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import.md

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    Denis Anikiev authored
    - Extended abbreviations
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    Import

    Import horizons

    This workflow is used if existing digital data define continuous horizons in the entire modelling area.
    Several horizons are stacked, the physical parameters between the interfaces are assumed to be constant.
    Users must use one file for each horizon.

    ???+ note Before we get started, here are a few tips to make sure the input works:

    **File formats**
    The following formats are possible: `*.xyz`,`*.csv` or [Geosoft binary grid format `*.grd`](https://surferhelp.goldensoftware.com/subsys/subsys_geosoft_grid_file_descr.htm). The `*.csv` file format is preferred.
    
    **Point types**
    The points defining the horizons may be gridded or irregularly distributed. Points with identical location but different z-values will be averaged (there will be a notice).

    ???+ warning But beware:

    - The points are interpreted to represent **point locations x, y, z**. They are not to be confused with **grid cells**, which are not used here, even in case of regularly gridded horizons.
    - Make sure that the files are read in such a way that they always start with the **top horizon**. The order (from top to bottom) is very important, because it directly controls the triangulation.
    - Have you prepared the "correct gravity field"?
    That means, do you want to calculate with a [FREE AIR](../glossary.md#free-air-gravity-anomaly) or with a [BOUGUER](../glossary.md#bouguer-anomaly) anomaly?
    In both cases a topography file must also be read in. Here you have to make sure that the model stations are located **OUTSIDE** of the model masses - otherwise the mathematics behind the algorithms are no applicable and the calculations will be incorrect.
    - Make sure that the units are correct: give densities in kg/m$^3$, gravity in mGal or 10$^{-5}$ m/s$^2$, depths and lengths in km or m.
    - And finally: did you prepare your model data files for a plane gravity calculation (use UTM, Gauss-Krüger coordinates) or for a spherical calculation (use geographic coordinates with latitude and longitude)?
    
    If all this is considered, it goes off, assuming that **IGMAS+** is installed correctly.

    This is how you can import the horizons:

    • Choose ++"File"++ --> ++"New Project"++ --> ++"Irregular/Regular Horizon (XY-Plane) Import"++ -> ++"Finish"++:

    • Choose the directory and the file(s) to be imported. Make sure to select all files for the model to be built, as later inclusion of additional horizons is not possible.

    • Press ++"Finish"++.

    • Now you see the following window:

      On the right, the input files are listed with the horizons from top to bottom. Below that the "Folder name" is displayed and below that the file type.

    • Press ++"Next"++

    • The Import Wizard lists all imported horizons (files) and orders them from top to bottom according to the value Zmin:

      Make sure, that the list corresponds to the stratigraphic column / layering in your modelling area. If necessary, change the order using the arrows on the right hand of the wizard.

      From left to right, the following information is displayed:

      • Name: This name will be used as the name of the body below the corresponding horizon (can be changed later).

      • # of points: Number of points to be read from file (for information only).

      • Area: Minimum x-coordinate, minimum y-coordinate, size in x-direction, size in y-direction (for information only).

      • Zmin Minimum depth of the horizon (for information only, the value is used to define the layer order).

      • Zmax Maximum depth of the horizon (for information only).

      • # of x-points, # of y-points These values are used to apply averaging of horizon vertices on regularly spaced locations. Default is 0 for irregular points and original number of points for grids (no averaging). All three coordinates (

        xx
        ,
        yy
        and
        zz
        ) will be averaged using the block average filter. Alternatively, user can use x-spacing and y-spacing to set up the grid for averaging (see below).

      • x-spacing, y-spacing Instead of setting number of points one can set desired spacing and the corresponding number of points will be automatically recalculated.

        ???+ note "Hint:" The last four columns can be used for filtering of highly oversampled horizons. For instance, a resolution of 100 m

        ×\times
        100 m for Moho depths is clearly an oversampling.

    • Press ++"Next"++

    • The next wizard defines the general model parameters derived from the loaded horizons:

      • Extend model borders: Use this checkbox, if the model should be extended laterally to reduce the border effect, and specify the model extension in Range.

        In our example we take default: 2192 km.

      • Minimum vertical distance: Minimum thickness of bodies. It is used only if the imported vertices have identical horizontal positions throughout all horizons or if the vertices are interpolated regularly on the sections (see Project Points (Mundry) below).

        In our example it is 2.2 m.

      • Z-Top. Depth of the upper limit of the model (plane, horizontal). Default 0, if no topography is given, otherwise maximum Zmin of all horizons.

        In our example model there is no topography.

      • Z-Button. Depth of the lower limit of the model (plane, horizontal) - defines the bottom of the model. Default: minimum
        ZminZ_{min}
        value of all horizons.

        In our example model we take biggest depth of 400 km (upper mantle).

      • Units. Make your choice depending on the data entered (depths, distances, grid spacing, etc.).

        In our example we use km.

      • Project Points (Mundry). Use this checkbox to interpolate irregularly spaced horizon vertices on the sections with the desired Distance.

        In our model, we wanted to re-interpolate the data ("even" grid spacing). For this purpose a procedure according to Mundry is used.

    • Press ++"Next"++

    • In this last wizard we are able to specify the area to be modelled and the position of the vertical sections.

      • The modelling area is the maximum area, which is covered by all horizons - indicated by a grey rectangle.
      • By default 5 vertical sections are suggested.
      • The numbers on the axes indicate coordinates in the example these are UTM coordinates.
      • The green circle :green_circle: defines the southwestmost point of the modelling area.
      • The red circle :red_circle: defines the northeastmost point of the modelling area.

      You may change the position of the circles by either clicking with the right mouse button on them (alphanumeric input); (an example for the coordinate input of the red point you can see here):

      or just dragging them. Both input options redefine the model boundaries, also change the spacing of the five specified vertical planes (dashed lines in the window between the coloured points.

      • Azimuth, N – E – S - W. Sometimes the horizontal direction of the vertical sections must be adapted to the gravity field to be examined, because the modeling should ideally always be as perpendicular as possible to the main strike of the anomaly - this ensures the greatest possible model gravity effect. You have the possibility to set a first rough adjustment of the direction via North - South - East - West.

      • West: the vertical sections run in N-S-direction

      • South: the vertical sections run E-W-direction.

      In our example from the beginning, the vertical sections are aligned in the west-east direction and count from south to north.

      If you want to rotate it even more precisely, use the numeric input in the azimuth window of the setting. In the example in the next figure, 283° (270° + 13°) was used:

      • Distance. The vertical sections to be created are indicated by dashed lines. Use the numeric input field to modify the distance between the vertical sections. Specification in km (as defined above for the input units). In the example, this would be approx. 317 km (316.85 km).

      • Count: Use the ++"<"++ and ++">"++ buttons to decrease or increase the number of layers.

        In the example, the number of vertical sections has been doubled; the distance between vertical levels is reduced accordingly to 133.94 km:

    • Press ++"Finish"++

      The model appears in the IGMAS+ main window, defined by the 10 vertical planes in the central part of the model and additionally a bounding section in the north and in the south - as it was entered earlier in the 2nd wizard window (above).

      The model can now be moved back and forth for viewing. Click into the model with the right mouse button and keep it pressed. In this combination, move the model in the window. Moving the mouse wheel changes the zoom.

      Note: The colours of the stratigraphic layers are set automatically by the program. You change them as shown below (refer to Colors). We still have no stations, no reference gravity field and the model densities loaded.

      But we can already have a quick look at the vertical sections. If you are interested, go straight to the item “show vertical cross sections” below and return later to this position.

    Import stations

    How to import reference gravity/gradient/magnetic field and topography/bathymetry?

    Use the File > Import > Stations

    Be sure to use the correct units and file type (.csv or .xyz)

    In this input window you have the chance to assign different input parameters to the individual columns X - Y - Z. Column Z could also contain gradients or a magnetic field size. "Measured z component" is selected correctly.

    Press Finish

    The stations are placed in red on top of the vertical cross sections.

    However, we do not yet have a basis for calculating the model gravity field. For this, two steps are necessary for preparation.

    (1) Triangulate the vertical cross section, which results in a true 3D structure.

    IGMAS+ offers the user two options: Press either in the TITLE BAR > Edit > Model - Triangulation

    or Press in the TOOL BAR

    Next you will see this wizard:

    Press Next

    … and get from the program the following information:

    Check the messages in the table. Here possible errors during triangulation are indicated, but at the same time it is pointed out that they will not be serious. This is a numerical instability in the visualization, which has no influence on the gravity calculation.

    PRESS Finish

    In the status information (below) the message will be shown that your model has no errors (green light), and we can proceed to calculate the modelled gravity field.

    To calculate the modelled fields, IGMAS+ offers two possibilities:

    Click in the TITLE BAR >Tools > Calculate Anomalies

    and select Calculate Anomalies.

    or

    press in the TOOL BAR

    and see the window:

    Select the field component you will calculate and then

    PRESS Finish

    Of course, the length of calculation time depends on the size of the model and the number of stations. Be patient with large models!

    The green "traffic light" of the "progress bar" in the “lower status line” gives you the certainty that everything has been calculated correctly.

    … then the time has come to see the modelled field and the model in perspective on the screen.

    Load a project

    First select “Open Project”.

    Open a version

    When opening a project, several versions are displayed. These are all versions that have been saved earlier.

    Select the following: