Open Pit Mining

Micro-seismic Monitoring of slopes

Careful geotechnical monitoring of open pit slopes has become important in modern planned-risk designs. Micro-seismic monitoring of such slopes can record and quantify detectable signatures of fracturing, located in 3-D space.

Potential Benefits of Micro-seismic monitoring of slope stability

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 micro-seismicity. 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 3-D picture of the rock mass is obtained unlike the 2-D 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.

Practical Implementation

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 3-dimensional spread of seismic sensors around the volume of rock that is of interest.

Usually two sensors are installed into each hole. These sensors are connected to a StandAlone QS data logger at the top of the hole. This station either communicates with a central computer via licence-free microwave radios or records data onto local hard disks and can be powered with a small solar panel.

Communications

Off-line
The local hard disks of each of the GS's are connected to the PRTS either in the field or at the office and seismic data is downloaded. Seismograms of common events are post-associated for off-line processing and interpretation. This hard disk medium is the cheapest form of communication for applications where real-time monitoring is not essential.
On-line
Licence-free microwave radios and repeaters can be used to permanently connect the GS network to the central site.

Power

The low power consumption of the GS (less than 1W/channel) allows use of an inexpensive solar panel and battery unit, making the seismic station completely independent.

Seismic Sensors

Each GS data acquisition unit can monitor and pre-process up to six seismic signals, in any combination of uni-, bi- or tri-axial seismic sensors. For each seismic channel up to 3 logical channels may be extracted with GS technology. Both geophones and accelerometers (piezoelectric and Force-Balance) are currently supported.

Seismic Data Processing & 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 analyzed 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.

This page last updated: January 2007