Passive seismic tomography using recorded microseismicity: Application to mining-induced seismicity

Abstract

Microseismicity is recorded during an underground mine development by a network of seven boreholes. After an initial preprocessing, 488 events are identified with a minimum of 12 P-wave arrival-time picks per event. We have developed a three-step approach for P-wave passive seismic tomography: (1) a probabilistic grid search algorithm for locating the events, (2) joint inversion for a 1D velocity model and event locations using absolute arrival times, and (3) double-difference tomography using reliable differential arrival times obtained from waveform crosscorrelation. The originally diffusive microseismic-event cloud tightens after tomography between depths of 0.45 and 0.5 km toward the center of the tunnel network. The geometry of the event clusters suggests occurrence on a planar geologic fault. The best-fitting plane has a strike of 164.7° north and dip angle of 55.0° toward the west. The study region has known faults striking in the north-northwest–south-southeast direction with a dip angle of 60°, but the relocated event clusters do not fall along any mapped fault. Based on the cluster geometry and the waveform similarity, we hypothesize that the microseismic events occur due to slips along an unmapped fault facilitated by the mining activity. The 3D velocity model we obtained from double-difference tomography indicates lateral velocity contrasts between depths of 0.4 and 0.5 km. We interpret the lateral velocity contrasts in terms of the altered rock types due to ore deposition. The known geotechnical zones in the mine indicate a good correlation with the inverted velocities. Thus, we conclude that passive seismic tomography using microseismic data could provide information beyond the excavation damaged zones and can act as an effective tool to complement geotechnical evaluations.

Attenuation tomography using recorded microseismicity in a mine

“…The maximum amplitudes of all recorded microseismic events are at least three orders of magnitude less than those of the mine blasts. So, their moment magnitudes are most likely to be negative (Barthwal and Van der Baan, 2019). The Nyquist frequency for the recorded data is 250 Hz; however, microseismic events with negative magnitudes are known to have corner frequencies greater 250 Hz possibly up to few kHz (Kwiatek et al, 2011;Eisner et al, 2013).…”

“…Microseismic monitoring is often employed in underground mines to detect anomalous seismicity to ensure safe mining operations. Recent studies (Westman et al, 2012;Barthwal and Van der Baan, 2019) have performed velocity tomography using the recorded microseismicity to obtain 3D velocity models of the mines. Similarly, the microseismic recordings can be used to obtain 3D seismic attenuation models that can provide additional insights about rock properties like lithology, fracture density, and fluid contents.…”

“…The data were recorded by a network of 7 boreholes each having 4 3C geophones at a sampling rate of 500 samples per second. These data have been studied previously by Van der Baan (2013, 2015) and Barthwal and Van der Baan (2019) who give details of the microseismic data processing and mine geology. Figure 1a shows a schematic North-South cross-section through the mine.…”

“…The ore deposition is along an unconformity contact between sandstone and the underlying basement rock gneiss at a depth of nearly 450 m. Figure 1a shows many faults in the basement rock and the overlying sandstones which may have lead to ore deposition due to fluids flowing through them (Fayek, 2013). There are two vertical shafts to access the horizontal tunnel networks at depths of 420 m, 465 m, 480 m, and 500 m. Figure 1b displays the 488 microseismic events located by Barthwal and Van der Baan (2019), the receiver locations used for data recording, and the 3D distribution of mapped faults. The two largest multiplet groups namely group 1 and group 3 containing 179 and 38 events are shown by red filled spheres and the green filled cubes respectively.…”

“…Red filled spheres show the event locations of multiplet group 1, green filled cubes correspond to the event locations of multiplet group 3, gray filled spheres show remaining event locations. Event locations are taken fromBarthwal and Van der Baan (2019).…”