Imaging technologies are routinely used within the medical field yet have not been adapted for use in the study of the earth. Of particular consequence is the study of seismicity, either naturally occurring or triggered by human activity. The rock mechanics community lacks a well-founded tool for imaging and assessing the potential for seismicity within a rock mass. Seismicity is a result of unstable equilibrium due to increased depth of excavation and/or increased extraction ratio. Both of these phenomena result in high-stress concentrations. Double-difference tomography, a recently developed technology, has been used by the PI to image stress redistribution within a rock mass more clearly than ever before. It is hypothesized that by combining an understanding of expected stress redistribution (through lithologic mapping, lab testing, and numerical modeling) with observed stress redistribution (from double-difference tomography) a forecast of subsequent seismicity can be made. This project will test this hypothesis using data from a deep, underground nickel mine.
Because this project is based upon a very novel hypothesis that has potential to transform existing abilities, the proposal is particularly well-suited for an NSF Early-concept Grant for Exploratory Research (EAGER). One graduate student will be supported by project funding and results will be disseminated through journal papers and conference proceedings.
Imaging technologies are routinely used within the medical field yet have not been adapted for use in the study of the earth. Of particular consequence is the study of seismicity, either naturally occurring or triggered by human activity. The rock mechanics community lacks a well-founded tool for imaging and assessing the potential for seismicity within a rock mass. Seismicity is a result of unstable equilibrium due to increased depth of excavation and/or increased extraction ratio. Both of these phenomena result in high-stress concentrations. Double-difference tomography, a recently developed technology, has been used by the PI to image stress redistribution within a rock mass more clearly than ever before. It is hypothesized that by combining an understanding of expected stress redistribution (through lithologic mapping, lab testing, and numerical modeling) with observed stress redistribution (from double-difference tomography) a forecast of subsequent seismicity can be made. This project tested this hypothesis using data from a deep, underground nickel mine. Seismicity at a deep hard rock mine can be a precursor to ground failure events. Seismicity data can be used in double-difference tomography, which produce tomograms showing velocity distributions in the rock mass that can be used to infer relative stress of the rock mass. The data set used for the double-difference tomography inversion was from Creighton Mine in Sudbury, Ontario, Canada, and consisted of two months of data averaging 150 microseismic events per day. Three separate studies were conducted to evaluate the applications of double-difference tomography on a deep hard rock mine. These studies produced mine scale tomograms, stope scale tomograms of two active stopes, and stope scale tomograms for a cluster of events. TomoDD was used for the tomographic inversion, with other commercial programs used to view the results. All three studies produced results consistent with prior mine knowledge and basic concepts of rock mass stress redistribution. Mine scale tomograms accurately displayed a low velocity where the mined ore body is known to be with adjacent high velocity, stope scale tomograms of the two stopes both correctly demonstrated a low velocity relaxed zone near the stope following a production blast, and stope scale tomograms of an event cluster displayed consistency in results for two clusters in periods before, during, and after each cluster. The three studies show that double-difference tomography is a promising tool for observing rock mass stress redistribution that provides a baseline evaluation for the potential uses of the technology in a deep hard rock mine. The overarching goal of the research is to provide a tool that can augment both numerical modeling results and underground geotechnical measurements to allow the mine operator to produce in the safest and most efficient manner possible.
