By Antonio Celis
Traditionally, geoscientists have translated their field data collection efforts into a digital version of a geological map, which is typically built using GIS software (e.g. ArcGIS). Consequently most companies will have large archives loaded with GIS information from their projects. With more companies now choosing to collect their field data using electronic devices it ultimately results in a large repository of GIS data.
The purpose of this blog is to help make use of this valuable data and share an efficient workflow to morph a 2D GIS geological map (without drillhole data) into a 3D geological model using Leapfrog Geo. The main part of this procedure is to assign a vertical component for the 2D map while providing a representative final interpretation of the underground geology.
The minimum data required for this procedure is:
- a Topography
- GIS lines representing lithological contacts or an image of a Geological Map
- Structural measurements
The proposed workflow for 3D mapping presented here is based on performing a combination of two main actions:
a) New Category Selection + New Guided Points representing expressions of lithologies. Veins and faults will follow a different treatment and therefore will be excluded from this procedure.
b) GIS Lines + Planar Structural Data to model expressions of veins and/or faults.
1. Points selection
The point file used for this step must correspond to the point file that represents the topography. Thus, if the topography is made from a mesh, its vertices would need to be extracted. The treatment of points can be broken into two steps, new category selection of points and new guided points.
The criteria for the category selection of points should be based on the expressions of lithologies on surface. Here it is essential to use either an image of a geological map or GIS data representing lithologies as a base layer to guide the category selection of points. It is important that even if GIS polygon files cannot be dynamically used for modelling in Leapfrog Geo, they can still be used as a base layer in a similar way If you choose to dynamically use GIS polygon files (which is not intended for this workflow), they will need to be translated into their GIS polyline equivalents, where each GIS line should match a lithological unit.
The new guided points step is required to establish “cross-cutting relations” between interior and exterior point selections representing each lithology. This step needs to be repeated only for those lithologies built using New Deposit or New Intrusion type surfaces. Units built using New Vein will be modelled strictly using GIS lines and planar structural data.
2. Surface Modelling
The following step creates New Deposit and New Intrusion type surfaces in the geological model using the guided points representing each lithology. Alternatively, the intrusion contact can be built using or adding the corresponding IS line.
It is possible that the New Deposit/New Erosion type surface that normally represent an overburden-bedrock contact will need some extra refinement. The best option to refine this surface is by adding its GIS line. However, if the GIS line needs to be edited (e.g. deletion of segments, adding polarity, etc.), you will need to create and use its polyline equivalent (dxf) and carry out the necessary adjustments. This may include adding polyline points to drag or pull the surface below the topography to give a more realistic look. Another option is to create this contact as an offset surface from the topography and edit with polyline points.
3. Vein Modelling
Because veins, dykes or faults are usually mapped as single GIS lines and vein surfaces require a hanging wall (HW) and a footwall (FW), a special dedicated workflow is required to model these units. The simplest method is to generate a new mesh from the GIS line representing the vein, name it HW and add the structural measurements (or any vertical component). Then make a copy of the mesh, name it FW and offset it to obtain a FW mesh separated from the HW mesh by the desired vein width. Finally, because it is not possible to build a vein surface using meshes, you need to extract the FW and HW meshes vertices (points) and use those to create the vein surface.
A realistic 3D model derived from a 2D GIS map will ultimately depend on the correct interpretation of an area’s and underground geology. Therefore, it is essential that users collect and integrate as much data as possible to ensure an objective interpretation. This mainly includes structural data (i.e. compass measurements) and GIS lines of lithological/alteration contacts. In addition, it is critical to assess that the resulting 3D model fits the regional geological context. It is always good practice to review literature of the study area, (i.e. geological survey reports, field data observations, etc.), and use any observations and additional data, (i.e. cross sections, images), describing contact relations between units as well as structural orientations.