By Lorraine Tam

Often, generating a continuous overburden surface with uniform thickness from limited drilling is difficult, especially when it is not logged in a consistent manner.  Simply extracting the bottom contact of drillholes may produce a patchy result which will require a significant amount of manual editing.

Project area.

Plan view of the project area. Drillhole collar locations are shown in red. The overburden surface (in yellow) does not extend to where there is no data.

2D slice through the model.

A 2D slice through the model. The overburden is patchy because where there is no data, it crosses above the topography (shown in light green).

If the overburden is known to be continuous throughout the project area and the minimum (or maximum) thickness is known, the surface can be created by offsetting the topography. There is a function in Leapfrog Geo allowing a surface to be offset through known points.

This workflow will explain how to extract the known overburden contact points from drillholes and offset the topography down to these points. This workflow assumes the geological model has already been created and that there is a topography mesh in the “Topographies” folder (shown with the mountain symbol).

How to create a continuous overburden surface.

Step 1 – Extract the contact points to the “Locations” folder

  • Right-click the Locations folder > New Contact Points.

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  • Select the column from which to extract the contacts. Click OK.
  • This will open up the New Contact Points window. In the Lithology tab (the first tab), select “Overburden” as the primary lithology. Select “Use contacts below” and all contacting lithologies will be displayed in the column on the left hand side. Ignored lithologies are those that exist in the dataset, but do not share a contact point with the primary lithology.
  • Click OK to extract the points.

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  • Drag the points (shown in white) into the scene to view them.

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Step 2 – Create the overburden mesh

  • Right click on the Meshes folder > New Mesh > From Offset Points.

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  • Under the “Mesh” drop-down list, select the “topography” mesh shown with the mountain symbol.
  • Under the “Points” drop-down list, select the contacts that were extracted in Step 1.
  • A minimum and maximum offset distance can then be set (if known).
  • Set the extents and a resolution.
  • Assign a name to this mesh (for example, “OVB from offset topo”). Click OK.

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  • Drag the mesh into the scene. Compared to the one that was created directly in the geological model, this one is less patchy as it now follows the overburden and includes the contacts if they fall within the distance limits. If no distance limits were specified, the mesh is offset based on the distances of the contacts from the topography.
Overburden mesh.

The overburden mesh created from the offset topography (3D view).

Overburden mesh 2.

The overburden mesh created from the offset topography (2D slice view with topography shown in light green).

Step 3 – Put the mesh into the geological model

  • In the geological model, right-click Surface Chronology > New Erosion > From Surface.

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  • Select the “OVB from offset topo” surface.
  • Set the First Lithology as “Overburden”. If using the “is Younger” drop-down, then set the Second Lithology as the older unit. Leave this as “Unknown” if the overburden is in contact with more than one unit. Change the name if desired and click OK
  • Double-click on Surface Chronology. Arrange the units in chronological order.
  • If there is more than one interpretation for a surface, check off only the one to be used (in this case that is the “Overburden from offset topo”).

Step 4 – Activate the surfaces

  • Double-click on Surface Chronology. Arrange the units in chronological order.
  • If there is more than one interpretation for a surface, check off only the one to be used (in this case that is the “Overburden from offset topo”).

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  • Click OK and allow the model to process.
  • View the results.

Results.

The resulting output volume should appear much more continuous than the original. Another advantage of using this workflow (rather than building from contact points directly within the geological model) is that the overburden is now generated using a mesh. If the model were to be faulted, the overburden would not be affected. To achieve the final surface, it may still be necessary to manually edit it using polylines or structural data to make interpretations where there is limited contact information.