geometry.md

Modification of Model Geometry

Manual model geometry changes

If changes to the geometry of the model become necessary, the user can make model adjustments manually by changing model vertices on the working sections and/or modifying the geometry of the bodies.

Changing model vertices

The actions Insert - Delete - Move vertices are described here.

Insert a vertex:

Press key I-key and navigate with left mouse button on the position of an interface/horizon > click.

Delete a vertex

Press shift key and navigate with left mouse button on the vertex > click

Be sure to see the coordinates of the vertex to delete on the screen:

Shift a vertex:

Press shift-key and navigate with left mouse button on the vertex to be shifted > click

Shift grouped vertices:

Press shift-key together with the key . Then define region of vertices to be shifted by open a window with left mouse button and move pressed left mouse button into the new position

Release the left mouse button. This is the result:

The model gravity field is automatically recalculated.

Split a body/polyhedron on a specific vertical cross section

Select "2D maps view" and mark in the map the cross section on which the density of the body should change (in ascending section number order). Here section 10 was selected; it is marked in red.

Alternatively, the vertical section can also be selected (1) in the OBJECT TREE or (2) by the "section up & down":

Click on the body to be divided (e.g. astenosphere) with the right mouse button and select "divide body".

The "divide body assistant" appears with the new name “Astenosphere new”.

Click "finish"

In the PROPERTY EDITOR (lower left window) the list of updated bodies with their properties appears (here “volume” and “density” are selected).

Note these important points:

(1) The newly inserted body (Astenosphere new) still has the same density as the old one. The color has been selected by the program and can be changed by the user (see section "Colors"). (2) Check the topology of the “new model”. Select Tools in the TITLE BAR and select “Check Topology”:

If now “error” is detected (normal case) continue with the next step. You may notice that the PROGRESS BAR (traffic light) indicates red light:

Explanation:

The model has been changed and both triangulation and modeled gravity are no longer correct. Start with:

(3) “Model – Triangulation” to the new model. Select Edit in the TITLE BAR and select “Model-triangulation”:

Alternatively, the icon can be used for model triangulation. Go to the TOOL BAR line and select .

(4) Re-calculate the gravity of the “new model”. Select Tools in the TITLE BAR and select “Re-calculate Anomaly”:

… and the PROGRESS BAR (traffic light) indicates green light.

If necessary, change density of the new body by “double click” on the density value and insert a new value: PROPERTY EDITOR**

> Body manager > double click” on the density of the new body.

Automated model geometry changes

Apart from the manual geometry modifications it is possible to use the automated geometry modifications through geometry optimization.

Geometry optimization is based on nonlinear optimization and specifically deals with adjusting the shape, position, and size of the model elements to minimize the difference between the observed data (e.g. gravity measurements) and the model predictions (the calculated potential field).

In IGMAS+ there are two types of automated geometry optimization:

• Spring-based geometry optimization, or space warping (read more in this post and see Alvers at al. (2023))
• Interface geometry optimization (read more in this release post)

Both methods are described in Inversion worklows.