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Source characterisation of microearthquakes in a fractured rock environment: A case study of Lift 2, Northparkes m

Source characterisation of microearthquakes in a fractured rock environment: A case study of Lift 2, Northparkes mine, Australia. George Begley 21/05/2010. Supervisors: James Wookey & Andreas Wuestefeld. Overview. Seismicity in mines, and seismic monitoring Source characteristics

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Source characterisation of microearthquakes in a fractured rock environment: A case study of Lift 2, Northparkes m

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  1. Source characterisation of microearthquakes in a fractured rock environment: A case study of Lift 2, Northparkes mine, Australia George Begley 21/05/2010 Supervisors: James Wookey & Andreas Wuestefeld

  2. Overview • Seismicity in mines, and seismic monitoring • Source characteristics • Introduction to Northparkes mine • Data processing and analysis • Summary

  3. Seismicity in mines • Microseismicity – broad term describing ~ML < 0 events, common in operating mines. • Represent a very low percentage of total energy released, but >99% by number of the total events • Less common larger, ~ML > 0 events that have the potential to cause damage. • Microseismicity – a mechanism to monitor a mine between the large events. • Recognising patterns of microseismicity leading up to the large events.

  4. Lift 1 Depth (m RL) a b Lift 2 Northing (m) Easting (m) Easting (m) passive Seismic monitoring • Many mines cover a large spatial extent, and can be complex in design. • Sources of opportunity rather than induced blasts. • Main objective is hazard analysis. • 14 seismic sensors returned useful results at Northparkes, the 3D array is displayed below

  5. Source characteristics • Location – distribution of events with relation to the area of active mining, as well as the different lithologies present. • Magnitude – spatial and temporal trends may provide information about the occurrence of (ML > 0) events, and more broadly about the evolution of the mine. • b-value – analysis of magnitude-frequency laws provide information between the magnitude and the total number of earthquakes in a given region ML = Log10 A (d) - Log10 A0 (d).

  6. Northparkes mine • Located in New South Wales Australia, ~ 300km NW of Sydney • Block caving operation mining a 24 million tonnes copper/gold reserve. • 4 main rock types: Quartz Monzonite Porphyry, Volcanics, Diorite and Biotite Quartz Monzonite. • Lift 2 is mining the E26 ore body expected to yield 600,000 tonnes of copper metal, and 20 tonnes of gold. Dataset • 13,232 events collected between 1st June 2004 – 28th February 2005. • Magnitude range from -2.7 ≤ ML ≤ 2.0, (House et al., 2000) Scott, K.M., 2005 (Hudyma et al., 2005)

  7. Lift 1 Lift 1 Lift 2 Lift 2 Magnitude distribution • Majority of ML < 0 events are located between the two lifts, within close proximity of active mining • ML ≥ 0 events are far more widely dispersed, and tend to occur away from the active mining. • Is this suggestive of different source mechanisms? Lift 1 Depth (m RL) • 14 out of 20 largest events, M ≥ 1.0 events, can be seen to occur within close proximity to geological contact Lift 2 Easting (m)

  8. Temporal trends in the frequency and magnitude of events Cumulative number of events Number of events Date Cumulative number of events Local Magnitude Date Production Date

  9. The Guternberg-Richter or Bi-modal Distribution, which is it? Linear Gutenberg-Richter model Bi-modal distribution model Log10 Frequency Log10 Frequency b-value = 1.31 b-value = 1.31 b-value = 1.17 Local Magnitude Local Magnitude Equation of line - logN = a -bML For very large regions there is no doubt that earthquakes of all magnitude scale according to the linear power law. Two populations of events however are not unexpected with seismicity in mines.

  10. Log10 Frequency b-value = 1.25 2nd b-value = 0.49 Local Magnitude Magnitude-frequency plot for Northparkes mine • Clear bi-modal distribution • Indicative of two distinct populations of seismic events • Type A, and Type B events • Likely indicative of two different source mechanisms. • 1.2-1.5 common value range when associated directly with mine blasting (Legge & Spottiswoode, 1987). • Very low b-values are often representative of fault-slip related seismicity, can be less than 0.8 (Hudyma et al., 2005). • Qualitatively linked to: the heterogeneity of the source material (Mogi, 1962), the state of stress (Scholtz, 1968) and changes in the fracture mechanism (Meredith & Atkinson, 1983). All 13,232 events Sensitivity of mine Has this always been the case for the mine, or has the mine evolved with time?

  11. Results for Northparkes mine 13/09/2004-29/10/2004 Unsettled production due to failure and maintenance 01/06/2004-13/09/2004 Pre-production to early production Log10 Frequency Log10 Frequency Local Magnitude Local Magnitude Log10 Frequency 29/10/2004-28/02/2005 Full production and connection of Lift 1+2 Local Magnitude

  12. Summary • Spatial-magnitude observations are indicative of 2 separate populations of seismic events, “Type A”, small events close to the mine face and “Type B”, larger events further away from the mine face. • Temporal trends compared with production levels support this, indicating that “Type A” events are directly related to mining activity, where as “Type B” events are not. • Magnitude-Frequency plots identify 2 separate populations of events also likely indicating two different source mechanisms for “Type A” and “Type B” events. • Not the case for the whole life of the mine, “Type 2” events only occur after a period of prolonged extraction, with a gradual introduction. • Introduction of “Type 2” events is likely related to the re-distribution of in-situ stress as a result of mass excavation. Movement occurs along pre-existing planes of weakness. • When regional de-stressing becomes too rapid for the energy release to remain gradual, relatively high magnitude (ML ≥ 0) events occur with movement along pre-existing planes of weakness (faults, discontinuities, geological contacts).

  13. N N Before November 2004 From November 2004 onwards (a) 0 100 200 0 100 200 (b) E-W E-W m Key -Location of events before November 2004 -Areas of event concentration from November 2004 onwards - Stress field ? ? m Two type seismicity, the onset of which is caused by the re-distribution of in-situ stress • Green circles represent “Type A” event and are present throughout the dataset • Red circles represent “Type B” events, introduced from mid October 2004 • With investigation into fracture orientations it may be possible to indentify the change in stress regime.

  14. Used to predict the return period, ad occurance rate of larger events from smaller events. • Event rates are highly sensitive to the b-value with a small error in the b-value leading to a large error in the prediction of larger events B = 1.0 B = 0.9 b-value 10 ML ≤ +2.0 events 10,000 ML ≤ -1.0 events 20 ML ≤ +2.0 events

  15. Block caving • Block cave mining is a mass mining method that allows for the bulk mining of large, relatively lower grade, ore bodies. • In general terms block cave mining is characterized by caving and extraction of a massive volume of rock which potentially translates into the formation of a surface depression Caldwell, J., 2005

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