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Hi, I’m Chris from Geosoft and in this video
we’ll walk through how to set up your project to run an inversion in Oasis montaj using VOXI.
The steps in this video include loading the VOXI menu,
setting up a new VOXI session, and
adjusting the parameters required to run an inversion.
After watching this video, we recommend watching our other videos on running inversions.
These include running an unconstrained inversion,
an inversion with an upper-bound constraint, followed by an inversion with gradient-weight constraints.
The data used in this video is the open source Reid-Mahaffy data from the Ontario Geological survey
flown at 200m nominal line spacing and 60m terrain clearance.
Before we can run an inversion, we’ll need some input files, which I’ve already prepared.
We begin with a Digital Elevation Model grid covering the area of interest to define the surface topography.
Alternatively, you can also use a constant value representing the average elevation above the standard datum.
Next we’ll need to create an outline defining the area of interest we want to model.
Here we’ll use a polygon to outline the area.
The other option is to use a geo-located voxel compiled from other sources.
Finally, we have a database containing observation coordinates along with potential field measurements.
The database must have a projected coordinate system defined.
These requirements are well documented in the Preparing your Data Best Practice Guide,
found in the Oasis montaj Technical Documentation section of the Geosoft website.
Before we can run an inversion, we’ll first need to load the VOXI Inversion menu.
To load the VOXI menu, click the load menu shortcut from the standard shortcut toolbar.
The Load Menu dialog is displayed.
Scroll down the list and select VOXI and Click Open.
VOXI Inversion is now displayed as a menu item in the Oasis montaj menu bar.
We are now ready to create a new VOXI session from the input files we have prepared
in order to run an inversion.
From the Oasis montaj menu bar, click VOXI Inversion and select New VOXI from Polygon.
The New VOXI from Polygon dialog is displayed.
This dialog collects the location and resolution information.
It also prompts you to define the session name, the polygon file that defines the Area of Interest, and the Digital Elevation Model grid file.
We will enter our VOXI session name and select the polygon file we want to use.
We will then select our DEM grid file.
If our selected polygon file does not have a coordinate system defined,
the Define button becomes active.
We can also click the Create polygon button to interactively create a new polygon to define our area of interest from an existing map.
We are deciding to keep the model resolution at 50 metres
as this is roughly equivalent to the flying altitude and a quarter of the line spacing of the data we are using.
At any point, you can click the question mark icon on the title bar to see all the various dialog options.
Ensure that the information and input files you have supplied are accurate.
Click OK.
The VOXI Viewer opens your session along with a 3D illustration displaying the mesh resolution, the Digital Elevation Model,
and the outlined area of the interest.
The 3D view of the constructed model provides the opportunity to visually inspect the model.
If it is not properly defined, for example if the mesh is too coarse or too fine,
if the terrain does not register correctly or the AOI polygon is not the right one,
we can modify the model before we add more data and proceed with modelling.
It also prompts you to add measurement data to the session.
If you are not satisfied with the model and you would like to modify any of the settings, click No.
We will click No for now so we can inspect the model before starting to add measurement data.
You can inspect the area of interest settings by expanding the Area of Interest in the Tree Viewer.
You can modify any of the specified settings for the mesh and the Digital Elevation Model.
Right-click the Mesh item under Area of Interest and select Modify.
The Modify Mesh Parameters dialog is displayed.
Here, you can modify the Active volume cell sizes that affect the model resolution.
Click More for further mesh parameters you can modify for the model.
Again, at any point, you can click the ? icon on the title bar to explore the available dialog options.
If you are satisfied with the parameters, click OK.
Similarly, you can also modify the DEM Grid.
Right-click the DEM item under Area of Interest and select Modify.
The Modify DEM Grid dialog is displayed.
You can modify the DEM type and also supply an alternate DEM grid, should you choose to do so.
If you are satisfied with the parameters, click OK.
We have now set up all the parameters to run our inversion.
This concludes our video on setting up your inversion project.
Next, we recommend watching our video on running an unconstrained inversion, followed by the video on upper-bound constraint inversions,
and then the video on gradient-weight constrained inversions.