====== Volume Calculations ======
This tutorial describes the volume calculation facilities in GeoScene3D based on surface layers (grids), solid layers and 3D property volumes, like resistivity volumes. We will also look into using regions to limit the horizontal extent on the volumes calculated.
==== Step 1. ====
Volume Calculations using surface layer (Grids)\\
In this example we will be looking at 2 surfaces, shown in Figure 1, named “Lag1” and “Lag2”.
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Figure 1. The example data used.
To calculate the volume spanned by these two surfaces, activate the tool “Grid Difference Calculation…” in the “Toolbox” menu, see Figure 2.
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Figure 2. Activating the volume calculation tool : “Grid Difference Calculation…”
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Figure 3. The "Grid difference Calculation" Tool enables you to calculate volumes spanned by 2D grid surfaces.
In the tool window, shown in Figure 3, we select the “Bottom_Layer_3” as the “Top Surface”, and “Top_Layer_5” as the “Bottom Surface”. Then press calculate, and the area, volume and average thickness of volume spanned by these two layers is calculated. The unit of the calculated values depends on the coordinate system used in the data. If you are using a metric based system, like UTM, the results are in m2 for area, m3 for volume and m for average Thickness. If using a US feet based, or other, this would then be your base.
==== Step 2. ====
Volume Calculations using surface layer (Grids) – constrained by Region.\\
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The previous section showed how to calculate volumes based on 2D grid layers. This calculation can be further refined and constrained to an area using regions.\\
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First you must digitize a region. This can be done in several ways, but here I will use the map. Open your map and activate the “Region Digitizer Tool”, shown in Figure 4.
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Figure 4. Region digitizer tool.
Now digitize a polygon on your map, by clicking on the map. You finalize the polygon by double-clicking the last point in the polygon. This brings up a dialog for naming the “region” you just digitized – select a unique name and press ok. You have now digitized a region. Deactivate the digitizing tool by pressing the “Default Pointer Tool” button on the map menu. See Figure 5.
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Figure 5. Deactivate the profile digitizer tool by selecting the default pointer tool.
The result of the digitizing is shown in both 2D map, 3D window and in the object managers Region group. See Figure 6.
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Figur 6. The digitized region, here named "Region1", is now available in the object manager and viable in 2D maps and the 2D windows.
==== Step 3. ====
Now follow the procedures from the last section to enable the tool to calculate volumes. In the final step – check the “Region” box and select the region you wish to use to constrain the volume calculation. See Figure 7.
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Figure 7. Calculating volumes and using a region for constrain.
The results are as described in the last section, only here confined by the extent of the region.\\
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Volume Calculations using solid layer (Grids)\\
Solid layers are volumes defined by a top surface and a bottom surface. The Solid Layer therefore by nature defines a “layer volume” which can be inspected in the “Object Properties of the Solid Layer, see Figure 8.\\
How to find solid layers Choose the “Object Properties” for the “Objects” in the Object Manager and find “Solid Layer” under “New Object”.
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Figure 8. Inspecting object properties allow you to check the volume, area and average thickness of the solid layer.
**Volume Calculations using 3D volumes and iso-surfaces (3D grids)**\\
3D Voxels can be generated in many different ways and have different values associated with them. Geophysical values, e.g. resistivity’s from an AEM survey, and Lithological unit values, e.g. Sand or clay, or geological unit, e.g. “Pliocene formation”. In Geoscene3D it is possible to calculate volumes based on these 3D voxels. Our example is a 3D resistivity grid from an TEM40 survey, shown in Figure 9.
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Figure 9. Vertical and horizontal slices in the 3D resistivity grid.
Bodies of similar resistivities can be isolated using iso-surfaces. Open the “Object properties” by double-clicking the 3D volume node in the object manager, and select the page named “Isosurface”. See Figure 10.
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Figure 10. Isosurface page in the object inspector for the 3D volume.
Now enable the isosurface by checking the “Visible” check box and change the iso value. This will generate an isosurface in the 3D volume, with the current value visualized as a body in 3D. See
{{:tutorials:themecalc:eca82acca014e4c03d0599f440c6187c.png}}Figure 11. Isosurface of the resistivity value 71 give a series of bodies in 3D. Inside the bodies are resistivity values larger than the cutoff value.
Now click the “Volume” button and the current volume for the bodies shown in 3D is calculated. See Figure 12.
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Figure 12. Calculate current volume by clicking the "Volume" Button.
To further narrow the volume calculation, surfaces can be used to constrain and further limits on volume calculations.\\
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Extra limits on values are enabled by checking the “Value Limits” check box on the iso-surface page. The limits themselves is edited on the page “Limits”, as shown in Figure 13.
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Figure 13. Filtering on values shown with a lower and an upper value.
The IsoSurface shows the value of 71 (figure 12), and by cutting off all values above 120 using limits, a new set of resistivity bodies are now shown, and a new volume calculation can be done by pressing the “Volume” button.
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Figure 14. Enabling Value Limits cuts off values above 120. Note the new volumes calculated by pressing the "Volume" button.
Finally, layer surfaces can by utilized to limit the bodies. Here we will make use of 2 layers and thereby calculate the volume of this resistivity body that lies within.\\
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On the “Limits” page, select the “Surface” section and select the surface to use. Here we want to calculate resistivity volumes between Terrain and KGL. See Figure 15.
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Figure 15. Setting up surfaces as limits on volume calculations.
On the “IsoSurface” page you now enable the “Surface Limits”, See Figure 16.
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Figure 16. Enabling the Surface Limits reflects directly on the resistivity bodies.
==== Result ====
Now update the volume calculation by pressing the “Update” button. See Figure 17.
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Figure 17. Update the volume calculation. This is now a calculation of the volume of resistivities between terrain and KGL. having a value above 82 ohmm. This could e.g. be the calculation of a sand body in a channel structure. The two figures illustrate the difference before and after the surface limits are added.
A final note – the values could be anything – e.g. a contaminant giving you an idea as to how much soil are infected or other values.