OR/18/010 Post-calculation checks and remediation for cross-sections
|Terrington, R L, Thorpe, S. 2018. Post-calculation checks of GSI3D Models V2. British Geological Survey Open Report, OR/18/010.|
General calculation issues
Check the calculation does not take overly long or does not finish — occasionally this can be seen if the certain units are not calculated or take a very long time to calculate (successful calculation indicated by aterisk * in top left corner of geological unit and a pop-up box indicating the calculation has finished). Figure 2 indicates where the calculation progress can be monitored.
A number of factors could contribute to the unsuccessful calculation of a model. These include:
- Wrongly attributed units in the ID GVS column (e.g. the numbering order has been corrupted or is no longer sequential)
- Java Memory runs out (check Java console for an indication of this). This could be due to large DTM sizes (generally those above 10mb) or large image sizes within the project (those above 2mb)
Wrongly attributed lines and missing envelopes
If when viewing the calculated cross-section, the correlation line exists but does not appear to be forming a calculated unit in cross-section then it is probably one of the factors below:
- The correlation line could be unattributed with anything from the GVS list and will appear to be attributed with NN (no name) or wrongly named as in Figure 3. Please re-name the line as applicable from the GVS
- The correlation line could be wrongly attributed from the list in the GVS and therefore not match the envelope defining its 2D distribution. Please re-name as applicable
- The envelope for the correlated line could be missing as in Figure 4. Please draw the necessary envelope required to match the correlation line in the cross-section
Envelopes not fully extended
For each of the units the cross-section correlations must be covered by an envelope for it to calculate and form a surface. Figure 5 shows an example where the geological unit has not been calculated to its full extent on the cross-section. Where this occurs, envelopes must be extended to cover the extent of the correlation on cross-section or the cross-section must be investigated to see if the correlation should actually exist there.
Lenses must have a base and top correlated in cross-section for the calculation. Only the Top lense needs to exist in the list of the geological units and have the envelopes drawn for it. A reason why a lense will not calculate is because the correlation lines are not tightly linked together in cross-section (i.e. the area in which the top lense line meets the base lense line). The envelope for the lense must also be tightly wrapped around the cross-section correlations. The snapping functions in GSI3D do not exist at present for snapping lense to cross-sections to lense outcrops so this must be done eye.
If the lense still does not calculate please check the GVS to make sure the top has a negative ID and the base a positive ID with the same number (e.g. Top Silt lense = -450, base silt lense = 450)
Below is an example of a calculated model section showing a lense that hasn’t been calculated correctly (Figure 6) and the areas in the map window and cross-section window to check the tightness of the correlation lines.
Missing envelope extensions
A common error that occurs is where the envelope has not been extended far enough to the end of where it has been correlated in cross-section. Figure 7 shows an example where the blue (water) envelope needs to be extended so the unit is calculated to its correct extent. This can be remediated by simply extending the envelope over the area of correlated section or changing the extent of the correlation in section
Missing correlation lines on section or missing ‘holes’ in the envelope
In some of the cross-sections, the geological unit may appear to be incorrectly calculated or have an incorrect geometry due to a correlation line not actually existing for that unit in cross-section but where an envelope does exist for that unit. The reasons for this include:
- Missing ‘holes’ in the envelope where the correlation has stopped mid-way through the section but the envelope still exists over this area Figure 8
- An envelope has been extended too far beyond the end of the section as in Figure 9
- The correlation needs to be added to the cross-section where the envelope exists in that area Figure 8 and 9
To remediate this issue, the surrounding geological data must be assessed and that is why either of these scenarios could exist as seen in figures below.
A common problem with thinner geological units (e.g. head, alluvium) is where there are not enough nodes on the cross-section correlation for the triangulation to calculate a full distribution of the unit. This will be shown in cross-section as ‘spiked’ triangular projections where the unit should be flat and continuous (Figure 10).
This can be remediated by adding nodes on the correlation line at the base of the spike/triangle where it spikes upwards as shown by the arrows in Figure 10. There is a Densify Line tool in GSI3D that can help you fix this quickly but this should be used with care as it makes future editing cumbersome and calculations take longer.
Further testing of the model the model triangulation can be achieved by creating new sections in the model and viewing the synthetic outputs. This will help indicate where ‘Helper’ sections might need adding, or where the calculation is poorly constrained.