By Lisa Swinnard

There are several ways to edit and modify an “Intrusion” surface within a GM; of them, value clipping is often overlooked. This document is part one of two and will outline the procedure for modifying intrusion surfaces by value clipping, describe the importance and relevance of volume points, and explain how the two are connected.

1. Modifying intrusion surfaces by Value Clipping

Step 1 – Create a new geological model and intrusion surface

  • Within the GM, create a new intrusion surface.
  • Edit the surface accordingly, add a trend if necessary.

Step 2  – Modify the value clipping

  • To modify the value clipping, double click the intrusion surface in the project tree.
  • In the dialogue box that appears, click on the surfacing tab and check the “Show additional surfacing options” box.

Value clipping. Dialogue box. Surfacing tab. Leapfrog 3D.

  • Once the box is checked, more tabs will appear. Click on the value clipping tab.
  • By default, an automatic value clipping already exists on the intrusion surface.

Value clipping. Dialogue box. Value clipping tab. Leapfrog 3D.

  • Clipping can be modified here; however, before doing so it’s important to understand what these values represent and how changing them will modify the surface.

2. Volume Points

Leapfrog constructs surfaces based on drillhole data and interpreted lines/points using a mathematical algorithm known as a Radial Basis Function (RBF). This algorithm relies on data points with known numeric values to interpolate surfaces in between said points. However, drillhole information is typically introduced into Leapfrog in interval tables that contain From-To intervals and lithology (etc.) codes, not numeric values.

In order for Leapfrog to understand this information, it must first convert the interval data to point data and that point data must have associated meaningful numeric data. This resultant point data is known as Volume Points. Every time a surface is created in Leapfrog from drillhole data, volume points are automatically generated. The distribution of Volume Points can also be modified, if necessary.

Step 1 – Accessing Volume Points

  • In the Project Tree, click the little black triangle beside the surface of interest in the GM. The object below the surface is the contact point’s object.

Assessing volume points

  • Drag the contact point’s object into the scene window. Select the points object in the Shapes list to activate the properties panel in the lower right hand corner. In the Properties panel there is a tick box to “Show volume points”.

Show volume points

Step 2 – Explanation of Volume Points

  • The red points have positive values and exist inside the unit of interest. The blue points have negative values and exist outside the unit of interest. The actual contact between two units is a green point with a value of 0.
  • These values represent actual distances away from a contact point along the drillhole trace.

Explanation of Volume Points

  • The spacing of the points is defined by a “Surface offset distance” and a “Background fill spacing”, which are automatically generated based on the lateral extent of the data.
  • Both these values can be changed by double clicking the points object to open the point generation window.

Point generation

  • The image below shows the results of the “Surface distance offset” and the “Background fill spacing”.

Results of the “Surface distance offset” and the “Background fill spacing”.

Definitions of “Surface distance offset” and “Background fill spacing”

The Surface offset distance parameter sets the top and bottom ends of the interval and affects how a surface behaves when it approaches a contact point. A smaller distance restricts the angles that an approaching surface can take. Another factor that affects the angles a surface will take is whether or not a trend has been applied to the surface.

The Background Fill Spacing parameter determines the approximate length of segments in the remaining intervals. If the remaining interval is not a multiple of the Background Fill Spacing value, Leapfrog Geo will automatically adjust the spacing to an appropriate value. A smaller value for Background Fill Spacing means higher resolution and, therefore, slightly smoother surfaces. However, computation can take longer.-Leapfrog Help File 3.

3. Volume Points and Value Clipping

The values that appear in the histogram on the Value Clipping page correspond to the generated volume point values. Below is a histogram example with no clipping. The positive values reflect the volume points that lie inside the unit of interest.

Volume Points and Value Clipping

For larger intrusions, there will be a decent spread of positive and negative values, as appears in the image above. However, in the case of very small intrusions over laterally extensive projects, this distribution will be heavily skewed towards the negative numbers as there will be significantly more values outside the intrusion (i.e. negative) than there will be inside (i.e. positive).

Value clipping affects the distribution of volume point values and this in turn affects how the surface is interpolated between two known contact points. When a clipping value is applied to the lower bound, all values lower than that set value will be reassigned to equal the set clipping value; the same is true for when an upper bound is set.

Volume clipping and value points
All values higher or lower than +/-21.66 equal +/- 21.66 when clipping is applied

Look out for the next part of this blog in the coming weeks. You can subscribe to the blog in the top right hand corner of this site, so you do not miss out!

3 thoughts on “Modifying intrusion surfaces – effect of value clipping on intrusions (part 1/2)”

  1. Was there ever a part 2 to this post? Something that describes what effect different clipping values have on the modelled volume? Can’t seem to find one anywhere…

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