Geological interpretations are customarily done on section plots using coloured pencils. These are then digitised into a
computer where the
interpretation may be modelled, then plotted again for verification or fine tuning, then digitised
again etc. The problem with this method is that it often results in double handling of data plus time being spent plotting
sections and copying or redoing an interpretation in various
section and plan orientations. It is often very difficult to visualise the orebody until the
interpretation has been completed, and then if it is perceived to be incorrect you have to start again. Another drawback of this method is that the geologist is always working in section planes to create the
interpretation, however the drillholes being used are
generally not in the same plane, resulting in an interpretation that only approximates the true geometry of the orebody. A
typical orebody interpretation using this method could take weeks or possibly even a few
months to be completed.
A quicker method would be to create the interpretation
onscreen using all the
information normally plotted. This negates the need to create a
lot of section plots until the
interpretation is completed. The geologist could rotate the
information to gain a true three dimensional picture of the data to be interpreted. This aids immensely in visualising the orebody before any interpretation has begun. The interpreted
strings may be snapped precisely to the drillholes giving a much more accurate approximation of the true geometry. The biggest saving comes in the time spent creating and fine tuning the interpretation. What once used to take weeks or months can now be done in days.
There are drawbacks to the onscreen interpretation method however. The first is that you may not be able to display all the information normally shown on a plotted section - it could degrade the performance of the computer to a frustrating degree. (The
increase in desktop computer performance means this is becoming less of an issue.) A bigger problem is that so much
information could clutter the screen making it difficult to
interpret the information.
Therefore you must learn to work with only the information
required for the particular task, and later display any other
information needed to complete the interpretation. The biggest problem is coming to terms with working in three dimensions
rather than in section planes or benches. Once this is overcome
however, the benefits in productivity are immense.
In this section we will look at the mechanics of onscreen
interpretation.Wewillrestrict ourselves to working in slices of spaceforthesakeofvisibility, and we will be interpreting in three dimensions. The method is simple but it does take some getting used to.
Creating the +nterpretation
To begin we must first load the drillholes onscreen and restrict our view to a slice of space
paralleltoadrillsection.
Exercise 2! – Creating an Interpretation
1. Load the database
thordemoa.dhd. Display the hole by LITH.
2. Load the layer SECTION. 3. Change view to one of the
section lines,i.e.select View>Create Section.
4. Set the slice width to 60 and select the section plane by line.
5. Select OK.
Figure 7-41 Create Section View panel
6. Select the line to change view.
7. Zoom in on the area of
interest. Your screen should look something like that shown below.
Figure 7-42 Screen image of cross section through
drillholes 8. Display a legend in a
convenient position to one side of the screen. Select Analyse>Display Legend.
Select DRILL and LITH.
Figure 7-43 Drillhole cross section and legend
9. Begin digitising one of the ore zones using the features we created earlier and
snapping to the lithological boundaries on the drillholes,
i.e. use the snap to point
icon. To use a feature for digitisingselect
Design/Create/Feature, and select the feature.
Figure 7-44 Snap to Point icon
10.Initially you should have
something that looks like the screen image below. The interpretation has been
snapped to the drillholes but it does not go past the outer drillholelimits.
Figure 7-45 First step of interpretation
11.To tidy up the ends of the interpretation we use one of the standard ENVISAGE
tools, namely
Design>Point Insert>Insert. 12.Select Design>Point
Insert>Insert. Select the object. Select the line to replace, i.e. the interp end, Figure 7-6.
Figure 7-46 Screen image of line which will have points
inserted
13.Once the insertion plane has been defined insert the
required points. Remember to insert points from S to E otherwise you will create crossovers in the string.
Figure 7-47 Screen image of inserted points
14.Cancel when finished
inserting points. Select Do not interpolate W. This
relates to the W-value and is irrelevant. The points will connect to form a high- lighted line. Selectretain.
Figure 7-48 Image of connected points
15.You will then be prompted to select the next line to
operation until the interp string is finished.
16.When finished you should end up with something like the screen image below.
Figure 7-49 Screen image of completed digitised interp
string
17.This looks OK but it can be made better by smoothing the line and inserting points at a given interval. This will also aid in the development of solid models later. To
smooth the string we apply a cubic spline using
Design/Attribute Edit/Apply Spline.
Figure 7-50 Screen image of splined string
18.The string now looks more natural, but this spline is only superimposed on the originalline. Ithasnot inserted any new points. Therefore we need to insert
extra points along this new line to maintain the shape when modelling. To do this
select Design>Attribute Edit>Insert Points On
Spline, . Set the step to be 5. Select the object to
insert points. The object will be highlighted showing the
resulting string. Select retain if it looks OK.
19.If you were to label the
points now you would see a great many more than we digitised. Trythis,notice the original points have not been altered.
We have now finished the first string in our interpretation. Reset the screen and rotate to view the final string, especially
the ends. You would now do much the same for all the other ore zones. You may find it easier to complete one section before moving to the next or you may want to follow an ore zone
through each section, it’s up to you.
Almost certainly you will be
required during the course of an interpretation to use most if not all of the other point editing options. It is a good idea to become familiar with each of
these so that you can choose the best option to use for each task. There is generally no right or wrong way to do something in
ENVISAGE but there are
certainly better and worse ways todoit.
Exercise 2# – Completing Interpretation
Try to complete the
interpretation you have started or add to the one supplied. Later we will use this
interpretation to create a solid model.