Ignore or Reverse Samples

The following information relates to the vein-from-samples and surface-from-samples commands.

The Create Vein Surface task is a focussed tool for the calculation of hanging wall (HW) and/or footwall (FW) surfaces that represent vein or vein-like lodes. Similarly, the Create Contact Surface task is used to generate contact surfaces between groups of contiguous categorical values.

This topic explains how the vein-from-samples sample editing commands let you reverse samples, or ignore hanging wall or foot wall positions by deactivating them.

Note: A Datamine eLearning course is available that covers functions described in this topic. Contact your local Datamine office for more details.

The vein-from-samples command assesses the overall orientation of an implied structure and will assign HW and FW points accordingly. This assessment involves determining which side of the trend surface a point lies, with the assumption being that all hanging wall points will be on one side and all foot wall points on the other, or pinching out will occur. In cases where the general direction of drilling is similar, the default assignment is typically acceptable.

However, in other cases (such as deeply dipping or vertical lodes, or where a structure has multiple implied anisotropic trends), there is an increased tendency for drillholes to intercept the trend surface from opposite sides and directions, meaning there is no consistent assignment of hanging wall and foot wall points. This poses a challenge to implicit modelling routines (but not ours).

Consider the example below where the trend surface is formed by holes in different alignments. The implied surface is vertical and the dip of hole descent varies from 70 to 120 degrees. Constructing a surface with the default point assignment will result in excessive pinching out as the hanging wall and footwall surfaces will become inverted in several places:

Where the underlying structure (based on the presence of drillholes in the vicinity) strongly implies a steeply-dipping vein, it makes sense to reverse samples in this case and if the general trend is for holes to penetrate the structure from opposing sides, the Auto Reverse All option could be useful as it will determine the mean plane of the intervals (near to vertical) and apply hanging wall and foot wall assignments appropriately.

Interactively editing samples is achieved using the Edit Samples command group.

Enable or disable a hanging wall (From) or foot wall (To) point by clicking either the On or Off buttons for the corresponding point type. This activates one of four point editing modes:
- HW on
- HW off
- FW on
- FW off

Reverse sample positions to:
- Swap hanging wall and foot wall positions - if done at both ends of the interval, this results in a full inversion of the interval.
- Convert the drillhole into a double-hanging-wall or double-foot-wall interval. This would be useful where a hole is aligned so that it intercepts the lode hanging wall or foot wall in two places, for example:
Note: sample reversal can't be performed if Ignore gaps is checked. This is because, if enabled, the first instance of a FROM position for the selected Value will always be an HW point, and the final TO position will always be a FW point. If you must reverse samples where intervals contain gaps, those gaps must be resolved first, possibly using the Assign Lithology task.

You can also try:

Auto Reverse All, which creates a trend plane through all sample intervals and determines how intervals align in relation to the normal of this plane, automatically reversing them if their direction is significantly different from that expected. For the majority of data, this step will force a distinct hanging wall and foot wall arrangement that closely follows the structure that is intuitively expected.

Note: automatic reversal is performed when an attribute value is first selected for modelling.