Re-entry Protocols for Seismically Active Mines
Rock Engineering 2009 - Rock Engineering in Difficult Conditions
The first part of the paper addresses the results of a survey on current re-entry practices at 18 seismically active mines. Some results from the survey are (Vallejos and McKinnon, 2008a): the majority of re-entry incidents are triggered by production blasts, resulting in classical Omori-style decay in seismic event rate over a number of hours. The distance from active mining that seismic events typically trigger re-entry restrictions is between 50-100 m. Over half of the mines use event decay rate analysis as their primary decision making parameter. The last of the above findings, motivated (Vallejos and McKinnon, 2008b, Submitted to Pure and Applied Geophysics) to study in detail the applicability of the Omori’s law, n(t)=K/(c+t)^p, as an equation for describing the event decay rate for mining induced aftershock sequences. The second part of the paper presents some of their results, which can be summarized as follows: the Omori equation can be adequately used to describe the decay rate of mining induced aftershock sequences. The analyzed sites presented p values that are normally distributed, most of them (98%) ranging from 0.4 to 1.6. The parameter K can be satisfactorily expressed as; K=k*N1hour, where k is an activity parameter and N1hour is the measured number of events occurring during the first hour after the principal event. The k values are in a well defined range. Theoretically k<=0.8, and empirically k?[0.2-0.5]. These findings enable the development of a real time re-entry protocol, as follows: The most probable p value can be assumed or as an alternative can be used as a normally distributed variable. Following the principal event, and after recording the number of seismic events during the first hour the value of K can be established with an associated level of confidence. Once the parameters p and K are set, the most probable expected decay event rate can be delineated with the Omori equation.
Vallejos, J., and McKinnon, .S. (2008a), Guidelines for development of re-entry protocols in seismically active mines, Proceedings of the 42st US Rock Mechanics Symposium, Golden Colorado, Paper ARMA/USRMS 08-97.
Vallejos, J., and McKinnon, .S. (2008b), Omori’s law applied to mining induced seismicity and re-entry protocol development, Submitted manuscript to Pure and Applied Geophysics Special Issue: Induced Seismicity.