Overview
GeoModeller mainly relies on foliations and interface/contact points to draw surface via cokriging interpolation of a 3D vector field. GeoModeller is able to consider the influence of foliations on the vector field anywhere in the model box. Consequently, it is possible to place foliations away from the interfaces and obtain geologically sound results (Figure 1).
Figure 1 – Foliations constrain the geometry of the interfaces regardless of their location.
However, GeoModeller is actually unable to fully incorporate this information automatically. GeoModeller discards the lithological support of observed foliations and records them as mere vectors with known orientations. The lithological support of foliations refers to the fact that foliations are taken off of actual outcrops. Therefore, foliations should not only be recorded as vector data to constrain the gradient of the vector field but also as a point data to constrain the interfaces. Not doing so can lead to absurd situations where an observed foliation is at a location where the model determines that another formation takes place (Figure 2, Figure 3 bottom right). Note that foliations orthogonally associated to interface points (Figure 3) are not concerned with this discussion as they typically are not genuine observations.
Figure 2 - Foliations of a formation may be located inside of another formation.
Figure 3 - Orthogonal foliations are often "miss-located", this is inconsequential.
The desired outcome is to somehow manage to extend the formations to their genuine foliations to honor the existence of their support.
For the sake of conciseness and simplicity, in the following sections, the type of foliation that is of concern will be referred to as “remote foliation”.
Single remote foliation case
In the case of a single remote foliation, all that is needed most of the time is to add a contact point on top of the remote foliation and recompute the model (Figure 4). This method assumes that the remote foliation is also a contact.
Figure 4 - Single remote foliation upgraded to associated contact point.
Multiple foliation case
Incorrect methods
When there are multiple remote foliations, applying the method described above on all remote foliations produces meaningless results regardless of assumptions (Figure 5).
Figure 5 - Multiple remote foliation are not solved by upgrading to associated points.
One might be tempted to apply the method on the remote foliation farthest to the original interface (Figure 6).
Figure 6 - Multiple remote foliations cannot be dealt with a single associated point.
However, this is ill-advised as it leads to many artefacts and doesn’t guarantee that all remote foliations be included in the formation.
Correct method
A completely different procedure is required to ensure that all remote foliations are appropriately included in the formation. The proposed method takes advantage of the fact that multiple formations share the same vector field when they are part of the same series.
For each remote foliation:
- Create a dummy formation
- Add the dummy formation to the series that contains the original formation
- Add a contact point on top of the remote foliation
- Attribute this contact point to the dummy formation
- Repeat for all remote foliations
Next all dummy formations should be ordered in the series as a function of distance to the original interface above or below the original formation (Figure 7, Bottom formation).
Figure 7 - Ordering of the dummy formations.
The next and final step is to anchor the farthest and nearest dummy formations (Figure 7, DummyBottom0 and DummyBottom6) on the existing geometry of the original formation to constrain the intercalary dummy formations. Some of the other dummy formations might need to be anchored in the same way if the geometry of the intercalary dummy formations is not constrained sufficiently.
Figure 8 - Successful integration of all remote foliations.
It is often observed that dummy formations are not visible because of the anchorage procedure described above (Figure 8). Dummy formations’ appearances may be harmonized with the original formation when it is not the case. If needed, tweaks in the variographic cokriging parameters may give a cleaner fit (Figure 9).
Figure 9 - Variographic parameters may be altered to achieve a cleaner fit.