Micro-Seismic Monitoring in Mines Benefits and Limitations

Abstract

ABSTRACT: Seismic monitoring is a useful tool in understanding the rockmass response to mining. The waveforms recorded by sensors contain information not only about where and when a seismic event occurred but also about the driving stresses, failure mechanism and the intervening rockmass. It is also good to understand the limitations of this tool as they should guide the system design, overall monitoring objectives and the analysis of the seismic data. This paper discusses what information can be extracted from a mine seismic system, how this information can be useful to the mine in understanding their rockmass interaction with mining, but also the limitations associated with this information. Only then can the data be interpreted correctly. 1 INTRODUCTION Seismic monitoring in a mining environment is a unique tool, in that it is continuous in time and can monitor a 3D volume. It is also a powerful tool, in that it can monitor ground motions over multiple orders of magnitude. For example, in the velocity domain, the sensors can measure ground motions from fractions of μm/s to tens of mm/s. The commonly used magnitude scales, which are logarithmic scales, typically extend from around −2.0 to +2.5 for mining applications, which is at least four orders of magnitude. The sources of the measured ground motions can be many and will be discussed below, but the main objective is to monitor sudden inelastic deformations as a result of stress changes in the rock mass due to mining. This gives a powerful monitoring tool, but it is still just one of many tools available to rock mechanic practitioners. One of the first questions when contemplating or designing a seismic system should be "What is the objective?". Mendecki et al. (1999) describes seismic monitoring as enabling the quantification of exposure to seismicity. They then list a number of specific objectives, namely location of potential rockbursts, prevention, control, warnings and back-analysis. Identifying the objectives is important, as it will determine such details as which sensors to use, how many and how dense a seismic array is needed and what type of alerts are required. This paper will not go into too much detail about seismic monitoring systems and array design, except where needed to clarify a concept. However, a short introduction to seismic sources and sensors is useful.