It is not often in geology that mineralisation or geological units behave in a consistent planar fashion. This article aims to give users an introduction to the application of structural trends and how they can be applied to a model to handle different situations.
What is a Structural Trend?
A structural trend is a generalisation of the global trend that allows changes in direction of continuity over a defined surface. Instead of being based on a plane like the global trend with the user defining the ellipsoid ratios, the structural trend is based on a surface. This surface can be any shape or orientation usually defined by geological constraints such as faulting, foliation etc. The surface is then effectively down-sampled to determine the local trend at each point on the mesh to give the user an anisotropy that varies throughout the defined space. This makes the structural trend perfect for geological units or mineralisation that is not planar. The structural trend does not determine the final surface; this is still done by the interpolant and the data points used. In Leapfrog the default interpolant type is isotropic, which lets us more easily visualise trends that are often hard to pick up when looking at raw data. Figures 1, 2 and 3 show the difference between an isotropic interpolant, global trend and a structural trend.
Types of Structural Trend
Leapfrog has three different types of structural trends which each behave slightly differently. Each trend type is covered in more detail with examples below. What they do have in common though is a strength and a range. The strength is similar to the ellipsoid ratio and dictates how much elongation or stretch you have in a certain direction. The range on the other hand is the distance from the mesh in which the trend has effect. To make it easier to visualise the trend, discs are displayed along the mesh. The size and shape of the discs represents the strength and orientation of the trend, and can range from circular to cigar shapes depending on the type of trend used.
Non-decaying trends are similar to global trends in that they have influence across the entire model but they can be built from a mesh or multiple meshes of any shape. A non-decaying trend is particularly useful in cases where all locations within the model need to follow the same trend with a consistent strength throughout the model. The discs in Figure 4 show a visual representation of the trend at each point throughout space.
Blended trends allow users to build a trend from multiple meshes specifying a range and strength for each mesh. If two or more of the trends intersect each other, Leapfrog will work out a combined trend. Figure 5 below shows the main trend built from the large blue mesh with two small meshes running on an angle to represent ore shoots. The discs in the image represent the trend at different locations. You will see in the centre where the ore shoots have influenced the main trend; the overall trend has been blended together to produce cigar shaped discs, which will give greater continuity in the direction of the ore shoots.
The blended trend is perfect for situations where two or more trends merge and you want to have a smooth transition from one to the other or show the combined effect.
Strongest Along Meshes
Strongest along meshes is a type of structural trend which allows users to create multiple trends that decay in strength back to the estimated global trend of the data based on a set range. This is particularly useful where there is strong geological continuity along a certain trend with intervals away from this behaving either more isotropically or in a planar manner. The images below are an example of the application of the strongest along meshes trend courtesy of First Quantum Minerals Exploration. The structural trend has been used for two main aspects of the Haquira East Project.
In the first case it was applied to the geological model to build a tightly overturned fold. Firstly, a mesh was hand-drawn with three polylines to roughly represent the orientation and position of the fold. From this mesh, a structural trend was created. The trend was then applied to the upper and lower surfaces of the folded formation which were created using intervals derived from drillhole intersects and geological contacts at the surface.
In the second case a series of structural trends were used to influence the interpolation of Cu grades. Trends were defined for each structure that had significant control of mineralisation and continuity. These trends were then applied to the interpolation of Cu values.
For further details on how to use structural trends contact your local support team and ask about our new Intermediate Leapfrog Geo training, which covers all aspects of structural trends and interpolation.