Tiling VOXI magnetics inversions

SeanWalker

Does anyone have a reliable workflow for tiling VOXI inversions (ie splitting a large area into smaller tiles and then merging the results)? I have a very irregular block and the sub-areas are different dimensions and therefore the VOXI from PLY gives me voxels with different depths/lin log cutoff etc. I am now manually editing each voxel after creating the VOXI (Mesh>Modify). Just wondering if anyone had a better method? I have the voxels overlap a bit and use the voxel merge utility but it still seems to have some seams.
Thanks
Sean

NickWilliams

Hi Sean, late reply, but hopefully still helpful.

I routinely tile VOXI MVI models. My workflow is usually:
1) Create a new master VOXI for the whole data area at the final desired cell size. Customise the mesh as desired then note all of the mesh parameters. Make sure topo covers the whole master mesh so that you don't clip the highest and lowest topo points. If you want to have a consistent data level for all tiles then add your data too, and note the regional trend removal parameters you use. I usually prefer to have the same level, but I can see situations where it might make sense to have different data levels. We don't submit this VOXI job.
2) I use a simple script to tile the PLY file into the desired number of rectangular tiles. It would be possible to manually draw appriopriate PLY rectangles. Important to 1) ensure outer edge of all tiles extends just past the PLY file limits, and 2) Overlap tiles by ~30-50% to ensure longer wavelengths near the edges of each tile are managed appropriately. Remember that it is also good to have the VOXI padding width match the depth of the model to manage long wavelength edge features. Using a script helps to ensure each tile is small enough to run on VOXI. 
3) For each tile's PLY, set up a new VOXI. Copy all of the mesh parameters exactly from the master mesh. It is critical that the discretization is the same to avoid joins in the merged VOXI. Add your data and if desired copy the master VOXI data trend parameters. Run each VOXI.
4) Merge or use Voxel math to join the unpadded resulting voxels.

Depending on the variability of the data across the area, you may want to apply the same or different data uncertainties. Likewise you may wish to use the same IGRF for each or different for each.

I find the results are amazingly seamless as long each each horizontal layer of cells is perfectly aligned with the master mesh. Sometime you see seams only outer-most faces of the voxel, but they do not extend internally.

Cheers,
Nick

PeterTschirhart

For point #2) if you don't mind switching between Geosoft and QGIS, QGIS has the tool Create Grid... which can be used to very easily generate a tiling grid of user defined tile widths and overlaps distances. You just need to have an idea of the voxel size you'll use beforehand so that you can work out how large your tiles can be. Additionally, if you have irregular boundaries you can intersect the tile grid with your project boundary to include (rather than perfect square or rectangles). The most cumbersome part is converting between vectors and ply files.