Velocity Modelling and Event Hypocentre Location in complex 3D velocity using Local Earthquake Tomography

In production data processing, event hypocentre locations are usually calculated by considering a homogeneous (constant) velocity within the volume of rock monitored. However, because of the presence of large excavation and significant variations in the stress distribution, the rock mass velocity is never homogeneous and its state can change rapidly. This can results in large discrepancies between the homogeneous velocity approximations and the true velocity distribution and can considerably hamper the location and characterization of mine induced microseismic activity. Local earthquake, or passive source, tomography provides an efficient way to estimate the 3D seismic velocity distribution and simultaneously refine estimates of microseismic event hypocentre locations. In addition, velocity model produced can be reused to directly locate microseismic events. Local earthquake tomography is a robust inversion method that exploits information readily available in the microseismic data, that is the P- and/or S-wave onset times. It requires no a priori knowledge of the rock mass composition and stress state and provides a comparatively easy way to estimate the 3D velocity distribution using solely seismic data. Here, we show how local earthquake tomography can be applied to calculate the 3D velocity distribution and improve event hypocentre locations using a data set obtained from automatically processing continuous microseismic recordings. We first demonstrate the capability of the method to estimate the 3D seismic velocity distribution and simultaneously correct the hypocentre location using a synthetic example. We then present and discuss results obtained from one block caving mine and one open stoping mine.
Mots clés : seismic event hypocenter location,stress,Seismicity,tomography,modelling,seismic velocity
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