Tracking cave shape development with microseismic data

Abstract

Microseismic monitoring is routinely used by block and panel caving mines worldwide to track cave shape development. The standard practice is to use the cloud of seismic event locations as a proxy for cave growth below. Careful seismic sensor array planning overcomes problems with event location reliability due to seismic-ray bending around the unknown cave. However, the aseismic gap-the (unknown) zone between actual cave back and bottom of the seismic event cloud-complicates estimation of cave geometry. More information than simple seismic event locations can be used to infer the cave shape and position. Since seismic events slip according to the stresses acting locally at the source locations, an analysis of source mechanisms yields indications of principal stresses around the cave. These stresses are the result of the virgin stresses perturbed by the cave shape, and this yields information about the cave shape. The apparent seismic wave arrivals under the straight-ray assumption are retarded when rays bend around the cave. These anomalous travel times are used to create 'spaghetti'-3D curved lines indicating the likely ray path followed. Analysis of these curved rays in space and time suggests where the cave is and where it is growing. All of this indirect information is now routinely used at Cadia East mine in Australia to estimate the cave shape, helping geotechnical engineers to better manage the cave. A direct verification of the estimated cave shape with an open hole drilled to the cave back in early 2017 provided some validation of these seismic data techniques.

Testing the Utilization of a Seismic Network Outside the Main Mining Facility Area for Expanding the Microseismic Monitoring Coverage in a Deep Block Caving

“…Growth of the cave that does not conform to the mine design will have an impact on its security, safety and economic aspects. Microseismics is a proven reliable method for monitoring cave propagation [26,32,33]. The conceptual model of stress redistribution for homogeneous rock masses during the undercutting and caving processes has been agreed upon and proven by empirical data based on the results of microseismic monitoring [10,33].…”

“…This synthetic model is based on a conceptual model that has been proven by empirical data based on observations and experiences at a number of mining sites using the block caving method [10,[33][34][35]. This model has also been validated by physical modeling conducted by previous researchers [32,36]. The growth and shape of the cave due to mining activities using the block caving method should be monitored and controlled.…”

Characterisation of seismic activity at a kimberlite block caving operation in a complex geological setting in Quebec, Canada

“…The aim of this is to investigate if the active source can help identify and track changes in the cave growth. Due to the ductile nature of kimberlite, the caving process is notoriously aseismic, unlike hard rock caving mines, where the cave propagation process generates clear microseismic events ahead of the yield zone that allow the progression of the cave to be monitored fairly easily (Lynch et al 2018). The use of an active seismic source in underground mines is still fairly experimental, although the methodology has been tested in a few hard rock mines.…”