Four tips for using the variogram map

Using the variogram map is conceptually pretty easy: you configure the best display from the source data and use that to identify the zones of greatest continuity in three dimensions. But in the real world things aren’t always so straightforward.

Here are four tips for understanding what’s really happening on the map:

Tip #4: What appears to be continuity … sometimes isn’t

Have you ever driven past a vineyard and noticed how the posts all seem to line up at certain angles? The same thing can happen with drillholes situated on a very regular drilling grid, and if these chance alignments coincide with a map direction they may create what appears to be a direction of strong continuity.


This mineral deposit is only weakly anisotropic. Much of the anisotropy visible in the map is concentrated around the first two lag intervals, but when you compare the directions to the drillholes in Vizex it is clear that they match the between-hole alignment

Always use your knowledge of your project area to validate any candidate directions, and be prepared to ignore any directions that are the result of chance drillhole alignments.

Tip #3: Near-vertical zones of continuity don’t mean your orebody is vertical

Sometimes the dip or pitch map shows a very steep zone of strong continuity. Don’t worry, your orebody isn’t standing on its end: it’s just another effect of your drillhole data.

When the map is in a vertical or near-vertical orientation (as the dip and pitch maps generally are), the downhole sample spacing usually dominates the vertical axis, but the between-hole spacing dominates the horizontal axis. And, in a typical mineral deposit the downhole spacing is much smaller than the between-hole spacing. The result? Way more downhole data than between-hole data, producing a zone of “continuity” that runs roughly parallel to the drilling direction.


This dip map suggests that the orebody has a nearly vertical dip, which contradicts the geological setting as shown in the Vizex cross section. This direction is driven by the predominantly vertical drilling, not the geology. The correct direction is the more subtle sub-horizontal one

The solution is straightforward: use your knowledge of your project area to validate the directions, and ignore any that don’t make sense.

Tip #2: Align Vizex to the map orientation to really understand what you’re seeing

The best way to really understand what the map is showing you is to view the data in Vizex with the display oriented parallel to the map plane. Along with providing an obvious reality check and helping you to understand your data, it also reveals the cause of any fake continuity from chance alignments or overabundance of data in any single direction.

In the Tip #3 image Vizex is aligned parallel to the variogram map, making it very easy to visualise the effect of the drilling.

For now you’ll have to work out the Vizex orientation yourself. Use the 3D View tab in the Vizex Display Limits dialog to set the Inclination to the dip of the plane minus 90°, and the Azimuth to the dip direction of the plane minus 180°. In future we’ll add a toolbar button to do this automatically.

Tip #1: Always click the Calculate True button before measuring a direction on the map

The interactive map simplifies its calculations to give you the fastest real-time updates as you refine its processing parameters. Clicking Calculate True forces the map to recalculate itself using the full dataset. The differences between the interactive and true maps are generally very small, but you want to ensure you are looking at the real data before measuring a direction. This is especially important if you make any interactive bandwidth adjustments.