[Qgis-user] How to get the thickness of a geol. layer from elevation isoline features

[Fernando M. Roxo da Motta]

On Tue, 3 Nov 2020 14:39:02 +0100, fantypus at web.de wrote:


> Hi all,

  Hi Eva,

  I am an exploration geophysicist and what is bellow is based on my
prior experience.

  I suppose those two lines are based on interpretation of profiles of
some sort (seismic, EMT, eletromagnetic, grav, or whatever).  My
question is:

  Are those lines just top and base of the layer of interest, or are
they crossing lines each one with the top and base of the layers?

  If those lines are in the same position containing just the top and
base of a geological layer it will be difficult, or just meaningless,
to extend those laterally to cover an area.  If you have just a 2D
profile of the underground it has no information of the surrounding
area.

  If those are two crossing lines, each one with the top and base depth
(or time) of the layer of interest, you may rasterize (interpolate) them
to cover an area. The decision on how meaningful this interpolation
will be depends on many factors.

  The ideal scenario would be to have a grid survey to be rasterized,
this would produce a result with more meaning.

  HTH


> 
> I have the following task:
> 
> I need to create a map with contour lines showing the thickness of
> one geological layer/horizon (actually not the true thickness, but
> the vertical extent is required, so if you drill down at one point,
> what is the distance between the upper and lower edge of the horizon).
> 
> The only data I have are two line features (shape files) of isolines
> with elevation values (in m NHN) of each the upper edge and lower
> edge of the horizon.
> 
> My approach would be something like this, but I can’t figure out how
> exactly to do it:
> • Create a raster from each line feature (did that, but only the
> lines are rastered, so would need a polygon to get a plane? How do
> that? How can I include the elevation values of the lines into the
> polygon?)
> • Interpolate the elevation values inbetween the isolines to receive
> a plane of elevation values (in my opinion would need a polygon to
> get a plane, but for interpolation input Points are needed? And which
> interpolation method? Also, some of the isolines are offset by
> faults, so need sharp edges in these areas, how to integrate that in
> the interpolation?)
> • Then calculate the vertical distance at each point in the area
> between the two planes to get the thickness (how, which tool does
> that?)
> • Create a raster showing the thickness throughout the area in colour
> range
> • Add contour lines showing the thickness of the horizon
> 
> Maybe there is even a better and probably much simpler way to do
> this. However, I am only a beginner in QGIS, python and statistics.
> So any help and easy to understand tips will be very much
> appreciated. I am using QGIS 3.10.11 by the way.
> 
> Best regards,
> 
> Eva
> 








  Roxo

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