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

Fri Nov 6 06:38:29 PST 2020

On Thu, 5 Nov 2020 14:20:18 +0100, fantypus at web.de wrote:

> 
> Hi Fernando M. Roxo da Motta,
> 
> Thanks for your answer. Maybe I wasn’t clear enough. The input data
> is not just two lines, it is two sets of line features. One set of
> elevation isolines for the top level of the horizon and one for the
> bottom level of the horizon, covering a small area of few square
> kilometres (i hope you can see the attached picture). This is all the
> data I have available. I already managed to get the vertical
> thickness of the layer, but I am still struggling with the
> interpolation going across tectonic faults and no data gaps as
> mentioned above. Do you have any experience in that matter?

  Hi Eva, I really didn't understand completely your problem at first,
but I think I do now with this image you sent.  😊

  The major problem, as I see, is the interpolation across boundaries,
being it geologic faults or no data area.

  I really don't have experience in this area, my experience is with
data acquisition and processing.  Never worked on interpretation. 

  I believe, although not really sure, that you may be able to
constrain the interpolation within the outer polygon or that it may
even not be necessary. At the very least, once interpolated the area
outside the boundaries may be clipped out using one of many clipping
tools available in QGIS.

  In the way I see it, the most difficult would be to get the fault
lines (the red ones in image, I suppose) as hard boundaries that would
block the interpolation across.

  Just brainstorming, if there is a way to restrain the interpolation
only within the boundaries of the outer polygon, perhaps the fault
lines could be transformed in very thin "holes" in the outer polygon(s)
in order to stop the interpolation there.

  I think that for geologists, and possibly other professionals, the
possibility to use specific lines as hard boundaries for interpolation
would be very useful.

  Sorry to not be of any help.  😞

> 
> Regards,
> 
> Eva
>  
>  
> Gesendet: Mittwoch, 04. November 2020 um 14:08 Uhr
> Von: "Fernando M. Roxo da Motta" <petro at roxo.org>
> An: qgis-user at lists.osgeo.org
> Betreff: Re: [Qgis-user] How to get the thickness of a geol. layer
> from elevation isoline features 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
> 
> --
> ---------------- Non luctari, ludare -------------------+ WYSIWYG
> Fernando M. Roxo da Motta <petro at roxo.org> | Editor?
> Except where explicitly stated I speak on my own behalf.| VI !!
> PU5RXO | I see text,
> ------------ Quis custodiet ipsos custodes?-------------+ I get text!
> 
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  Roxo

-- 
---------------- Non luctari, ludare -------------------+ WYSIWYG
Fernando M. Roxo da Motta <petro at roxo.org>              | Editor?
Except where explicitly stated I speak on my own behalf.|  VI !!
                PU5RXO                                  | I see text,
------------ Quis custodiet ipsos custodes?-------------+ I get text!