → Export → Stations:

Select the desired file type, e.g. [csv | xyz] - Comma Separated Values for exporting in the CSV format and click Save.

References

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1. Przybycin, A. M., Scheck-Wenderoth, M., & Schneider, M. (2015). Assessment of the isostatic state and the load distribution of the European Molasse basin by means of lithospheric-scale 3D structural and 3D gravity modelling. International Journal of Earth Sciences, 104(5), 1405-1424. doi:10.1007/s00531-014-1132-4 ↩↩↩↩↩

2. Szymanski (Przybycin), A., Scheck-Wenderoth, M., Schneider, M., & Anikiev, D. (2024). MOLA: 3D lithospheric-scale structural model of the European Molasse basin (Version 1). Zenodo. doi:10.5281/zenodo.10869954 ↩↩↩↩

3. BGI (2012). The International Gravimetric Bureau. In: Drewes H, Hornik H, Adam J, Rozsa S (eds) The Geodesist’s handbook 2012. (International Association of Geodesy). J Geodesy 86(10). doi:10.1007/s00190-012-0584-1 ↩

4. Pavlis N. K., Holmes S. A., Kenyon A. C., Factor J. K. (2012). The development and evaluation of the Earth Gravitational Model 2008 (EGM2008). J. Geophys. Res. 117:B04406. doi:10.1029/2011JB008916 ↩

5. Mundry, E. (1970). "Zur automatischen Herstellung von Isolinienplänen". In: BEIH. GEOL. JB. 98, pp. 77–93. ↩