Stress-Release Seismic Source for Seismic Velocity Measurement in Mines

Abstract:

Accurate seismic event locations are needed to delineate roles of mine geometry, stress and geologic structures in developing rockburst conditions. Accurate absolute locations are challenging in mine environments with rapid changes in seismic velocity due to sharp contrasts between individual layers and large time-dependent velocity gradients attending excavations. Periodic use of controlled seismic sources can help constrain the velocity in this continually evolving propagation medium comprising the miners’ workplace. With a view to constructing realistic velocity models in environments in which use of explosives is problematic, a seismic source was developed subject to the following design constraints: (i) suitable for use in highly disturbed zones surrounding mine openings, (ii) able to produce usable signals over km-scale distances in the frequency range of typical coal mine seismic events (~10-100 Hz), (iii) repeatable, (iv) portable, (v) non-disruptive to mining operations, and (vi) safe for use in potentially explosive gaseous environments. Designs of the compressed load column seismic source (CLCSS), which generates a stress, or load, drop normal to the surface of mine openings, and the fiber-optic based source-initiation timer are presented. Tests were conducted in a coal mine at a depth of 500 m (1700 ft) and signals were recorded on the surface with a 72-ch (14 Hz) exploration seismograph for load drops of 150-470 kN (16-48 tons). Signal-to-noise ratios of unfiltered signals ranged from ~200 immediately above the source (500 m (1700 ft)) to ~8 at the farthest extent of the array (slant distance of ~800 m (2600 ft)), suggesting the potential for use over longer range. Results are compared with signals produced by weight drop and sledge hammer sources, indicating the superior waveform quality for first-arrival measurements with the CLCSS seismic source.