The model geometry is automatically created each time a new project is created with File/NewProject. The geometries can later be rebuilt by selecting Edit/CreateGeometries menu option and choosing a project file as input. MigriX will load the horizons (layers) and their fault-traces and create non-vertical fault planes from the traces and store the result in a binary geometry file (mirg), after which the project is ready to be opened and viewed. The resulting geometry consists of the boundaries (horizons) between the layers, with faults incorporated as non-regular elements. The irregular geometries consists of fault insides elements and fault neighbours elements.
The layer model can be modified by selecting the Modify Project or Edit Geological Events menu options. MigriX will ask the user if geometries shall be re-generated afterwards. Model layers can be added and removed using the Add/delete options in the rightmost column. To split a layer evenly, select the Interpolate by Age option from the dropdown menu in the Depth column.
Mathematical grid operations such as between and inbetween can be used to to perform more advanced layer splitting, e.g. use the operation |lowerlayer.gri|upperlayer.gri|0.7|between| to create a layer that is placed at 70% down into the isopach between two layers. The makedepth.Ma keyword can be used to refer to an on-the-fly computed horizon where the depth is computed as a linear interpolation between the one above and below based on the Age of the horizons. A “grid-file” named e.g. Horizon_159.Ma refers to the horizon of the 159Ma layer and one use of this notation is to combine it with max to ensure that a deeper layer is not crossed: |Paleocene.gri|Horizon_65.Ma|max| ensures that the Paleocene horizon does not go below the 65Ma horizon (the depth values in both horizons are here negative).
Validation of geometries
Crossing horizons can cause problems for the migration modelling, e.g. resulting in incorrect layer geometries or areas with zero thickness. The layer model is built from the shallowest layer and downwards, meaning that older layers cutting younger layers, will be eroded at the the intersection with the younger layer. TheModel/QA structure plots can be used to identify such problems in the layer model. The Lost.thickness property shows the difference in depth between the input depth map (Horizon) and the model depth. The Layer.above and Layer.below properties can be used to identify which layers that are intersecting others. Another possibility for QA the layer model is to load the input horizons into gridmaps and visualize them together with the layer model for a more direct visual comparison.
Sediment surface
MigriX uses the seabed topography and terrain above mean sea level to complete the geometrical description when burial reconstruction is done for timesteps before 0 Ma. Negative numbers are used for seabed below MSL and positive numbers above MSL. Constant values and/or grids can be input by setting the Histories/SedimentSurface parameter. In order to avoid inconsistencies in the interpolation of depths between timesteps, the sediment surface can only be specified at times corresponding to horizons.
MigriX requires that you define the sediment surface for all Horizons/Ages that are “Neighbours in Age” to the simulation time-steps used. When all simulation time-steps are at horizon ages, then only values for these Horizons have to be specified. This is because MigriX interpolates linearly between the simulation time-steps and only creates the full geometries at the simulation time steps.
Erosion
MigriX can use the amounts of erosion to calculate the burial history. The erosion histories are defined by the Histories/Eroded parameter and can be edited using the Histories Editor. This eroded thickness is cumulative, so it can account for multiple erosion events back in time. If there is a single erosion event from e.g. 5 to 2 Ma which follows a depositional event from 10 to 5 Ma, one would typically use the same Horizon for the ages 10, 5 and 2 Ma, and with Eroded = erosion.gri for 10 and 5 Ma and zero at 2 Ma. When the amount of erosion is specified, one would expect that the value for the sediment surface is zero or positive at that location (erosion normally would not occur below MSL), but MigriX does not enforce this on the model.
Faults
Faults are incorporated in the geometrical description when you create the geometries. Faults must be input as fault traces (closed polygons for both sides of the fault), typically as polygon files or digitized in MigriX. One file is supplied for each layer with theProperties/Input/Geomodel/Faults parameter. MigriX tries to auto-map areas that are inside and outside the faults, but users can override this mapping using the Geomodel/FaultsInside parameter and with options described in the Faults documentation. Once the fault geometries have been properly defined, the faults may be assigned fault seal properties and used in the migration simulations.
