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Coda polarization properties of high-frequency microseismic events

BERNARD PRICE INSTITUTE OF GEOPHYSICAL RESEARCH UNIVERSITY OF THE WITWATERSRAND WITS, JOHANNESBURG 2050, South Africa
ROCK MECHANICS LABORATORY CHAMBER OF MINES OF SOUTH AFRICA, JOHANNESBURG, South Africa
ROCKBURST RESEARCH PROJECT WESTERN DEEP LEVELS LIMITED, CARTONVILLE, South Africa

Abstract

Earthquake prediction research would benefit if a technique was available to continuously monitoring the physical characteristics of a volume of rock which immediately surrounds a seismic source. This study employs the polarization properties of microseismic events in order to estimate the volume containing the near-source scatterers associated with the coda arrivals.

A three-component accelerometer installed for research purposes approximately 180 m ahead of an active stope in a gold mine was located at a depth of 3,200 m. Microseismic events that were analyzed occurred within 300 m of the accelerometers, and these events had an average seismic moment of 10¹² dyne-cm, with an average P-wave corner frequency close to 1.5 kHz. In general, the seismograms showed a strong acceleration coda following the direct P and S arrivals. A polarization filter was utilized to investigate the linearity of these coda signals, and this in turn was used to estimate the location of the scattering sources for these P-coda waves. The small values for the deflection angles (deflection is defined relative to the backazimuth from the receiver to the source) that were recorded for the strong P-coda pulses suggested that the region of the scattering was close to the seismic source. The series of initial strong pulses associated with the P-coda waves (5 msec) resulted largely from forward scattering caused by velocity fluctuations; these variations occurred over a length greater than 1 m. After these initial pulses, a second partition of the P-coda wave consisted of backscattered energy, and these pulses were attributed to velocity and density fluctuations over a length much smaller than 1 m. Results of the analyses suggest that the volume containing the scatters can be approximated by a sphere of radius 35 m that encompasses the seismic source.

Two parameters, F~₂ and DP, were investigated, and these enabled a comparison to be made of the degree of polarization between different records. The degree of polarization F₂(t) changes from having a value one, for linear polarization, to a value zero for the unpolarized signal. The second parameter DP was defined as the ratio of the integral of the square of the ground velocity multiplied by the polarization filter [sin²(2 arctan F₂(t))] to the integral of the square of the ground velocity. This parameter facilitated the removal of the unpolarized signal from DP. The analyses of 25 events yielded an average value for F~₂ of 0.57 ± 0.06 for the P coda and 0.47 ± 0.05 for the S coda. The corresponding values for DP were 0.66 ± 0.08 for the P coda and 0.42 ± 0.11 for the S coda. The P-coda was found to have a high degree of polarization over a significant portion of the coda; this was not the case for the S coda.

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