One of the articles appearing in IM‘s October issue looks at deeper open pits, and slope monitoring in particular. Micro-seismic monitoring of open-pit slopes can record and quantify detectable signatures of fracturing, located in 3D space, ISS International reports.
ISS says the potential benefits of micro-seismic monitoring of slope stability include:
1. Micro-seismic activity of the slope as a whole, when taken as a proxy for macroscopic slope stability, yields a useful indication of when particular mining activities are affecting slope stability, and by how much
2. Micro-seismic monitoring can give an indication whether a particular known geological structure is seismologically active or not. If many seismic events occur on or near the structure, then it is likely that the structure is slipping. Indeed, planes of weakness defined by seismic events may indicate previously unknown geological structures that may be confirmed by other geotechnical means
3. From the seismic event locations and sizes, it is possible to infer relative surface movements resulting from this microseismicity. Surface movement is also recorded by conventional surveying of prisms. Comparisons between these two forms of data has revealed that the seismic data may be able to indicate regions of surface movement 30-45 days before these movements are seen on the surface. This delay would depend on rock properties and the locations of the seismic events.
“The usefulness of micro-seismic monitoring stems from the fact that cracks are located wherever they occur, and so a 3D picture of the rock mass is obtained unlike the 2D picture obtained with conventional surface monitoring. It is not so much a short-term slope-failure warning technique (like a trip wire), but rather a system for longer term understanding of where rock weaknesses are occurring.”
Modern micro-seismic arrays are installed behind specific slopes in long inclined holes. The holes are either drilled from access points on the slope (ramps or berms) or, if necessary, from outside the pit. The design of the array must result in a 3D spread of seismic sensors around the volume of rock that is of interest.
For seismic data processing and interpretation, the seismograms of every seismic event that triggers multiple stations are transmitted to the seismic controller and recorded on a user-specific computer on the Local Area Network (LAN). The seismic data is regularly transferred to ISS’s offices. Here the seismic data is processed and quality controlled according to ISO 9001 procedures. The seismic data is then analysed and a standard report and Jdi project regularly submitted to the mine geotechnical engineer. In this way all seismological tasks are accomplished off-site leaving the geotechnical engineer free to do his job.
Jdi is ISS’s package for advanced visualisation and analysis in the Microsoft Windows and Linux environment. Jdi was originally developed as a tool to facilitate the integration of modelled and seismic data but has grown to incorporate many features which are more generally applicable.
SoftRock Solutions is another specialist in slope monitoring and recently successfully completed trials of the new Trimble S8 super fast monitoring instrument and reports that “this instrument has a long range capability which will take over from our estimed Geodimeter ATS.
“Trimble FineLockTM technology is a smart tracker sensor with a narrow field of view. A Trimble S8 Total Station equipped with FineLock technology will detect a target without interference from surrounding prisms, achieving more reliable accuracy and greater density of mounted targets. FineLock technology is also offered as a Long-Range FineLock option reaching up to 2,500 m with 1 cm accuracy.
“The Trimble S8 Total Station is built on Trimble’s most advanced total station platform It offers 1” angular accuracy and EDM precision of 1 mm + 1 ppm, plus the best features available from technology today to ensure unsurpassed efficiency, productivity, and profitability.
“Trimble MagDriveTM servo technology: Survey or monitor targets up to 40% faster. Frictionless motion reduces wear and tear for worry-free 24/7 operation and less maintenance Silent movement ensures unobtrusive operation in urban or residential settings
“Trimble MultiTrackTM technology: Choose between passive and active tracking. The Trimble MultiTrack Target ensures you always find and lock to the correct target fast. Nearby reflective surfaces, including other prisms, will not disrupt jobs.”