Simple_Basin.md



Simple Basin
This example contains a detailed description on how to create a simple model of a sedimentary basin based on synthetically generated gravity anomaly data using interactive geometry modification in IGMAS+.

Description

Input data
To create the input gravity dataset we used a simple model of a sedimentary basin with densities of 2300 kg/m^3 for the sediments and 2700 kg/m^3 for the basement.
A set of 1938 gravimeters are placed on a flat Earth's surface above the sedimentary basin.
The Figure below shows the measured gravity (Bouguer anomaly with values in the range from approximately -33 to 0 mGal:


  A simulation of a typical Bouguer anomaly above a sedimentary basin measured with a regularly spaced set of stations on a flat Earth's surface.


Download
The input data required for this example as well as the resulting two models are available for download here.
The share contains 3 files:


Simple_Basin_Measured_Gravity.csv: the input gravity data in CSV format

Simple_Basin_2D.zip: the resulting 2D IGMAS+ model together with intermediate timeline steps (for comparison)

Simple_Basin_3D.zip: the resulting 3D IGMAS+ model together with intermediate timeline steps (for comparison)

The input gravity data file Simple_Basin_Measured_Gravity.csv is in CSV format and has 4 data columns: "x" "y" "z" "calculated z component" and 1938 data rows corresponding to stations.
The values in columns are delimited with space.
The two models are zip archives with IGMAS+ projects. Simply unpack and load projects in IGMAS+.

Modelling
!!! abstract "Goal"
The goal of this modelling example is to determine the depth and extent of sedimentary basin based on the input gravity data and known densities.
The modelling is carried out in three steps:


1D interpretation: Perform a quick initial estimate of the maximal depth of the sediments using the simple Bouguer Plate.

2D interpretation: Build a 2D model through the gravity minimum. Use a single section with mirrors, its direction should be West-East, which is perpendicular to the North-South striking of the anomaly. Use IGMAS+ for this task.

3D interpretation: Extend the 2D model in IGMAS+ to a 3D model by adding more sections north and south of the 2D model. Do not use more than 5 sections in total.


1D interpretation
We start with a quick initial estimate of the maximum sediments depth using a simple Bouguer Plate approach.
The formula for Bouguer Plate correction,
\delta g_{max} = 2\pi G \Delta\rho h_{max},
reformulated for h_{max} reads
h_{max} = \frac{\delta g_{max}}{2\pi G \Delta\rho},
where:


\delta g_{max} is the maximum absolute gravity anomaly value

\Delta\rho is the density difference between the basement and the sediments

G is the gravitational constant.

!!! question
What is the approximate maximum sediment thickness based on the quick 1D estimate?


2D interpretation
Here we build a 2D model through the gravity minimum. We use a single West-East oriented section, perpendicular to the North-South strike of the anomaly.

1. Import the measured anomaly

Start IGMAS+

Open the import dialogue: ++"File"++ --> ++"Import"++ --> ++"Stations"++
Select the measured anomaly file Simple_Basin_Measured_Gravity.csv

Specify the unit of the coordinates: "km"
Program will automatically generate a model domain with 8 km depth for the area covered by stations (see Figure below), but without any working sections.


  A 3D view of the model in IGMAS+ created after loading of the measured gravity.


2. Add a working section
Add a single working section along y=10 km (West-East):

Open sectioning dialogue: ++"Edit"++ --> ++"Add Sections"++
Adjust the ++"Count"++ of sections to 1
Press ++"Preview"++
Click ++"right mouse button"++ on the South West node (bottom-left white dot) of the section area rectangle (see Figure below)
Leave -3 in field X, change field Y value to 10 and click ++"OK"++
Click ++"Finish"++


  Adding a single West-East oriented section in IGMAS+


3. Mirror the section
Now add mirrors to the working section:


Select the section in the Object Tree and add 100 (km) to "mirror+" and "mirror-" in the Section Mirror dropdown as shown on the right
This creates a pseudo-2D model extending 100 km on either side of the profile
Triangulate the model using ++"Edit"++ --> ++"Model Triangulation"++
Click  to clip to model bounds and view the entire model (see Figure below).


  A pseudo-2D model created using section mirrors


4. Assign densities
Now create bodies for the sediments and the basement and assign the densities:


In the Body Manager Tab use ++"Add Parameter"++ and select "Density"
Rename the "new_body" body to "Basement" (double click the name to rename it)
Add a body with name "Sediments" using ++"Add Body"++
Assign a density of 2.7 t/m^3 to the "Basement" and 2.3 t/m^3 to the "Sediments"
Assign the "reference" body with the density of 2.7 t/m^3 to avoid any edge effect


5. Define the "Sediments" body and adjust its geometry
There is still just one body ("Basement"), no "Sediments" body in the triangulated geometry.

Add the 2D view with  or ++"Add View"++ --> ++"2D View"++


You must now cut off the upper part of the model block to build the "Sediments" body. There are several possibilities. The simplest is to insert 2 vertices on the surface to the left and right of the centre (A and B) and then move the central vertex upwards (see Figure on the right side)

To insert a vertex, hold ++i++ and click ++"left mouse button"++ on the border of a polygon. A new vertex will appear in this position.
To shift a vertex, hold ++shift++ and drag the desired vertex with ++"left mouse button"++


Now divide the polygon between the new vertices A and B by holding ++d++ and  dragging the ++"left mouse button"++ (see Figure on the right side)
Double ++"left mouse button"++ click on the upper part, use ++"right mouse button"++ and select ++"Set Body"++ --> "Sediments"
New triangulation is necessary after this step: ++"Edit"++ --> ++"Model Triangulation"++


Now insert some points along the interface between "Sediments" and "Basement": hold ++i++ and click ++"left mouse button"++ (see Figure on the right side).


Move the new vertices to form a basin structure: hold ++shift++ and drag the desired vertex with ++"left mouse button"++ (see Figure on the right side)
Delete or shift down the central uppermost vertex. To delete, hold ++i++ and click ++"left mouse button"++ on the undesired vertex
Again, new triangulation is necessary: ++"Edit"++ --> ++"Model Triangulation"++
Now there are both "Basement" and "Sediments" bodies in the triangulated geometry and both have assigned densities.


6. Fit the anomaly
Now calculate the anomaly of the model using  or ++"Tools"++ --> ++"Calculate Anomalies"++.

To fit the calculated anomaly to the measured better, adjust the positions and/or add/remove the subsurface vertices
A constant shift may remain due to additional stations in the file (see Figure below)


  Constant shift between the calculated and measured anomaly

You can switch off Auto Shift (see Figure below):

Select "Gravity: z-component" under "Model" --> "Fields" in the Object Tree.
Open Property Editor Tab

Uncheck "Auto Shift" checkbox