1. Back Analysis on Mag 2.4 along De Hoek Structure Moab Khotsong Mine
CF Moller
18 September 2009

2. Agenda
Background
Location of Moab Khotsong Mine
Mining Layout (Scattered Mining)
Problem statement
Calibration Results
Varying K ratio
Horizontal stress rotation
Could it have been a Zuiping Structure?
Vertical stress rotation
How was the analysis used?
The way forward

3. Location

5. Mining Layout

6. Isometric View of Current Mining operations

7. The Problem - Location of Magnitude 2.4 Seismic Event??
De Hoek Fault
Local Mag 2.4 event 19 April :43:13
Current Mining

8. The Problem - Location of Magnitude 2.4 Seismic Event??
De Hoek Fault

9. Calibration Methodology
K-ratios of the stress field were varied systematically.
Selected horizontal and vertical stress was rotated
k-ratios will be referred to as XxxYyy were xx will be the k-ratio in the x direction and yy the k-ratio in the y direction.
The x and y reference in the k-ratio is related to the South West Down coordinate system.
The objective of this exercise was to establish why a M2.4 event occurred on this structure. To this end a calibration exercise using the numerical modelling was done where both the k-ratios of the stress field were varied systematically. When this had been completed the direction of the horizontal and vertical stress was rotated to obtain the best fit (both for the magnitude and the spatial position) for the seismic event that occurred.
The vertical stress tensor was maintained at but the k-ratio was varied in the x and the y direction. σH (y direction) was changed from 0.5 to 0.9 and σh (x direction) changed from 0.5 to 0.1. Next using these selected k-ratios the direction of σH was rotated. This was done to quantify whether rotation of the horizontal stress field would influence the size and position of the positive ESS lobes. σH rotated to 3200, 450 and 230 using k-ratios identified in the previous exercise. In addition to this the maximum principal stress was direction was rotated to normal to reef for 00,100, 220 and 450rotation in the σH direction.
For brevity the different k-ratios will be referred to as XxxYyy were xx will be the k-ratio in the x direction and yy the k-ratio in the y direction. The x and y reference in the k-ratio is related to the South West Down coordinate system used on Moab Khotsong Mine

10. Results from K Ratio Rotation
25 models where run and the ESS data stripped and represented in a column graph of weighted ESS. Y was varied between 5-9 and X was varied between 1-5. This was done because all evidence points to a north south direction for sigma 2. Reduction in k ratio results in an increase in ESS for X and Y
Evaluate all possible K ratios to determine the worst case scenario.

11. Results from K Ratio Rotation
Five good correlations
All discarded due to poor correlation with underground stress measurements
K-ratio was approximately 0.78 for σ2.
What to do know?
The actual direction of σ2 was not well understood Due to the k-ratio of X03Y09 and X03Y07 were chosen. The spatial correlation for both these ratios is good but the magnitude correlations were poor and for this reason and the fact stress direction was not well understood the direction of σ2 was varied in an attempt to get a better magnitude correlation.

12. Results from Sigma 2 Rotation

13. Results from Sigma 2 Rotation

14. Results from Sigma 2 Rotation

15. Results from Flat Dipping Zuiping Type Fault
Having established that a seismic event could have resulted from only the De Hoek structure other Jersey type structures where looked at as a possible source for the seismic event.
It was found that the source of the event plotted in line with the structure but according to the Geologist on Moab Khotsong Mine none of the structures extend beyond the De Hoek Fault.
The contact of the shallow dipping fault was found to be too far from the source of the seismic event to be responsible for the event. No positive ESS lobes were found along this structure.

16. Results from Rotating Sigma1 Normal to Reef
There are some good correlations
Underground investigation indicates a vertical to reef orientation for sigma 1
Rotation of sigma 1 normal to reef results in reduction of ride lobes
Although some of the results have apparently good correlations with the magnitude 2.4 seismic event that occurred along the De Hoek Fault the direction of σ1 was kept at vertical and not rotated to normal to the reef as indicated by the Groundwork report.
The σ1 orientation was rotated normal to the reef for X03Y07, X03Y08 and X03Y09 for the un-rotated σ2 directions from 3.1 and the rotations (100, 220 and 450).
The results from rotating σ1 to normal to the reef showed an expected reduction in the modelled magnitude of possible events due to the clamping forces created by the increased normal stress along the De Hoek Fault that dips and strikes sub parallel to the reef.

17. Results from Rotating Sigma1 Normal to Reef

18. Results from Rotating Sigma1 Normal to Reef

19. Results from Rotating Sigma1 Normal to Reef

20. K-ratios provide a reasonable explanation
Conclusion
K-ratios provide a reasonable explanation
Rotation of the stress field provides a better solution
Large Lobe?
σ1 direction was normal to the reef plane.
Most likely stress field existing on the vicinity of the De Hoek fault to be X03Y07 with a 450 rotation and σ1 normal to the reef plane
The variation in the k-ratios does to some extent provide a reasonable explanation for the seismic event that occurred. However the rotation of the stress field provides a better solution with respect to the spatial position of the event and the magnitude of the event. This solution, although plausible does however indicate a large lobe to the west of the m2.4 event position where no seismic event occurred.
Due to this further investigation was done and it was found that stress mapping done at Moab Khotsong Mine indicated that the σ1 direction was normal to the reef plane. Using this information the four possible scenarios for the seismic event identified above where modified to simulate these scenarios with σ1 direction normal to reef plane.
The results for both X03Y07 and X03Y09 with 450 rotations horizontal rotation with σ1 normal to the reef plane indicate that the lobes to the west have decreased while still providing a reasonable explanation for the M2.4 seismic event. When taking this into account and 0.78 measured k-ratio described in appendix 2 it can be concluded that the most likely stress field existing on the vicinity of the De Hoek fault to be X03Y07 with a 450 rotation and σ1 normal to the reef plane

21. Life of Mine Model of Moab Khotsong

22. Ride Results from LOM Model
Area of interest

23. Points of Interest
It was found that the potential for seismicity along the De Hoek Fault is larger than originally thought. It is recommended that dynamic modelling be done for all seismic events along the De Hoek Fault that has the potential for damage.

24. The way forward
Dynamic Modelling of points of interest
High resolution seismic monitoring (9 accelerometers)
Japanese project
Strain measurements
Acoustic emission sensors
Installation of a seismic source

25. The way forward