How to perform Dynamic Anisotropy in Surpac

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Grade estimation for undulating or folded deposits can often be challenging. A variety of solutions can be used to reduce the bias in the search parameters during block model estimation, one of which is dynamic anisotropy interpolation.

Dynamic anisotropy interpolation is an estimation method which takes into consideration the local variation of the domain orientation into the block estimation.

Why use dynamic anisotropy?

 

Dynamic Anisotropy – Before and After

 

The traditional estimation process is to use a single search ellipsoid within a domain. But when this grade continuity within the domain is folded, the estimation can be biased by the misalignment of the search definition to the direction of the grade continuity. A common approach was to create “subdomains”.

Note that for complex orebodies, dozen of subdomains may be required, hence “subdomaining” may be time consuming and an error prone process. For each subdomain, a variogram will be calculated and modeled. You also may not have sufficient samples in a subdomain to model a decent variogram.

From Surpac 6.7.1 onwards, it is possible to perform dynamic anisotropy to define the search parameters for Inverse distance, Nearest Neighbour, Ordinary Kriging and Simple Kriging.

What is the process for dynamic anisotropy in Surpac?

This process uses mainly block model attributes to store the orientation of the search ellipsoid and consider these parameters during the block value estimation.

1. Create a trend surface

This surface should honor the different variations in dip and dip direction (trend) of the mineralization. The trend surface can be created in numerous ways:

  • In case of an upper and lower surface are defining the orebody, you can use “Surfaces> Advanced options> Maths between surfaces” to create the mid-plane (HW & FW, or stratified deposits).
  • In case the domain information is already in the database, extract the composite by Geology for each domain at the mid-point of intersection intervals. “Database > Composite> By geological constraints” and form a DTM of this point data.
  • Digitize on a section basis the lines of grade continuity and create the trend DTM surface.

Note: Smoothing of local angles may be required so please do this carefully.

Using the mid plane as trend plane for grade continuity inside the solid

 

2. Create the trend outlines

Use Surfaces > DTM file functions > Create Trend outlines from DTM triangles to create a string file from the trend DTM surface. The output file is a string file which will mimic each triangle as a polygon with dip in d1 and dip direction in d2.

Create Trend Outlines from DTM triangles

3. Add block model attributes

Add appropriate block model attributes to store value for dip and dip_dir. Make sure the direction selected honor your search ellipsoid definition. Next, assign value from the polygons to the block model:

  • Across strike with “Block model> Estimation> Assign value from polygons”; or
  • Use “File tools> Extract centroids from polygons”, then use Nearest Neighbour with the polygons centroids
Assign value from polygons to block model

 

4. Select attributes during block model estimation

Select the attributes from Step 3 in the Search parameters form during the block model estimation. For the estimation of each block, the ellipsoid will be set exactly as per the values in the orientation attributes for each block.

Set Search Parameters – Ellipsoid Orientation
Estimation with and without dynamic anisotropy

 

The use of dynamic anisotropy or local anisotropy can have a large local impact in the estimation outcomes. Thus, care should be taken when defining the local variability when removing spurious angles and/or when applying smoothing.

Dynamic anisotropy should mainly be based on the structural geology rather than the artefact of the domain wireframe. GEOVIA Surpac provides the effective tools to perform dynamic anisotropy in a conventional geostatistical framework.

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Contact us at GEOVIA.Info@3ds.com if you need any further assistance or have questions on dynamic anisotropy and I will be glad to help you out!

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Claver Gnamien

Claver Gnamien

Mining Knowledge Consultant, MAusIMM at Dassault Systèmes
Based in Perth, Australia, Claver has over 10 years of mining geology experience, specializing in the areas of grade control and reconciliation, resource estimation, block modelling, planning, supervision, logging, hydrogeology, and geotechnical engineering. He is also knowledgeable in the exploration of basic and precious metals, having previously worked as a Senior Mine Geologist and Geotechnical Engineer in Côte d'Ivoire, West Africa.