Microseismic event locations and source mechanisms using dominant guided waves recorded in an underground potash mine

Abstract

Microseismic event locations and moment tensors in underground mines can provide insights into the subsurface deformation and the current state of stress. However, reliable estimation of these source parameters is rather challenging due to the high-frequency waveforms and low signal-to-noise ratio for negative magnitude events. We study microseismicity in an underground potash mine in Saskatchewan, Canada, recorded between March 1 and June 30, 2021, by a network of broadband seismometers. The active mining is carried out in low-velocity evaporites at depths of approximately 1 km below the ground level. The theoretical dispersion curves show that guided waves in the form of leaky P and P-SV/SH normal modes can exist in a 1D velocity model representing the mine geology. These guided waves are detected as high-energy dispersive arrivals on the seismograms recorded at the underground receivers. We locate the events using the arrival times of the guided waves and their mean group velocities. Most (∼80%) of the detected events cluster around the mine layout between depths of 0.95 to 1.05 km. Next, we compute moment tensors for 92 events using waveforms of guided phases. The moment tensors show non-double couple components with only 28 events having double-couple percentages greater than 50%. These events occur near the mined-out cavities with source mechanisms corresponding to layer delamination in the roof and floor or pillar yield related to the closure of cavities. No abnormal microseismicity is detected away from the mine levels in the more competent carbonate rocks above or below the evaporite formations. Thus, guided waves enable the detection of microseismic events up to large distances and can provide high-resolution event locations and moment tensor inversion. These can be interpreted in the context of local geology and mining activities to identify the dominant factors affecting microseismicity.