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properties.md

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    Modification of Physical Properties

    We already knew this action when it came to changing the density/susceptibility of a new body (see also "Split a body/polyhedron on a specific vertical cross section").

    Change densities/susceptibilities

    Select in the window of BODY MANAGER / PROPERTY Editor (bottom left) and select Body manager.

    The names of the existing model bodies are displayed with their selected colors. Then follow from left to right "voxel factor", "density [t/m^3]", "volume voxels [km^3]" and "volume polyhedron [km]". What of this information is displayed, the user can specify

    by clicking the small in the right corner.

    The various parameters can be changed in their horizontal position: Click with the left mouse button on the parameter to be moved and drag it to the desired position with the mouse button lowered.

    Double left mouse click on the body (here upper mantle) where the density/susceptibility is to be changed and type in the new density alpha-numerically.

    Note: The resulting change in model gravity is displayed immediately, that is, the density change is performed automatically by the program.

    The definition of „reference density“ is import because it minimizes the edge effect” of the model. Always set the reference density so that the edge effect is a minimum! Try this out on your model.

    Density Inversion

    Besides the possibility to change densities directly by the user, IGMAS+ also offers the possibility of an automatic calculation (inversion).

    ???+ warning "Attention:" The inversion does not make sense if all or very many densities are to be changed in a model. The system of equations would then have too many parameters, would be overdetermined and the result would be meaningless. You have to try it out! One or two density values should be allowed for an inversion (e.g. 2 values out of 10).

    The MMSE method is used in IGMAS+. MMSE stands for “Minimum Mean Square Error” and utilizes the mean square approach and Gaussian random variables within a statistical framework (refer to Haase (2014) for details).

    Select in the TITLE BAR “Tools” among other important program activities – we already know klick on “Parameter inversion (MMSE). Here is what you see:

    The next window opens:

    First, one has the possibility to select the density-inversion by means of Gravity and/or Gradients”; with “Magnetic” one would make an inversion of susceptibilities (no susceptibilities exist in this example); refer to figure above.

    To modify the error given for the field(s) (here 0.2 mGal) refer to scenario “Change error of measured fields” in last wizard.

    At the right side of the above window, under the tab "IGMAS effect", the inversion of densities of polyhedra is to be selected (figure above). To set and/or to modify the standard deviations for the different densities in the menu above (STD: 5.0 t/m3) refer to scenario “Change standard deviation of densities/susceptibilities” below. For the inversion it will be sufficient in most cases to leave the values (STD) unchanged.

    Otherwise, the densities of the voxel cube “Voxel Effect” can also be inverted (next figure).

    To set and/or to modify the voxel factor (here 1.0) and the variance/STD 5.0) for the different bodies in the menu above refer to scenario “Change standard deviation of densities/susceptibilities” below. For the inversion it will be sufficient in most cases to leave the values (“voxel factor” and STD) unchanged. The “voxel factor” is explained in detail in the scenario “Voxelcube).

    Below under “settings” you can switch off or switch on the of geological bodies listed above.

    Invert the densities of polyhedra “4Astenosphere” and “reference” (under IGMAS Effect). All bodies under “Voxel Effect” are disabled now:

    Click Next and the result will be presented in the next window

    Click Finish to accept the new densities

    The upper panel lists the densities before and after the inversion and the lower statistics panel shows the standard deviations (before/after) and the Pearson correlation coefficients. Since the correlation after the inversion is better than before, the result of the inversion was accepted.

    In the other case, by clicking on "Previous", the user would have the possibility to select other bodies, respectively to change the standard deviations and errors.

    Attention:

    To change standard deviations and errors, the inversion must be canceled. It is not possible to change the above parameters in the inversion window.

    Change error of measured fields

    Click in the OBJECT TREE on “Gravity z-component”. Then select Property editor

    And select one of the icons :

    Click on “error gz” and then on “Value”. Double click with left mouse button

    Enables the user to input a new value for the error of measured gravity field.

    Change standard deviation of densities/suceptibilities

    Click in the OBJECT TREE on “Gravity z-component”. Then select Body Manager.

    You will see a window like this; (be aware: this here is only a snippet):

    In the next step it is important to move the displayed parameters so that the items to be changed are visible. To do this, the window can be enlarged (keep the left mouse button pressed on the right edge of the BODY MANAGERS and drag it larger...).

    Then, by clicking the ICON above right corner, a selection of the parameters can be made. Now it is important that Standard deviation is selected - for the density or for the susceptibility.

    This results in the next screen:

    … and after moving/sorting the parameters by pressing left mouse button and moving them into another position in the BODY MANGER, we see:

    To select a new Standard deviation for the density/susceptibility of one of the geological bodies > click on the body > and then on “Add Parameter”

    In the small window that is displayed you may select the correct units (here t/m^3) and the type you will change: “Density Standard Deviation”:

    ... finally double click with the left mouse button on body and include the new value:

    In the same way, the other parameters can be changed - for example, the "Voxel Factor", which can also be inverted. In this case, the "Voxel Effect" tab must be activated in the "Inversion" window (see "DENSITY INVERSION" above).