The object of the study was to determine whether there is a difference in the seismic response of the rockmass between 11-day mining cycle and full calendar operations - FULCO.
A literature survey was useful to gain insight in the time dependent rockmass response to mining. This helps to understand how the time of day distribution of seismic events can be affected by changes in mining operations. The literature survey did not contribute to the development of particular procedures for the difference in seismic hazard or risk between 11- day fortnight operations and FULCO.
We developed the simplistic concept of Seismic Exposure (SE). We define the hourly hazard as the average number of events greater than magnitude 1. The liability is the average number of workers underground during that hour. The average daily risk (daily SE) is the product of the hazard and the liability. The full risk for the period under consideration would then be the daily SE multiplied with the number of working days. This can be normalised by production for comparison purposes.
The general seismic hazard is described by conventional seismic hazard statistics. One advantage of a Gutenberg-Richter fit to the data is that the presence or otherwise of one large event need not influence the seismic hazard parameters. This helps to overcome a major problem with seismic hazard back analysis, namely the issue of single very large events that may or may not be included in a seismic events population because of a small change in the spatial or temporal filter parameter.
We introduced here an extension to the Gutenberg-Richter statistics by combining it with Energy-Moment statistics. Combining Gutenberg-Richter statistics, E-M statistics and empirically derived relations between strong ground motion and stress drop, the area is calculated over which strong ground motions could have exceeded a damaging threshold. This is the Potential Damage Area (PDA). The advantage of this parameter is that the hazard is expressed in a simple scalar number of m2 which allows conventional arithmetic calculations, e.g. normalising by production.
The outcome of the study is tabulated below. It shows quite variable production figures for the different cases. Since, for the deep, hard rock mines, seismic hazard generally increases with production, normalising the hazard and risk parameters by production is considered fair. We express the risk in terms of daily Seismic Exposure and the general hazard in terms of PDA. The final comparison between 11-day fortnight and full calendar operations is then done by multiplying the daily SE with the number of production days and dividing it by the total centares mined. The Potential Damage Area is expressed as a percentage of the area mined.