Introduction
At an iron ore open pit mine in Southern Africa, a MSR Modular was deployed for safety critical monitoring of a pit slope above a production area. The MSR Halo, a Synthetic Aperture Radar (SAR) was deployed alongside the MSR Modular, a Real Aperture Radar (RAR) which was focussed on a specific area of interest at the time. It was found that outside of the scan envelope of the MSR Modular, ground deformation was taking place in another area of the pit. This case study outlines the data captured during the deployment period of the MSR Halo which is a 360 SAR. An image of the pit slope .dtm (DXF) and the live view synthetic map displacement data is illustrated in Figure 1 for a selected portion of the entire scan envelope, whereby only the area of interest is highlighted in this study.
Identification of the Area of Interest
For a 24 hour review period of the collected data for Area 1 (A1), transitional (acceleration and regression cycles) accumulation profiles were identified for which trend plot values are provided in Figure 2. The distance to the pit slope is 325 m at the centre of Area 1. A single point/pixel was also allocated at the centre of the concentration of movement to track the highest level of movement being measured by MSR Halo (Point 1, P1). Towards the end of the collected data, there was a notable deceleration trend. This may be due to the stop of production activity during night operations, which indicates the pit slope could be susceptible to excavation at the toe of the slope.
Note that the movement convention is positive, that is, above the zero line on the trend plot which indicates
*An observational scale is one that is outside of the alarm threshold application and shows the displacement shows and average velocity at increments defined by the user to show as much detail as is required.
Alarm Threshold Application and Trend Plot Observations
Alarm threshold values were applied based on the observed accumulation rate of displacement for Area 1 which was also applied to Point 1. The alarm thresholds were applicable for both trend plot alarm exceedance and an area threshold (6 m x 6 m = 36 m2) criterion (Figure 3).
Note that the alarm thresholds are set for a 24 hour period. The alarm exceedance instructions are an example and may be configured as per the user’s requirements and operational processes and procedures.
Based on the assessment of the synthetic map, trend plots, and the alarm exceedance warnings, an Alert Level Rating Synthetic Map was generated for this specific dataset and period of observational time (Figure 4).
A time period of 24 hours is applied to the displacement with a one hour time window utilised for the average velocity. Whilst the displacement synthetic map illustrates over 20 mm of accumulation in a 24 hour period with clear patterns of displacement data, the average velocity is below 1 mm/hr, thus, the synthetic map data illustration is less than Level 1.
The area was visually inspected after the concentration of movement had been detected by MSR Halo and data had been captured for the aforementioned time period. Tension cracks were noted on nearby berms in the vicinity of the area of interest, which was of a poor, highly jointed rock mass.
Conclusion
In summary:
- The displacement and average velocity values provided information of the anticipated accumulation and the associated speed for a 24 hour monitoring period.
- The transitional (progressive and regressive cycle) accumulation profile was identified for Area 1 indicating continued displacement, with alternating periods of acceleration and deceleration.
- The alarm threshold values provided the foundation for an Alert Level Rating scale to be applied to the synthetic map, outside of the observational scale used for visual assessment of the synthetic maps.
- Should the linear movement trend change, the alarm threshold values will need to be updated to represent a higher level of accumulation.
- The average velocity trend plot provides detailed data pertaining to the slope response as deformation is accumulated, which can be utilised to assist with operational control of the working area below Area 1.
