Guest blogger Yafet Birhanu shares with us one common bottleneck problem faced when modelling the weathering profile in many exploration projects. That is, ignoring the effect of topography on the weathering profile.
Enough attention has been given for producing the most realistic weathering profile in gently undulating topography or in situation where there is good drill hole sampling. The conventional approach of producing weathering profile by interpolation of contact points, however, cannot be used in projects with ragged topography and sparse drill hole data. To tackle this problem, an approach is designed and demonstrated using Leapfrog Mining software. Please read the article and share your opinions with us below.
The weathering profile or the base of oxidation surface is an important surface to model for a number of reasons, but it is mainly used to classify materials into specific gravity domains for the purposed of resource estimation.
The base of oxidation is produced from the fresh-oxide contact under topography sampled by drill holes. Weathering surface and topography usually have a simple geometric relationship in many regions. The base of the weathering surface commonly follows the undulation of topography and is below the topography surface unless it is exposed by erosion.
Millions of topographic point data can be collected using LIDAR or other surveying methods. In contrast, the amount of raw data we can obtained for the weathering surface is limited to the number of drillholes. The figure below shows a cross-section of topography represented by surface and depth of weathering represented by points along drillholes.
Usually weathering surface is generated by interpolating the contact points, but this approach does not consider the topographic undulation. In some areas, the weathering surface can cross the topographic surface and be then located above the topography. An example of such unrealistic weathering profile is represented in a blue surface in the figure below.
The illustration below shows a topography and weathering profile outlines with drill hole trace and fresh-oxide contacts on a cross-section. Two possible weathering profiles are drawn. Figure (a) shows equivalent of a weathering profile interpolated from the fresh-oxide contacts. Figure (b) hows a weathering profile constructed using the fresh-oxide contacts following the undulation of topography.
The weathering profile shown in (a) above is the most common type used in many projects. This approach is tolerable only in gentle topography. In areas where topography is rugged or undulating a valid approach should be followed to construct a weathering profile. The result of the second approach which honours fresh-oxide contact and follows topography (governed by the undulations in the topography) is illustrated in figure (b).
The following outline summarises the major steps needed to produce a realistic weathering profile using fresh-oxide contacts and topography.
A) Identify vertical depth and XY coordinate of base of weathering points
The 'Project Vertically Onto Mesh' tool of Leapfrog Mining is found by right clicking on point features which is oxide-fresh contact in our case. Using this tool project the points to the topography and export the projected point to csv file. Two Z values are found in the CSV file; the difference is the vertical depth of the points.
B) Import x, y and vertical depth CSV file to Leapfrog Mining
Import the prepared csv file by taking the vertical depth as z value instead of elevation. Using surface interpolant of LFM, interpolate the point data to create surface. Cross-sectional profile of the result displayed below shows positive depth value of oxide-fresh contacts throughout project area.
C) Prepare Grid Points
The easiest way to produce weathering surface at this step is to deduct depth interpolated surface prepared at step B) from topographic surface. In LFM there is no one click step to perform arithmetic operations on surfaces as there is Boolean operation for wireframes/solids.
Prepare dense Grid Points at a specific elevation. We project these points vertically to the two surfaces and acquire respective Z values of the two surfaces.
Below we can see 3D representation of grid points created at a specific elevation. A cross-sectional view shows the relative positions of the three features(topography, grid points and positive depth surface) easily.
Once the grid points are created, use the Project Vertically Onto Mesh tool to vertically project the Grid Points to the topography surface and the positive depth surface. Export the projected points to csv and merge the data into one spreadsheet by matching x and y values to calculate the Z value differences from the two surfaces. In other words the mentioned process deducts positive depth values from the topography and assigns the result to the location of grid points.
D) Produce the weathering surface from the z difference
Import the csv file prepared in step C) and interpolate surface using the z difference. The output is the most realistic weathering surface represented in green color as shown below.
The green surface is the weathering profile interpolated crossing fresh-oxide contact and following undulation of topography.
As can be seen in the cross sectional figures, the blue surface is produced from simple interpolation of the fresh-oxide contact and is not governed by topographic undulation. See also another cross-section below to examine the difference between the three surfaces. Red: topography, Blue: interpolation of fresh-oxide contacts and Green: Weathering profile from fresh-oxide contact and topographic effect.
The most important factors to produce a more realistic weathering profile are the number of sample points, sample distribution and technique of interpolation. The methodology described above explains an useful technique which takes into consideration the topographic undulation.
(GIS specialist and Geologist)