[Folds modelisation problem] - Bomal-sur-Ourthe (Belgium)

[SneakerShot]

Hello,

I am in an internship and I need to create a geological model of the Bomal-sur-Ourthe area (EPSG:3812: xmin = 731000, xmax: 733150, ymin = 617600, ymax = 619500). The area to be modeled is quite complex (a succession of synclines and anticlines with faults). I used QGIS to overlay the geological map layers on the digital terrain model (DTM, see files). I then sampled the point layers and gave them coordinates (x, y, z), an orientation, a dip, a polarity, and an order of formation. I then exported this into two CSV files, which you will find below. When I implement them in the Gempy Python script, the folds in the subsurface are not connected to the layers that outcrop at the surface, and the folds are poorly modeled (I get strange shapes). However, I modeled fold hinges and cuts between the hinges to force Gempy to create a folded relief. According to the geological map, I should normally obtain, on a NW-SE section, the right edge of a syncline, followed by an anticline and ending with the left edge of an anticline. I don't know what else to try to fix the problem. What do you think the problem is?

Thank you in advance for your time.

Here are the files:

Projet_modèle_géol.ipynb
orientations_2.csv
interfaces_2.csv
MNT_Bomal_5m.tif

[NilsChudalla]

Hi @SneakerShot,

quite often, when the modelled geometry is too complex, you are lacking information in the depth and where you have the most complexity. I will have a look at it in the next days (I have not opened your script yet). If you have cross sections based on the map (the Walloon geological survey commonly has those), those would be a great aid. If you have cross sections at hand, check out GemGIS. The Gempy specific functions there might be outdated, but the data extraction from GIS software for input for Gempy should still be up to date! :)

Kind regards
Nils

[SneakerShot]

Hello,

Thank you for your very quick reply. If I understand you correctly, I need to create more cross-sections (parallel to the axis of my folds) to force GemPy to model my folds correctly. Unfortunately, no geological cross-section covers my area of ​​interest. I can only provide you with cross-sections from two geological maps adjacent to mine :

• au Nord de ma carte : https://geologie.wallonie.be/files/ressources/geologie/planches/49-1-2_Tavier_Esneux.jpg
• au Sud-Ouest de ma carte : https://geologie.wallonie.be/files/ressources/geologie/planches/54-7-8_Aye_MarcheenFamenne.jpg

These cross-sections represent the folded relief, which should be very similar to what I have in my area of ​​interest, as they represent the continuation of the folds at Bomal or the same fold zone with the same fold axis. If I don't have access to a true cross-section of my area of ​​interest, is it still worthwhile to use GemGIS ?

Kind regards

[NilsChudalla]

Hi @SneakerShot,

the advice I gave you was quite general. I built a model North-East of yours and had that issue there (that's why I was familiar with the Walloon geological survey).

Looking at your data, there is a more urgent fix required: It seems as if you picked points along the exterior of the polygons of your map.

With the green layer, I am not sure which sampling strategy was used. But what you should be aiming for, is the bottom contact of each layer. So something like this:

My drawing is obviously without proper knowledge, what makes sense in your case (edit: in terms of summarizing your current data into a surface to be modeled). After doing this, you might notice, if some layers are unconformaties and if you should utilize more serieses than one you are currently using.

I hope this helps
Nils

[SneakerShot]

Hello, what software or Python script do you use to visualize points in 3 dimensions? Is it GemGIS? I sampled my surface layers on my digital terrain model and then created underground points based on the elevations of the digital terrain model. I don't understand how I could have created points outside my DEM because I only sampled on it.

[NilsChudalla]

This is the default pyvista plotter (after installing gempy-viewer, run gpv.plot_3d(geomodel) or check the associated video :) If you want to use it, you might have to install python locally and not use collab. To my knowledge, pyvista allows only the static view, that you already saw in the collab notebook.

Please ignore my initial tip. Please focus on the time regarding the sampling from the map. Gempy tries to connect surface points of the same surface. If you have the outline of a geological unit in your input data, Gempy will still follow what it was supposed to do and draw mostly horizontal layers or some very weird stuff.

[NilsChudalla]

GemGIS was meant as an interface between GIS software and Gempy; reading shapefiles and so on turning them into tables of the required input shape... But it also includes functions to sample from DEMs :)

[SneakerShot]

Thank you for your advices, i will try it today !

[NilsChudalla]

Enjoy! Feel free to share the result, if you want some feedback :)

When selecting surface points, it helps to ask yourself, if the selected points represent the true bottom contact of the geological body you are modeling. If not, is it a fault or an unconformity? Faults would be their own structural element.